Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J31/00—Cathode ray tubes; Electron beam tubes
  • H01J31/08—Cathode ray tubes; Electron beam tubes having a screen on or from which an image or pattern is formed, picked up, converted, or stored
  • H01J31/26—Image pick-up tubes having an input of visible light and electric output
  • H01J31/28—Image pick-up tubes having an input of visible light and electric output with electron ray scanning the image screen
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J29/00—Details of cathode-ray tubes or of electron-beam tubes of the types covered by group H01J31/00
  • H01J29/02—Electrodes; Screens; Mounting, supporting, spacing or insulating thereof
  • H01J29/10—Screens on or from which an image or pattern is formed, picked up, converted or stored
  • H01J29/36—Photoelectric screens; Charge-storage screens
  • H01J29/39—Charge-storage screens

Definitions

• the present invention relates to scanning image tubes especially those employed in video cameras. It is directed particularly to those devices which benefit from or require a sensor-target to have a large area capacity to perform optimally in an application such as to image the chest or abdomen of an adult with X-rays.
• Low velocity tubes have typically featured better detective quantum efficiency and contrast than high velocity tubes.
• High velocity tubes have, on the other hand, typically featured better lag performance (less lag) and spatial resolution and permitted the use of larger capacity layers in the sensor-target than low velocity tubes. Such characteristics are discussed in 1. the Electronics Engineers' Handbook, Second Edition, edited by Donald G. Fink, 1982;
• the first operates "without" gain and is exemplified by the vidicon, Plumbicon, Chalnicon, Saticon,
• Newvicon and Silicon vidicon The second operates "with” gain and is used in situations where there is insufficient light to operate “without” gain.
• These tubes are exemplified by the image orthicon, SEC and SIT tubes.
• the unique feature of this latter group is the incorporation of a front end intensifier structure designed to provide image charge multiplication.
• an imaging section In both classes of tubes, it is observed that there is included an imaging section and an electron beam scanning section. It should be noted, however, that in the "no gain” tube, the imaging section comprises simply a disc-like layer of material. It serves the dual function of being a photon sensor and a target on which to store a layer of electronic charge. For this reason, it is referred to herein as the sensor-target.
• the electron beam scans the inside surface of the sensor-target in a raster scanning fashion. It scans adjacent parallel lines of the sensor-target one after another. The electrons arrive at the sensor-target surface with low energies, and in particular, too low to cause any secondary electron emission. In the process, it deposits electrons uniformly across the surface and drives it to approximately the potential of the electron gun chathode which is usually at or near ground. Secondly, the electron beam generates a time varying video signal as it scans through the raster. This results from modification of the charge on the sensor target as a result of projection of the optical image onto the input sensor surface of the image tube. The process occurs on a successive pixel by pixel basis as electrons lost on the target surface through image formation are replaced by the scanning electron beam.
• the imaging section incorporates a medium with two functions; a sensor responsive to the incident radiation to be imaged and an insulating target having a resistivity on the order of 10 12 ohm cm.
• the high resistivity is essential to maintaining electron charge storage and immobility on the inner scanned surface of the sensortarget during a raster scanning period.
• the resultant electronic image is "readout" by the scanning electron beam.
• the tubes "with gain” have a forefront intensifier type structure which separates the sensor from the target. As a result, the sensor changes from a vidicon photoconductor to a photoelectron emitter.
• the scanner image tube In general, numerous factors must be considered if the scanner image tube is to perform effectively and optimally. .Besides the sensor-target's capacity and distributed capacity, other factors include the energy of the electron beam, its resistance and current, as well as the modulation transfer function (MTF) of the tube's imaging components. The latter depends on such factors as target thickness, the lateral displacement of charge stored on the sensor-target inner surface during a raster period, and beam current discharge characteristics.
• MTF modulation transfer function
• a high detective quantum efficiency (DQE) of preferably 100% is a desired feature for an optimal scanner image tube.
• DQE of 100% corresponds to a sensor having a quantum efficiency of 100%; output noise limited by the photon noise at the input where the image is projected initially—i.e. photon noise, should dominate other electronic sources of noise; and the MTF high enoughto assure that the dynamic range matches the imaging requirement throughout the necessary spatial frequency spectrum.
• x- ray intensifiers have been developed with diameters up to 22 inches. Such intensifiers are than optically coupled to a conventional sized TV camera. These systems are used in fluoroscopy and diagnosis. They suffer from intrinsically poor spatial resolution compared to that of the x- ray sensor. This results from multiplication of component MTF's. Poor spatial resolution manifests itself medically in two ways, i.e., in reduced diagnostic performance and increased dose to the patient. These deficiencies must be overcome.
• a "high velocity" beam video tube differs from the “low velocity” beam in that the electrons of the scanning beam are made to strike the inner surface of the sensor-target with sufficient energy to cause secondary electron emission.
• the electron bombarded surface gives off secondary electrons, which are collected by "collecting" electrodes.
• the scanned surface becomes positively chargedbecause of the lost secondary electrons, and the surface voltage rises until at equilibrium, the scanning beam is essentially equivalent to the collected beam.
• the senor When the sensor is then exposed to incident light, there is a loss of positive charge in a manner that causes an electronic image reproduction of the incident irradia tion image.
• the scanning beam then is able to replace the positive charge lost through secondary electron emission until the equilibrium potential is reached. In the process of replacing lost charge, it generates a corresponding video signal on a pixel-by-pixel basis which effectively creates the time varying video signal that is similar to that from the "low velocity" tube.
• the Iconoscope in the United States, and its counterpart, the Emitron, in Great Britain, were the first video tubes to incorporate the features of "high velocity" scan and the principle of charge storage.
• the Iconoscope suffered from the low sensitivity and poor efficiency resulting in large part from poor collection and redistribution effects of secondary electrons.
• a "high velocity" tube with a photoconductive sensor target was demonstrated in experimental tubes in the early 1950's. The purpose was to develop a new concept which would overcome the problems of lag and limited sensitivity. It was discovered that under certain conditions of operation, the tube could be made to operate with a high capacity target and provide improved lag performance. Furthermore, the higher capacity (thinner) targets also offered superior spatial resolution, and in theory, were expected to provide superior quantum efficiency. Redistribution effects were reduced in that the photoemitted electrons did not exist. The secondary electrons generated during scanning were less of a problem in one mode of operation and worse in the other. Shading continued to be bothersome, but again on a reduced scale due to better collector design.
• Diagnostic radiology requires large area imaging with attendant sensor-target capacity and distributed capacity both prohibitively large with "state of the art” video tubes.
• Solid state sensor panels employed in X-ray imaging are under development but suffer from other disadvantages. Some such sensor panels provide that readout be performed on the same side of the panel that is exposed. Moreover, the panel undergoes a cycling of voltages to respectively effectuate charging, reading, writing, erasing, and recharging. Accordingly, the panel must be transported from one station to another, rendering rapid imaging impossible. When such a system is automated, a large mechanical transporter is incorporated to move the panel from the exposure platform to its readout station. Disadvantages include cost, space occupied by the system, and the time involved to complete the process.
• the present invention improves over prior art scanner image tubes by overcoming various disadvantages and shortcomings set forth hereinbefore and by incorporating features that provide for more optimal operation.
• the present invention has as an object reducing, if not eliminating, the negative effects of beam resistance and capacitance in a low velocity image tube. Moreover, the present invention has as another object the realization of advantages relating to both low velocity beams and high velocity beams in a scanner image tube. That is, the present invention provides for good lag performance, for good spatial resolution, and for the use of large capcity layers in the sensor-target as well as providing for a high DQE and good contrast.
• a scanner image tube useful in radiology, especially in applications requiring a large area sensor—such as imaging a human chest.
• a scanner image tube is provided that can be operated in near real-time or in snap shot applications.
• diagnostic radiology the large majority of X-ray pictures are simply "snap-shots" as exemplified by the simple chest radiograph.
• Most procedures in angiography require repetition rates up to 7.5 frames/sec, while studies of the adult heart and coronary arteries can require real time imaging, i.e., 30 frames/sec.
• This capacity increases with the area of the sensor-target in conventional "low velocity" and "high velocity” video tubes.
• the area and distributed capacity associated with an individual preamplifier attached to a stripe can be sharply reduced.
• This approach requires multiplexing or a separate preamplifier per electrode stripe for optimal performance with the total given by the number of raster lines divided by the number of raster lines per stripe.
• Prior high velocity beam tubes have produced secondary electron emission that result in the backscattering of some of these emitted electrons back onto the sensor during scanning, causing image quality degradation.
• the present invention can employ a high velocity beam without such attendant degradation.
• Lag arises from the failure of the beam to return the surface of a target to the charged potential after a single scan. In viewing an image, undesirable smear results from lag.
• the invention thereby directs a more copious flow of electrons to effect readout and recharge of the target than is produced by the source.
• a scanner image sensor target including a first electrode; a second grounded electrode, said first electrode being positive relative thereto; a first solid state layer; a second layer; and means for a raster scanning beam of irradiation; wherein said first layer is sandwiched between said first electrode and said second layer,- and wherein said second layer is sandwiched between said first layer and said second electrode, and wherein said raster scanning means directs irradiation beams into said second layer through said second electrode, said second layer generating electrons which are conveyed toward the interface with the first layer for storage thereat; the electrons stored at the interface forming an electron layer displaced from said second electrode, the electrons of said electron layer being combinable with holes generated in said first layer in response to imaging irradiation passing through said first electrode into said first layer; the combining of holes from said first layer with electrons from the said second layer at the interface forming an electronic image thereat.
• an imaging pattern of irradiation in a given spectral ban strikes the first layer in the image section which generates free electrons free electrons and holes.
• the holes in a pattern analogous to the pattern of irradiation, drift toward the interface under the influence forming an electronic image thereat. Accordingly, the free electrons drift away from the interface toward the first electrode.
• the holes combine with electrons at the interface to form an electronic image thereat. Scanning with the beam results in readout and recharging at the same time.
• This structure avoids the negative effects of charge redistribution from both photoemission and secondary electron emission which doomed the success of the Iconoscope for such uses.
• the two electrodes represent plates of a capacitor and the first layer and the second layer from the dielectric therebetween.
• the second layer is subject to local charge multiplication where a scanning beam of electrons strikes and generates more electrons (or electron-hole pairs).
• a beam striking the second layer can produce almost a short circuit between the second electrode and the interface along a path determined by the beam—resulting in a low beam resistance. It is thus an object of the invention to increase tube performance and adaptability by reducing beam resistance alone and in conjunction with reducing capacitance by the utilization of striped electrodes.
• DELST Displaced Electron Layer in the Sensor-Target
• the beam can cause sufficient charge multiplication in the second layer along a path between the second electrode and a pixel at the interface—to provide practically a short circuit therebetween. It is, therefore, an object to permit an increase in the capacity of the sensor target by effectuating charge multiplication up to breakdown in the second layer in response to the irradiating thereof by the beam.
• This mechanism is designed to permit use of the high area capacity sensor-target with minimal lag.
• the selected material is responsive to the beam irradiation and generates electrons that charge or recharge an electron layer displaced from the beam section electrode.
• the selected material is subject to local avalanche breakdown triggered by the beam on a pixel-by-pixel basis. The breakdown provides a substantial short circuit through a pixel across the layer of selected material. It ensures an adequate supply of electrons for all applications.
• Figure 1 is a side-view illustration of a first embodiment of a scanner image tube employed in the invention.
• Figure 2 is a front view illustration of a sensortarget employed in the embodiment of Figure 1.
• FIG. 3 is an illustration of stripe electrodes each connected to respective circuitry in accordance with the invention.
• Figure 4 is an illustration showing a plurality of stripe electrodes being scanned by a fan beam according to the invention.
• Figure 5a is an illustration showing reduction in positive charge along" a stripe electrode when exposed to input radiation.
• Figure 5b is an illustration showing secondary electrons, generated during readout, being drawn away from the stripe which has undergone a reduction in positive charge.
• Figure 6 is an illustration of an embodiment of the invention including a displaced electron layer sensortarget (DELST).
• Figures 7, 8 and 9 are illustrations showing the effects of impinging input irradiation and a scanning beam on a DELST.
• Figure 7 shows the beam striking one surface of a sensor-target, and
• Figure 3 shows radiation impinging the opposite surface of the sensor-target.
• Figure 9 shows readout.
• Figure 10 is an illustration of a modification to the DELST shown in Figure 6.
• Figure 11 is an illustration of an embodiment of the invention including a DELST and an optical scanning beam.
• Figure 12 is an illustration of a prioximity focussed intensifier device employed in the invention.
• Figure 12a illustrates a generalized arrangement that utilizes the channel multiplier of Figure 12.
• Figures 13 and 14 are illustrations showing embodiments of the invention including channel multipliers.
• the essence of the invention is to provide a large capacity video type imager that is able to function in a manner that overcomes the customary limitations imposed by large target and large distributed capacities.
• One version makes use of parallel stripe electrodes spread across the sensor-target surface to offset large distributed capacity and in a further preferred embodiment, a "high velocity" beam is used in overcoming the problem of large target capacity.
• the stripe elec trodes are particularly important when the cross-sectional area of the sensor is required to be large, as in the case of diagnostic radiology.
• the invention offers freedom of design even in the case of small dimension sensor applications of using high dielectric constant materials for the sensor-target to increase target capacity.
• the advantage derived is in the target-sensor being able to store a larger charge, and thereby offer the opportunity to increase the dynamic range of the imager.
• a scanning beam with stripe electrodes provides the means for achieving maximum spatial resolution for the large dimension sensor, since readout occurs at the primary sensor-target. It avoids the intrinsic limitations of conventional systems used, for example, in diagnostic radiology.
• X-ray intensifiers are designed with a large degree of demagnification, and then have their output optically coupled to a video camera tube. The result is a substantial loss of resolution to the extent where the system is limited principally to fluroscopy and to intravenous angiography.
• a purpose of this invention is to provide a large area X- ray video imager able to meet the needs of diagnosis as distinct from fluoroscopy, for most of the procedures used in the practice of diagnostic radiology.
• Another version uses super "high velocity” electrons (kilovolt energies) and video display type electronoptics. This type of operation eliminates much of the usual interelectrode capacitance and minimizes the total distributed or shunt capacity. It effectively reduces "gain” requirements on the sensor and also reduces, if not eliminates, the need for stripe electrodes. Furthermore, the effective RC time constant is reduceable by offsetting large target capacitance with a dramatic decrease in beam resistance.
• another version uses a laser scanner to generate photoelectrons in a semiconducting target for charging and recharging a storage surface. It effectively eliminates interelectrode capacity, provides reduced beam impedance, and permits operation with a large target capacity.
• the "super high velocity" scanning electron beam and the laser scanner versions use a basic two layer structure, comprising a sensor layer adjacent to a target layer. In one mode of operation, the target layer must have the high resistivity associated with video charge storage as exemplified by SEC and EBIC targets in conventional "low velocity" scan tubes. Positive charge created during the scanning process is stored on the target surface adjacent to the inner surface of the sensor.
• the sensor layer must absorb incident radiation and convert it into representative charge which is transported to the interface between the sensor and target, where it discharges proportionately, the beam induced stored charge. since the stored charge layer is displaced from the outer (scanned) surface where it resides in a typical video tube operation to the inner surface at the interface, the structure is referred to as a "Displaced Electron Layer Sensor-Target,” abbreviated as DELST.
• DELST Displayed Electron Layer Sensor-Target
• the tube 100 has essentially the same structure for operation as a "low velocity” or a "high velocity” vidicon.
• the front end comprises a supporting member 102 on which is deposited a conducting layer electrode 104 and a photoconducting layer 106.
• the front end member 102 is glass and 104 is a light transparent, electrically conducting layer suitable for applying a bias voltage from source 122 to the outer surface of the photoconductor 106.
• the photoconductive layer 106 is selected to be responsive to the light.
• the structure 102 can be metal known to be transparent to the radiation.
• the metal combines the role of the supporting member 102 and the transparent electrode 104.
• the photoconductor 106 is then selected to be responsive to X-rays.
• the materials sensitive to the other known types of radiaton (gamma, alpha, ionic, cosmic, beta, neutrons, etc.) are well known so that the tube may be made responsive to substantially, if not all, of the various types of radiant energy.
• the sensor-target 106 comprises an efficient absorber of X-ray radiaton, Csl, which efficiently converts the incident X-ray into light emission.
• the light is absorbed by an adjacent layer of light sensor photoconductor with a transparent conducting electrode at the interface between the light emitter and the photoconductor to apply the bias voltage for the photoconductor.
• the electrode 104 is connected to a conventional video readout circuit 120 that includes a voltage source 122 which biases the electrode 104 relative to ground and a capacitor 126 which carries the video signal to a preamp.
• the resistance 124 serves as a load resistor to develop the video signal.
• an electron gun cathode 110 which provides electrons to form an electron beam 114 and which is directed by electron-optics 116 to scan a raster on the inner surface of the photoconductor 106.
• the tube can operate either as a "low velocity” or a "high velocity” vidicon.
• the biases for "low velocity” operation are selected so that the energy of the beam electrons is too low when arriving at the inner-scanned surface of the photoconductive layer 106 to cause any secondary electron emission.
• Typical for a vidicon would be the electrode bias 122 set at 30 volts and the gun cathode bias V c set at ground.
• the grid 118 is called the "decelerating grid” and has the function of slowing down the electrons so that their arrival of 106 will be with energies too low to cause any secondary electron emission. Accordingly, its voltage is positive with respect to ground, and typically about 1000V.
• bias settings for "high velocity” operation are selected to ensure that tube performance is governed by secondary electron emission.
• the scanned surface of layer 106 acquires a positive equilibrium potential such that the net target current equals zero.
• the equilibrium voltage V o is determined by the collector voltage V g and lies a few volts above it.
• V g the potential across layer 106 depends upon V t applied to electrode 104. Accordingly, the photocurrent can be made to flow in either direction according to whether V t is larger or smaller than the equilibrium potential V o .
• Reference to Figure 2 shows the circular sensortarget 206 identified in Figure 1 as 106. Traced on this target is a raster, whose vertical and horizontal dimensions, are taken equal (a square) for simplicity.
• the typical sensor-target capacity for a small video tube using a low velocity vidicon target using antimony trisulfide is on the order of 1000-4000 picofarads, although special tubes can be found with larger values. Assume for simplicity that a l"xl" raster is used and that the target capacity is 1000 picofarads. If this tube were enlarged to provide a 16"xl6" raster to meet the needs of diagnostic radiology, the capacity would grow in proportion to the increased area and be as large as 256,000 picofarads.
• the distributed capacity can be expected to grow, at worst, in a manner proportional to the increased area.
• the distributed capacity of the small size "low velocity" tube is in the range of 2-25 picofarads, depending upon the manufacturer. Assuming a favorable value of 5 picofarads for the small tube, the large area tube could have a distributed capacity of about 1,280 picofarads. The implication of these large capacitance values can be discerned by the associated capacitive lag and the preamplifier noise.
• the scanning beam instead of acting as a constant current source, acts as a resistance of the order of 10 7 ohms.
• the distributed (interelectrode-shunt) capacity is related to preamplifier noise through the expression
• the preamplifier noise current grows by a factor of 256 and the preamp noise becomes 67.6 na.
• a "state of the art" preamplifier has been reported for the small sized, low velocity tube with a current noise as low as 0.5 na. Good preamplifiers are available at about 1 na. Relative to the 1 na value, the large area tube illustrates a value almost 70 times larger and a corresponding reduction in the signal to noise ratio.
• the “high velocity” beam reduces the effect of target capacity on lag. The reason can be explained in terms of tube conductance.
• the current flowing into the target (I t ) is given by
• T effective temperature of the cathode responsible for the energy distribution of electrons in the beam. It is in excess of 1000°K.
• e electronic charge
• k the Boltzman constant
• Dresner shows that the beam conductance is given by:
• N(v) the energy distribution of the secondary electrons produced at the sensor-target
• N I b N where N represents the number of electrons effectively collected .
• the dependence of distributed capacity on the area of the sensor-target is dealt with in accordance with the present invention by using stripe electrodes as illustrated in Figure 3.
• the raster lines 302 are shown simply as parallel horizontal lines.
• the shaded stripes 304 represent electrodes placed one next to the other, spaced so that their separation distance is appreciably less than the vertical dimension of a pixel.
• Attached to each electrode is a preamplifier circuit exemplified by the load resistors 324, the target bias sources and the coupling capacitors 326.
• the distributed capacity is now associated with the area of each electrode stripe and the wiring to the preamp.
• the length of. each stripe as shown in Figure 3 is slightly longer than the length of a raster line.
• the width encompasses as many raster lines as device capacity and design allows. This number may differ for "low” versus "high” velocity operation because of a "secondary electron” redistribution problem inherent to conventional "high velocity” tubes.
• the number of preamplifiers depends upon the number of stripes given by the vertical raster, height divided by a stripe width. For a 16" raster height, and a 1/16" stripe width, the number of preamplifiers equals 256.
• a vertical set of stripes may be employed in place of the horizontal stripes. Such an arrangement would require the use of digital to analog conversion and storage so that the picture can be reassembled at the end of each frame.
• stripe electrodes to minimize distributed capacity is achievable at the expense of adding a large number of preamplifiers. Although, in view of modern integrated circuit techniques such numbers are acceptable, this number can be reduced by exposing each stripe separately, reading out the raster lines beneath each stripe, and then repeating the process for successive stripes. This is a procedure compatible with diagnostic radiology where an X-ray fan beam parallel to the stripes can be made to rotate so that each stripe is exposed and readout in succession, as shown in Figure 4. This approach permits replacing preamplifiers by switches or multiplexing to the extant that any increase in distributed capacity can allow. Shown in Figure 4 are the electrode stripes 404, their separation 408, the sensor- target shown here as one layer 406, the X-ray source 402 and the fan beam 410.
• FIG. 4 is the overlaying support structure shown in Figure 1 as 102.
• the scanner 412 is identified as the means to charge the inner-scanned surface of the sensor-target.
• the invention described to this point can apply equally well to the large area "high” or “low” velocity tubes.
• the sensor-target stripe capacity and the distributed capacity can be made essentially the same as for those found in conventional, small dimension video tubes.
• this invention demonstrates that the "low velocity" electron beam scanner can be used for X-ray diagnostic radiology, when coupled to this invention comprising electrode stripes with associated preamplifiers and can be further enhanced by using a translating fan beam and associated switching or multiplexing circuits.
• the high velocity beam offers the opportunity for increased target capacity without a corresponding increase in distributed capacity.
• This approach permits extending the dynamic range of the video sensor, since such range depends upon the magnitude of the charge that can be stored on the target, and the ability of the electron gun to read out totally that charge.
• the "high velocity" tube offers this improvement over the "low velocity” tube and without suffering additional lag.
• a porous layer of trisulfide target of 1000 picofarads could be replaced by a thin amorphous layer of 10,000 picofarads permitting a corresponding increase in the magnitude of the signal that can be managed.
• the thinner layer offers improved spatial resolution from the target with increased sensitivity.
• the principal disadvantage of the "high velocity” tube is in the redistribution of the secondary electrons. When secondary electrons emerge from a surface, they emerge in a range of velocities and angles according to the expression
• R v 2 cos ⁇ sin0 (6) where: R is the distance traveled from the point of emission on the surface of the target to the point of landing, if not collected by the mesh d is the target to mesh spacing m is the electron mass e is the electron charge v is the electron velocity
• ⁇ is the emergent angle of an electron trajectory relative to the normal to the surface
• Vt is the potential of the scanned surface of the sensor-target
• V c is the potential of the collector
• the number of secondary electrons generated varies according to each pixel potential, being more or less according to the degree of potential change experienced by the sensor-target.
• the capa ⁇ itive output signal current is largely neutralized by the return of the secondaries to other target areas, thus limiting the output signal to 25% of the level possible if all secondaries were collected.
• the raster lines adjacent to three stripes can be scanned in a manner designed to erase charge stored as a result of prior imaging or redistributed secondary elec trons.
• a new imaging charge distribution occurs along the lines within the stripe which results in the potential along the lines being depressed relative to these in the two surrounding stripes. Accordingly, there is a strong electric field set-up between the lines within the center stripe and those within the neighboring stripes. Then on scan readout of the lines within the exposed center stripe, the secondary electrons are drawn off either toward the collector or toward the adjacent strip and few electrons fall back on the lines within the exposed stripe.
• the signal readout is optimized by minimizing the dramatic signal loss associated with charge redistribution in conventional operation of a high velocity tube.
• the approach is illustrated in Figure 5 with three stripes 502, the sensor-target 504 and four scan lines per stripe 506. Each scan line is represented by a shaded region supporting a charge distribution.
• the center stripe shows a reduced positive charge distribution because of exposure in Figure 5a by X-rays 510.
• the procedure to expose and read out a full raster involves repeating the process shown in Figure 5 in sequence until all stripes are individually exposed and read out.
• An estimate of the total cycle time using this procedure can be provided for diagnostic radiology. Assume 256 stripes with each stripe exposed for 3 msec, so that the total time required to expose a 16 "long" surface scanned by a translating fan beam equals .768 sees. Assume also that each stripe and, therefore each line is scanned by the electron beam three times for the cycle required to minimize the effect of secondary election redistribution. Let the dwell time of the beam on a pixel be the same as for commercial television, that is about 10 -7 sec.
• readout time equals .075, .300 and 1.20 seconds. If, to these numbers, is added the total exposure time of 0.768 seconds, the total times to acquire an image for viewing and/or processing become 0.84, 1.07 and 1.97 sees respectively.
• Another procedure for reducing the effects of redistrubution is to switch biases on the stripes. In this case, the stripe to be exposed is made to be negative compared to the others. The sequence of events is then erase, expose and read out a stripe. During readout, the secondary electrons generated are collected by the mesh and the surrounding stripes. The magnitude of the signal is then determined by the exposure reduced only by the few electrons that might fall back on exposed lines.
• the number of stripes and associated preamplifiers is based on the assumption that the distributed capacity grows in proportion to the area of the sensor target. This is not necessarily the case in comparison with target capacity which clearly grows directly proportionate to area. With “high velocity” operation, considerably more target capacity can be tolerated, since the price of "increased lag” does not apply as it does for "low velocity”. The price for large distributed capacity is incrased electronic noise associated with the preamplifier. If, for example, the distributed capacity can be made to grow slowly with area, then the stripe widths can be increased, the number of stripes and correspondingly the number of preamplifiers reduced. Gain before readout remains an important factor in determining ultimate design of an X-ray sensitive video camera tube.
• the principal mechanism for obtaining such gain is with a luminiscent layer such as Csl, which generates about 1000 light photons per absorbed X-ray photon.
• a luminiscent layer such as Csl
• Csl which generates about 1000 light photons per absorbed X-ray photon.
• ideal conversion would offer 15 to 20 times more light photons.
• the consequences would be a similar improvement in signal and the signal to noise ratio (relative to distributed capacity induced noise).
• an X-ray sensitive photoconductor offering a similar range in gain from photons directly into charge carriers would provide the same advantages in signal and the signal to noise ratio.
• the design can trade off more raster lines per stripe (that is, wider stripes) and fewer preamplifiers versus some loss of image contrast due to recording an increased number of scattered events.
• Channel multipliers may also be employed to reduce the number of stripes required.
• the "high velocity" tube discussed above has been described with the usual raster dimensions of a conventional video tube.
• the principle of operation applies even better to a line sensor shape, as compared to an area sensor-raster shape.
• a line sensor for example, can be matched to an X-ray fan beam of radiation.
• the fan beam and line video sensor can then be translated in unison in a direction perpendicular to the plane defined by the fan beam and line sensor, to provide an image covering the area defined by the length of the sensor and the distance it is moved.
• This technique is well established in diagnostic radiology, but suffers in part from the nature of the sensors used in that application.
• Both the "high and low" velocity beam operation can operate with a long sensor, whose width is determined by resolution requirements and the number of raster lines required for an application.
• the simplest case is a single raster scan line along the sensor length.
• the device In the low velocity case, the device will have low shunting capacity and target capacity limited by the usual requirement on the RC time constant and preamplifier noise.
• the secondary electron redistribution effects become negligible by proper design of the collecting mesh and the device offers the opportunity to use a much larger target capacity, for superior dynamic range.
• Additional raster lines can be managed within the capacity limits imposed by an application. The advantages gained relate to decreased power requirements imposed on the radiation source (as for X-rays), and more speed in acquiring an image.
• the line scanner also offers options in its shape. It can be straight or curved. The latter could have applications, for example, in X-ray radiology to image a cylindrical object with a point X-ray source as in nondestructive testing. An example in medicine is to X-ray the breast with close proximity to the chest wall.
• This approach to a line scanner applies with equal validity to versions described below for DELST WITH A HIGH VELOCITY SCANNING BEAM and DELST WITH A LASER SCANNING BEAM.
• a device that can offer the advantages of the "high” and “low” velocity beam tubes without secondary electron redistribution effects, is the displaced electron layer sensor-target. It can be designed to function with either an electron beam or a laser beam.
• a first embodiment displaced electron layer sensor-target (DELST) 600 according to the invention is illustrated. It includes a sandwich structure 602 formed of a first electrode 604, a first layer 606, a second layer 603, and a second electrode 610. The first layer 606 is sandwiched between the first electrode 604 and the second layer 608. The second layer 608 is sandwiched between the first layer 606 and the second electrode 610. Between the first layer 606 and the second layer 608 is an interface surface 612.
• the first electrode 604 is connected to a conventional readout circuit 620 that includes a source 622 which positively biases the first electrode 604 relative to ground and a capacitor 623 which carries the video signal to a preamp.
• the second electrode 610 is connected to ground.
• DELST also shown in Figure 6 are the exposure side of the DELST indicated by imaging irradiation 630 directed to the electrode 604, and the charge-read side of the DELST indicated by a scanning beam 650 directed to electrode 610.
• the operation of DELST can be decribed in the following steps: 1. Use a scanning beam of particles or photons to penetrate electrode 610 and cause the generation of electrons in layer 608. Under the influence of the applied electric field due to the bias 622, the electrons flow to the interface surface where they are stopped and stored.
• layer 608 which makes possible overcoming the disadvantages of low velocity tubes. This occurs because this layer 606 permits operation with a scanning beam of high velocity particles or an intense scanning beam of photons, without suffering the imagedegradation of the prior art. In effect the electron storage surface 612 is now buried between layer 606 and 608 and effectively isolated from the source of any scanner electrons, or of unwanted secondary electron emission.
• Electrode 604 is transparent to the incident radiation comprising the image of some object. If the irradiation is light, then the electrode 604 must be transparent to light. If it is X-rays, then electrode 604 must be transparent to these X-rays.
• Layer 606 correspondingly must be responsive to the nature of the incident radiation. If the irradiation is in the visible range, it must be a photoconductor responsive to light. Similarly, it mast have a spectral response matched to the spectra of the irradiation throughout the spectra of interest, which can range from high energy gammas through the ultraviolet, visible and into the near infrared. As one moves to sufficiently long wavelengths in the infrared, this approach must be modified to accommodate the sensors' lower resistivity by providing cooling as needed. Nevertheless, even infrared responsive devices are derivable from DELST sensortargets.
• Appropriate materials for layer 606 could include, for example, properly doped germanium or silicon.
• the scanner beam 650 as shown in Figure 6 can be derived from high velocity particles or photons. Particles would be used in combination with a layer 608 selected to provide charge multiplication. This would permit a relatively low beam current of high velocity particles to cause a large current scanning beam to flow in the layer 608, suitable for charging surface 612 and readout with low lag. Most commonly, the particles in the beam would be high velocity electrons with sufficient energy to cause the required charge multiplication in layer 608. However, energetic beams of other particles such as ions and alphas could also be used for scanning.
• the scanning beam is derived from photons
• their source could be from devices such as flying spot scanners using incoherent sources of radiation, or coherent laser raster scanners.
• the essential requirement is that the 'flux of photons be sufficiently intense to generate sufficient numbers of electrons in layer 608 so that the charging and reading out functions can be managed properly.
• the layer 608 is made of a photoconducting material whose spectral response is matched to the beam radiation spectrum.
• FIG. 7 An example that can serve to il lustrate a DELST structure and its operation is shown in Figures 7, 8 and 9 for imaging with light irradiation and scanning with a high velocity electron beam.
• the layer 708 When the scanning beam projects energetic electrons through electrode 710, the layer 708 comprises a charge multiplication layer. Accordingly, the layer 708 generates more electrons than are incident thereupon.
• Various materials and mechanisms are known which provide this charge multiplication effect.
• One mechanism is referred to as electron bombardment induced conductivity (EBIC).
• EBIC electron bombardment induced conductivity
• Typical EBIC materials include semiconducting glass, magnesium oxide, and silicon. When such materials are struck, or bombarded, by electrons of high enough energy, electron-hole pairs are generated which exceed the number of incident electrons.
• a second mechanism relates to seocndary electron conductivity (SEC).
• SEC seocndary electron conductivity
• KC1 Potassium Chloride
• Another type of known charge multiplier is the channel multiplier commonly found in second generation image intensifier tubes.
• the ideal multiplier layer 708 would simply provide a short circuit between electrode 710 and the interface surface 712.
• Figure 7 is shown the flow of multiplied electrons in an SEC type target, 708, which, under the influence of the electric field, drift toward the interface where they are stored.
• the high velocity beam electrons 750 penetrate layer 708 bypassing through electrode 710, they trigger an avalanche of electron flow, affecting a short circuit flow toward the interface 712.
• Each picture element (pixel) on the interface 712 charges with electrons successively as- the beam scans through a raster with each avalanche terminating as the voltage drop between 710 and 712 becomes too small to sustain breakdown. This would ensure the maximum number of electrons available in minimizing time for storage and signal readout.
• Figure 7 is designed to show the beginning of electon deposition at the interface 712 as the raster scan is initiated. Note that although layer 708 is responsive to the high energy beam of electrons 750, it is not responsive to input imaging irradiation 730 passing through electrode 704 and into layer 706. This scanning process results in a uniform deposition of electrons at the interface surface 712.
• the input layer 706 is similar to the photoconductive sensor-target of the vidicon type tubes. It can have any of the detailed structures exemplified by the Vidicon, Saticon, Chalnicon, Newvicon, and Silicon Vidicon.
• the bias voltage 622 On absorption of incident imaging radiation, electrons and holes are created which drift in opposite directions due to the internal electric field established by the bias voltage 622. This is illustrated in Figure 8. There is shown a charged interface 814 and the movement of holes drifting toward the interface 812. On arrival, they remove stored electrons by recombination. This results in the uniform distribution of electrodes at 312 being modified to form an electronic image reproduction of the optical image.
• Readout of the electronic image 916 is illustrated in Figure 9, where the high velocity electron beam 950, in collaboration with the multiplier layer 908, is shown replacing the interface layer of electrons removed in the imaging process. This is done on a pixel-by-pixel basis during a raster scan, and results in a video signal being picked up by the preamplifier circuitry 920.
• the input electrode 904 is electrically conductive while being transparent to the imaging light.
• the scanned electrode 910 is also electrically conducting while permitting of the scanning electrons into layer 908.
• the process of imaging and readout has been described in sequence. In fact, the process can be managed either in sequence or concurrently, similar to these possibilities for conventional video tubes.
• the forming of the electron layer may be thought of as charging or recharging the target uniformly to a predefined equilibrium voltage.
• electrons generated in layer 908 are deposited on the interface surface 912. Ideally they do not penetrate layer 906, at least to the extent that significant video signal is lost.
• This is managed by the selection of materials in layers 906 and 908 and their treatment during deposition so as to form a blocking layer of the surface 912 preventing movement of electrons from layer 908 into layer 906.
• Such layers are well known in the art.
• the electron density pattern is read out over line 924 as the beam 950 scans. This is shown in Figure 9. Specifically, when the beam generates electrons which are directed toward a portion (i.e. pixel) of the interface 912 where no electron-hole combination has occurred, there is no variation detected at electrode 904. The voltage at the line 924 does not change. When the generated electrons recharge a portion (or pixel) that has lost electrons due to recombination, a surge of current—depending on the level of recombination—is detected at line 924.
• the first layer 906 and the second layer 908 together represent the dielectric between two capacitor plates—namely the two electrodes 904 and 910. Interposed between the two plates (i.e. the two electrodes 904 and 910) is the electronic image layer at the interface 912— displaced from the second electrode 910. Between the interface 912 and the second electrode 910 is a voltage drop ⁇ V.
• the second layer 908 of a material that undergoes avalanche breakdown or other such electronic multiplication when bombarded by electrons from a beam, the voltage drop ⁇ V approaches zero between the second electrode 910 and the interface 912 where breakdown has occurred. The minimum for ⁇ V is determined by the voltage at which avalanching or charge flow can no longer be sustained.
• the electronic breakdown is localized. Specifically, breakdown occurs where the beam 950 causes electrons to be generated at a particular time. Hence, as the beam 950 strikes the second layer 908, avalanche breakdown occurs along a path in the second layer 903 from the second electrode 910 to the interface 912 along which electrons are generated and flow to the interface. Such a path represents a substantial short, and the localized resistance can momentarily approach zero. With a low intensity, high velocity beam 950, successive portions of the interface 912—whereat target charge is or is not stored— can be simultaneously read out and recharged as required.
• the present invention displaces the image forming electron layer from the equivalent of the second electrode 910—where it typically is positioned—to the interface 912 and achieves charging (or recharging) not with electrons from a low current beam subject to high beam resistance but rather with a larger flow of current generated from layer 908 electrons that avoid the source of conventional beam resistance.
• the reduction in beam resistance permits the sensor-target capacity to be increased without adversely affecting lag.
• the increased sensor capacity may be reflected in a larger area sensor and/or a a thinner sensor.
• the larger area sensor permits use of the present invention in a broad range of applications.
• the increased thinness permits (a) extended dynamic range where the sensor can support increased charge density and (b) improved spatial resolution.
• stripe width and stripe separation may be employed in further overcoming capacitance.
• the target performs as an SEC layer, for example, electron flow induced by the "high velocity" electron beam flows away from the interface, causing the inner surface of the target to become positively charged.
• the sensor is an n type material such as CdS, the absorption of radiation causes electrons to flow toward the interface and discharge the positive surface of the interface. In this mode of operation, the sensor need not store charge and can have a somewhat reduced resistivity. It now has the opportunity to also provide gain, since space charge limitations need not prevail.
• biases can be applied to the stripes, with the video signal picked-off the opposite surface electrode. This offers the advantage of individual bias control, which can be important for optimization in obtaining uniform response, as well as unusual applications.
• Another arrangement has stripe electrodes on each side of the DELST target, so that individual bias and preamplifier stripes are paired to ensure optimum performance.
• the stripes can be placed on either the sensor or target surface, being required only to be transmissive to the irradiation at their surfaces. Referring now to Figure 10, a modification is applied to the DELST in 602.
• a light emitting X-ray sensor such as a cesium iodide layer 1000 is positioned between input X-radiation and the first electrode 1004 for another embodiment of DELST 1002.
• the cesium iodide layer 1000 converts X-ray photons into light photons which pass through the first electrode 1004 to strike the first sensor-target layer 1006.
• the second layer 1008 is a charge multiplication layer that forms an interface region 1012 with the first layer 1006.
• An electronic image forms at the interface 1012— displaced from the second electrode 1010—as described in the previous embodiments.
• the electron beam has a comparatively low current of one microamp, and the charge multiplicaiton layer has a gain of 200.
• the target is a conventional material having protions, or pixels, thereof which discharge within 10 seconds.
• An estimate of the charge ratio can be obtained from the beam diameter, the dwell time of the beam, exposure, and gain.
• the specific example is characterized by a beam diameter A b of .016 mm; an input flux of radiation directed toward the first electrode of 2 x 10 5 phontons/mm 2 ; a sensor detective quantum efficiency (DQE) of 50%; and a pixel area A p of .02 mm 2 .
• the number of holes generated per pixel to remove stored electrons is calculated from the expressions
• the flux 2 X 10 5 photons/mm 2 corresponds to the flux available in a typical low light level condition. Also, this flux corresponds to the X-ray flux available in diagnostic radiology in an exposure of 1 mR of 50-60 KeV photons.
• N q The number of electrons per pixel available for charging N q is then determined from:
• the scanner image tube includes a light scintillator in the form of a cesium iodide layer. Accordingly, the effect of the cesium iodide layer must be accounted for in the above calculations.
• the cesium iodide layer is known to generate on the order of 1000 light photons for each absorbed X-ray photon. These light photons are absorbed by the sensor, e.g., the first layer 1006 of Figure 10, whereupon electron-hole pairs are generated. At a quantum effici ency of 100%, 1000 absorbed electron-hole pairs are generated in response to the 1000 absorbed light photons.
• the number of electrons that combine with the holes conveyed to the interface 1012 through the first layer 1006 thus increases by a factor of 10 3 .
• the number of electrons that can combine with holes equals 2 X 10 6 .
• the charge:discharge ratio i.e. the ratio of (a) electrons directed to a pixel from scanning versus (b) electrons lost through the recombination, reduces to 6.7 x 10 3 .
• the high charge:discharge ratio results in rapid and total readout and recharge. It is further noted that the ratio can be further increased by increasing the charge density of the beam or increasing the gain in the second layer. In this regard, silicon has been reported with gains as high as 3000.
• asuitable high resistivity photoconductive first layer 1006 that is responsive directly to the X-rays may be employed—thus obviating the need for the additional cesium iodide layer 1000.
• a consequence of the high charge:recharge ratio is that the leading edge of the scanning beam is able to effectively cause discharge.
• the full beam diameter is not necessary for recharge, and the effective MTF of the beam's contribution to spatial resolution is improved.
• FIG. 11 shows a further embodiment of the invention.
• a laser scanner image DELST 1102 includes the first layer 1106 sandwiched between the first electrode 1104 and the second layer 1108.
• the second layer 1108 is sandwiched between the first layer 1106 and the second electrode 1110.
• Between the first layer 1106 and the second layer 1108 is an interface region 1112 whereat an electronic image is formed.
• a laser beam 1150 scans the second electrode 1110 which is transparent to laser radiation.
• the first layer 1106 and the second layer 1108 are each photoconductive layers of high resistivity.
• the first layer 1106 is responsive to incident radiation, e.g., X-radiation, and does not respond to laser radiation.
• the second layer 1108 is responsive to the laser input but is insensitive to the incident radiation.
• photoconductive layers 1106 and 1108 can be selected and their thicknesses adjusted to absorb the radiation passing through their adjacent electrodes 1104 and 1110, respectively. Such layers ensure that radiation entering a layer through an electrode blocks any transmission to the opposite layer and thus each layer is exposed to only the radiation intended.
• a light blocking layer can be interposed at surface 612, while maintaining the prerequisite electronic properties prescribed for DELST operation.
• the interface 1112 represents an electron layer displaced from the second electrode 1110. Electrons generated in the second layer 608 are blocked at the interface 1112. Electrons and holes are generated in the first layer 1106, the electrons moving to the first electrode 1104 with the holes combining with electrons at the interface region 1112 to define a distribution of charges along the electron layer.
• the second layer 1108 may provide gain, if desired. However, the number of electrons generated by the laser scanner 1150 may create a large enough charge:discharge ratio to enable readout and recharge following electron-hole combining at the interface 1112.
• the incident radiation may be light, X-rays, or other radiation.
• the first layer 1106 is selected to be responsive thereto—such materials being known in the art.
• a sample laser scanner image DELST operates as follows with specified parameters.
• a 10mw laser beam provides 10 5 photons/sec.
• ⁇ 6000 Angstroms, this equals 3 10 16 photons/sec.
• the number of photons avai lab le at a pixel with dwel l time t of 10 sec is :
• the number of photoel ectrons created by photoconduction in the s econd layer 1008 is gi ven by :
• n e 3 X 10 9 electrons.
• n e 3 x 10 8 electrons.
• the number of electrons, ⁇ e is compared to the photon irradiation effects at the input to the imaging sensor.
• a typical low light level condition is irradiation with 10 photons/mm 2 .
• a pixel of .15mm by .15mm would be exposed to about 10 3 photons/mm 2 .
• the ratio becomes 10 3 3 x 10 5 . In both cases, we have assumed conservatively that the laser beam and pixel diameters are essentially the same.
• the laser scanner imager 1102 may include a cesium iodide layer (see Figure 10) to convert X-radiation into radiation matched to the first layer (if the first layer is not responsive to X-radiation).
• the Csl light photon gain provides a factor of 1000 which reduces the ratio to 3 x 10 3 or 3 x 10 2 for 100% and 10% efficiency, respectively.
• a conventional low velocity electron beam operating with light and a similar pixel size and dwell time, and a beam current of 2 micoamps provides a charge-discharge ratio of 10 3 .
• This ratio it is observed, is quite small compared to present invention embodiments having light radiation input and is, in fact, comparable to the ratio for X-ray input.
• the charge:discharge ratio may be enhanced as desired by increasing the laser power, increasing the dwell time, and/or providing gain in the second layer 1108 as in the electron beam embodiments.
• photoconductive layer is used in a generic sense to include one or more layers of material having structures such as intrinsic, p-n, and/or p-i-n layers to provide a photoconductive effect with associated properties such as high impedance, good spatial resolution, and good speed of response. It applies also to heterojunctions from dissimiliar materials.
• the present invention achieves high spatial resolution and an enhanced MTF.
• the fixed potential electrodes combine with the high charging electron numbers to greatly reduce electron beam shot noise of prior art devices, sincedepleted charges are completely replaced.
• the senor need not face space charge limited performance and can provide gain as described earlier for the "high velocity" electron beam scanned DELST.
• the large area DELST application is typified by diagnostic radiology, and in particular to X-ray imaging of the adult chest or abdomen.
• the conventional X-ray film size for such applications is 14" x 17," or 238 square inches.
• This large area provides a daunting problem for imaging with a low or a high velocity image tube.
• In the low velocity tube it is necessary to overcome large target and distributed capacities.
• In a conventional high velocity tube where bias is in the order of a hundred volts, distributed capacity is as much a problem, and in addition charge redistribution effects must be minimized if not eliminated.
• the structure of DELST is designed to eliminate the effects of lag from target capacity and low sensitivity from secondary electron redistribution.
• the effect of distributed capacity can be minimized when using "super high velocity” type electron beam scanning (kilovolt range), which makes use of display electron-optics.
• the field and suppressor meshes of low velocity tubes which are close to the target surface and are principal sources of interelectrode capacity and thus are a source of preamplifier noise, are eliminated.
• the remaining distributed capacity related to electrodes are associated with electrodes on the inner wall of the tube, and as such is small. Furthermore, its increase with tube dimensions approaches linearity with increasing sensortarget diameter, rather than its square when proportional to area.
• stripe electrodes were implemented as a solution to the problem of distributed capacity in dealing with conventional "low” and “high” velocity tubes. This approach can also be applied to the DELST structure to the extent that capacity remains a problem in any particular application.
• the first is to use a photoconductive 606, as in Figure 6 responsive to the incident photons or as layer 1006 in Figure 10 responsive to the light from the X-ray sensor 1000, to provide substantial photoconductive gain.
• the second way to provide gain is to use an intensifier placed before the DELST structure.
• an intensifier placed before the DELST structure.
• forefront intensifiers have been incorporated in the image orthicon, image isocon, SEC and SIT tubes, with target gains ranging from as little as 2 or 3, to as much as 3000.
• the use of a DELST structure to replace these targets requires that layer 606 in Figure 6 be responsive to imaging energetic electrons, such as materials used for EBIC and SEC targets.
• forefront intensifiers can also include Generation I, II and III intensifiers optically or fiber optically coupled to the DELST target, wherein the layer 606 would be photoconductive.
• Channel multipliers incorporated into proximity focussed intensifiers are also applicable when coupled to the input layer 606.
• Channel multipliers have also been made to be directly responsive to incident radiation and can avoid in some cases the need for a photoemissive photosensor arranged with proximity focussing before the input surface of the multiplier.
• the output surface would traditionally be placed for proximity focussing of exiting electrons to impinge on layer 606 through the electrode 604, where layer 606 would comprise an EBIC or an SEC material.
• the channel multiplier can be eliminated and the simplier, less expensive one or two stage proximity focussed intensifier can suffice.
• a simple one stage device would consist of a photoemitter placed before and in close proximity to electrode 604, which would be selected to permit electrons to pass through into ther EBIC or SEC layer 606.
• the geometry of proximity focussed intensifier devices shown in Figure 12 is particularly advantageous for application to large area devices, such as required for diagnostic radiology.
• There a typical sensor would be a layer of Csl, on which was deposited a photoemitter such as CsSb. The emergent photoelectrons could then impact electrode 1204 directly or through a channel multiplier.
• the total structure would comprise a metal cap 1220 transparent to incident X-rays 1230, whose inner surface would support the Csl and CsSb layers 1222 and 1224 respectively and be proximity focussed through the vacuum space 1226 to the DELST structure as shown in Figure 12 — an electrode 1225 is located between the Csl and CsSb layers and is held at a negative potential relative to electrode 1204. With this approach, electrode 1204 would permit photoelectrons to pass through an impact layer
• the raster scanner 1240 could be a source of a high velocity electron beam 1242 which would pass through electrode 1210 and impact layer 1208 functioning as an electron multiplier.
• the interface 1212 supports the electron layer for image reproduction as described earlier. In this arrangement, the bias is reversed to expedite electron flow through layer 1206 to the interface.
• the raster scanner 1240 could be a source of laser radiation 1242, which would pass through electrode 1210 and be absorbed by a photoconductor in layer 1208. Again an interface surface 1212 serves to support the electronic imaging layer.
• the bias In arrangements with the channel multiplier serving to provide gain of an imaging signal, the bias must be arranged so that the first electrode 1204 is biased negatively with respect to electrode 1210. This ensures that the photo-emitted and multiplying electrons drift toward the interface. Furthermore, holes generated in a semiconducting solid state EBIC layer 1208 created by the electron beam or laser beam from the scanner, will also drift toward the interface, as required to discharge the imaging electrons in generating a video signal. The mechanism differs with the SEC type of layer, where the interaction at the interface only involves electron flow discharging through the SEC layer.
• the channel multiplier when operating with a relatively low gain requirement offers the potential for the very useful design advantage.
• the channel multiplier need not be self supporting.
• the input window can be used to support all device layers, and thereby offers more flexibility in acquiring an appropriate multiplier structure.
• Multipliers can be designed to function for example, without the difficult requirements imposed in large area applications for rigidity and sturdiness. Since they would essentially lie against the sensor which in turn would be supported by the input window, the multipliers need not be self supporting. They could be optimized for spatial resolution and gain. This could be done with far less concern for mechanical properties the multiplier must possess as in proximity focussing or whether it be made of metal, glass or any other suitable substance. Fragility and rigidity simply become far less severe requirements with this design. Examples of input windows that could apply here would be metal discs as used in X-ray imaging or glass discs as used in imaging with light.
• the rear surface of the device could also provide a source of structural support.
• the rear surface need only be selected for optical transmission matched to the spectrum of the scanning photons.
• the rearwindow could be a disc of glass for transmission of the light beam from a scanning laser.
• the front and rear windows both provide structural support and in effect permit the construction of a sturdy sandwich-like large area video sensor. Between the front and rear windows would be placed in effect three layers: In succession from the inside surface of the front window to the inside surface of the rear window, they would comprise the sensor layer structure, the channel multiplier and the solid state layer.
• FIG. 12a A generalized arrangement that utilizes the channel multiplier is shown in Figure 12a.
• This figure illustrates the channel multiplier arranged to be separated from the scanned layer 1208' and the photoemitter 1224' by a vacuum space 1225'.
• the separation oneither side if sufficiently wide, permits incorporation of electron optics as used in the Generation I image intensifier, and if sufficiently thin, can function with proximity focussing.
• the photoemissive layer 1224' can be deposited on the input side of the channel multiplier. Another option is to place the channel multiplier in contact with layer 1208'.
• the device can function with any of these arrangements, i.e., with or without separation on either side of the channel multiplier relative to the appropriate layer 1224' or 1208', and thereby offers the opportunity to select the arrangement that best fits any one application. For example, in large diameter X-ray imaging, the opportunity to support the channel multiplier against another surface reduces the need to make the channel plate rigid and inflexible.
• layers 1206 and 1208 are desirable for maximum versatility, i.e., either could be a photoconductor or a charge multiplying layer.
• the designer given the nature of the incident radiation, the cross-sectional area of the device and its imaging requirements can seek to optimize system performance by judicious choice of all components in Figure 12.
• Gain achieved through the use of photoconductive gain in layer 1006 of Figure 10 is the simplest and most desirable method and should be used whenever possible. It reduces the number of components required for the device and thereby its size and cost.
• layer 1006 in Figure 10 can be managed with a material such as ZnCdS or even CdS, which have been developed with high resistivities.
• a material such as ZnCdS or even CdS, which have been developed with high resistivities.
• These characteristics are important for DELST operation, since even though reamining distributed capacity is reduced, any residual capacity induced preamplifier noise can be overcome by this magnitude of photoconductive gain.
• the resultant performance would not suffer from any distributed capacity induced preamplifier noise.
• the distributed capacity is expected to grow, at worst, approximately linearly with diameter. Accordingly, an increase in diameter should see a 20-fold increase in noise associated with going from a 1 inch to a 20 inch tube. This could well be in the order of "state of the art" preamplifier noise, so that only a small gain, if any, need be included. Nevertheless, for unforseen circumstances, the order of gain available provides solutions to any distributed capacity problems that might arise.
• the DELST offers signal current in principal up to hundreds of micoramperres and with more gain to milliamperes.
• the result is that the preamplifiers attached to stripe electrodes become much less sophisticated and far less expensive.
• it permits reduction in stripe numbers through the use of wider stripes and/or multiplexing.
• it makes the use of simultaneous multiple fan beam exposures easily possible for diagnostic radiology, permits more scan lines per stripe as well as wider stripes.
• the number of fan beams is thus determined by the spacing required between fans to ensure that scanner from one fan does not fall on the sensor-target exposed by an adjacent fan; and also by the length of the sensor-target being scanned. Thus, for a length L for the sensor-target and a spacing S, the number of fans become L/S.
• a specific example illustrates the performance possible from a DELST designed for diagnostic radiology. Assume a worst case with the following conditions where the distributed capacity is taken equal to a stripe capacity:
• DELST performance can be predicted as follows:
• Electrode stripes 100 c.
• Stripe width 4mm d.
• Raster area 400 x 400 mm 2 g.
• Tstripe 3x10 -3 20x4x10 -5
• the imaging rate is the reciprocal of the above, and with four fan beams, approaches 10 images per second.
• Most of diagnostic radiology is carried out below 7.5 frames/sec. Only for studies of the heart and coronary arteries is there a need for real time imaging. Thus with only four fan beams, one exceeds the requirement in speed for at least 90% of diagnostic radiology.
• the imaging, raster rate can approach 8 frames/second and include rejection of scattered X-rays.
• the layers 606 and 608 in Figure 6 can be thought of as two capacitors in series.
• the capacity betweenelectrode 604 and 610 is less than that of the sensortarget, the extent depending upon the thickness and dielectric constant of layer 608.
• Reduced capacity means reduced noise associated with the preamplifier.
• the extent to which this is possible depends upon the extent of increase in bias 622 required to maintain the proper internal electric fields in layers 606 and 608; the ability to maintain a well defined electron beam of small diameter in 608 to hold spatial resolution, maintaining sufficient charge storage for the desired dynamic range and maintaining prerequisite performance as an electron multiplier.
• ZnCdS and/or CdS can serve as the photoconductive layer 606, and have dielectric constants that are in the range of 8.37-9.4.
• the multiplying layer 608 used with an electron beam might comprise KC1 with a dielectric constant of 4.64.
• an example of materials selection could be a
• An advantage to using KC1 as an electron multiplying layer or Sb 2 S 3 as the photoconductor responsive to the laser radiation is that they are relatively insensitive to X-rays. Accordingly, they would not be activated by incident X-rays 630 as would be used in diagnostic radiology.
• ZnCdS and CdS have been used as X-ray sensors in the past.
• any X-rays penetrating the X-ray sensor layer of Csl 1000 in Figure 10, and absorbed in the sensor-target layer would only have a favorable effect in total photoresponse. Since the layer 606 here, however, is only a few microns thick, X-ray absorption will be small and any effect on overall device performance will be small.
• FIG. 13 the signal electrode is 1304 and the ground electrode is 1310 as described heretofore.
• Layer 1306 is a photoconductor responsive to the incident radiation 1330 passing through 1304.
• layer 1308 is now a channel multiplier placed between layer 1306 and electrode 1310.
• a raster scanner 1340 sends a high velocity beam of elec trons through electrode 1310 and with proper bias voltage applied to 1304 (not shown) causes a displaced charge to be deposited at the interface 1312.
• the scanner and channel multiplier replace the lost charge and in the process generate a video signal which is picked up by the preamplifier 1322 through the coupling capacitor 1320 *
• the channel plate can now be millimeters thick versus microns thick for designs described earlier, and correspondingly cause a drop in DELST capacity ranging from a few hundreds to a thousand or more.
• the multiplier need not operate with dramatically high, gain, and the range of 1,000 to 10,000 should prove adequate for most cases.
• Figure 14 shows the same basic scheme making use of the channel multiplier, but incorporating a laser raster scanner 1440 and a scanning laser beam 1442 passing through electrode 1414 to be absorbed by a photoemitter 1410 deposited on the channel multiplier 1408. Photoelectrons emitted from 1410 enter the channels of 1408, are multiplied and deposited on the interface 1412. On exposure by imaging radiation 1430, the video signal generated by scanning is picked up by preamplifier 1422 through the coupling capacitor 1420.
• the photocondutor 1406 is made of a material responsive to the spectrum of the incident radiation 1430.
• the option to use the channel multiplier on the imaging side of the interface 1312 and 1412 for increased electronic image gain has been described in section 5.
• the capacity is reduced in the same manner as described above. It can be reduced further by adding the vacuum separation 1226 with proximity focussing for example.
• diagnostic radiology requires that for certain procedures, fluoroscopy be carried out prior to obtaining a diagnostic quality radiograph.
• the former is managed with a low X-ray exposure to minimize dose to the patient. It serves the dual purposes of providing the radiologist wtih a preliminary viewing of the body anatomy and the positioning of the X-ray imaging apparatus. A radiograph is then obtained with a much larger exposure, which is essential to obtaining a diagnostic quality image. Accordingly, fluoroscopy requires a much higher level of gain sufficient to ensure that the resultant image is of sufficient quality to meet its purposes.
• the diagnostic exposure on the other hand, being much larger requires less gain in DELST to provide a suitable radiograph.
• the potential difference applied across electrodes 1004 and 1010 combined with the resistance of layers 1006 and 1008, plus that associated with the interface layer 1012, determines the voltage drop across each of the layers and the interface.
• the potential differences across the electrodes 1004 and 1010 are selected to provide the necessary gains in 1006 during fluoroscopy and diagnostic imaging, and a third potential difference for the appropriate voltage drop across 1008 during raster scanning for readout.
• the layer 1008 require a much larger voltage drop to provide an optimum scan readout, than would exist during exposure conditions set-up to optimize gain in 1006.
• the electrodes 1004 and 1010 potential difference could be switched to a higher value to accommodate the requirements imposed by layer 1008 for optimum readout.
• a substantial benefit in using the channel multiplier to substantially reduce capacity is in the opportunity to use fewer and wider stripe electrodes. This leads to more scan lines per stripe, fewer preamplifiers and the opportunity to readout and erase scattered events before readout within one stripe width. These factors lead to overall improved imaging frame rates and operational simplicity. Even real time imaging can be designed into a DELST system given all the options avail ⁇ able to incorporate into a system.
• biases have been arranged so that the first electrode is either positive or negative with respect to the second electrode taken as ground. This results in a potential difference across the DELST structure such that it causes charge carriers to move in a preferred direction compatible with a specific DELST structure. It is clear that this description is presented for simplicity; that, in fact, the grounded electrode could be the first electrode and that the second electrode could be at a postive or negative potential relative to ground; that, in fact, neither electrode need be grounded to support a preferred potential difference, and that any bias arrangement need only be accompanied by proper electronic circuitry to transmit the video signal to preamplifier.

Landscapes

• Transforming Light Signals Into Electric Signals (AREA)

Applications Claiming Priority (4)

Publications (2)

Family

ID=27086194

Family Applications (1)

Country Status (3)

Families Citing this family (1)

Citations (2)

Family Cites Families (11)

• 1985
  • 1985-05-14 CA CA000481518A patent/CA1242037A/fr not_active Expired
  • 1985-05-14 WO PCT/US1985/000874 patent/WO1985005527A1/fr not_active Application Discontinuation
  • 1985-05-14 EP EP19850902791 patent/EP0183777A4/fr not_active Withdrawn

Patent Citations (2)

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events