WO1989007257A1 - Fiber optic beam-imaging apparatus and method - Google Patents

Abstract

Apparatus for generating a reduced-scale optical image of an x-ray beam pattern or the like. The apparatus includes an optical fiber assembly (38) which forms a two-dimensional input array and whose output ends form a reduced-scale two-dimensional output array. A beam converter (34, 36) in the apparatus converts the irradiation beam (29) to a corresponding electron beam, and this beam is converted to a two-dimensional light image of the beam which is directed onto the input array. In one embodiment, the optical fibers form a fiber assembly (38) constructed of an array of multifiber reducers which are tapered on progressing between input and output ends.

Description

FIBER OPTIC BEAM-IMAGING APPARATUS AND METHOD

Field of the Invention

This invention relates to diagnostic imaging devices. More particularly, it relates to such imaging devices for producing a real-time image from a high-energy irradiating beam directed through an object. In other aspects, this invention relates to tapered cross-sectional area fiber optic elements and a system for their fabrication as well as a system for forming clad plastic preforms from which they may be fabricated.

Background of the Invention

This invention is particularly well suited to external beam treatment verification, low-energy x-ray diagnostic applications, and quality assurance in radiotherapy. The discussion and description which follows is specifically directed to this application. However, it will be understood that the invention is also applicable to other imaging systems, such as imaging mechanical parts for quality control or inspection. Other applications presently using film to image x-ray or charged particle beams are also applicable.

In radiation therapy, a beam modifying block which absorbs the treating radiation is placed between the patient and the radiation source. This block has an aperture or port which allows the radiation to be directed directly at the patient in a limited treatment region. Errors in the placement of the port or the alignment or intensity of the radiation source can result in suboptimal treatments. It is therefore desirable to obtain feedback on the location of the port and on the actual dose of the radiation applied to the patient. Conventional feedback is provided by the use of radiographic film exposed for a short initial duration for localization, or for the entire duration of treatment for verification. This is a popular method because conventional treatment tables have a space usually limited to four to six inches between the platform on which the patient lies and the radiation-absorbing shield or beam stopper disposed opposite the radiation source to absorb the radiation extending beyond the patient.

The use of radiographic film has several limitations. Among them is that the resulting image is of poor quality. Further, a significant amount of time is involved in the processing of the exposed film to produce an image of the treatment radiation, as well as the time required by the treating physician to study the film, before deciding on what corrective or compensating action should be taken. In the localization mode, the approach cannot ensure that there is no further misalignment during treatment. Certainly, in the verification mode, if the imaged treatment involved errors, those errors already exist and cannot be neutralized. This deficiency is further compounded because the portal film verification process usually limits such a procedure to once a week. It is thus desirable to provide real-time feedback so that any desired corrective measures can be taken rapidly to avoid exposing the patient to a full treatment radiation dosage that is incorrect.

Several such systems have been proposed or developed. For instance, a real-time feedback system was generally proposed by Hubener et al. in "Computed Tomography in Radiotherapy Treatment Planning: Necessity or Pseudo-Accuracy?", British Journal of Radiology, Sup plement 15, March 28-30, 1979, pp. 186-189. They suggest the use of a detector array for measuring the absorption of radiation passing through the patient, similar to the CT detector system. No specific structure of the detector array is proposed.

Other systems have been more recently proposed. Leong in "Use of Digital Fluoroscopy as an On-line Verification Device in Radiation Therapy", Phys. Med. Biol., 1986, Vol. 31, No. 9, pp. 985-992, discloses a record and verify system which combines a fluoroscopic technique combined with digital image processing. The system specifically uses an E-2 fluorescent screen to convert x-ray images into light images. The light images are detected by a video camera which receives the image as reflected by a mirror large enough to image the irradiated area of interest on the fluorescent screen. This system thus requires a substantial amount of space below the patient, where space is not generally available, as has been mentioned. In those treatment machines with beam stoppers, such space is generally not available. Even where there is no beam stopper, the bulkiness of the device renders its adaptation to the treatment machine impractical.

Other systems use an image intensifier tube to generate an optical image for direct viewing by a camera. This system is variously described in U.S. Patent Nos. 4,590,518, issued to Fenster et al. for "Arrangement for Processing Data in Digital Fluorographic Systems"; 4,595,949, issued to Fenster et al. for "Systems and Methods for Translating Radiation Intensity into Pixel Values"; and 4,649,559, issued to Wang for "Digital Radiography Device". These systems show the placement of monitoring devices in line with the radiation source and patient, thereby also requiring a substantial amount of space for imaging. Summary of the Invention

It is one general object of the invention to provide apparatus which overcomes the above-mentioned limitations in x-ray and other high-energy imaging procedures.

A more specific object of the invention is to provide such an apparatus which is compatible with the limited space available in typically x-ray treatment machines and the like. Yet another object of the invention is to provide such an apparatus which can also be designed for low-energy x-ray diagnostic applications.

Providing a novel fiber-optic assembly for use in such an apparatus is yet another object of the invention

The apparatus of the invention is used in generating a reduced-scale optical image of an beam pattern produced by directing a high-energy irradiating beam, such as a gamma- or x-ray beam, through an object along a predetermined path. The apparatus includes an assembly of optical fibers which have input end regions disposed in a two-dimensional input array which encompasses the area of the beam pattern, and output ends which are disposed in a reduced-scale two-dimensional output array. A beam converter in the apparatus functions to convert the irradiating beam which passes through the object to an electron beam whose density distribution is directly related to the impinging irradiating beam. This electron beam is converted to a pattern of visible light which is carried by the fibers to their output ends, to form a reduced-scale image of the irradiating beam pattern.

In one embodiment the individual fibers in the array have tapered cross-sectional areas, on progressing from their input to output ends, and are arranged so that the fiber input ends form the input array, and the fiber output ends, the output array. Preferably, the fibers are formed of optically conductive polymer. In a preferred embodiment, the fibers are arranged in groups of fused, tapered fibers, forming optical reducers, each having input and reduced-area output ends. The reducers, which form another aspect of the invention, are then arranged to form the fiber arrays in the fiber assembly.

In a related embodiment, for use in generating a high-resolution image of the irradiating beam, the optical fiber assembly is formed as an array of reducers, as above, and the reducers are formed as an array of fiber bundles. The fiber bundles, in turn, are each formed as an array of optical fibers. The array of fiber bundles making up each reducer are drawn and tapered, in fused form, to produce a reduced cross-sectional area on progressing from the reducer's input to output end.

In the above embodiments, the beam converter includes a metal plate which covers the input array. The electron beam is converted to a corresponding light beam by a fluorescent screen disposed between the metal plate and the array.

In a second general embodiment, the input array in the optical fiber assembly is formed by longitudinally extending input regions of the planar array of parallel fibers, adjacent the input ends of the fibers, and the output array is formed as a two-dimensional array of the output ends of the fibers. The beam converter includes a metal strip extending across the end regions of the fibers, substantially transversely thereof, for converting associated strip regions of the irradiating beam to strip regions of electron beams. The electron beam strips are converted to corresponding light intensity signals in the fibers by (a) a fluorescent screen strip disposed between the metal strip and the two-dimensional input array, for converting the strip regions of electron beams to a firstwavelength radiation beam strip, and (b) a fluorescent dye contained in the input regions of the fibers, for convert ing the first-wavelength radiation beam strip to a visible light within the fibers. The metal strip and fluorescent screen strip are moved as a unit, along the length of the fiber input region, to generate in the fiber input region of each fiber, an instantaneous light intensity which is directly related to the intensity of the irradiation beam which is impinging on the metal strip in the region between the beam and that fiber.

This embodiment further includes an image processor for generating from the instantaneous light intensity received at the output of each fiber, as the metal strip and fluorescent screen are moved along the lengths of the fiber input regions, a two-dimensional optical image corresponding to the intensity of the irradiating beam impinging on the two-dimensional input array.

In a third general embodiment, the input array in the optical fiber assembly is also formed by longitudinally extending input regions of a planar array of parallel fibers, and the output array, as a two- dimensional array of the output ends of the fibers. The beam converter in this embodiment is a metal plate covering the input array, for forming a related electron beam, and this beam is converted to light signals in the optical fibers by one or more fluorescent species in the fibers. To generate the image of the irradiation beam, the input array is rotated with respect to the irradiated object, and the instantaneous light intensity signals from each fiber, as the fiber array is rotated with respect to the object, are processed to construct the desired image. In other aspects, this invention provides a method for manufacturing tapered fiber optic bundles or reducers of the type used herein. These reducers or bundles include an assembly of optical fibers, each tapered so as to have a larger cross-sectional diameter at one end of the reducer, as compared to the cross-sectional diameter at the second end of the reducer. This process includes a first step of securing a preform which consists essentially of an assembly of optical fibers having crosssectional diameters corresponding to the cross-sectional diameters of the optical fibers at the first end of the reducer. Generally, this securing places the preform in a vertical position. The second step involves applying heat to a portion of the length of the preform with a movable heater element. During this heating, an end-to-end tension is applied to the preform. When the heating reaches the point that the preform becomes plastic, the tension is maintained constant so as to taper the preform through the heating zone and draw a fiber having the desired smaller second cross-sectional diameter from the preform. The movable heater is moved along the preform to obtain the desired fiber diameter and tapered fiber optic reducer. Once this is formed, the heating element is withdrawn from the preform, and the fiber optic reducer is allowed to cool.

The preforms which are formed into the taper are preferably themselves bundles of clad fibers, such that when the taper is formed, a plurality of fibers are themselves tapered. In preferred embodiments, each of these fibers is a clad material. That is, each fiber comprises a core surrounded by a concentric outer coating. In another embodiment of this invention, these clad preforms are formed. Preferably, this outer coating is continuous around the core when viewed cross sectionally. In another embodiment of this invention, clad preforms from which these clad fibers are made, are formed. In this embodiment, these clad preforms, which comprise a core surrounded by an outer coating, are formed. The core generally comprises an optically transmissive plastic material and the cladding comprises a second plastic material generally of lower index of refraction than the core material. These clad preforms are generally in the form of billets and are formed in a cladding oven by the process of fitting a preshaped body of core material inside a preshaped hollow sleeve of cladding material to give a sleeved core, placing this sleeved core within an open-ended pressurable fixture which encloses the outer surface of the sleeve in an open-ended configuration; placing this fixture in an evacuatable oven equipped with means for applying pressure to the ends of the sleeved core contained within the cladding fixture through the open ends of the fixture; applying heat to the cladding fixture and contained sleeved core and applying pressure to the ends of the sleeved core through the open ends of the fixture for a time adequate to cause the core and cladding sleeve to be compressed and form a single clad preform billet; and thereafter cooling the cladding fixture and clad preform and removing the preform from the fixture. The clad preform billet so formed may then be drawn into a fiber. These clad fibers may be tapered in accord with the invention or these fibers may be aggregated into bundled preform for forming high-resolution bundles of tapered fibers.

These and other advantages and features of the invention will be apparent from a review of the drawings and the following detailed description of the preferred embodiments .

Brief Description of the Drawings

Figure 1 is a schematic side view of an imaging system including an apparatus made according to the invention. Figure 2 is a perspective view of an optical fiber assembly constructed according to the invention. Figure 3 is a perspective view of an optical fiber reducer from the Figure 2 assembly.

Figure 3a is an enlarged prespective view of a typical clad optical fiber reducer from the Figure 2 assembly.

Figure 4 is a fragmentary perspective view of a fiber bundle used in forming a high-resolution reducer.

Figure 4a is an enlarged perspective view of a fiber bundle formed of clad fibers. Figure 5 a partial view similar to Fig. 1, showing a second preferred embodiment of the invention.

Figure 6 is an enlarged perspective view of the apparatus of Fig. 5.

Figure 7 is a further enlarged fragmentary perspective view of a portion of the apparatus of Fig. 5.

Figure 8 is a cross section taken along line 8-8 in Fig. 7.

Figure 9 is a simplified fragmentary view of the output end of the transmission optical fiber array of Fig 5.

Figure 10 is a simplified perspective view of a third preferred embodiment of the invention.

Figure 11 is a frontal view of a system for drawing tapered fibers . Figure 12 is a frontal view of a mechanism for moving the oven which makes up part of the system shown in Fig. 11.

Figure 13 is a series of views of a vacuum fixture in exploded "story board" format, illustrating its assembly for use in preparing clad billets of preform material for forming tapered fibers.

Figure 14 is a frontal view of an apparatus useful for forming the clad billets of material which incorporates the vacuum fixture of Fig. 13. Figure 15 is a perspective view of a fiber array formed into a taper in accord with the invention. Detailed Description of the Invention

Referring initially to Fig. 1, an apparatus 20 made according to the invention is shown schematically in a radiotherapy system 22 for treating a patient 24. System 22 includes a source 26 of x-ray radiation directed during treatment along an incident beam path 28. The irradiating beam, indicated at 29, may also be a gamma-ray or other high-energy radiation beam. Although it is not shown, a radiation block is normally placed over the patient. The block has a port which allows a limited cross-sectional area of the radiation to pass into and through the patient. The alignment and dosage of the treatment is obtained by sensing the intensity distribution of the beam after it has passed the patient.

The patient is typically disposed on a bed or platform 30 below which is a limited amount of space for positioning sensing or imaging equipment. The imagesensing and reduction portions of apparatus 20, shown enlarged in Fig. 1, will fit within this space. The apparatus includes a light-tight box 32. Disposed along a surface portion of the box is a plate-like beam converter 34, conventionally made of a sheet or foil of metal, such as tantalum, which converts the impinging irradiating beam into electrons, and therefore in essence, into an electron beam having an electron density distribution directly related to the intensity distribution of the incident beam.

The electrons created in the photon conversion reactions produce light in a sheet-like, fluorescent screen 36. This screen is preferably about 0.1 mm thick, but can be up to 1 mm or greater in thickness. The preferred screen has an active rare earth phosphor such as Gd₂O₂S:Tb. Other phosphors which would be suitable are La₂O₂S:Tb or CaWO₂. These fluorescent screens can be purchased from Eastman Kodak Co. Alternately a sheet of plastic scintillator can be used instead of the screen to give light output closely related to the dose in water. This could be very useful for dosimetric verification or quality assurance purposes. These plastic scintillators are commercially available from companies such as Bicron of Cleveland, Ohio and Nuclear Enterprises of Scotland. Screen 36 is also referred to herein as means for converting the electron beam produced by the converter to a pattern of visible light which is directly related to the irradiation beam impinging on the beam converter. The top surface 36a of the fluorescent screen, as viewed in the figure, is disposed adjacent the lower surface of converter 34. The bottom surface 36b confronts the input end of an optical fiber assembly 38, which will now be described.

As seen best in Figure 2, fiber assembly 38 is formed of an n × m image array of fiber reducers, such as reducer 40, which is detailed in Figure 3. Although the assembly illustrated forms a rectangular and preferably square input and output square array, it will be appreciated that a variety of other input and/or output array shapes, such as an oval or circular array, are possible. The assembly has an input end 42 whose area encompasses the area of the beam pattern, and a reduced-scale output end 44 whose area is preferably between about 1/25 to 1/200 of the area of the assembly's input end. The assembly functions to carry the light image formed by screen 36 to a location remote from the screen, where the image output is in reduced scale. As seen in Figure 1, assembly 38 has an approximately right-angle bend between its input and output ends.

With reference now to Figure 3, reducer 40, which is exemplary, is formed of a j × k fiber array of optical fibers, such as fibers 46. The fiber reducer is produced, in accordance with one method, by first forming a j × k image array of uniform-area optical fibers, e.g., square optical fibers 46 whose sides are between about 1-2 mm, and having lengths between about 25-100 cm. Such fibers may be formed, for example, by drawing out relatively large-dimension square optical light pipes or preforms to the desired uniform cross-sectional size. The total number of fibers forming the fiber array is preferably between about 100-400, and a preferred array is a 15 × 15 square array, i.e., having 225 fibers. This array, with 1.5 mm square fibers, would thus have side dimensions of about 2.3 cm, and a surface area of about 5-6 cm².

The fiber array is then placed in mold or fixture whose inner wall dimensions are just slightly larger than the dimensions of the array. The array is now heated in the fixture to a temperature which allows the fibers to fuse into a multifiber unit. Here it is noted that the fibers in the fused unit retain their individualfiber cladding, and thus continue to function as discrete light carriers. The fused unit is now preferentially heated along its length so that when drawn out, it will have a progressively reduced cross-sectional area on progressing from one end to another, and more particularly, from a larger input end 48 formed by the input ends of the fibers making up the reducer, to a smaller output end 50 formed by the output ends of the fibers. As indicated above, the area of the output end is typically 1/25 to 1/200 of the area of the input end. In one preferred embodiment, the input end has an area of about 6 cm², and the output end, an area of about 0.06 cm ².

The reducers forming assembly 38 may be bent in row or column units by heating the multi-reducer array of straight-fiber reducers, or the reducers may be individually bent and then fitted together to form the assembly. It can be appreciated from Figure 1 that the individual reducers forming each "column" of the fiber assembly will have different lengths and bend curvatures.

The optical fibers forming each reducer are preferably made of plastic, since they can be made less expensively than reducers made from glass fibers. Plastic fibers from which the fiber reducer can be formed can be obtained from Fibre Optics Development Systems (Santa Barbara, CA). Glass fiber reducers can be obtained from such companies as Galileo of Sturbridge, Mass. or Reichert Fiber Optics of Southbridge, Mass. In the present embodiment, the fibers have a square cross section, as shown in Figure 3, although the image reduction could also be done with circular fibers or other shapes such as pentagons, hexagons, or the like. The fibers in the reducers have a turning radius of about 2 cm to 5 cm. The optical aperture of the "untapered" portion of each optical fiber is such as to transmit light whose angle of incidence, at the fiber input end, is within a cone of about 20 degrees. Because the fiber width at the output region has been substantially reduced, this cone of transmitted light will actually be much smaller, e.g., about 2 degrees when the output end fiber width is about one-tenth that of the fiber input end. Thus, besides providing for substantial space reduction, the fiber reducers also act as collimators to admit only that light that is entering essentially normal to the fiber ends. Thus, there is little blurring of the resultant image due to crossover of the light between scintillator regions defined by the fiber inlet ends.

A fraction of the scintillation light produced above each fiber is light piped through the fiber reducers, the output of which is viewed by a lens 52. When the screen or plastic scintillator sheet produces light in the green-red spectral region, a blue filter 54 is preferably placed between the output end of the assembly and lens 52 to filter out unwanted background light which is predominantly in the blue region of the visual spectrum.

The lens couples the optical signal transmitted through assembly 38 to a video camera 56. Figure 1 shows light rays from the fiber assembly passing through filter 54, lens 52 and into camera 56. The camera may be of the conventional vidicon, CCD or CID type. Further, instead of using a lens, the image may be coupled to the camera by proximity focusing, wherein optical fibers from a fiber optic coupler are placed directly on the fiber reducer array 38 at one end and onto the sensor array of the camera at the other end, or by coupling the output end of the fiber assembly directly onto the camera sensor array. The camera output is digitized, processed by a small computer or microprocessor 58 using conventional video signal processing programs such as that described in the prior art for signal generation and enhancement, or such as that sold under the proprietary name of Data Translation Frame Grabber. The digitized signal from processor 58 is displayed on a monitor 60, and is stored in the image processor memory for later use. This resultant image is available within a few seconds from the time the incident irradiation beam is directed through the subject. In applications requiring resolution of very low contrast images, cross-talk between fibers may degrade the resolution. Cross-talk can be prevented, in one embodiment, by making alternate fibers out of nontransmissive, i.e., black plastic. This prevents light from traveling between adjacent light-transmissive fibers. However, this approach inherently cuts down on the sharpness of the resulting image, since half the image information is removed. To some extent this loss may be compensated for by image-enhancing software that is available. An alternative approach is to coat each optical fiber with a second cladding of non-transmissive material, e.g., black plastic. Here each optical fiber will have a clear, light-transmissive cladding, and an outer dark, non-transmissive cladding. The nontransmissive cladding can be formed conveniently on the large-dimension optical fiber which is drawn to form the relatively small-dimension fibers used in forming the individual reducers. Although this is a more expensive solution, it allows more of the scintillation light to be transmitted.

Figure 3A shows how each fiber 46, 46A, etc. in reducer 40 can be formed with a cladding 45 and core 47, if desired.

A second source of resolution degradation which occurs is low level scintillation and/or Cherenkov emission resulting from electrons penetrating into the fiber reducer. This background can be greatly reduced by using an optical filter 54 as mentioned above. If proximity focusing is used, this filter must be very thin, of the order of 0.1 mm. If significant background is present, the fluorescent screen or plastic scintillator is chosen as mentioned such that it emits in the green-red region of the optical spectrum. Since the scintillation and/or Cherenkov background emitted from the fiber reducers is predominately in the blue, a long wavelength pass filter which filters out blue light effectively reduces the background from these sources.

Further background reduction can be attained by blackening the large input end of several fibers in each reducer, such as illustrated in Figure 3, which shows fiber ends 62. The darkened ends effectively block light transmission in the selected fibers. This provides in those optical fibers, a light signal which represents the background light being produced in and transmitted by the "clear-end" fibers. The image processor is designed to distinguish between the two and to subtract the background light from that produced in the active fibers.

A further alternative approach for obtaining background levels for the embodiment shown in Fig. 1 is to take an image when beam 29 is not being transmitted. This reading then represents the electronic system background, or non-radiation induced background. Radiation-induced background can also be measured by removing the scintillator screen when the beam is being transmitted and to use this as the background signal. Conventional digital image enhancement programs can readily subtract the background frame from the active signal frame to eliminate the background. These alternatives allow all of the fibers to transmit a beam image signal. Figure 4 shows a portion of a reducer 64 designed for a fiber assembly capable of high-resolution imaging of a radiation beam. The reducer is formed of a j × k array of fiber bundles, such as bundle 66, where each bundle has approximately the same dimensions as the individual fibers in the above reducer 40. More particularly, a preferred reducer contains between about 100-400 fiber bundles, and each bundle has an input end which is about 1.5 mm on a side, and an output end which is about one-tenth that dimension. With continued reference to Figure 4, bundle 66, which is representative, is composed of a c × d array of optical fibers, such as fibers 68. To form a fiber bundle, the optical fibers, which are preferably square fibers about 1-2 mm on a side, are arranged in a desired array, such as a square array containing between about 100-400 fibers, and these are fused as above. The fused block is then drawn down to a very small cross section, preferably about 1-2 mm on a side. The bundles, each of which contains between 100 and 400 individual fibers, are then combined to form a j × k bundle array which will, in effect, substitute for the individual fibers used in form ing reducer 40, to increase the total number of optical fibers in the reducer, and therefore the reducer resolution, by a factor of 100-400. Of course, the fiber bundles could be composed of a smaller number of fibers, such as 5-100 fibers, which would produce a corresponding decrease in the total number of reducer fibers.

If desired, each of the fibers in the bundle may be clad so as to give a configuration as shown in Fig. 4A. A second preferred embodiment of the invention is shown in Figs. 5-9. In this embodiment, an optical image-generating apparatus 70 has a light-tight box 72, a strip photon converter 74, or beam converter, a fluorescent strip 76, a fiber assembly 78, a lens 80 and a video camera 81. The microprocessor and monitor are eliminated for simplicity of illustration.

The fluorescent strip 76 consists, in one embodiment, of a linear array of short segments of plastic fluorescent optical fibers 77 which are positioned to extend preferably normal to input regions of the fibers in assembly 78. An enlarged fragmentary portion of the strip is seen in Figure 7. The fiber segments making up the strip contain a primary fluorescent dye with a fluorescent emission in the ultraviolet or blue in response to the electrons generated in the strip itself and by the strip photon converter 74 placed directly above the fluorescent

2 strip 76. The segments are nominally 1 mm in cross section and 1-5 mm in height. The fluorescent strip could also be made of a thin strip of fluorescent screen as described in the first embodiment. In this case a 1 mm × 40 cm strip of fluorescent screen would be located directly below the strip photon converter, as was the linear array of scintillating fibers.

The fiber assembly is composed of an array of optical fibers, such as fibers 82, and preferably square polymer fibers having side dimensions of between about 1-2 mm, as above, although other cross-sectional shapes and dimensions .may be suitable. An enlarged fragmentary portion of the assembly is seen in Figure 7. The fibers are arranged at their input regions in a parallel or side-by- side planar array, such that longitudinally extending input regions of the fibers, such as input regions 84, form an input array 86 whose area encompasses the area of the irradiation beam. The fibers are arranged at their output ends in groups of stacked fibers, as shown in Figure 6, which form a two-dimensional array of fiber output ends. In a preferred embodiment, the total number of fibers forming the input array is between about 100- 1,000, where the width dimension of the individual fibers is preferably between about 1-2 mm.

A two-dimensional output array 88 (Figure 9) in the assembly is formed by stacking groups of the optical fibers in an n × m arrangement, as indicated in Figure 6. For example, where the assembly consists of 400 fibers, 1 mm in cross section, the output array can be formed by stacking 20 groups of 20 planar fibers, to form a square 20 × 20 fiber array. The fibers are preferably not tapered between their input and output end, so that the side dimension of the output array is 2 cm in this example.

The fibers forming the assembly are doped with a secondary or "waveshifter" fluorescent dye which responds mainly to light emitted from the fluorescent strip 76. The secondary dye is chosen so that its absorption band is well matched to the emission band of the fluorescent strip and so that it re-emits this light at longer wavelength. The fluorescent strip 76 and overlying converter strip are mounted at opposite ends to frame 90 and supported on a movable carriage and driver (indicated by arrow 96, which indicates the back-and-forth directions of movement of the two strips across the input array). The driver and movable carriage, which are entirely conventional, are also referred to herein as moving means. During operation, the two movable strips travel as a unit across the input array of the fiber assembly, with the fluorescent strip emitting a light-beam strip, in response to excitation by electrons, which is then absorbed by the dye in the assembly fibers, as indicated in Figure 8. This dye acts as a waveshifter to emit isotropically the desired light, a portion of which travels down the secondary fibers to the fiber output 88. It will be appreciated that the fibers in the fiber assembly need to contain internal fluorescent doping material in order to convert a portion of the light emission striking the fibers at substantially right angles into light that will propagate within the fibers.

Summarizing, the fluorescent screen strip functions to convert the strip regions of electron beams produced by the beam converter into a first-wavelength radiation beam strip. The secondary fluorescent dye in the assembly fibers functions to convert the first-wavelength beam strip into visible light emissions within the fibers. The moving means functions to move the metal strip and fluorescent strip as a unit along the lengths of the input regions of the assembly fibers, to generate in the fiber input regions, an instantaneous light intensity which is directly related to the intensity of the radiation beam which is impinging upon the metal strip. The fluorescent screen, secondary fluorescent dye, and moving means are also referred to herein as converting means for converting the electron beam image produced by the irradiating beam into a reduced scale optical image. The data obtained with the fluorescent strip 76 in a single x position gives a linear array of y values. The fluorescent strip is swept along the length of the fibers in array 86 - - i . e . , along the x axis . To obtain 1 mm resolution, data must be taken at 1 mm intervals. As with the first embodiment, the resolution of low contrast images may be degraded by background scintil lation caused by electrons or x-rays traversing the assembly fibers, thus giving a signal in addition to the signal from the fluorescent strip resulting from the incident beam. This background can be determined by optically isolating several of the fibers in the secondary fiber array from emission from the primary scintillator strip. This can be done by applying a black coating to the outside of these fibers. The output of these fibers can be read out and subtracted from the secondary fiber signal, thus removing the background. A second method for removing background is to remove the scintillator strip and expose the remaining apparatus to the radiation beam. This background signal can then be subtracted from the total signal as in the first embodiment. In some applications, the second embodiment of this invention can be simplified by leaving out the fluorescent strip, and simply scanning the converter strip 74 along the assembly fiber input regions. This would require the addition of both primary and secondary dyes to the core material in the fibers. All other aspects of the instrument remain unchanged. As the converter is scanned across the assembly fiber input regions, the radiation beam will be converted into electrons in the converter, thereby resulting in enhanced excitation of the scintillator immediately below the converter. Here the converting means would include the primary and secondary dyes contained in the assembly fibers, as well as the above moving means.

Reference is now made to a third embodiment of the invention as shown in Fig. 10 without relation to a patient or x-ray beam. This embodiment, shown as an apparatus 110 which substitutes for apparatus 20 in Fig. 1 or apparatus 70 in Fig. 5. In fact, this embodiment is more closely similar to that shown in Fig. 5. It includes a planar fiber assembly 112 whose optical fiber arrangement is substantially identical to that of assembly 78 in apparatus 7.0. In particular, assembly 112 includes an input array 113 formed by the input regions 115 of the fibers in a parallel side-by-side configuration, and an output array 122 formed by stacking groups of the fibers. Assembly 112 differs from assembly 86, however, in that the fibers 114 making up the assembly contain both the above primary and secondary fluorescent dyes. It is to be understood that a sheet-like photon converter would also be disposed above this assembly, preferably in a fixed position so that the electrons produced are directly relatable in physical position to the impinging x-ray beam.

The output array of assembly 112 is coupled to a digital camera as described for the embodiment of Fig. 5. The output array is divided into groups, such as group

120, for forming a rectangular output array 122 for viewing by a camera 126, through a lens 127. These components are mounted on a turntable 128 controllably driven, by suitable moving means (indicated by arrow 129) for rotation about a vertical axis 130.

For a single location of the ribbon, the signal obtained from each fiber is the line integral, or sum, of the signal resulting from penetration of the beam all along each fiber. The entire scintillator array - camera system is then rotated about axis 130. Data is acquired at many angular orientations. Algorithms such as those used for computed tomography scanners would provide image reconstruction.

The converting means in this embodiment thus includes the primary and secondary fluorescent dyes in the assembly fibers, and the moving mass for rotating turntable 128.

It will be appreciated that variations may be made in the foregoing embodiments without departing from the scope of the invention. In particular, it will be appreciated that the device can be readily adapted for imaging a particle beam. Here, since the beam particles can produce direct scintillation of a primary fluorescent dye, the beam converter in the above-described embodiments would not be required. The tapered fiber optic fibers or bundles useful in this invention can be prepared from preforms in a novel manner using the apparatus 200 shown in Figure 11. Unlike prior art processes, the invention process involves holding the fiber preform in a fixed position and moving a heater element gradually along the preform to melt it. Presently available machines used to draw optical fibers from preforms generally employ a flexible chain which lowers the preform into an oven. This system is inherently incapable of reliably positioning the preform. Reliable positioning is necessary to insure even heating. The presently available systems also do not provide for constant tension on the fiber during heating, nor do they permit the progress of the tapering to be viewed directly. The method and system of the present invention remedy these defects.

The present method and system involve holding one end of the preform in a fixed position while pulling on the other end of the preform while sliding an oven along the preform's length. This permits tension to be applied to the preform and further allows a steady positioning of the preform and resulting fiber in the heating zone. In addition, the progress of the operation can be viewed and a constant temperature maintained at the preform-to-fiber interface. A general overview of the fiber optic forming apparatus 200 is shown in Figure 11. The fiber optic substrate is preform 204. Preform 204 has a larger cross section at one end and is drawn to the fiber optic 204a of reduced cross section. The preform is held at one end by an XYZ positioning device 207. The other end of the preform extends through a heater/oven 211. The oven softens the preform such as at a temperature of about 250°C This softening allows the preform to stretch into a prefiber and the lower portion of the preform to drop away. The prefiber is coupled to tractor pulling device 205 and a tension is drawn on the prefiber by tractor pulling device 205. As the oven softens and melts a region of the preform, the tension applied via tractor pulling device 205 effects a taper and draws out the preform into a fiber. The lower portion of the preform drops off as shown at 203 leaving the fiber of reduced diameter 204a.

The preform 204 can be a single body of fusable plastic fiber optic-forming material. It also can be a two-component material comprising a center core and an exterior cladding. In a preferred embodiments, it can be a multifiber preform made up of a plurality of prefibers bonded together. Each of these prefibers can be a single component or can be clad. In a preferred embodiment, each of the prefibers is about 1.5 mm² in cross section. These prefibers are formed into a solid preform rod.

The oven 211 is slidably mounted on slide rods 210 and 210A through eyes 202 and 202A. Oven 212 is held in position and moved along the preform 204 by a lead screw mechanism detailed in Figure 12. A plexiglass enclosure 209 surround the system and permits direct viewing of the progress of the taper development and permits the drawing environment to be controlled such as by flowing dry nitrogen into the box and excluding moisture and airborne particles . In operation, after the lower part of the preform drops off, as shown in 203, the fiber optic (preferably multifiber fiber optic) of reduced diameter 204a is placed threaded through the tractor pulling device 205. The oven positioning device shown in Figure 12 moves the oven gradually away from the tractor pulling device 205. This causes new regions of the preform to be fused by the heat- of the oven. The tractor pulling device 205 draws out the softened and fused preform into a taper and into the fiber of reduced diameter 204a. By controlling the temperature of the oven, the speed at which the oven is moved along the preform and the speed at which the tractor pulling device pulls the fiber from the fused area, the size of the fiber approaches the desired fiber opticr size. The oven 211 is caused to move away from the pulling device 205 on rods 210 and 210A. This movement is very slow to maintain a constant supply of plastic fiber optic preform in the heater region. Preferably the device keeps a constant tension through the tractor pulling device on the fiber. The caliper 212 monitors the resulting fiber size, and the drawing speed is varied until the desired size is obtained. When the desired size is obtained, the oven 212 (typically a heater band) is moved out of the drawing region back onto the drawn fiber. The preform is left to cool in this configuration while the fiber is held under tension. Cooling is facilitated by passing a stream of cool gas through the oven region. The resulting tapered reducing fiber optic is then removed from the oven.

As shown in Figure 11, in a preferred embodiment, the various elements are arranged vertically. In this preferred configuration the preform holding device 207 is mounted above the oven and the tractor pulling device 205 is mounted beneath the oven. In this configuration, in use, the oven moves upward toward the XYZ positioning device and gravity assists the drawing. Figure 12 shows a preferred embodiment of a mechanism to effect the motion of the oven. Again, this is shown in the context of upward and downward motion. The oven mount 225 is threaded along the longitudinal lead screw 220 and the longitudinal screw is connected to a turning mechanism (not shown) wherein the connection is maintained and controlled by two clutches, 221 and 221A. The progress of the longitudinal screw may be further controlled by a boule or preform holder 222. The exact side-to-side position of the heater is precisely regulated by the slide bars 210 and 210A along which the heater unit slides through eyes 202 and 202A.

Thus, according to the method of this invention, the fiber preform is held in constant position at one end and under tension at its other end while a heater element is moved along its length to differentially melt a region and form that region into a taper.

The tapered product so formed can appear as shown in Figure 15 as number 46. As can be seen, 46 is a bundle of fibers having a large end 42 and small end 44. In practice, this bundle would be cut at the hatch marks to give a new large end 42a which would be polished to an optical surface.

As noted previously, in preferred embodiments of this invention, the individual fibers used herein are clad. That is, they have a light-transmissive core surrounded by an outer surface which has differing optical properties. Figures 11 and 12 illustrates one way to form these materials from clad preforms or billets. This invention additionally provides a new method and device for forming these clad preforms or billets. Turning to Figure 13, a block or loaf of optically transmissive plastic 130 is formed. This block has a defined shape, including a cross section 132 and length L. This will become the core of the clad material. It is fitted within a hollow sleeve of cladding material 134. The cross section 136 of the hollow opening of sleeve 134 corresponds to the cross section of core 130 so that the core may be slid inside. These dimensions should be closely tailored so that the space between the core and the cladding is relatively minimal. This sleeve with its enclosed core then fits within the void of hollow vacuum fixture 138. The cross section 140 of this hollow is sized to receive the outside dimensions of sleeve 134. The length L of the core, the length L' of the sleeve, and the length L" of the vacuum fixture are all substantially identical. Fixture 138 may be somewhat longer than the other two components, if desired. Fixture 138 is formed of a solid material capable of good heat transfer and also capable of withstanding substantial pressure. Aluminum or other metals are preferred materials of construction. The vacuum fixture loaded with core 130 and sleeve 134 is placed inside closable pressure fixture 142. Pressure fixture 142 has an interior cavity 144 having interior dimensions somewhat larger than the exterior dimensions of fitting 138 so that fixture 138 may fit inside. Vacuum fixture 142 is equipped with an O-ring seal and a replaceable door 148, which is sealably bolted to the opening of the fixture, thereby forming an enclosed pressure-tight box separately shown as 150. Vacuum fixture 142 is equipped with pressure rams or plungers 152 and 154. These plungers appear at opposite ends of the fixture and have plunger heads sized to fit into the end cavities of fixture 138. Thus when these two pressure rams move inwardly on their shafts 156 and 158, respectively, they impinge upon and compress the body of sleeving material 134 and core material 130 contained within fixture 138. Turning now to Figure 14 the use of this cladding fixture in the cladding process of this invention is illustrated. In Figure 14 three pressure fixtures, 150, 150a, and 150b are illustrated mounted within oven 160. In use, a vacuum supplied by vacuum pump 162 is applied to the interior of each of the three fixtures. The oven 160 is heated gradually from room temperature to about 125ºC. This takes about one hour. After about two hours, the core and cladding materials contained within the vacuum fixtures are heated to a point that they are becoming plastic and flowable. Pressure is then applied to the plungers via shafts 156, 158, 156a 158a, 156b and 158b via drive units 164 and 166, 164a and 166a, and 164b and 166b, respectively. These drive units can be motorized or can be pneumatic or hydraulic. A pressure is raised to about 1600 psi and should be a slow, steady application of pressure. Preferably, the pressure is increased from about 1000 to 1600 psi over a 3- to 5-minute period. Pressure is held constant at this 1600 lb level for about one-half an hour. The rams may gradually move inward during this period as the two plastics flow and fill. Then the heat is turned off, and the vacuum is turned off. The three vacuum fixtures are allowed to cool to room temperature. No additional pressure is applied and as the system cools, the plastic in the vacuum chambers shrinks, thereby automatically releasing the pressure. Thereafter, the vacuum fixtures are removed and opened, the plungers are retracted, and the fixtures such as 138, are withdrawn from the vacuum fixtures. The plastic contained within the fixture 138 may then be removed from the fixture 138. The product so formed is a perfectly concentric core surrounded by a sleeve.

This sleeved product will typically be several inches in cross-section. It can be drawn to some smaller size either to form a single unit preform for tapering or, more preferably, drawn further to a pre-fiber size having a cross-sectional area of about 1-2 mm² for forming into a multi-fiber preform.

The cladding conditions just described, are exemplary. Any dimensions which will give rise to a suitable ratio of cladding material to core material may be used. Similarly, any shape, for example circular, octagonal, pentagonal, square or rectangular, may be used. Typical forming temperatures can range from about 100ºC to about 300°C and maybe higher, if the materials used will permit. So too the forming pressure may range from about 800 psi to about 3000 psi, or preferably from about 1000 to about 2000 psi. Typical forming times may be from about 5 minutes to several hours . Longer times could be used, if desired.

While this invention has been described with reference to certain preferred embodiments, it will be appreciated that it could be varied in many ways. For example, one is not required to use the cladding process herein described to form the preforms. Similarly, the fiber tapering process described herein is merely a preferred method. Other methods could be used, if desired. Similarly, the tapered fibers so formed could be used in embodiments other than the particular devices shown herein. Accordingly, the invention may be modified or applied in ways beyond those shown specifically in this application. The invention is as defined by the following claims.

FIBER OPTIC BEAM-IMAGING APPARATUS AND METHOD

Field of the Invention

This invention relates to diagnostic imaging devices. More particularly, it relates to such imaging devices for producing a real-time image from a high-energy irradiating beam directed through an object. In other aspects, this invention relates to tapered cross-sectional area fiber optic elements and a system for their fabrication as well as a system for forming clad plastic preforms from which they may be fabricated.

Background of the Invention

This invention is particularly well suited to external beam treatment verification, low-energy x-ray diagnostic applications, and quality assurance in radiotherapy. The discussion and description which follows is specifically directed to this application. However, it will be understood that the invention is also applicable to other imaging systems, such as imaging mechanical parts for quality control or inspection. Other applications presently using film to image x-ray or charged particle beams are also applicable.

In radiation therapy, a beam modifying block which absorbs the treating radiation is placed between the patient and the radiation source. This block has an aperture or port which allows the radiation to be directed directly at the patient in a limited treatment region. Errors in the placement of the port or the alignment or intensity of the radiation source can result in suboptimal treatments. It is therefore desirable to obtain feedback on the location of the port and on the actual dose of the radiation applied to the patient. Conventional feedback is provided by the use of radiographic film exposed for a short initial duration for localization, or for the entire duration of treatment for verification. This is a popular method because conventional treatment tables have a space usually limited to four to six inches between the platform on which the patient lies and the radiation-absorbing shield or beam stopper disposed opposite the radiation source to absorb the radiation extending beyond the patient.

The use of radiographic film has several limitations. Among them is that the resulting image is of poor quality. Further, a significant amount of time is involved in the processing of the exposed film to produce an image of the treatment radiation, as well as the time required by the treating physician to study the film, before deciding on what corrective or compensating action should be taken. In the localization mode, the approach cannot ensure that there is no further misalignment during treatment. Certainly, in the verification mode, if the imaged treatment involved errors, those errors already exist and cannot be neutralized. This deficiency is further compounded because the portal film verification process usually limits such a procedure to once a week. It is thus desirable to provide real-time feedback so that any desired corrective measures can be taken rapidly to avoid exposing the patient to a full treatment radiation dosage that is incorrect.

Several such systems have been proposed or developed. For instance, a real-time feedback system was generally proposed by Hubener et al. in "Computed Tomography in Radiotherapy Treatment Planning: Necessity or Pseudo-Accuracy?", British Journal of Radiology, Sup plement 15, March 28-30, 1979, pp. 186-189. They suggest the use of a detector array for measuring the absorption of radiation passing through the patient, similar to the CT detector system. No specific structure of the detector array is proposed.

Other systems have been more recently proposed. Leong in "Use of Digital Fluoroscopy as an On-line Verification Device in Radiation Therapy", Phys. Med. Biol., 1986, Vol. 31, No. 9, pp. 985-992, discloses a record and verify system which combines a fluoroscopic technique combined with digital image processing. The system specifically uses an E-2 fluorescent screen to convert x-ray images into light images. The light images are detected by a video camera which receives the image as reflected by a mirror large enough to image the irradiated area of interest on the fluorescent screen. This system thus requires a substantial amount of space below the patient, where space is not generally available, as has been mentioned. In those treatment machines with beam stoppers, such space is generally not available. Even where there is no beam stopper, the bulkiness of the device renders its adaptation to the treatment machine impractical .

Other systems use an image intensifier tube to generate an optical image for direct viewing by a camera. This system is variously described in U.S. Patent Nos . 4,590,518, issued to Fenster et al. for "Arrangement for Processing Data in Digital Fluorographic Systems"; 4,595,949, issued to Fenster et al. for "Systems and Methods for Translating Radiation Intensity into Pixel Values"; and 4,649,559, issued to Wang for "Digital Radiography Device". These systems show the placement of monitoring devices in line with the radiation source and patient, thereby also requiring a substantial amount of space for imaging. Summary of the Invention

It is one general object of the invention to provide apparatus which overcomes the above-mentioned limitations in x-ray and other high-energy imaging procedures.

A more specific object of the invention is to provide such an apparatus which is compatible with the limited space available in typically x-ray treatment machines and the like. Yet another object of the invention is to provide such an apparatus which can also be designed for low-energy x-ray diagnostic applications.

Providing a novel fiber-optic assembly for use in such an apparatus is yet another object of the invention

The apparatus of the invention is used in generating a reduced-scale optical image of an beam pattern produced by directing a high-energy irradiating beam, such as a gamma- or x-ray beam, through an object along a predetermined path. The apparatus includes an assembly of optical fibers which have input end regions disposed in a two-dimensional input array which encompasses the area of the beam pattern, and output ends which are disposed in a reduced-scale two-dimensional output array. A beam converter in the apparatus functions to convert the irradiating beam which passes through the object to an electron beam whose density distribution is directly related to the impinging irradiating beam. This electron beam is converted to a pattern of visible light which is carried by the fibers to their output ends, to form a reduced-scale image of the irradiating beam pattern.

In one embodiment the individual fibers in the array have tapered cross-sectional areas, on progressing from their input to output ends, and are arranged so that the fiber input ends form the input array, and the fiber output ends, the output array. Preferably, the fibers are formed of optically conductive polymer. In a preferred embodiment, the fibers are arranged in groups of fused, tapered fibers, forming optical reducers, each having input and reduced-area output ends. The reducers, which form another aspect of the invention, are then arranged to form the fiber arrays in the fiber assembly.

In a related embodiment, for use in generating a high-resolution image of the irradiating beam, the optical fiber assembly is formed as an array of reducers, as above, and the reducers are formed as an array of fiber bundles. The fiber bundles, in turn, are each formed as an array of optical fibers. The array of fiber bundles making up each reducer are drawn and tapered, in fused form, to produce a reduced cross-sectional area on progressing from the reducer's input to output end.

In the above embodiments, the beam converter includes a metal plate which covers the input array. The electron beam is converted to a corresponding light beam by a fluorescent screen disposed between the metal plate and the array.

In a second general embodiment, the input array in the optical fiber assembly is formed by longitudinally extending input regions of the planar array of parallel fibers, adjacent the input ends of the fibers, and the output array is formed as a two-dimensional array of the output ends of the fibers. The beam converter includes a metal strip extending across the end regions of thefibers, substantially transversely thereof, for converting associated strip regions of the irradiating beam to strip regions of electron beams. The electron beam strips are converted to corresponding light intensity signals in the fibers by (a) a fluorescent screen strip disposed between the metal strip and the two-dimensional input array, for converting the strip regions of electron beams to a firstwavelength radiation beam strip, and (b) a fluorescent dye contained in the input regions of the fibers, for convert ing the first-wavelength radiation beam strip to a visible light within the fibers. The metal strip and fluorescent screen strip are moved as a unit along the length of the fiber input region, to generate in the fiber input region of each fiber, an instantaneous light intensity which is directly related to the intensity of the irradiation beam which is impinging on the metal strip in the region between the beam and that fiber.

This embodiment further includes an image processor for generating from the instantaneous light intensity received at the output of each fiber, as the metal strip and fluorescent screen are moved along the lengths of the fiber input regions, a two-dimensional optical image corresponding to the intensity of the irradiating beam impinging on the two-dimensional input array.

In a third general embodiment, the input array in the optical fiber assembly is also formed by longitudinally extending input regions of a planar array of parallel fibers, and the output array, as a twodimensional array of the output ends of the fibers. The beam converter in this embodiment is a metal plate covering the input array, for forming a related electron beam, and this beam is converted to light signals in the optical fibers by one or more fluorescent species in the fibers. To generate the image of the irradiation beam, the input array is rotated with respect to the irradiated object, and the instantaneous light intensity signals from each fiber, as the fiber array is rotated with respect to the object, are processed to construct the desired image. In other aspects, this invention provides a method for manufacturing tapered fiber optic bundles or reducers of the type used herein. These reducers or bundles include an assembly of optical fibers, each tapered so as to have a larger cross-sectional diameter at one end of the reducer, as compared to the cross-sectional diameter at the second end of the reducer. This process includes a first step of securing a preform which consists essentially of an assembly of optical fibers having crosssectional diameters corresponding to the cross-sectional diameters of the optical fibers at the first end of the reducer. Generally, this securing places the preform in a vertical position. The second step involves applying heat to a portion of the length of the preform with a movable heater element. During this heating, an end-to-end tension is applied to the preform. When the heating reaches the point that the preform becomes plastic, the tension is maintained constant so as to taper the preform through the heating zone and draw a fiber having the desired smaller second cross-sectional diameter from the preform. The movable heater is moved along the preform to obtain the desired fiber diameter and tapered fiber optic reducer. Once this is formed, the heating element is withdrawn from the preform, and the fiber optic reducer is allowed to cool. The preforms which are formed into the taper are preferably themselves bundles of clad fibers, such that when the taper is formed, a plurality of fibers are themselves tapered. In preferred embodiments, each of these fibers is a clad material. That is, each fiber comprises a core surrounded by a concentric outer coating. In another embodiment of this invention, these clad preforms are formed. Preferably, this outer coating is continuous around the core when viewed cross sectionally. In another embodiment of this invention, clad preforms from which these clad fibers are made, are formed. In this embodiment, these clad preforms, which comprise a core surrounded by an outer coating, are formed. The core generally comprises an optically transmissive plastic material and the cladding comprises a second plastic material generally of lower index of refraction than the core material. These clad preforms are generally in the form of billets and are formed in a cladding oven by the process of fitting a preshaped body of core material inside a preshaped hollow sleeve of cladding material to give a sleeved core, placing this sleeved core within an open-ended pressurable fixture which encloses the outer surface of the sleeve in an open-ended configuration; placing this fixture in an evacuatable oven equipped with means for applying pressure to the ends of the sleeved core contained within the cladding fixture through the open ends of the fixture; applying heat to the cladding fixture and contained sleeved core and applying pressure to the ends of the sleeved core through the open ends of the fixture for a time adequate to cause the core and cladding sleeve to be compressed and form a single clad preform billet; and thereafter cooling the cladding fixture and clad preform and removing the preform from the fixture. The clad preform billet so formed may then be drawn into a fiber. These clad fibers may be tapered in accord with the invention or these fibers may be aggregated into bundled preform for forming high-resolution bundles of tapered fibers.

These and other advantages and features of the invention will be apparent from a review of the drawings and the following detailed description of the preferred embodiments.

Brief Description of the Drawings

Figure 1 is a schematic side view of an imaging system including an apparatus made according to the invention. Figure 2 is a perspective view of an optical fiber assembly constructed according to the invention. Figure 3 is a perspective view of an optical fiber reducer from the Figure 2 assembly.

Figure 3a is an enlarged prespective view of a typical clad optical fiber reducer from the Figure 2 assembly.

Figure 4 is a fragmentary perspective view of a fiber bundle used in forming a high-resolution reducer.

Figure 4a is an enlarged perspective view of a fiber bundle formed of clad fibers. Figure 5 a partial view similar to Fig. 1, showing a second preferred embodiment of the invention.

Figure 6 is an enlarged perspective view of the apparatus of Fig. 5.

Figure 7 is a further enlarged fragmentary perspective view of a portion of the apparatus of Fig. 5. Figure 8 is a cross section taken along line 8-8 in Fig. 7.

Figure 9 is a simplified fragmentary view of the output end of the transmission optical fiber array of Fig 5.

Figure 10 is a simplified perspective view of a third preferred embodiment of the invention.

Figure 11 is a frontal view of a system for drawing tapered fibers. Figure 12 is a frontal view of a mechanism for moving the oven which makes up part of the system shown in Fig. 11.

Figure 13 is a series of views of a vacuum fixture in exploded "story board" format, illustrating its assembly for use in preparing clad billets of preform material for forming tapered fibers.

Figure 14 is a frontal view of an apparatus useful for forming the clad billets of material which incorporates the vacuum fixture of Fig. 13. Figure 15 is a perspective view of a fiber array formed into a taper in accord with the invention. Detailed Description of the Invention

Referring initially to Fig. 1, an apparatus 20 made according to the invention is shown schematically in a radiotherapy system 22 for treating a patient 24. System 22 includes a source 26 of x-ray radiation directed during treatment along an incident beam path 28. The irradiating beam, indicated at 29, may also be a gamma-ray or other high-energy radiation beam. Although it is not shown, a radiation block is normally placed over the patient. The block has a port which allows a limited cross-sectional area of the radiation to pass into and through the patient. The alignment and dosage of the treatment is obtained by sensing the intensity distribution of the beam after it has passed the patient.

The patient is typically disposed on a bed or platform 30 below which is a limited amount of space for positioning sensing or imaging equipment. The imagesensing and reduction portions of apparatus 20, shown enlarged in Fig. 1, will fit within this space. The apparatus includes a light-tight box 32. Disposed along a surface portion of the box is a plate-like beam converter 34, conventionally made of a sheet or foil of metal, such as tantalum, which converts the impinging irradiating beam into electrons, and therefore in essence, into an electron beam having an electron density distribution directly related to the intensity distribution of the incident beam.

The electrons created in the photon conversion reactions produce light in a sheet-like, fluorescent screen 36. This screen is preferably about 0.1 mm thick, but can be up to 1 mm or greater in thickness. The preferred screen has an active rare earth phosphor such as Gd₂O₂S:Tb. Other phosphors which would be suitable are La₂O₂S:Tb or CaWO₂. These fluorescent screens can be purchased from Eastman Kodak Co. Alternately a sheet of plastic scintillator can be used instead of the screen to give light output closely related to the dose in water. This could be very useful for dosimetric verification or quality assurance purposes. These plastic scintillators are commercially available from companies such as Bicron of Cleveland, Ohio and Nuclear Enterprises of Scotland. Screen 36 is also referred to herein as means for converting the electron beam produced by the converter to a pattern of visible light which is directly related to the irradiation beam impinging on the beam converter. The top surface 36a of the fluorescent screen, as viewed in the figure, is disposed adjacent the lower surface of converter 34. The bottom surface 36b confronts the input end of an optical fiber assembly 38, which will now be described.

As seen best in Figure 2, fiber assembly 38 is formed of an n × m image array of fiber reducers, such as reducer 40, which is detailed in Figure 3. Although the assembly illustrated forms a rectangular and preferably square input and output square array, it will be appreciated that a variety of other input and/or output array shapes, such as an oval or circular array, are possible. The assembly has an input end 42 whose area encompasses the area of the beam pattern, and a reduced-scale output end 44 whose area is preferably between about 1/25 to 1/200 of the area of the assembly's input end. The assembly functions to carry the light image formed by screen 36 to a location remote from the screen, where the image output is in reduced scale. As seen in Figure 1, assembly 38 has an approximately right-angle bend between its input and output ends.

With reference now to Figure 3, reducer 40, which is exemplary, is formed of a j × k fiber array of optical fibers, such as fibers 46. The fiber reducer is produced, in accordance with one method, by first forming a j × k image array of uniform-area optical fibers, e.g., square optical fibers 46 whose sides are between about 1-2 mm, and having lengths between about 25-100 cm. Such fibers may be formed, for example, by drawing out relatively large-dimension square optical light pipes or preforms to the desired uniform cross-sectional size. The total number of fibers forming the fiber array is preferably between about 100-400, and a preferred array is a 15 × 15 square array, i.e., having 225 fibers. This array, with 1.5 mm square fibers, would thus have side dimensions of about 2.3 cm, and a surface area of about 5-6 cm².

The fiber array is then placed in mold or fixture whose inner wall dimensions are just slightly larger than the dimensions of the array. The array is now heated in the fixture to a temperature which allows the fibers to fuse into a multifiber unit. Here it is noted that the fibers in the fused unit retain their individualfiber cladding, and thus continue to function as discrete light carriers. The fused unit is now preferentially heated along its length so that when drawn out, it will have a progressively reduced cross-sectional area on progressing from one end to another, and more particularly, from a larger input end 48 formed by the input ends of the fibers making up the reducer, to a smaller output end 50 formed by the output ends of the fibers. As indicated above, the area of the output end is typically 1/25 to 1/200 of the area of the input .end. In one preferred embodiment, the input end has an area of about 6 cm², and the output end, an area of about 0.06 cm².

The reducers forming assembly 38 may be bent in row or column units by heating the multi-reducer array of straight-fiber reducers, or the reducers may be individually bent and then fitted together to form the assembly. It can be appreciated from Figure 1 that the individual .reducers forming each "column" of the fiber assembly will have different lengths and bend curvatures.

The optical fibers forming each reducer are preferably made of plastic, since they can be made less expensively than reducers made from glass fibers. Plastic fibers from which the fiber reducer can be formed can be obtained from Fibre Optics Development Systems (Santa Barbara, CA). Glass fiber reducers can be obtained from such companies as Galileo of Sturbridge, Mass. or Reichert Fiber Optics of Southbridge, Mass. In the present embodiment, the fibers have a square cross section, as shown in Figure 3, although the image reduction could also be done with circular fibers or other shapes such as pentagons, hexagons, or the like. The fibers in the reducers have a turning radius of about 2 cm to 5 cm. The optical aperture of the "untapered" portion of each optical fiber is such as to transmit light whose angle of incidence, at the fiber input end, is within a cone of about 20 degrees. Because the fiber width at the output region has been substantially reduced, this cone of transmitted light will actually be much smaller, e.g., about 2 degrees when the output end fiber width is about one-tenth that of the fiber input end. Thus, besides providing for substantial space reduction, the fiber reducers also act as collimators to admit only that light that is entering essentially normal to the fiber ends. Thus, there is little blurring of the resultant image due to crossover of the light between scintillator regions defined by the fiber inlet ends.

A fraction of the scintillation light produced above each fiber is light piped through the fiber reducers, the output of which is viewed by a lens 52. When the screen or plastic scintillator sheet produces light in the green-red spectral region, a blue filter 54 is preferably placed between the output end of the assembly and lens 52 to filter out unwanted background light which is predominantly in the blue region of the visual spectrum.

The lens couples the optical signal transmitted through assembly 38 to a video camera 56. Figure 1 shows light rays from the fiber assembly passing through filter 54, lens 52 and into camera 56. The camera may be of the conventional vidicon, CCD or CID type. Further, instead of using a lens, the image may be coupled to the camera by proximity focusing, wherein optical fibers from a fiber optic coupler are placed directly on the fiber reducer array 38 at one end and onto the sensor array of the camera at the other end, or by coupling the output end of the fiber assembly directly onto the camera sensor array. The camera output is digitized, processed by a small computer or microprocessor 58 using conventional video signal processing programs such as that described in the prior art for signal generation and enhancement, or such as that sold under the proprietary name of Data Translation Frame Grabber. The digitized signal from processor 58 is displayed on a monitor 60, and is stored in the image processor memory for later use. This resultant image is available within a few seconds from the time the incident irradiation beam is directed through the subject. In applications requiring resolution of very low contrast images, cross-talk between fibers may degrade the resolution. Cross-talk can be prevented, in one embodiment, by making alternate fibers out of non-transmissive, i.e., black plastic. This prevents light from traveling between adjacent light-transmissive fibers. However, this approach inherently cuts down on the sharpness of the resulting image, since half the image information is removed. To some extent this loss may be compensated for by image-enhancing software that is available. An alternative approach is to coat each optical fiber with a second cladding of non-transmissive material, e.g., black plastic. Here each optical fiber will have a clear, light-transmissive cladding, and an outer dark, non-transmissive cladding. The nontransmissive cladding can be formed conveniently on the large-dimension optical fiber which is drawn to form the relatively small-dimension fibers used in forming the individual reducers. Although this is a more expensive solution, it allows more of the scintillation light to be transmitted.

Figure 3A shows how each fiber 46, 46A, etc. in reducer 40 can be formed with a cladding 45 and core 47, if desired.

A second source of resolution degradation which occurs is low level scintillation and/or Cherenkov emission resulting from electrons penetrating into the fiber reducer. This background can be greatly reduced by using an optical filter 54 as mentioned above. If proximity focusing is used, this filter must be very thin, of the order of 0.1 mm. If significant background is present, the fluorescent screen or plastic scintillator is chosen as mentioned such that it emits in the green-red region of the optical spectrum. Since the scintillation and/or Cherenkov background emitted from the fiber reducers is predominately in the blue, a long wavelength pass filter which filters out blue light effectively reduces the background from these sources.

Further background reduction can be attained by blackening the large input end of several fibers in each reducer, such as illustrated in Figure 3, which shows fiber ends 62. The darkened ends effectively block light transmission in the selected fibers. This provides in those optical fibers, a light signal which represents the background light being produced in and transmitted by the "clear-end" fibers. The image processor is designed to distinguish between the two and to subtract the background light from that produced in the active fibers.

A further alternative approach for obtaining background levels for the embodiment shown in Fig. 1 is to take an image when beam 29 is not being transmitted. This reading then represents the electronic system background, or non-radiation induced background. Radiation-induced background can also be measured by removing the scintillator screen when the beam is being transmitted and to use this as the background signal. Conventional digital image enhancement programs can readily subtract the background frame from the active signal frame to eliminate the background. These alternatives allow all of the fibers to transmit a beam image signal. Figure 4 shows a portion of a reducer 64 designed for a fiber assembly capable of high-resolution imaging of a radiation beam. The reducer is formed of a j × k array of fiber bundles, such as bundle 66, where each bundle has approximately the same dimensions as the individual fibers in the above reducer 40. More particularly, a preferred reducer contains between about 100-400 fiber bundles, and each bundle has an input end which is about 1.5 mm on a side, and an output end which is about one-tenth that dimension. With continued reference to Figure 4, bundle 66, which, is representative, is composed of a c × d array of optical fibers, such as fibers 68. To form a fiber bundle, the optical fibers, which are preferably square fibers about 1-2 mm on a side, are arranged in a desired array, such as a square array containing between about 100-400 fibers, and these are fused as above. The fused block is then drawn down to a very small cross section, preferably about 1-2 mm on a side. The bundles, each of which contains between 100 and 400 individual fibers, are then combined to form a j × k bundle array which will, in effect, substitute for the individual fibers used in form ing reducer 40, to increase the total number of optical fibers in the reducer, and therefore the reducer resolution, by a factor of 100-400. Of course, the fiber bundles could be composed of a smaller number of fibers, such as 5-100 fibers, which would produce a corresponding decrease in the total number of reducer fibers.

If desired, each of the fibers in the bundle may be clad so as to give a configuration as shown in Fig. 4A. A second preferred embodiment of the invention is shown in Figs. 5-9. In this embodiment, an optical image-generating apparatus 70 has a light-tight box 72, a strip photon converter 74, or beam converter, a fluorescent strip 76, a fiber assembly 78, a lens 80 and a video camera 81. The microprocessor and monitor are eliminated for simplicity of illustration.

The fluorescent strip 76 consists, in one embodiment, of a linear array of short segments of plastic fluorescent optical fibers 77 which are positioned to extend preferably normal to input regions of the fibers in assembly 78. An enlarged fragmentary portion of the strip is seen in Figure 7. The fiber segments making up the strip contain a primary fluorescent dye with a fluorescent emission in the ultraviolet or blue in response to the electrons generated in the strip itself and by the strip photon converter 74 placed directly above the fluorescent strip 76. The segments are nominally 1 mm² in cross section and 1-5 mm in height. The fluorescent strip could also be made of a thin strip of fluorescent screen as described in the first embodiment. In this case a 1 mm × 40 cm strip of fluorescent screen would be located directly below the strip photon converter, as was the linear array of scintillating fibers.

The fiber assembly is composed of an array of optical fibers, such as fibers 82, and preferably square polymer fibers having side dimensions of between about 1-2 mm, as above, although other cross-sectional shapes and dimensions .may be suitable. An enlarged fragmentary portion of the assembly is seen in Figure 7. The fibers are arranged at their input regions in a parallel or side-by- side planar array, such that longitudinally extending input regions of the fibers, such as input regions 84, form an input array 86 whose area encompasses the area of the irradiation beam. The fibers are arranged at their output ends in groups of stacked fibers, as shown in Figure 6, which form a two-dimensional array of fiber output ends. In a preferred embodiment, the total number of fibers forming the input array is between about 100- 1,000, where the width dimension of the individual fibers is preferably between about 1-2 mm.

A two-dimensional output array 88 (Figure 9) in the assembly is formed by stacking groups of the optical fibers in an n × m arrangement, as indicated in Figure 6. For example, where the assembly consists of 400 fibers, 1 mm in cross section, the output array can be formed by stacking 20 groups of 20 planar fibers, to form a square 20 x 20 fiber array. The fibers are preferably not tapered between their input and output end, so that the side dimension of the output array is 2 cm in this example.

The fibers forming the assembly are doped with a secondary or "waveshifter" fluorescent dye which responds mainly to light emitted from the fluorescent strip 76. The secondary dye is chosen so that its absorption band is well matched to the emission band of the fluorescent strip and so that it re-emits this light at longer wavelength. The fluorescent strip 76 and overlying converter strip are mounted at opposite ends to frame 90 and supported on a movable carriage and driver (indicated by arrow 96, which indicates the back-and-forth directions of movement of the two strips across the input array). The driver and movable carriage, which are entirely conventional, are also referred to herein as moving means. During operation, the two movable strips travel as a unit across the input array of the fiber assembly, with the fluorescent strip emitting a light-beam strip, in response to excitation by electrons, which is then absorbed by the dye in the assembly fibers, as indicated in Figure 8. This dye acts as a waveshifter to emit isotropically the desired light, a portion of which travels down the secondary fibers to the fiber output 88. It will be appreciated that the fibers in the fiber assembly need to contain internal fluorescent doping material in order to convert a portion of the light emission striking the fibers at substantially right angles into light that will propagate within the fibers.

Summarizing, the fluorescent screen strip functions to convert the strip regions of electron beams produced by the beam converter into a first-wavelength radiation beam strip. The secondary fluorescent dye in the assembly fibers functions to convert the firstwavelength beam strip into visible light emissions within the fibers. The moving means functions to move the metal strip and fluorescent strip as a unit along the lengths of the input regions of the assembly fibers, to generate in the fiber input regions, an instantaneous light intensity which is directly related to the intensity of the radiation beam which is impinging upon the metal strip. The fluorescent screen, secondary fluorescent dye, and moving means are also referred to herein as converting means for converting the electron beam image produced by the irradiating beam into a reduced scale optical image. The data obtained with the fluorescent strip 76 in a single x position gives a linear array of y values. The fluorescent strip is swept along the length of the fibers in array 86 - - i . e . , along the x axis . To obtain 1 mm resolution, data must be taken at 1 mm intervals. As with the first embodiment, the resolution of low contrast images may be degraded by background scintil lation caused by electrons or x-rays traversing the assembly fibers, thus giving a signal in addition to the signal from the fluorescent strip resulting from the incident beam. This background can be determined by optically isolating several of the fibers in the secondary fiber array from emission from the primary scintillator strip. This can be done by applying a black coating to the outside of these fibers. The output of these fibers can be read out and subtracted from the secondary fiber signal, thus removing the background. A second method for removing background is to remove the scintillator strip and expose the remaining apparatus to the radiation beam. This background signal can then be subtracted from the total signal as in the first embodiment. In some applications, the second embodiment of this invention can be simplified by leaving out the fluorescent strip, and simply scanning the converter strip 74 along the assembly fiber input regions. This would require the addition of both primary and secondary dyes to the core material in the fibers. All other aspects of the instrument remain unchanged. As the converter is scanned across the assembly fiber input regions, the radiation beam will be converted into electrons in the converter, thereby resulting in enhanced excitation of the scintillator immediately below the converter. Here the converting means would include the primary and secondary dyes contained in the assembly fibers, as well as the above moving means.

Reference is now made to a third embodiment of the invention as shown in Fig. 10 without relation to a patient or x-ray beam. This embodiment, shown as an apparatus 110 which substitutes for apparatus 20 in Fig. 1 or apparatus 70 in Fig. 5. In fact, this embodiment is more closely similar to that shown in Fig. 5. It includes a planar fiber assembly 112 whose optical fiber arrangement is substantially identical to that of assembly 78 in apparatus 7.0. In particular, assembly 112 includes an input array 113 formed by the input regions 115 of the fibers in a parallel side-by-side configuration, and an output array 122 formed by stacking groups of the fibers. Assembly 112 differs from assembly 86, however, in that the fibers 114 making up the assembly contain both the above primary and secondary fluorescent dyes. It is to be understood that a sheet-like photon converter would also be disposed above this assembly, preferably in a fixed position so that the electrons produced are directly relatable in physical position to the impinging x-ray beam.

The output array of assembly 112 is coupled to a digital camera as described for the embodiment of Fig. 5. The output array is divided into groups, such as group

120, for forming a rectangular output array 122 for viewing by a camera 126, through a lens 127. These components are mounted on a turntable 128 controllably driven, by suitable moving means (indicated by arrow 129) for rotation about a vertical axis 130.

For a single location of the ribbon, the signal obtained from each fiber is the line integral, or sum, of the signal resulting from penetration of the beam all along each fiber. The entire scintillator array - camera system is then rotated about axis 130. Data is acquired at many angular orientations. Algorithms such as those used for computed tomography scanners would provide image reconstruction.

The converting means in this embodiment thus includes the primary and secondary fluorescent dyes in the assembly fibers, and the moving mass for rotating turntable 128.

It will be appreciated that variations may be made in the foregoing embodiments without departing from the scope of the invention. In particular, it will be appreciated that the device can be readily adapted for imaging a particle beam. Here, since the beam particles can produce direct scintillation of a primary fluorescent dye, the beam converter in the above-described embodiments would not be required. The tapered fiber optic fibers or bundles useful in this invention can be prepared from preforms in a novel manner using the apparatus 200 shown in Figure 11. Unlike prior art processes, the invention process involves holding the fiber preform in a fixed position and moving a heater element gradually along the preform to melt it. Presently available machines used to draw optical fibers from preforms generally employ a flexible chain which lowers the preform into an oven. This system is inherently incapable of reliably positioning the preform. Reliable positioning is necessary to insure even heating. The presently available systems also do not provide for constant tension on the fiber during heating, nor do they permit the progress of the tapering to be viewed directly. The method and system of the present invention remedy these defects.

The present method and system involve holding one end of the preform in a fixed position while pulling on the other end of the preform while sliding an oven along the preform's length. This permits tension to be applied to the preform and further allows a steady positioning of the preform and resulting fiber in the heating zone. In addition, the progress of the operation can be viewed and a constant temperature maintained at the preform-to-fiber interface. A general overview of the fiber optic forming apparatus 200 is shown in Figure 11. The fiber optic substrate is preform 204. Preform 204 has a larger cross section at one end and is drawn to the fiber optic 204a of reduced cross section. The preform is held at one end by an XYZ positioning device 207. The other end of the preform extends through a heater/oven 211. The oven softens the preform such as at a temperature of about 250°C. This softening allows the preform to stretch into a prefiber and the lower portion of the preform to drop away. The prefiber is coupled to tractor pulling device 205 and a tension is drawn on the prefiber by tractor pulling device 205. As the oven softens and melts a region of the preform, the tension applied via tractor pulling device 205 effects a taper and draws out the preform into a fiber. The lower portion of the preform drops off as shown at 203 leaving the fiber of reduced diameter 204a.

The preform 204 can be a single body of fusable plastic fiber optic-forming material. It also can be a two-component material comprising a center core and an exterior cladding. In a preferred embodiments, it can be a multifiber preform made up of a plurality of prefibers bonded together. Each of these prefibers can be a single component or can be clad. In a preferred embodiment, each of the prefibers is about 1.5 mm² in cross section. These prefibers are formed into a solid preform rod.

The oven 211 is slidably mounted on slide rods 210 and 210A through eyes 202 and 202A. Oven 212 is held in position and moved along the preform 204 by a lead screw mechanism detailed in Figure 12. A plexiglass enclosure 209 surround the system and permits direct viewing of the progress of the taper development and permits the drawing environment to be controlled such as by flowing dry nitrogen into the box and excluding moisture and airborne particles. In operation, after the lower part of the preform drops off, as shown in 203, the fiber optic (preferably multifiber fiber optic) of reduced diameter 204a is placed threaded through the tractor pulling device 205. The oven positioning device shown in Figure 12 moves the oven gradually away from the tractor pulling device 205. This causes new regions of the preform to be fused by the heat- of the oven. The tractor pulling device 205 draws out the softened and fused preform into a taper and into the fiber of reduced diameter 204a. By controlling the temperature of the oven, the speed at which the oven is moved along the preform and the speed at which the tractor pulling device pulls the fiber from the fused area, the size of the fiber approaches the desired fiber opticr size. The oven 211 is caused to move away from the pulling device 205 on rods 210 and 210A. This movement is very slow to maintain a constant supply of plastic fiber optic preform in the heater region. Preferably the device keeps a constant tension through the tractor pulling device on the fiber. The caliper 212 monitors the resulting fiber size, and the drawing speed is varied until the desired size is obtained. When the desired size is obtained, the oven 212 (typically a heater band) is moved out of the drawing region back onto the drawn fiber. The preform is left to cool in this configuration while the fiber is held under tension. Cooling is facilitated by passing a stream of cool gas through the oven region. The resulting tapered reducing fiber optic is then removed from the oven.

As shown in Figure 11, in a preferred embodiment, the various elements are arranged vertically. In this preferred configuration the preform holding device 207 is mounted above the oven and the tractor pulling device 205 is mounted beneath the oven. In this configuration, in use, the oven moves upward toward the XYZ positioning device and gravity assists the drawing. Figure 12 shows a preferred embodiment of a mechanism to effect the motion of the oven. Again, this is shown in the context of upward and downward motion. The oven mount 225 is threaded along the longitudinal lead screw 220 and the longitudinal screw is connected to a turning mechanism (not shown) wherein the connection is maintained and controlled by two clutches, 221 and 221A. The progress of the longitudinal screw may be further controlled by a boule or preform holder 222. The exact side-to-side position of the heater is precisely regulated by the slide bars 210 and 210A along which the heater unit slides through eyes 202 and 202A.

Thus, according to the method of this invention, the fiber preform is held in constant position at one end and under tension at its other end while a heater element is moved along its length to differentially melt a region and form that region into a taper.

The tapered product so formed can appear as shown in Figure 15 as number 46. As can be seen, 46 is a bundle of fibers having a large end 42 and small end 44. In practice, this bundle would be cut at the hatch marks to give a new large end 42a which would be polished to an optical surface.

As noted previously, in preferred embodiments of this invention, the individual fibers used herein are clad. That is, they have a light-transmissive core surrounded by an outer surface which has differing optical properties. Figures 11 and 12 illustrates one way to form these materials from clad preforms or billets. This invention additionally provides a new method and device for forming these clad preforms or billets. Turning to Figure 13, a block or loaf of optically transmissive plastic 130 is formed. This block has a defined shape, including a cross section 132 and length L. This will become the core of the clad material. It is fitted within a hollow sleeve of cladding material 134. The cross section 136 of the hollow opening of sleeve 134 corresponds to the cross section of core 130 so that the core may be slid inside. These dimensions should be closely tailored so that the space between the core and the cladding is relatively minimal. This sleeve with its enclosed core then fits within the void of hollow vacuum fixture 138. The cross section 140 of this hollow is sized to receive the outside dimensions of sleeve 134. The length L of the core, the length L' of the sleeve, and the length L" of the vacuum fixture are all substantially identical. Fixture 138 may be somewhat longer than the other two components, if desired. Fixture 138 is formed of a solid material capable of good heat transfer and also capable of withstanding substantial pressure. Aluminum or other metals are preferred materials of construction. The vacuum fixture loaded with core 130 and sleeve 134 is placed inside closable pressure fixture 142. Pressure fixture 142 has an interior cavity 144 having interior dimensions somewhat larger than the exterior dimensions of fitting 138 so that fixture 138 may fit inside. Vacuum fixture 142 is equipped with an O-ring seal and a replaceable door 148, which is sealably bolted to the opening of the fixture, thereby forming an enclosed pressure-tight box separately shown as 150. Vacuum fixture 142 is equipped with pressure rams or plungers 152 and 154. These plungers appear at opposite ends of the fixture and have plunger heads sized to fit into the end cavities of fixture 138. Thus when these two pressure rams move inwardly on their shafts 156 and 158, respectively, they impinge upon and compress the body of sleeving material

134 and core material 130 contained within fixture 138. Turning now to Figure 14 the use of this cladding fixture in the cladding process of this invention is illustrated. In Figure 14 three pressure fixtures, 150, 150a, and 150b are illustrated mounted within oven 160. In use, a vacuum supplied by vacuum pump 162 is applied to the interior of each of the three fixtures. The oven 160 is heated gradually from room temperature to about 125°C. This takes about one hour. After about two hours, the core and cladding materials contained within the vacuum fixtures are heated to a point that they are becoming plastic and flowable. Pressure is then applied to the plungers via shafts 156, 158, 156a 158a, 156b and 158b via drive units 164 and 166, 164a and 166a, and 164b and 166b, respectively. These drive units can be motorized or can be pneumatic or hydraulic. A pressure is raised to about 1600 psi and should be a slow, steady application of pressure. Preferably, the pressure is increased from about 1000 to 1600 psi over a 3- to 5-minute period. Pressure is held constant at this 1600 lb level for about one-half an hour. The rams may gradually move inward during this period as the two plastics flow and fill. Then the heat is turned off, and the vacuum is turned off. The three vacuum fixtures are allowed to cool to room temperature. No additional pressure is applied and as the system cools, the plastic in the vacuum chambers shrinks, thereby automatically releasing the pressure. Thereafter, the vacuum fixtures are removed and opened, the plungers are retracted, and the fixtures such as 138, are withdrawn from the vacuum fixtures. The plastic contained within the fixture 138 may then be removed from the fixture 138. The product so formed is a perfectly concentric core surrounded by a sleeve.

This sleeved product will typically be several inches in cross-section. It can be drawn to some smaller size either to form a single unit preform for tapering or, more preferably, drawn further to a pre-fiber size having a cross-sectional area of about 1-2 mm² for forming into a multi-fiber preform.

The cladding conditions just described, are exemplary. Any dimensions which will give rise to a suitable ratio of cladding material to core material may be used. Similarly, any shape, for example circular, octagonal, pentagonal, square or rectangular, may be used. Typical forming temperatures can range from about 100°C to about 300ºC and maybe higher, if the materials used will permit. So too the forming pressure may range from about 800 psi to about 3000 psi, or preferably from about 1000 to about 2000 psi. Typical forming times may be from about 5 minutes to several hours. Longer times could be used, if desired.

While this invention has been described with reference to certain preferred embodiments, it will be appreciated that it could be varied in many ways. For example, one is not required to use the cladding process herein described to form the preforms. Similarly, the fiber tapering process described herein is merely a preferred method. Other methods could be used, if desired. Similarly, the tapered fibers so formed could be used in embodiments other than the particular devices shown herein. Accordingly, the invention may be modified or applied in ways beyond those shown specifically in this application. The invention is as defined by the following claims.

FIBER OPTIC BEAM-IMAGING APPARATUS AND METHOD

Field of the Invention

This invention relates to diagnostic imaging devices. More particularly, it relates to such imaging devices for producing a real-time image from a high-energy irradiating beam directed through an object. In other aspects, this invention relates to tapered cross-sectional area fiber optic elements and a system for their fabrication as well as a system for forming clad plastic preforms from which they may be fabricated.

Background of the Invention

This invention is particularly well suited to external beam treatment verification, low-energy x-ray diagnostic applications, and quality assurance in radiotherapy. The discussion and description which follows is specifically directed to this application. However, it will be understood that the invention is also applicable to other imaging systems, such as imaging mechanical parts for quality control or inspection. Other applications presently using film to image x-ray or charged particle beams are also applicable.

In radiation therapy, a beam modifying block which absorbs the treating radiation is placed between the patient and the radiation source. This block has an aperture or port which allows the radiation to be directed directly at the patient in a limited treatment region. Errors in the placement of the port or the alignment or intensity of the radiation source can result in suboptimal treatments. It is therefore desirable to obtain feedback on the location of the port and on the actual dose of the radiation applied to the patient. Conventional feedback is provided by the use of radiographic film exposed for a short initial duration for localization, or for the entire duration of treatment for verification. This is a popular method because conventional treatment tables have a space usually limited to four to six inches between the platform on which the patient lies and the radiation-absorbing shield or beam stopper disposed opposite the radiation source to absorb the radiation extending beyond the patient.

The use of radiographic film has several limitations. Among them is that the resulting image is of poor quality. Further, a significant amount of time is involved in the processing of the exposed film to produce an image of the treatment radiation, as well as the time required by the treating physician to study the film, before deciding on what corrective or compensating action should be taken. In the localization mode, the approach cannot ensure that there is no further misalignment during treatment. Certainly, in the verification mode, if the imaged treatment involved errors, those errors already exist and cannot be neutralized. This deficiency is further compounded because the portal film verification process usually limits such a procedure to once a week. It is thus desirable to provide real-time feedback so that any desired corrective measures can be taken rapidly to avoid exposing the patient to a full treatment radiation dosage that is incorrect.

Several such systems have been proposed or developed. For instance, a real-time feedback system was generally proposed by Hubener et al. in "Computed Tomography in Radiotherapy Treatment Planning: Necessity or Pseudo-Accuracy?", British Journal of Radiology, Sup plement 15, March 28-30, 1979, pp. 186-189. They suggest the use of a detector array for measuring the absorption of radiation passing through the patient, similar to the CT detector system. No specific structure of the detector array is proposed.

Other systems have been more recently proposed. Leong in "Use of Digital Fluoroscopy as an On-line Verification Device in Radiation Therapy", Phys. Med. Biol., 1986, Vol. 31, No. 9, pp. 985-992, discloses a record and verify system which combines a fluoroscopic technique combined with digital image processing. The system specifically uses an E-2 fluorescent screen to convert x-ray images into light images. The light images are detected by a video camera which receives the image as reflected by a mirror large enough to image the irradiated area of interest on the fluorescent screen. This system thus requires a substantial amount of space below the patient, where space is not generally available, as has been mentioned. In those treatment machines with beam stoppers, such space is generally not available. Even where there is no beam stopper, the bulkiness of the device renders its adaptation to the treatment machine impractical.

Other systems use an image intensifier tube to generate an optical image for direct viewing by a camera. This system is variously described in U.S. Patent Nos . 4,590,518, issued to Fenster et al. for "Arrangement for Processing Data in Digital Fluorographic Systems"; 4,595,949, issued to Fenster et al. for "Systems and Methods for Translating Radiation Intensity into Pixel Values"? and 4,649,559, issued to Wang for "Digital Radiography Device". These systems show the placement of monitoring devices in line with the radiation source and patient, thereby also requiring a substantial amount of space for imaging. Summary of the Invention

It is one general object of the invention to provide apparatus which overcomes the above-mentioned limitations in x-ray and other high-energy imaging procedures.

A more specific object of the invention is to provide such an apparatus which is compatible with the limited space available in typically x-ray treatment machines and the like. Yet another object of the invention is to provide such an apparatus which can also be designed for low-energy x-ray diagnostic applications.

Providing a novel fiber-optic assembly for use in such an apparatus is yet another object of the invention

The apparatus of the invention is used in generating a reduced-scale optical image of an beam pattern produced by directing a high-energy irradiating beam, such as a gamma- or x-ray beam, through an object along a predetermined path. The apparatus includes an assembly of optical fibers which have input end regions disposed in a two-dimensional input array which encompasses the area of the beam pattern, and output ends which are disposed in a reduced-scale two-dimensional output array. A beam converter in the apparatus functions to convert the irradiating beam which passes through the object to an electron beam whose density distribution is directly related to the impinging irradiating beam. This electron beam is converted to a pattern of visible light which is carried by the fibers to their output ends, to form a reduced-scale image of the irradiating beam pattern.

In one embodiment the individual fibers in the array have tapered cross-sectional areas, on progressing from their input to output ends, and are arranged so that the fiber input ends form the input array, and the fiber output ends, the output array. Preferably, the fibers are formed of optically conductive polymer. In a preferred embodiment, the fibers are arranged in groups of fused, tapered fibers, forming optical reducers, each having input and reduced-area output ends. The reducers, which form another aspect of the invention, are then arranged to form the fiber arrays in the fiber assembly.

In a related embodiment, for use in generating a high-resolution image of the irradiating beam, the optical fiber assembly is formed as an array of reducers, as above, and the reducers are formed as an array of fiber bundles. The fiber bundles, in turn, are each formed as an array of optical fibers. The array of fiber bundles making up each reducer are drawn and tapered, in fused form, to produce a reduced cross-sectional area on progressing from the reducer's input to output end.

In the above embodiments, the beam converter includes a metal plate which covers the input array. The electron beam is converted to a corresponding light beam by a fluorescent screen disposed between the metal plate and the array.

In a second general embodiment, the input array in the optical fiber assembly is formed by longitudinally extending input regions of the planar array of parallel fibers, adjacent the input ends of the fibers, and the output array is formed as a two-dimensional array of the output ends of the fibers. The beam converter includes a metal strip extending across the end regions of the fibers, substantially transversely thereof, for converting associated strip regions of the irradiating beam to strip regions of electron beams. The electron beam strips are converted to corresponding light intensity signals in the fibers by (a) a fluorescent screen strip disposed between the metal strip and the two-dimensional input array, for converting the strip regions of electron beams to a firstwavelength radiation beam strip, and (b) a fluorescent dye contained in the input regions of the fibers, for convert ing the first-wavelength radiation beam strip to a visible light within the fibers. The metal strip and fluorescent screen strip are moved as a unit along the length of the fiber input region, to generate in the fiber input region of each fiber, an instantaneous light intensity which is directly related to the intensity of the irradiation beam which is impinging on the metal strip in the region between the beam and that fiber.

This embodiment further includes an image processor for generating from the instantaneous light intensity received at the output of each fiber, as the metal strip and fluorescent screen are moved along the lengths of the fiber input regions, a two-dimensional optical image corresponding to the intensity of the irradiating beam impinging on the two-dimensional input array.

In a third general embodiment, the input array in the optical fiber assembly is also formed by longitudinally extending input regions of a planar array of parallel fibers, and the output array, as a twodimensional array of the output ends of the fibers. The beam converter in this embodiment is a metal plate covering the input array, for forming a related electron beam, and this beam is converted to light signals in the optical fibers by one or more fluorescent species in the fibers. To generate the image of the irradiation beam, the input array is rotated with respect to the irradiated object, and the instantaneous light intensity signals from each fiber, as the fiber array is rotated with respect to the object, are processed to construct the desired image. In other aspects, this invention provides a method for manufacturing tapered fiber optic bundles or reducers of the type used herein. These reducers or bundles include an assembly of optical fibers, each tapered so as to have a larger cross-sectional diameter at one end of the reducer, as compared to the cross-sectional diameter at the second end of the reducer. This process includes a first step of securing a preform which consists essentially of an assembly of optical fibers having crosssectional diameters corresponding to the cross-sectional diameters of the optical fibers at the first end of the reducer. Generally, this securing places the preform in a vertical position. The second step involves applying heat to a portion of the length of the preform with a movable heater element. During this heating, an end-to-end tension is applied to the preform. When the heating reaches the point that the preform becomes plastic, the tension is maintained constant so as to taper the preform through the heating zone and draw a fiber having the desired smaller second cross-sectional diameter from the preform. The movable heater is moved along the preform to obtain the desired fiber diameter and tapered fiber optic reducer. Once this is formed, the heating element is withdrawn from the preform, and the fiber optic reducer is allowed to cool. The preforms which are formed into the taper are preferably themselves bundles of clad fibers, such that when the taper is formed, a plurality of fibers are themselves tapered. In preferred embodiments, each of these fibers is a clad material. That is, each fiber comprises a core surrounded by a concentric outer coating. In another embodiment of this invention, these clad preforms are formed. Preferably, this outer coating is continuous around the core when viewed cross sectionally. In another embodiment of this invention, clad preforms from which these clad fibers are made, are formed. In this embodiment, these clad preforms, which comprise a core surrounded by an outer coating, are formed. The core generally comprises an optically transmissive plastic material and the cladding comprises a second plastic material generally of lower index of refraction than the core material. These clad preforms are generally in the form of billets and are formed in a cladding oven by the process of fitting a preshaped body of core material inside a preshaped hollow sleeve of cladding material to give a sleeved core, placing this sleeved core within an open-ended pressurable fixture which encloses the outer surface of the sleeve in an open-ended configuration; placing this fixture in an evacuatable oven equipped with means for applying pressure to the ends of the sleeved core contained within the cladding fixture through the open ends of the fixture; applying heat to the cladding fixture and contained sleeved core and applying pressure to the ends of the sleeved core through the open ends of the fixture for a time adequate to cause the core and cladding sleeve to be compressed and form a single clad preform billet; and thereafter cooling the cladding fixture and clad preform and removing the preform from the fixture. The clad preform billet so formed may then be drawn into a fiber. These clad fibers may be tapered in accord with the invention or these fibers may be aggregated into bundled preform for forming high-resolution bundles of tapered fibers.

These and other advantages and features of the invention will be apparent from a review of the drawings and the following detailed description of the preferred embodiments.

Brief Description of the Drawings

Figure 1 is a schematic side view of an imaging system including an apparatus made according to the invention. Figure 2 is a perspective view of an optical fiber assembly constructed according to the invention. Figure 3 is a perspective view of an optical fiber reducer from the Figure 2 assembly.

Figure 3a is an enlarged prespective view of a typical clad optical fiber reducer from the Figure 2 assembly.

Figure 4 is a fragmentary perspective view of a fiber bundle used in forming a high-resolution reducer.

Figure 4a is an enlarged perspective view of a fiber bundle formed of clad fibers. Figure 5 a partial view similar to Fig. 1, showing a second preferred embodiment of the invention.

Figure 6 is an enlarged perspective view of the apparatus of Fig. 5.

Figure 7 is a further enlarged fragmentary perspective view of a portion of the apparatus of Fig. 5. Figure 8 is a cross section taken along line 8-8 in Fig. 7.

Figure 9 is a simplified fragmentary view of the output end of the transmission optical fiber array of Fig 5.

Figure 10 is a simplified perspective view of a third preferred embodiment of the invention.

Figure 11 is a frontal view of a system for drawing tapered fibers. Figure 12 is a frontal view of a mechanism for moving the oven which makes up part of the system shown in Fig. 11.

Figure 13 is a series of views of a vacuum fixture in exploded "story board" format, illustrating its assembly for use in preparing clad billets of preform material for forming tapered fibers.

Figure 14 is a frontal view of an apparatus useful for forming the clad billets of material which incorporates the vacuum fixture of Fig. 13. Figure 15 is a perspective view of a fiber array formed into a taper in accord with the invention. Detailed Description of the Invention

Referring initially to Fig. 1, an apparatus 20 made according to the invention is shown schematically in a radiotherapy system 22 for treating a patient 24. System 22 includes a source 26 of x-ray radiation directed during treatment along an incident beam path 28. The irradiating beam, indicated at 29, may also be a gamma-ray or other high-energy radiation beam. Although it is not shown, a radiation block is normally placed over the patient. The block has a port which allows a limited cross-sectional area of the radiation to pass into and through the patient. The alignment and dosage of the treatment is obtained by sensing the intensity distribution of the beam after it has passed the patient.

The patient is typically disposed on a bed or platform 30 below which is a limited .amount of space for positioning sensing or imaging equipment. The imagesensing and reduction portions of apparatus 20, shown enlarged in Fig. 1, will fit within this space. The apparatus includes a light-tight box 32. Disposed along a surface portion of the box is a plate-like beam converter 34, conventionally made of a sheet or foil of metal, such as tantalum, which converts the impinging irradiating beam into electrons, and therefore in essence, into an electron beam having an electron density distribution directly related to the intensity distribution of the incident beam.

The electrons created in the photon conversion reactions produce light in a sheet-like, fluorescent screen 36. This screen is preferably about 0.1 mm thick, but can be up to 1 mm or greater in thickness. The preferred screen has an active rare earth phosphor such as Gd₂O₂S:Tb. Other phosphors which would be suitable are La₂O₂S:Tb or CaWO₂. These fluorescent screens can be purchased from Eastman Kodak Co. Alternately a sheet of plastic scintillator can be used instead of the screen to give light output closely related to the dose in water. This could be very useful for dosimetric verification or quality assurance purposes. These plastic scintillators are commercially available from companies such as Bicron of Cleveland, Ohio and Nuclear Enterprises of Scotland. Screen 36 is also referred to herein as means for converting the electron beam produced by the converter to a pattern of visible light which is directly related to the irradiation beam impinging on the beam converter. The top surface 36a of the fluorescent screen, as viewed in the figure, is disposed adjacent the lower surface of converter 34. The bottom surface 36b confronts the input end of an optical fiber assembly 38, which will now be described.

As seen best in Figure 2, fiber assembly 38 is formed of an n × m image array of fiber reducers, such as reducer 40, which is detailed in Figure 3. Although the assembly illustrated forms a rectangular and preferably square input and output square array, it will be appreciated that a variety of other input and/or output array shapes, such as an oval or circular array, are possible. The assembly has an input end 42 whose area encompasses the area of the beam pattern, and a reduced-scale output end 44 whose area is preferably between about 1/25 to 1/200 of the area of the assembly's input end. The assembly functions to carry the light image formed by screen 36 to a location remote from the screen, where the image output is in reduced scale. As seen in Figure 1, assembly 38 has an approximately right-angle bend between its input and output ends.

With reference now to Figure 3, reducer 40, which is exemplary, is formed of a j × k fiber array of optical fibers, such as fibers 46. The fiber reducer is produced, in accordance with one method, by first forming a j × k image array of uniform-area optical fibers, e.g., square optical fibers 46 whose sides are between about 1-2 mm, and having lengths between about 25-100 cm. Such fibers may be formed, for example, by drawing out relatively large-dimension square optical light pipes or preforms to the desired uniform cross-sectional size. The total number of fibers forming the fiber array is preferably between about 100-400, and a preferred array is a 15 × 15 square array, i.e., having 225 fibers. This array, with 1.5 mm square fibers, would thus have side dimensions of about 2.3 cm, and a surface area of about 5-6 cm².

The fiber array is then placed in mold or fixture whose inner wall dimensions are just slightly larger than the dimensions of the array. The array is now heated in the fixture to a temperature which allows the fibers to fuse into a multifiber unit. Here it is noted that the fibers in the fused unit retain their individualfiber cladding, and thus continue to function as discrete light carriers. The fused unit is now preferentially heated along its length so that when drawn out, it will have a progressively reduced cross-sectional area on progressing from one end to another, and more particularly, from a larger input end 48 formed by the input ends of the fibers making up the reducer, to a smaller output end 50 formed by the output ends of the fibers. As indicated above, the area of the output end is typically 1/25 to 1/200 of the area of the input end. In one preferred embodiment, the input end has an area of about 6 cm², and the output end, an area of about 0.06 cm².

The reducers forming assembly 38 may be bent in row or column units by heating the multi-reducer array of straight-fiber reducers, or the reducers may be individually bent and then fitted together to form the assembly. It can be appreciated from Figure 1 that the individual reducers forming each "column" of the fiber assembly will have different lengths and bend curvatures.

The optical fibers forming each reducer are preferably made of plastic, since they can be made less expensively than reducers made from glass fibers. Plastic fibers from which the fiber reducer can be formed can be obtained from Fibre Optics Development Systems (Santa Barbara, CA). Glass fiber reducers can be obtained from such companies as Galileo of Sturbridge, Mass. or Reichert Fiber Optics of Southbridge, Mass. In the present embodiment, the fibers have a square cross section, as shown in Figure 3, although the image reduction could also be done with circular fibers or other shapes such as pentagons, hexagons, or the like. The fibers in the reducers have a turning radius of about 2 cm to 5 cm. The optical aperture of the "untapered" portion of each optical fiber is such as to transmit light whose angle of incidence, at the fiber input end, is within a cone of about 20 degrees. Because the fiber width at the output region has been substantially reduced, this cone of transmitted light will actually be much smaller, e.g., about 2 degrees when the output end fiber width is about one-tenth that of the fiber input end. Thus, besides providing for substantial space reduction, the fiber reducers also act as collimators to admit only that light that is entering essentially normal to the fiber ends. Thus, there is little blurring of the resultant image due to crossover of the light between scintillator regions defined by the fiber inlet ends.

A fraction of the scintillation light produced above each fiber is light piped through the fiber reducers, the output of which is viewed by a lens 52. When the screen or plastic scintillator sheet produces light in the green-red spectral region, a blue filter 54 is preferably placed between the output end of the assembly and lens 52 to filter out unwanted background light which is predominantly in the blue region of the visual spectrum.

The lens couples the optical signal transmitted through assembly 38 to a video camera 56. Figure 1 shows light rays from the fiber assembly passing through filter 54, lens 52 and into camera 56. The camera may be of the conventional vidicon, CCD or CID type. Further, instead of using a lens, the image may be coupled to the camera by proximity focusing, wherein optical fibers from a fiber optic coupler are placed directly on the fiber reducer array 38 at one end and onto the sensor array of the camera at the other end, or by coupling the output end of the fiber assembly directly onto the camera sensor array. The camera output is digitized, processed by a small computer or microprocessor 58 using conventional video signal processing programs such as that described in the prior art for signal generation and enhancement, or such as that sold under the proprietary name of Data Translation Frame Grabber. The digitized signal from processor 58 is displayed on a monitor 60, and is stored in the image processor memory for later use. This resultant image is available within a few seconds from the time the incident irradiation beam is directed through the subject. In applications requiring resolution of very low contrast images, cross-talk between fibers may degrade the resolution. Cross-talk can be prevented, in one embodiment, by making alternate fibers out of nontransmissive, i.e., black plastic. This prevents light from traveling between adjacent light-transmissive fibers. However, this approach inherently cuts down on the sharpness of the resulting image, since half the image information is removed. To some extent this loss may be compensated for by image-enhancing software that is available. An alternative approach is to coat each optical fiber with a second cladding of non-transmissive material, e.g., black plastic. Here each optical fiber will have a clear, light-transmissive cladding, and an outer dark, non-transmissive cladding. The nontransmissive cladding can be formed conveniently on the large-dimension optical fiber which is drawn to form the relatively small- dimension fibers used in forming the individual reducers. Although this is a more expensive solution, it allows more of the scintillation light to be transmitted.

Figure 3A shows how each fiber 46, 46A, etc. in reducer 40 can be formed with a cladding 45 and core 47, if desired.

A second source of resolution degradation which occurs is low level scintillation and/or Cherenkov emission resulting from electrons penetrating into the fiber reducer. This background can be greatly reduced by using an optical filter 54 as mentioned above. If proximity focusing is used, this filter must be very thin, of the order of 0.1 mm. If significant background is present, the fluorescent screen or plastic scintillator is chosen as mentioned such that it emits in the green-red region of the optical spectrum. Since the scintillation and/or Cherenkov background emitted from the fiber reducers is predominately in the blue, a long wavelength pass filter which filters out blue light effectively reduces the background from these sources.

Further background reduction can be attained by blackening the large input end of several fibers in each reducer, such as illustrated in Figure 3, which shows fiber ends 62. The darkened ends effectively block light transmission in the selected fibers. This provides in those optical fibers, a light signal which represents the background light being produced in and transmitted by the "clear-end" fibers. The image processor is designed to distinguish between the two and to subtract the background light from that produced in the active fibers.

A further alternative approach for obtaining background levels for the embodiment shown in Fig. 1 is to take an image when beam 29 is not being transmitted. This reading then represents the electronic system background, or non-radiation induced background. Radiation-induced background can also be measured by removing the scintillator screen when the beam is being transmitted and to use this as the background signal. Conventional digital image enhancement programs can readily subtract the background frame from the active signal frame to eliminate the background. These alternatives allow all of the fibers to transmit a beam image signal. Figure 4 shows a portion of a reducer 64 designed for a fiber assembly capable of high-resolution imaging of a radiation beam. The reducer is formed of a j × k array of fiber bundles, such as bundle 66, where each bundle has approximately the same dimensions as the individual fibers in the above reducer 40. More particularly, a preferred reducer contains between about 100-400 fiber bundles, and each bundle has an input end which is about 1.5 mm on a side, and an output end which is about one-tenth that dimension. With continued reference to Figure 4, bundle 66, which is representative, is composed of a c × d array of optical fibers, such as fibers 68. To form a fiber bundle, the optical fibers, which are preferably square fibers about 1-2 mm on a side, are arranged in a desired array, such as a square array containing between about 100-400 fibers, and these are fused as above. The fused block is then drawn down to a very small cross section, preferably about 1-2 mm on a side. The bundles, each of which contains between 100 and 400 individual fibers, are then combined to form a j × k bundle array which will, in effect, substitute for the individual fibers used in form ing reducer 40, to increase the total number of optical fibers in the reducer, and therefore the reducer resolution, by a factor of 100-400. Of course, the fiber bundles could be composed of .a smaller number of fibers, such as 5-100 fibers, which would produce a corresponding decrease in the total number of reducer fibers.

If desired, each of the fibers in the bundle may be clad so as to give a configuration as shown in Fig. 4A. A second preferred embodiment of the invention is shown in Figs. 5-9. In this embodiment, an optical image-generating apparatus 70 has a light-tight box 72, a strip photon converter 74, or beam converter, a fluorescent strip 76, a fiber assembly 78, a lens 80 and a video camera 81. The microprocessor and monitor are eliminated for simplicity of illustration.

The fluorescent strip 76 consists, in one embodiment, of a linear array of short segments of plastic fluorescent optical fibers 77 which are positioned to extend preferably normal to input regions of the fibers in assembly 78. An enlarged fragmentary portion of the strip is seen in Figure 7. The fiber segments making up the strip contain a primary fluorescent dye with a fluorescent emission in the ultraviolet or blue in response to the electrons generated in the strip itself and by the strip photon converter 74 placed directly above the fluorescent strip 76. The segments are nominally 1 mm² in cross section and 1-5 mm in height. The fluorescent strip could also be made of a thin strip of fluorescent screen as described in the first embodiment. In this case a 1 mm × 40 cm strip of fluorescent screen would be located directly below the strip photon converter, as was the linear array of scintillating fibers.

The fiber assembly is composed of an array of optical fibers, such as fibers 82, and preferably square polymer fibers having side dimensions of between about 1-2 mm, as above, although other cross-sectional shapes and dimensions may be suitable. An enlarged fragmentary portion of the assembly is seen in Figure 7. The fibers are arranged at their input regions in a parallel or side-by-side planar array, such that longitudinally extending input regions of the fibers, such as input regions 84, form an input array 86 whose area encompasses the area of the irradiation beam. The fibers are arranged at their output ends in groups of stacked fibers, as shown in Figure 6, which form a two-dimensional array of fiber output ends. In a preferred embodiment, the total number of fibers forming the input array is between about 100-1,000, where the width dimension of the individual fibers is preferably between about 1-2 mm.

A two-dimensional output array 88 (Figure 9) in the assembly is formed by stacking groups of the optical fibers in an n × m arrangement, as indicated in Figure 6. For example, where the assembly consists of 400 fibers, 1 mm in cross section, the output array can be formed by stacking 20 groups of 20 planar fibers, to form a square 20 × 20 fiber array. The fibers are preferably not tapered between their input and output end, so that the side dimension of the output array is 2 cm in this example.

The fibers forming the assembly are doped with a secondary or "waveshifter" fluorescent dye which responds mainly to light emitted from the fluorescent strip 76. The secondary dye is chosen so that its absorption band is well matched to the emission band of the fluorescent strip and so that it re-emits this light at longer wavelength. The fluorescent strip 76 and overlying converter strip are mounted at opposite ends to frame 90 and supported on a movable carriage and driver (indicated by arrow 96, which indicates the back-and-forth directions of movement of the two strips across the input array). The driver and movable carriage, which are entirely conventional, are also referred to herein as moving means. During operation, the two movable strips travel as a unit across the input array of the fiber assembly, with the fluorescent strip emitting a light-beam strip, in response to excitation by electrons, which is then absorbed by the dye in the assembly fibers, as indicated in Figure 8. This dye acts as a waveshifter to emit isotropically the desired light, a portion of which travels down the secondary fibers to the fiber output 88. It will be appreciated that the fibers in the fiber assembly need to contain internal fluorescent doping material in order to convert a portion of the light emission striking the fibers at substantially right angles into light that will propagate within the fibers.

Summarizing, the fluorescent screen strip functions to convert the strip regions of electron beams produced by the beam converter into a first-wavelength radiation beam strip. The secondary fluorescent dye in the assembly fibers functions to convert the firstwavelength beam strip into visible light emissions within the fibers. The moving means functions to move the metal strip and fluorescent strip as a unit along the lengths of the input regions of the assembly fibers, to generate in the fiber input regions, an instantaneous light intensity which is directly related to the intensity of the radiation beam which is impinging upon the metal strip. The fluorescent screen, secondary fluorescent dye, and moving means are also referred to herein as converting means for converting the electron beam image produced by the irradiating beam into a reduced scale optical image. The data obtained with the fluorescent strip 76 in a single x position gives a linear array of y values. The fluorescent strip is swept along the length of the fibers in array 86 - -i . e . , along the x axis . To obtain 1 mm resolution, data must be taken at 1 mm intervals. As with the first embodiment, the resolution of low contrast images may be degraded by background scintil lation caused by electrons or x-rays traversing the assembly fibers, thus giving a signal in addition to the signal from the fluorescent strip resulting from the incident beam. This background can be determined by optically isolating several of the fibers in the secondary fiber array from emission from the primary scintillator strip. This can be done by applying a black coating to the outside of these fibers. The output of these fibers can be read out and subtracted from the secondary fiber signal, thus removing the background. A second method for removing background is to remove the scintillator strip and expose the remaining apparatus to the radiation beam. This background signal can then be subtracted from the total signal as in the first embodiment. In some applications, the second embodiment of this invention can be simplified by leaving out the fluorescent strip, and simply scanning the converter strip 74 along the assembly fiber input regions. This would require the addition of both primary and secondary dyes to the core material in the fibers. All other aspects of the instrument remain unchanged. As the converter is scanned across the assembly fiber input regions, the radiation beam will be converted into electrons in the converter, thereby resulting in enhanced excitation of the scintillator immediately below the converter. Here the converting means would include the primary and secondary dyes contained in the assembly fibers, as well as the above moving means .

Reference is now made to a third embodiment of the invention as shown in Fig. 10 without relation to a patient or x-ray beam. This embodiment, shown as an apparatus 110 which substitutes for apparatus 20 in Fig. 1 or apparatus 70 in Fig. 5. In fact, this embodiment is more closely similar to that shown in Fig. 5. It includes a planar fiber assembly 112 whose optical fiber arrangement is substantially identical to that of assembly 78 in apparatus 7.0. In particular, assembly 112 includes an input array 113 formed by the input regions 115 of the fibers in a parallel side-by-side configuration, and an output array 122 formed by stacking groups of the fibers. Assembly 112 differs from assembly 86, however, in that the fibers 114 making up the assembly contain both the above primary and secondary fluorescent dyes. It is to be understood that a sheet-like photon converter would also be disposed above this assembly, preferably in a fixed position so that the electrons produced are directly relatable in physical position to the impinging x-ray beam.

The output array of assembly 112 is coupled to a digital camera as described for the embodiment of Fig. 5. The output array is divided into groups, such as group

120, for forming a rectangular output array 122 for viewing by a camera 126, through a lens 127. These components are mounted on a turntable 128 controllably driven, by suitable moving means (indicated by arrow 129) for rotation about a vertical axis 130.

For a single location of the ribbon, the signal obtained from each fiber is the line integral, or sum, of the signal resulting from penetration of the beam all along each fiber. The entire scintillator array - camera system is then rotated about axis 130. Data is acquired at many angular orientations. Algorithms such as those used for computed tomography scanners would provide image reconstruction.

The converting means in this embodiment thus includes the primary and secondary fluorescent dyes in the assembly fibers, and the moving mass for rotating turntable 128.

It will be appreciated that variations may be made in the foregoing embodiments without departing from the scope of the invention. In particular, it will be appreciated that the device can be readily adapted for imaging a particle beam. Here, since the beam particles can produce direct scintillation of a primary fluorescent dye, the beam converter in the above-described embodiments would not be required. The tapered fiber optic fibers or bundles useful in this invention can be prepared from preforms in a novel manner using the apparatus 200 shown in Figure 11. Unlike prior art processes, the invention process involves holding the fiber preform in a fixed position and moving a heater element gradually along the preform to melt it. Presently available machines used to draw optical fibers from preforms generally employ a flexible chain which lowers the preform into an oven. This system is inherently incapable of reliably positioning the preform. Reliable positioning is necessary to insure even heating. The presently available systems also do not provide for constant tension on the fiber during heating, nor do they permit the progress of the tapering to be viewed directly. The method and system of the present invention remedy these defects.

The present method and system involve holding one end of the preform in a fixed position while pulling on the other end of the preform while sliding an oven along the preform's length. This permits tension to be applied to the preform and further allows a steady positioning of the preform and resulting fiber in the heating zone. In addition, the progress of the operation can be viewed and a constant temperature maintained at the preform-to-fiber interface. A general overview of the fiber optic forming apparatus 200 is shown in Figure 11. The fiber optic substrate is preform 204. Preform 204 has a larger cross section at one end and is drawn to the fiber optic 204a of reduced cross section. The preform is held at one end by an XYZ positioning device 207. The other end of the preform extends through a heater/oven 211. The oven softens the preform such as at a temperature of about 250°C. This softening allows the preform to stretch into a prefiber and the lower portion of the preform to drop away. The prefiber is coupled to tractor pulling device 205 and a tension is drawn on the prefiber by tractor pulling device 205. As the oven softens and melts a region of the preform, the tension applied via tractor pulling device 205 effects a taper and draws out the preform into a fiber. The lower portion of the preform drops off as shown at 203 leaving the fiber of reduced diameter 204a.

The preform 204 can be a single body of fusable plastic fiber optic-forming material. It also can be a two-component material comprising a center core and an exterior cladding. In a preferred embodiments, it can be a multifiber preform made up of a plurality of prefibers bonded together. Each of these prefibers can be a single component or can be clad. In a preferred embodiment, each of the prefibers is about 1.5 mm² in cross section. These prefibers are formed into a solid preform rod.

The oven 211 is slidably mounted on slide rods 210 and 210A through eyes 202 and 202A. Oven 212 is held in position and moved along the preform 204 by a lead screw mechanism detailed in Figure 12. A plexiglass enclosure 209 surround the system and permits direct viewing of the progress of the taper development and permits the drawing environment to be controlled such as by flowing dry nitrogen into the box and excluding moisture and airborne particles. In operation, after the lower part of the preform drops off, as shown in 203, the fiber optic (preferably multifiber fiber optic) of reduced diameter 204a is placed threaded through the tractor pulling device 205. The oven positioning device shown in Figure 12 moves the oven gradually away from the tractor pulling device 205. This causes new regions of the preform to be fused by the heat of the oven. The tractor pulling device 205 draws out the softened and fused preform into a taper and into the fiber of reduced diameter 204a. By controlling the temperature of the oven, the speed at which the oven is moved along the preform and the speed at which the tractor pulling device pulls the fiber from the fused area, the size of the fiber approaches the desired fiber opticr size. The oven 211 is caused to move away from the pulling device 205 on rods 210 and 210A. This movement is very slow to maintain a constant supply of plastic fiber optic preform in the heater region. Preferably the device keeps a constant tension through the tractor pulling device on the fiber. The caliper 212 monitors the resulting fiber size, and the drawing speed is varied until the desired size is obtained. When the desired size is obtained, the oven 212 (typically a heater band) is moved out of the drawing region back onto the drawn fiber. The preform is left to cool in this configuration while the fiber is held under tension. Cooling is facilitated by passing a stream of cool gas through the oven region. The resulting tapered reducing fiber optic is then removed from the oven.

As shown in Figure 11, in a preferred embodiment, the various elements are arranged vertically. In this preferred configuration the preform holding device 207 is mounted above the oven and the tractor pulling device 205 is mounted beneath the oven. In this configuration, in use, the oven moves upward toward the XYZ positioning device and gravity assists the drawing. Figure 12 shows a preferred embodiment of a mechanism to effect the motion of the oven. Again, this is shown in the context of upward and downward motion. The oven mount 225 is threaded along the longitudinal lead screw 220 and the longitudinal screw is connected to a turning mechanism (not shown) wherein the connection is maintained and controlled by two clutches, 221 and 221A. The progress of the longitudinal screw may be further controlled by a boule or preform holder 222. The exact side-to-side position of the heater is precisely regulated by the slide bars 210 and 210A along which the heater unit slides through eyes 202 and 202A.

Thus, according to the method of this invention, the fiber preform is held in constant position at one end and under tension at its other end while a heater element is moved along its length to differentially melt a region and form that region into a taper.

The tapered product so formed can appear as shown in Figure 15 as number 46. As can be seen, 46 is a bundle of fibers having a large end 42 and small end 44. In practice, this bundle would be cut at the hatch marks to give a new large end 42a which would be polished to an optical surface.

As noted previously, in preferred embodiments of this invention, the individual fibers used herein are clad. That is, they have a light-transmissive core surrounded by an outer surface which has differing optical properties. Figures 11 and 12 illustrates one way to form these materials from clad preforms or billets. This invention additionally provides a new method and device for forming these clad preforms or billets. Turning to Figure 13, a block or loaf of optically transmissive plastic 130 is formed. This block has a defined shape, including a cross section 132 and length L. This will become the core of the clad material. It is fitted within a hollow sleeve of cladding material 134. The cross section 136 of the hollow opening of sleeve 134 corresponds to the cross section of core 130 so that the core may be slid inside. These dimensions should be closely tailored so that the space between the core and the cladding is relatively minimal. This sleeve with its enclosed core then fits within the void of hollow vacuum fixture 138. The cross section 140 of this hollow is sized to receive the outside dimensions of sleeve 134. The length L of the core, the length L' of the sleeve, and the length L" of the vacuum fixture are all substantially identical. Fixture 138 may be somewhat longer than the other two components, if desired. Fixture 138 is formed of a solid material capable of good heat transfer and also capable of withstanding substantial pressure. Aluminum or other metals are preferred materials of construction. The vacuum fixture loaded with core 130 and sleeve 134 is placed inside closable pressure fixture 142. Pressure fixture 142 has an interior cavity 144 having interior dimensions somewhat larger than the exterior dimensions of fitting 138 so that fixture 138 may fit inside. Vacuum fixture 142 is equipped with an O-ring seal and a replaceable door 148, which is sealably bolted to the opening of the fixture, thereby forming an enclosed pressure-tight box separately shown as 150. Vacuum fixture 142 is equipped with pressure rams or plungers 152 and 154. These plungers appear at opposite ends of the fixture and have plunger heads sized to fit into the end cavities of fixture 138. Thus when these two pressure rams move inwardly on their shafts 156 and 158, respectively, they impinge upon and compress the body of sleeving material 134 and core material 130 contained within fixture 138. Turning now to Figure 14 the use of this cladding fixture in the cladding process of this invention is illustrated. In Figure 14 three pressure fixtures, 150, 150a, and 150b are illustrated mounted within oven 160. In use, a vacuum supplied by vacuum pump 162 is applied to the interior of each of the three fixtures. The oven 160 is heated gradually from room temperature to about 125°C. This takes about one hour. After about two hours, the core and cladding materials contained within the vacuum fixtures are heated to a point that they are becoming plastic and flowable. Pressure is then applied to the plungers via shafts 156, 158, 156a 158a, 156b and 158b via drive units 164 and 166, 164a and 166a, and 164b and 166b, respectively. These drive units can be motorized or can be pneumatic or hydraulic. A pressure is raised to about 1600 psi and should be a slow, steady application of pressure. Preferably, the pressure is increased from about 1000 to 1600 psi over a 3- to 5-minute period. Pressure is held constant at this 1600 lb level for about one-half an hour. The rams may gradually move inward during this period as the two plastics flow and fill. Then the heat is turned off, and the vacuum is turned off. The three vacuum fixtures are allowed to cool to room temperature. No additional pressure is applied and as the system cools, the plastic in the vacuum chambers shrinks, thereby automatically releasing the pressure. Thereafter, the vacuum fixtures are removed and opened, the plungers are retracted, and the fixtures such as 138, are withdrawn from the vacuum fixtures. The plastic contained within the fixture 138 may then be removed from the fixture 138. The product so formed is a perfectly concentric core surrounded by a sleeve.

This sleeved product will typically be several inches in cross-section. It can be drawn to some smaller size either to form a single unit preform for tapering or, more preferably, drawn further to a pre-fiber size having

2 a cross-sectional area of about 1-2 mm for forming into a multi-fiber preform.

The cladding conditions just described, are exemplary. Any dimensions which will give rise to a suitable ratio of cladding material to core material may be used. Similarly, any shape, for example circular, octagonal, pentagonal, square or rectangular, may be used. Typical forming temperatures can range from about 100ºC to about 300°C and maybe higher, if the materials used will permit. So too the forming pressure may range from about 800 psi to about 3000 psi, or preferably from about 1000 to about 2000 psi. Typical forming times may be from about 5 minutes to several hours . Longer times could be used, if desired.

While this invention has been described with reference to certain preferred embodiments, it will be appreciated that it could be varied in many ways. For example, one is not required to use the cladding process herein described to form the preforms. Similarly, the fiber tapering process described herein is merely a preferred method. Other methods could be used, if desired. Similarly, the tapered fibers so formed could be used in embodiments other than the particular devices shown herein. Accordingly, the invention may be modified or applied in ways beyond those shown specifically in this application. The invention is as defined by the following claims.

Claims

It is Claimed :

1. Apparatus for generating a reduced-scale optical image of an beam pattern produced by directing a gamma-ray or x-ray irradiating beam through an object along a predetermined path, comprising an assembly of optical fibers which have input end regions disposed in a two-dimensional input array which encompasses the area of the beam pattern, and output ends which are disposed in a reduced-scale two-dimensional output array, a beam converter for converting the irradiating beam which passes through the object to an electron beam whose density distribution is directly related to the impinging irradiating beam, and means for converting the electron beam produced by the converter to a pattern of visible light which is carried by said fibers to their output ends, to form a reduced-scale image of the beam pattern produced.

2. The apparatus of claim 1, wherein the fibers in said assembly have tapered cross-sectional areas, on progressing from their input to output ends, and are arranged so that the fiber input ends form the input array, and the fiber output ends, the output array.

3. The apparatus of claim 2, wherein the fiber assembly is composed of an image array of fiber reducers having input and output ends which make up the input and output arrays, respectively, where each reducer is formed of a fiber array of fused optical fibers, and where the fibers forming each reducer are drawn, in fused form, to produce the reduced cross-sectional area on progressing from the reducer's input to output end.

4. The apparatus of claim 3, wherein the image array of fiber reducers has an approximately right angle bend between the input and output arrays.

5. The apparatus of claim 3, wherein the crosssectional area of reducers is reduced between about 25-200 fold between input and output ends.

6. The apparatus of claim 5, wherein each reducer includes between about 100-400 fibers, the area of each reducer's input end is between about 3-10 cm², and the area of each reducer's output end in between about 0.03 to 0.1 cm².

7. The apparatus of claim 2, for generating a high-resolution image of the irradiating beam, wherein the fiber assembly is composed of an image array of fiber reducers having input and output ends which make up the input and output arrays, respectively, where each reducer is formed of a bundle array of fused optical fiber bundles, each bundle is formed of a fiber array of optical fibers, and the array of fiber bundles making up each reducer has been drawn, in fused form, to produce a reduced cross-sectional area on progressing from the reducer's input to output end.

8. The apparatus of claim 2, wherein the fibers are light-transmissive polymer fibers.

9. The apparatus of claim 2, wherein said beam converter includes a metal plate which covers said input array, and said converting means includes a fluorescent screen disposed between the metal plate said input array for converting the electron beam produced by the metal plate to a light image which is directly related to the irradiating beam image which impinges on the metal plate.

10. The apparatus of claim 4, wherein the height of the fiber assembly, as measured along an axis perpendicular to the input array, is substantially less than the side dimensions of the area of the beam pattern.

11. The apparatus of claim 1, wherein the input array in said fiber assembly is formed by longitudinally extending input regions of the fibers arranged in a sideby-side manner, and oriented in one direction, said output array is formed as a two-dimensional array of the output ends of the fibers, said beam converter includes a metal strip extending across the input regions of the fibers, substantially transversely thereof, for converting associated strip regions of the irradiating beam to strip regions of electron beams, and said converting means includes (a) a fluorescent screen strip disposed between the metal strip and the two-dimensional input array for converting the strip regions of electron beams to a firstwavelength radiation beam strip, (b) a fluorescent dye contained in the input regions of the fibers, for converting the first-wavelength radiation beam strip to a visible light within the fibers, and (c) means for moving the metal strip and fluorescent screen strip as a unit along the lengths of the fiber input region, to generate in the fiber input region of each fiber, an instantaneous light intensity which is directly related to the intensity of irradiation beam which is impinging on the metal strip in the region between the beam and that fiber.

12. The apparatus of claim 11, which further includes an image processor for generating from the instantaneous light intensity received at the output of each fiber, as the metal strip and fluorescent screen are moved along the lengths of the fiber input regions, a two-dimensional optical image corresponding to the intensity of the irradiating beam impinging on the two-dimensional input array.

13. The apparatus of claim 12, wherein said fluorescent-screen strip is composed of a linear array of optical fibers.

14. The apparatus of claim 1, wherein the input array in said assembly is formed by longitudinally extending input end regions of the fibers arranged in a side-by-side manner, and oriented in one direction, said output array is formed as a two-dimensional input array of the Output ends of the fibers, said beam converter includes a metal plate covering the input array, for converting the irradiating beam to a related electron beam, and said converting means includes (a) a fluorescent dye contained in the input regions of the fibers, for converting the electron beam to a visible light within the fibers, (b) means for rotating the two-dimensional input array with respect to the object irradiated, and (c) means for processing the instantaneous light intensity signals from each fiber, as the fiber array is rotated with respect to the object, to construct the reduced-scale image of the irradiation beam impinging on the metal plate.

15. The apparatus of claim 1, wherein selected ones of the fibers do not transmit light resulting from the irradiating beam, and these fibers provide a measure of the background intensity of light transmitted in light-transmitting fibers.

16. An optical fiber assembly for transmitting a light image formed in one plane to a second plane, in reduced scale, comprising an image array of fiber reducers having input and output ends which make up input and output arrays, respectively, where each reducer is formed of a fiber array of fused optical fibers, the fibers forming each reducer are drawn, in fused form, to produce a reduced cross-sectional area on progressing from the reducer's input to output ends, and which contains a substantially right-angle bend between input and output ends.

17. The fiber assembly of claim 16, wherein the fibers are formed of a light-transmissive polymer.

18. A high-resolution optical fiber assembly for transmitting a light image formed in one plane to a second plane, in reduced scale, comprising an image array of fiber reducers having input and output ends which make up input and output arrays, respectively, where each reducer is formed of a bundle array of fused optical fiber bundles, each bundle is formed as an array of optical fibers, and the reducers are drawn, in fused form, to produce a reduced cross-sectional area on progressing from the reducer's input to output ends.

19. A method for manufacturing a fiber optic reducer which reducer comprises a body of optically transmissive plastic material tapered so as to have a larger first cross-sectional area at its first end as compared to the second cross-sectional area of its second end, which process comprises: obtaining a preform of said optically transmissive material, said preform having first and second preform ends and an elongate length along an axis, the cross-sectional area of the preform being substantially constant along the axis and corresponding to the first cross-sectional area of the first end of the reducer, securing the first end of the preform to a holding device, heating and stretching a portion of the preform adjacent to the the second end of the preform to form a reduced cross-section prefiber, coupling the prefiber to means for applying a pulling tension along the axis relative to the holding device and placing a region of the length of the preform intermediate the first and second ends in a heating zone, said heating zone being moveable along the axis of the preform, heating said region of the preform in the heating zone to above the plastic deformation temperature of the optically transmissive material while applying a pulling tension to the second end of the preform thereby drawing the preform to a reduced cross-sectional area, moving the heating zone along the axis of the preform toward the first end, said moving being at a rate gradual enough, to permit continued drawing of the preform to said second cross-sectional area and formation of a reducer tapering from said first cross-sectional area to said second cross-sectional area, and cooling the reducer so formed.

20. The method of claim 19 wherein said preform comprises a plurality of optical fibers arrayed parallel to one another and parallel to said axis such that the reducer comprises a plurality of optical fibers tapering along said axis.

21. The method of claim 20 wherein said holding device is positioned above said means for applying tension such that the drawing is downward.

22. The method of claim 21 wherein the heating zone is moved upwards during drawing.

23. The method of claim 19 wherein the heating zone is moved from the reducer during cooling.

24. A device for manufacturing a fiber optic reducer having a body of optically transmissive plastic material tapered so as to have a larger first crosssectional area at its first end as compared to the second cross-sectional area of its second end, from a preform of said optically transmissive material, said preform having first and second preform ends and an elongate length along an axis, the cross-sectional area of the preform being substantially constant along the axis and corresponding to the first cross-sectional area of the first end of the reducer which device comprises: means for securing the first end of the preform to a holding device, means for coupling the second end of the preform to means for applying a pulling tension along the axis relative to the holding device, a heating zone capable of heating said preform to above the plastic deformation temperature of the optically transmissive material and placable around a region of the length of the preform intermediate the first and second ends in a heating zone, said heating zone being moveable along the axis of the preform, and means for moving the heating zone along the axis of the preform toward the first end, said moving being at a rate gradual enough to permit continued drawing of the preform to said second cross-sectional area and formation of a reducer tapering from said first cross-sectional area to said second cross-sectional area.

25. The device of claim 24 additionally comprising means for measuring a dimension of said second cross sectional area.

26. The device of claim 25 additionally comprising means for storing a standard dimension for said second cross-sectional area and means for comparing the observed dimension with said standard and adjusting the drawing conditions to minimize differences between the observed dimension and the standard dimension.

27. A method for forming a clad plastic body comprising obtaining a sized elongate block of plastic core said core having a known length and a known cross sectional dimensions, inserting said core in a sleeve of cladding plastic, said sleeve having a length at least as long as the length of said core and internal cross-sectional dimensions sized to closely receive said core thereby forming a sleeved core, placing said sleeved core in a fixture enclosing said sleeve and permitting the application of pressure on the ends of said sleeved core axially along its length, heating said sleeved core and said fixture to a temperature at which at least one of said plastic core and said cladding plastic are flowable while applying a pressure to the ends of said sleeved core thereby cladding said sleeve to said core to give a clad core, cooling said fixture and said clad core, and removing said clad core form said fixture.

28. The method of claim 27 wherein a vacuum is applied to said fixture during said heating.

29. A system for forming a clad plastic body from a sized elongate block of plastic core said core having a known length and a known cross sectional dimensions and a sleeve of cladding plastic, said sleeve having a length at least as long as the length of said core and internal cross-sectional dimensions sized to closely receive said core thereby forming a sleeved core, comprising a heat transmissive fixture sized to enclose said sleeve and equipped with means for the application of pressure on the ends of said sleeved core axially along its length, means for heating said sleeved core and said fixture to a temperature at which at least one of said plastic core and said cladding plastic are flowable while applying a pressure to the ends of said sleeved core thereby cladding said sleeve to said core to give a clad core.

30. The system of claim 29 additionally comprising means for applying vacuum to said fixture.