US3001098A - X-ray image intensifying device - Google Patents

Description

P 1961 R. J. SCHNEEBERGER 3,001,098

X-RAY IMAGE INTENSIFYING DEVICE Filed March 17, 1954 Ilil'l'lll'l'lll'l'lli INVENTOR WITNESSES.

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United States Paten Q 3,001,098 X-RAY IMAGE INTENSH YING DEVICE Robert J. Schueeberger, Pittsburgh, Pa., assignor to Westmghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Mar. 17, 1954, Ser. No. 416,879 3 Claims. (Cl. 31511) My invention relates to X-ray image Producing systems and in particular releases to a single tube which is adapted to receive on its input screen a relatively faint 7 formation that is unattainable from X-ray pictures on I photographic plates; but, in views .of anything except relatively thin body members, fluoroscopic images are so faint that desired details are only visible after the radiologist has adapted his eyes for around half an hour in a darkened room. The wastefulness of time incident to this procedure has recently resulted in the development of an X-ray image intensifying tube described in Mason and Colnnan Patent 2,523,132, issued September 19, 1950, assigned to the present assignee, in which the faint X-ray image on its fluorescent input screen is reproduced with several hundredfold increase of brightness on'its output screen. I

It is inevitable in fluoroscopy that a number of images of portions of the object viewed are superposed on the image screen, and these are produced with diiferent in tensities depending upon their X-ray transparency and depth below the surface. What appears is, in fact, a jumble of superposed separate shadows. Many times in using fluoroscopic images, the observer is primarily interested in distinguishing one particular organ or area from others, and probably the most efiective agent for this purpose is a selective control of contrasts in the image. It is likely that the particular organ desired will appear as a shadow of one particular shade, and if contrasts can be made high in the neighborhood of that particular shade contrasts are depreciated at all other levels of light intensity, the image of the desired organ is likely to appear clearly while those producing both brighter and darker shadows fuse into their surroundings and virtually disappear. My present invention is adapted to this purpose in that it employs components which make it possible to accentuate contrasts over certain shade ranges and suppress them over other ranges.

For many purposes, also, it is desirable to be able to transmit the output picture of the .fluorescent image to one or several distant points. For example, doctors in distant hospitals, or medical students in distant classrooms may thus be permitted to view the fluoroscopic picture of a patient under X-ray examination. 'My pres- Patented Sept. 19, 1961 varied to emphasize contrasts at certain desired levels and suppress them at other levels.

Another object is to provide a new and improved apparatus for producing light images which have space idlisltdribution corresponding to X-ray or other radiation Still another object is to provide an X-ray image intensifier screen in which secondary electron emission is used to reduce the effect on the output image of periodic pulsations present in the X-ray beam. 7

Another object is to provide a new andimproved arrangement for reproducing on distant screens pictures of X-ray or other radiation fields. ,7 1

Another object is to provide a novel and improved gpg of pickup tube for televising pictures of radiation el s.

' Still another object is to provide in a single envelope an electronic device capable of transforming a spatially distributed radiation field into signals adapted to control a television transmitter.

Yet another object is to provide a single envelope containing the elements of an X-ray image reproducer and a television pickup.

, Other objects ofmy invention will be apparent to those skilled in the. art upon reading the following description taken in connection with the attached drawings, in which:

FIG. 1 is a schematic view in longitudinal section of a tube embodying the principles of my invention;

FIG; 2 is a view in section at enlarged scale of part of the input screen of FIG. 1; i V FIG. 3 is a view similar to FIG. 1 of a portion of a target screen of FIG. 1;

FIG. 4 is a view in section at enlarged scale of a modified form of my invention which employs a different target; and 7 FIG. 5 is a view in section at enlarged scale of the latter.

Referring in detail to FIG. 1, a vacuum-tight enclosure 1, which may if desired be of glass, has a cathode screen Z'near one end which may comprise a thin glass wall 3 coated on its inner concave face with a thin layer of transparent conducting material 4 and the latter coated with a photo-emissive material, such as cesiated antimony 5. The convex surface is coated with a layer 6 of fluorescent material such as zinc-cadmium sulphide. When an X-ray field is projected onto layer'fithrough the wall of enclosure 1, the light generated therein is transmitted through the conductive layer 4 and glass wall 3 and generates at the surface of the photo-emissive layer 5 an cut invention makes this possible using ordinary existing One object of my invention is, accordingly, to provide a new and improved method of producing light images which are replicas of X-ray images. 7

Another object is to provide a picture reproducing system in which contrast between picture areas of nearly the same shade-level may be selectively controlled and electron image which is a replica of the X-ray field or picture incident on layer 6. An electron lens system symbolized by electrodes 7 and 8, provided with suitable leads 10 and 11 for connection to voltage sources, focuses a contracted replica of this electron image ona target '12 which is shown schematicallyin section in FIG. 3.

The target 12 comprises a screen of metal wire 13 which has. a surface layer 29 of potassium chloride and has a large ratio of open space to a solid area supporting a layer 14 of aluminum thin enough to be substantially transparent to the electrons focused on it by the electron lens system aforesaid. The other face of the aluminum layer 14 is coated with a thin layer 15 of an insulating dielectric such as arsenic trisulphide which is briefly rendered substantially conductive over a small area around a spot to which an electron passing through the aluminum layer 14 has penetrated.

The face 15 of the target is'scanned by an electron beam 16 directed upon it by a conventional .cathode ray gun 17 comprisinga cathode 18 focussing electrodes 19, 20, deflecting coils 21, focussing coil 22, and aligning coil 23 of usual forms. The aluminum layer 14is connected through a resistor 24 to thepositive terminal 25 of direct current source (not shown) which has its negative terminal connected to cathode 18. The output signal to a conventional television transmitter (not shown) isderived from the resistor 24. Any kinescope may receive the signals from this transmitter and will produce a picture of the light image generated in the fluorescent layer 6 by the X-rays.

The mode of operation of the above arrangement appears to be substantially as follows. The X-ray field striking the fluorescent layer 6 generates therein a light image which penetrates to the photo-emissive layer 5 and causes emission from its surface of an electron image which duplicates the space-distribution of the X-ray field. The electron image is accelerated through the electron lens system 7 .and 8 and penetrates the aluminum layer 14 to produce in the insulating layer 15 what may perhaps be thought of as a conductivity-image duplicating the space-distribution of the electron image and so of the X-ray image.

When the electron beam 16 scans the face of resistance layer 15, it brings its entire outer surface to the same potential as cathode 18. When no X-rays strike input screen 6, under which condition there is no conductivity image in insulating layer 15, the outer face of layer 15 thus stands uniformly at the potential of cathode 18, While the aluminum layer 14 on its inner face stands uniformly at the potential of positive terminal .25, and the current in output resistor 24 is substantially zero. However, when an X-ray image is projected onto input screen .6, the conductivity image previously mentioned exists over the area of insulating layer 15, and the respective areas on the outer surface of layer 15 rise to a fraction of the potential of positive terminal 25 which correspond the variations of the conductivity image over its surface. When now the electron beam 16 strikes any particular elemental area of layer 15 in its scanning movement, it substantially instantaneously deposits electrons enough on the outer surface of that elemental area to recharge it to the potential of cathode 18. A corresponding charge current, I Z, flows through output resistor 24 and may be used, like that in the output resistor of conventional television pickup tubes, to modulate a television transmitter and send out signals which duplicate the X-ray field on the screens of distant kinescopes. This image may be brightened, enlarged and varied in contrast by standard television techniques.

By employing in the transmission channel between resistor 24 and the reproducing kinescope (not shown) an adjustable non-linear amplifier, the contrasts in the image viewed may be controlled by the operator. Contrast between any two closely similar levels of shade in the picture is proportional to the slope of the output voltage vs. input voltage characteristic of the amplifier, and by adjusting the latter in ways well known in the electronics art so that the characteristic has a steep slope for those shade intensities which are characteristic of the organ which it is desired to emphasize and a small or zero slope for shade intensities both greater and less than those of the desired organ, the latter will stand out clearly against a nearly uniform background, as is pointed out in R. L. Longinis Patent 2,692,299 issued October 19, 1954, entitled Image Contrast intensifier and assigned to the present assignee.

In addition to the above-mentioned charge current I Z, the current in resistor 24 has another component -I,,( l'-6) where 5 is the total electron yield (secondaries plus back scattered electrons) from the side of the target on which the high energy bombarding electrons are incident. Thus o Since the I,,(1-5)-component is directly proportional to the total amount of light falling on the photocathode, and if this component were an appreciable fraction of total I, any time variation in the total light would result in either a stationary or a moving bar-pattern superimposed on the kinescope picture. Variations which were synchronous would appear stationary, while non-synchronous variations would appear as a moving pattern. Light from a screen excited by X-rays from an X-ray tube energized by the usual non-filtered full-wave power supply would fluctuate times per second and would produce a stationary double bar pattern on the standard 60-field-per-second raster.

Fortunately, it is possible to make the effective 6 of the screen unity at bombarding voltages of the order of 15 to 20 kilovolts by evaporating on the bombarded side a layer 29 of the proper thickness of potassium chloride, so that the I (l-6)-componcnt approaches zero as 6 approaches unity. Referring again to FIG. 1, operating electrode 8 at a voltage which is always positive with respect to the target will insure that all of the secondary and back-scattered electrons from the target will be collected. With targets with amplifications of 25 or more, the above-mentioned potassium chloride or a metal such as gold evaporated on the bombarded side of the supporting screen 13 would produce a 6 near enough to unity so that spurious patterns would be too dim to be noticed. It is for this reason that the wire screen 13 is gold coated. The electrode 8 is accordingly given a voltage which is always positive relative to screen 13 to collect secondary electrons ejected therefrom by such electrons from cathode-layer 5- as do not pass through the openings in screen 13 into impact onaluminum layer 14.

The conductivity acquired by insulating layer 15 is found to make possible a great amplifying action in that I have found the current flow from the electron beam through the layer 15 as a result of penetration by the bombarding electrons through aluminum layer 14 to be as high as a thousand times the bombarding electron current, this result having been attained with a voltage of fifty between terminal 25 and cathode 18, a layer 15 of arsenic trisu'lphide two microns thick, and the bombarding electrons accelerated through a twenty kilovolt potential. The electron-bombardment-induced condue =tivity in the insulating layer 15 has, therefore, a very high amplification which is very valuable.

It can likewise be shown that there are great advantages in making the area of the image as focused on target l2 as small as possible relative to the X-ray image on fluorescent screen 6. Thus, the variations of voltage from elementaryarea to elementary .area over the face of target layer 15 and the ratio of output current from target layer 15 to the leakage current therein increase with the decrease in target area. The physical size of the magnetic elements 21, 2.2 and 23 for focussing the electron beam decrease with area of target layer 15. Furthermore, the rate of light: decay in fluorescent materials, the rate of conductivity decay in photoconductors, and the rate of decay of -electron-bombardment-induced conductivity in dielectrics, such as target 15 of FIG. 1, are each increased by increasing intensity perunitarea ofthe exciting energy. Rapid decay of the output image is desirable where moving 'objects are being pictured. Hence, for this fourth reason, small target-electrode areas are advantageous.

However, decreasing the area of the target layer 15 decreases resolution in the image seen, the dimension tolerances of the electron-optical system and the permissible diameter'of the scanning beam, but the smallest size of the target layer 15 which these limitations will admit of should be used.

FIG. 4 is an image tube similar to FIG. 1 except that the target screen comprisesa thin glass plate coated on one side with a transparent conducting material like tin oxide, the plate being covered on the side facing the X-ray field with an electron phosphorsuch as zinc sulfide, and on "the side facing the scanning beam with a photoconductive layer such as antimony trisulphide. FIG. 5 shows the glass plate 31, tin oxide coatings 32 and 3 3, electron phosphor coating -34 of zinc sulphide, and photoconductive layer '35 of selenium 0r antimony 'trisulphide.

The other parts of FIG. 4 being similar to the parts bearin-g similar reference numerals in FIG. 1 are believed to require no separate description.

In the FIG. 4 arrangement, the electron image focused on the phosphor layer 34 excites a light image in the latter which is projected through the glass plate 31 and tin oxide layers 32 and 33 onto photoconductive layer 35 where it produces a conductivity image just like the one described as produced in the resistance layer 15 of FIG. 1. The action of this conductivity image when scanned by the electron beam is the same as already described in explaining FIG. 1. All the advantages pointed out above as resulting from making the target 12, 14, 15 in FIG. 1 as small as practicable are attained by restricting the area of the target 31 to 35 in FIG. 4.

It will be noted that since the only function of the fluorescent layer 6 is to project a light field on the photoemissive layer 5, it is possible by simply omitting the layer 6 to focus a light picture directly on the photo-emissive layer 6. The resulting tube an improved form of television image-pickup tube. This change can be made in the case of both the FIG. 1 and FIG. 4 tubes.

While I have mentioned antimony trisulphide as the material exhibiting the property of electron-bombardment conductivity, other materials, both inorganic and organic, showing this property in marked degree are known in the art, such as selenium, cadmium sulphide, sulfur, aluminum oxide and anthracene.

I claim as my invention:

1. A11 image reproducing device comprising a photoelectrioally-emissive screen, a target comprising a metallic support mesh, a thin electron permeable layer of electrically conductive material positioned on the side of said mesh remote to said screen, a layer of material having a property altered by electron impact supported on said electron permeable layer, an electron-lens system for accelerating electrons from said screen into impact upon said support, said metallic mesh having a surface layer of a material which emits secondary electrons copiously when struck by said electrons on the side facing said screen.

2. An image reproducing device comprising a photoelectrically-emissive screen, a target comprising a metallic support mesh, a thin electron permeable layer of electrically conductive material positioned onthe side of mesh remote to said screen, a layer of material having a property altered by electron impact supported on said electron permeable layer, an electron-lens system for accelerating electrons from said screen into impact upon said support, said metallic support mesh having a surface layer of potassium chloride on the side facing said screen to cause the total electron yield from the bombarded surface of said target to approach the number of electrons bombarding said surface from said screen.

3. In combination with a vacuum tube, a photo-electrically emissive input screen adjacent one end thereof, a target comprising a thin electron permeable layer of electrically conductive material, a layer of resistance material Which exhibits the property of electron bombardment induced conductivity supported on said electron permeable layer on the side thereof remote with respect to said input screen, an electron lens system accelerating and focusing electrons emitted by said input screen onto said electron permeable layer and secondary electron emissive means provided on the bombarded surface of said electron permeable layer to cause the electron yield from the bombarded surface of said target to approach the number of electrons bombarding said surface from said input screen.

References Cited in the file of this patent UNITED STATES PATENTS 2,555,424 'Sheldon June 5, 1951 2,555,545 Hunter JuneS, 1951 2,587,830 Freeman Mar. 4, 1952 2,683,832 Edwards et a1. July 13, 1954 2,617,954 Rose Nov. 11, 1952 2,776,387 Pensak Jan. 1, 1957 FOREIGN PATENTS 990,402 France June 6, 1951 668,727 Great Britain Mar. 19, 1952

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