PATENT SPECIFICATION C») 1 605 m (21) Application No. 47351/77 (22) Filed 14 Nov. 1977 (31) Convention Application No. 741430 (32) Filed 12 Nov. 1976 (31) Convention Application No. 763637 (32) Filed 28 Jan. 1977 in (33) United States of America (US) (44) Complete Specification published 16 Dec. 1981 (51) INT CL3 H01J 31/50 (52) Index at acceptance HID 17C I8C 18LX 18LY 344A2A 4A2Y 4A4 4B44F2C4F2E 4F2Y 4H1X 4HY 4K2B 4K2C 4K2E 4K2Y 4K3B 9C1X 9C1Y 9FX 9 FY 9Y

(54) X-RAY IMAGE INTENSIFIER TUBE AND RADIOGRAPHIC CAMERA INCORPORATING SAME

(71) We, DIAGNOSTIC INFORM- usually around 350,000 to 700,000 erg/cmJ-R 45 ATION, INC., a corporation organised and or about 50,000 to 100,000 cd-sec/mJ-R, existing under the laws of the State of which is about 5,000 to 10,000 times the California, United States of America, of 246 conversion efficiency of the old-time 5 Sobrante Way, Sunnyvale, California 94086, fluoroscopic screen. Part of this United States of America, do hereby intensification is obtained as true electronic 50 declare the invention for which we pray that gain, which is about 50 to 100 times over the a patent may be granted to us, and the old-time fluoroscopic screen. Another method by which it is to be performed, to be factor of 100 gain is obtained through the 10 particularly described in and by the 100 fold area minification of the image of following statement:— the output screen. 55 The invention pertains to medical x-ray The image quality of such conventional apparatus, and more particularly to an x-ray inverter type image intensifier tube is image intensifier tube for medical x-ray reasonably adequate for fluoroscopic use, 15 diagnostic use, and to a radiographic but is far short of the requirement for camera using such a tube. radiographic use. The requirements for 60 A common present day x-ray image radiographic use are established by the intensifier tube is of the electrostatically conventional film-screen system, which focused inverter type with a 100 fold area demands a 20% modulation transfer 20 minified output image size. This function response at between 2 to 3 line conventional inverter type x-ray image pairs per millimeter. 65 intensifier tube typically has a convexly Such conventional film-screen systems curved, six to nine inch diameter input x-ray are commercially available in speeds" sensitive screen which converts the x-ray ranging from 250 R"1 to 8000 R"1. The speed 25 image into a light image which, in turn, is is defined as the reciprocal of the x-ray converted into which are then exposure in terms of roentgens, R, to the 70 accelerated and electrostatically focused film-screen system to result in a net optical onto an output image screen which is 100 density of 1.0 on the processed film. The times smaller in area than the input screen, spatial resolving ability of the film-screen 30 being typically 0.6 inches to 1.0 inches in system is generally inversely proportional to diameter. The displayed image on' the the speed of the system. That is, the higher 75 output screen can be optically magnified the spatial resolving ability the lower the and coupled to other systems for speed of the system. radiographic or fluoroscopic purposes. For While film-screen systems have desirable 35 example, for radiographic purposes, the system speed qualities, they have the image is optically coupled to a film camera drawback that they require taking full size 80 or a photographic film. For fluoroscopic photos which are difficult to store and purposes, the image can be displayed either which are becoming increasingly more by using a system of mirrors and lenses for expensive due to the rising cost of the 40 direct viewing or by using a closed circuit halide x-ray film. Also, the film cannot be television camera and monitor for remote monitored during exposure to control the 85 viewing. dosage or timing. The conversion efficiency of such a A recent article published by C. B. conventional image intensifier system is Johnson in the Proceedings of the Society of 2 1,604,2 2 Photo Optical Instrumentation Engineers, output brightness of the tube starts to Volume 35, pages 3—8 (1973), become sublinear in response with respect hypothetical^ suggests that an x-ray to the input x-ray dose rate. The sublinear sensitive proximity type image intensifier response becomes worse at higher x-ray 5 may be designed with an x-ray sensitive dose rate. This undesirable feature reduces 70 conversion screen on one side of a glass contrast discrimination during fluoroscopy support and a on the other and is virtually useless for radiography. side of the glass support. However, the Again, it is unknown whether a large format article gives no specifics concerning the beyond six inches in' • diameter, self- 10 critical parameters or what might be used as supporting and with uniform gain, MCP can 75 the x-ray sensitive conversion screen. How be fabricated. this image intensifier can be designed to The Millar proximity type image result in high conversion efficiency or high intensifier tube has a glass envelope and an resolution was also not discussed. inwardly concave, titanium input window. 15 A proximity device using a The window is described as being 0.3 mm 80 michrochannel plate (MCP) both as the thick. Materials such as titanium, aluminum primary x-ray sensitive conversion screen and beryllium cause undesirable scattering and as an multiplication device was of the x-rays which reduces the image described by S. Baiter and his associates in quality. Furthermore, because of the 20 Radiology, Volume 110, pages 673—676 relatively high porosity and low tensile 85 (1974), and by Manley et al. in U. S. Patent strength properties of such materials, they no. 3,394,261. According to an article cannot be made with the optimum thickness published by J. Adams in Advances in to maximize their x-ray transmissive Electronics and Electron Physics, Volume properties. Still another problem with tubes 25 22A (Academic Press, 1966), pages 139— constructed with such materials for the 90 153, this type of device has a very low input window and glass for tube envelope is quantum detection efficiency in the in joining the window to the tube envelope. practical medical diagnostic x-ray energy The materials have such dissimilar thermal range of 30—100 Kev. The device gain of expansion properties, among other 30 the Baiter article was first reported to be differences, as to preclude their practical 95 2 20—3 cd-sec/m -R which is too low to be commercial use in a large format device. useful as a radiographic or fluoroscopic It is an aim of the present invention to device. A higher gain device described in provide an x-ray image intensifier tube the same Baiter article exhibited excessive which offers high quality image resolution 35 noise. There is a real question whether a at the output display screen and provides a 100 practical self-supporting MCP plate with substantially full size image at the display uniform gain can be constructed with screen. current technology to sizes beyond five to According to the present invention, an x- * six inches in diameter which is not of ray sensitive image intensifier tube 40 sufficient size to produce an output useful comprises an evacuated tube envelope 105 for radiographic purposes. having an input window comprising at least Another approach involving proximity one of iron, chromium and nickel for design was taken by I.C.P. Millar and his receiving an x-ray image; a substantially flat associates and their results were published in scintillator screen as herein defined 45 1) IEEE Transactions on Electron Devices, adjacent the input window for converting HO Volume ED—18, pages 1101—1108 (1971), the x-ray image into a light pattern image, a and 2) Advances in Electronics and Electron substantially flat photocathode layer Physics, Volume 33A, pages 153—165 parallel to and adjacent the scintillator (1972). screen for emitting photoelectrons in a 50 Millar's approach again involves the use pattern corresponding to the light pattern H5 of a micro-channel plate (MCP). In this image, a substantially flat, display device, however, the MCP is used purely as screen parallel to and spaced from the an electron multiplication device and not as photocathode layer, there being no tube an x-ray conversion screen. The conversion components present in the space between 55 factor for Millar's tube is reported to be the photocathode layer and the display 120 around 200,000 cd-sec/m2-R, which is above screen and means for applying an or higher than needed for fluoroscopic electrostatic voltage to the display screen purposes, but is far too high for and the photocathode layer so as to create radiographic purposes. However, the an electric field therebetween to accelerate 60 output brightness of Millar's tube also the pattern of photoelectrons toward the 125 exhibits strong dependence on the display screen along substantially parallel, photocathode current density. At around a straight trajectories to impinge upon the photocathode current density of 5xl0~" output display screen; the paths of such amperes/cm or at the equivalent x-ray input parallel straight trajectories being governed 65 dose rate of around 0.6xl0~3R/sec, the solely by the electrostatic voltage applied to 130 3 1,604,3 3 the photocathode layer and the display image to be created at the display screen. screen, the image created at the display Aso, shallowness of the depth of field of the 65 i. ; screen being substantially equal in size to electron and the external optics is that of the x-ray image received at the input avoided. In use, the electrical field in the 5 window. The tube envelope is preferably space between the input and output screen metallic, enabling the basic components of of the image intensifier tube of the present the tube to be kept at a neutral potential, invention is quite high compared to those 70 : and avoiding spureous emissions. used in a conventional inverter type tube of The invention also provides a the kind described above, and the cathode 10 radiographic camera incorporating an region field strength is normally about 100 intensifier tube as defined above and having times higher than that of such a known an optical recording medium, and an optical inverter type tube. Thus, the tube of the 75 lens system adapted for focusing the image present invention is less sensitive to external presented on the output display screen as a magnetic fields and defocussing problems 15 reduced image on the recording medium. encountered when subjected to bursts of high In preferred embodiments, the spacing intensity, short millisecond duration pulses. between the photocathode layer and the Furthermore, in preferred embodiments 80 output display screen is between 6 mm to 25 of the invention, the metallic tube envelope mm such that when an electrostatic and all of the basic tube components except 20 potential of 10,000 volts to 60,000 volts is the scintillator-photocathode screen may 6e applied between them from an external held at a neutral potential with respect to source, the tube has a linear response with the output display screen. This assists in 85 respect to input x-ray dose rates in excess of avoiding spurious electron emission, 0.06 R/sec. The brightness gain (conversion resulting in a clearer display. 25 efficiency) in such embodiments is typically in the range of 500 to 20,000 cd-sec/m-R, The tube of the invention offers an and the thickness of the scintillator is in the improved performance compared to known range of 50 to 600 Microns, whereby high x- intensifier tubes in several different ways. 90 ray utilization, high gain, high image quality For example, much higher gain and patient 30 and low field emission are simultaneously dose reduction can be achieved by using a obtained. thicker (200 to 600 micron) input x-ray to light conversion screen and still having The image created on the display screen acceptable image resolution. Another 95 is preferably viewed through a high Z glass example is to provide a radiographic output window which reduces x-ray back camera by obtaining a very high image 35 scatter and further protects the operator of resolution at the output screen through the the tube from the x-rays. Typically, a collar use of a 50 to 100 microns thick scintillator of iron-nickel alloy is fitted to the output screen and a narrower (6 to 10 mm) 100 window and welded to the tube envelope for photocathode to display screen gap spacing. mounting the output window in the tube This output display can then be 40 envelope. photographed. Although the image intensifier tube used in the preferred embodiment of the In the preferred embodiment of a invention has a wholly flat or planar input x- radiographic camera according to the 105 ray sensitive screen, it may be slightly invention, reduction type optics focus the 45 curved for the purpose of increasing the full size output display onto photographic mechanical strength of the screen, as long film which is smaller in diagonal dimension as the trajectories of the accelerated than the output display screen. The film electrons remain substantially parallel and sensitivity (G) is defined as the reciprocal of 110 incident light energy in ergs per square straight. The term "substantially flat" as 2 50 applied to the input screen in this centimeter (erg/cm ) which is required to specification, means either wholly flat, or produce a net density of 1.0. More specifically the film sensitivity is chosen to slightly curved to that extent. The tube is 2 quite thin and compact in size compared to be in the range of 5 to 100 cm /erg. The 115 a conventional image intensifier system. image intensifier tube is chosen to have a conversion efficiency (C) in the range of 55 The input area can be square, rectangular or 2 circular in shape in the various 1,000 to 30,000 erg/cm -R or, if the output phosphor is green emitting, in the range of embodiments. As discussed above, in a 2 conventional inverter type image intensifier 140 to 4,300 cd-sec/m -R. The fractional 120 tube the input screen is limited to a circular light energy (T) emitted by the output 60 disc shape and is commonly outwardly screen which is collected by the optics Mid curved. which is transferred to the photographic film can be approximated by the The fact that the output image is the same relationship: 125 size as the input image enables a very sharp 3 1 604 101 4 t curved input screen of the other prior art T= image intensifier tubes. 4f2(l+m)2 Still another advantage is that the x-ray 65 sensitive area input format size of the where, camera can be expanded without sacrificing t=transmission of the optical system, image quality as would happen with certain f=the f number of the optical system, and conventional inverter type image intensifier 5 m=magnification of the image, of ratio of systems. A still further advantage is that it 70 image to object size, and can be easily photo-timed with a sensing is approximately in the range of lxlO-1 to device directly monitoring the output image ] xlCT3. In this embodiment, the total speed to obtain consistent exposure on each film. of the camera (S=CTG) in the medical Unlike the proximity x-ray image 10 diagnostic region of the x-ray spectrum, i.e., intensifiers heretofore discussed, the x-ray 75 30—100 Kev, is in the range of 100 to 10,000 image intensifier tube of the present R~\ In a preferred embodiment, where the invention achieves high conversion conversion efficiency (C) is in the range of efficiency without requiring the use of 3,000 to 14,000 erg/cm-R, the system speed additional multiplication means or non- -1 15 (S) is in the range of 500 to 5,000 R . . linear responding components, i.e., a micro- 80 The x-ray sensitive photographic camera channel plate between the output phosphor according to one embodiment of the screen and the photocathode. As a result, invention is thus designed to have a system the x-ray image intensifier tube of the speed which is optimal to take maximum present invention is mechanically simpler, 20 advantage of the amount of information more reliable and can exhibit a linear 85 provided by the incident x-ray quanta such response with respect to input x-ray dosages that the recorded image will have a in excess of 0.06 R/sec, the dosage used for balanced image quality for the x-ray medical diagnostic purposes. information. The image quality of the Intensifier tubes of the invention are 25 photographs produced m accordance with relatively simple in design and can be of 90 the invention is as good as that of light weight and compact. For example, conventional cassette film-screen systems, when the tube is used in a direct view, which is not achievable with conventional fluoroscopic mode, the physician can have inverter type image intensifier systems. easy access to the patient for palpation and 30 However, with the camera of the invention, can observe the effects of palpation without 95 smaller than full size films can be used with having to turn away from the patient, as is no loss of x-ray information. This allows for necessary in the present day systems having a significant reduction in required storage an inverter type image intensifier coupled to space for the developed films. Also, the a television display. 35 camera can be modified by a beam splitting In other embodiments of the invention, 100 mirror to simultaneously generate a second such as for use in teaching institutions, for photograph of the x-ray information. A example, it may be desirable to provide second optical system, placed off axis, may remote displays of the output of the x-ray also be used to generate the second image intensifier tube's large output display 40 photograph. screen which is quite easily coupled to a 105 One of the most important features of the silicon intensifier target (SIT) tube type camera of the preferred embodiment is that closed circuit television system for remote a long focal length, in excess of 100 mm viewing or for video recording. optics in the preferred embodiment, the A particular advantage of the invention is 45 non-minified output image size, and small that the x-ray sensitive area input format 110 aperture optical system give the camera size of the system can be expanded without greater tolerance for thermal expansions, sacrificing image quality as would happen dimensional changes, etc. than certain with conventional inverter type image conventional image intensifier x-ray camera intensifier systems. 50 which is extremely sensitive to such It will be appreciated from the above that 115 • changes. Also, the optical system can be the invention offers a panel type x-ray folded so that the camera can be made more image intensifier tube of rugged design for compact, which is an important feature in a medical diagnostic purposes which can cramped radiological examination room, minimize the danger of injury to the 55 than can a conventional system of patient resulting from implosion of the tube. 120 comparable input format size. The input format may be square, Moreover, the image intensifier of the rectangular or circular or otherwise freely camera allows stereo x-ray photographs to shaped, the format size being expandable to be produced with no image distortion. This 17x17 inches. Further, the intensifier tube 60 is primarily due to the fact that the input ray need not be sensitive to the effects of 125 conversion screen is substantially fiat voltage drifts, external magnetic fields, and (planar) as opposed to the conventional field emission. 1,605,127 An x-ray radiographic camera also The brightness gain of the image by the embodying the invention may have long tube 18 is due partly to the electron focal length optics to increase the acceleration and partly to the result of dimensional stability tolerance of the electronic image minification. This is the 5 system. result of reducing the image generated on 70 The invention will now be described by the scintillator screen 22 down to a way of example and with reference to the relatively small image at the output display accompanying drawings, wherein:— screen 28. The reduced image on the display Figure 1 is a diagrammatic illustration of screen 28 is too small however, to allow 10 a conventional inverter type image direct viewing without optical aids. 75 intensifier x-ray tube; Moreover, the quality of the image is Figure 2 is a diagrammatic illustration of reduced both by the quality of the electron an x-ray image intensifier tube embodying optics and by the quality of the output the invention; phosphor screen in the electronic image 15 Figure 3 is a detailed vertical view, in minification, and by the subsequent 80 section, of the image intensifier tube of enlarging of the output image onto the film Figure 2; or onto the monitor screen by the closed Figure 4 is an enlarged, vertical view, circuit television system. illustrating a cross-section of a portion of Another disadvantage is that because of 20 the image intensifier tube of Figure 3; the considerable curvature of the 85 Figure 5 is a vertical, sectional view, scintillator screen 22, there is a spatial taken generally along the line 5—5 in Figure distortion produced in the image due to x- 3; ray projection of the curved surface and due Figure 6 is a diagrammatic illustration of to the field configuration in the tube. Still 25 an x-ray radiographic camera embodying another problem is that because of the weak 90 the invention; and field near the cathode region and the multi- Figure 7 is a graph relating the design electrode arrangement 24, the tube 18 is parameters of the x-ray radiographic extremely sensitive to external magnetic camera for a commercially available fields and voltage drifts among the 30 photographic film. electrodes. Both of these factors can cause 95 A conventional inverter type x-ray image distortion and unsharpness in the produced intensifier tube is illustrated in Fig. 1. A x- image. ray source 10 generates a beam of x-rays 12 Yet another problem is that because of which pass through the patient's body 14 the greatly minified output image and the 35 and casts a shadow image onto the face of a short focal length optics 30, any change in 100 camera system 16. The camera system the positioning of the elements of the includes a conventional inverter type image optical system with respect to the intensifier 18. The tube 18 has photosensitive layer of the camera tube or an outwardly convex input window 20 and a the output screen 28, will render the image 40 correspondingly convex scintillator screen out of focus. This can result from vibration 105 and photocathode assembly 22. The or from thermal expansion. purpose of this scintillator screen, as is well One other major disadvantage of this known to those skilled in that art, is to known system is that because of the curved . convert the x-ray shadow image into a light glass window 20 which is necessary to 45 image, which, in turn, is immediately withstand the pressures due to the vacuum 110 converted by the photocathode layer into a inside the tube 18 and the already very weak pattern of electrons. This pattern of field strength in the cathode region, the electrons is electrostatically accelerated by system is limited to approximately nine a set of electrodes 24 and anode 25 near the inches in input format for optimum 50 display screen 28 and is focused by this set performance. Any greater diameter input 115 of electrodes 24 and anode 25 to form an will necessitate a much higher tube voltage image on the small output screen 28. The and a thicker input window which would " electrodes 24 and anode 25 are connected cause increased problems due to ion spots to a high voltage source 26 whose other lead inside the tube and x-ray transmission and 55 is connected to the scintillator and scattering in the input window. Even in the 120 hotocathode screen assembly 22. The tube conventional sized tubes, there is also, of Eody is made of insulating glass. The image course, the danger to the patient and the at the output display screen 28 is magnified radiologist that the tube might fracture by a short focal length optical system 30 and causing an implosion and resulting ejection 60 is projected onto suitable recording media, of the glass fragments. 125 such as film 32. The image could also be An embodiment of the present invention projected onto the sensitive area of the is illustrated in Fig. 2 in the form of a panel closed-circuit television camera for display shaped proximity type x-ray image on a closed circuit monitor in a fluoroscopic intensifier tube. The image intensifier tube 65 mode. 34 comprises a metallic, typically type 304 130 4 1 604 101 6

stainless steel, vacuum tube envelope 36 thereby reduce distortion due to field and a metallic, inwardly concave input emission. No other elements such as a window 38. The window 38 is made of a microchannel plate, for example, are specially chosen metal foil or alloy metal interposed between the output phosphor 5 foil in the family of iron, chromium, and screen and the photocathode screen. The 70 nickel, and in some embodiments, use of such non-linear devices (with respect additionally combinations of iron or nickel to input x-ray dosage) cause distortion in together with cobalt or vanadium. These and of themselves but they also increase the elements are not customarily recognized in deleterious field emission effects since some 10 the field as a good x-ray window material in of the elements of the microchannel plate 75 the diagnostic region of the x-ray spectrum. must operate at different electrostatic By making the window thin, down to 0.1 potentials with respect to the output display mm in thickness, we have been able to screen and thereby become sources for achieve high x-ray transmission with these spurious electron emission. 15 materials and at the same time obtain the It should be noted that no focussing takes 80 desired tensile strength. In particular, a foil place in the tube 34 as opposed to the prior made of 17—7 PH type of precipitation art type tube 18 in Fig. 1. The screen 40, the hardened chromium nickel stainless steel is photocathode layer 42 and the display utilized in the preferred embodiment. This screen 44 are parallel to each other. Also, 20 alloy is vacuum tight, high in tensile the gap spacing between the photocathode 85 strength and has very attractive x-ray 42 ana the display screen 44 are relatively properties: high transmission to primary x- long, in the range of 6—25 millimeters, rays, low self-scattering, and reasonably, thereby reducing the likelihood of field absorbing with respe„ct to patient scattered emission and at the same time keeping the 25 x-rays. Tlie window 38 is concaved into the electrostatic defocusing to a tolerable level, 90 tube like a drum head. that is, around 2.0 to 5.0 line pairs per The use of materials which are known for millimeter. high x-ray transmission such as beryllium, Furthermore, the applied voltage across aluminum and titanium for example can the gap between photocathode layer 42 and 30 cause undesirable scattering which is the display screen 44 is in the range 10,000 95 apparent in some prior art proximity type, to 60,000 volts (10 to 60 Kv) which is higher x-ray image intensifier devices. than in Millar's tube, described earlier in One purpose of having a metallic window this application. In addition, the non- 38 is that it can be quite large in diameter focussing nature of the field avoids the ion 35 with respect to the prior art type of convex, spot problem which plagues inverter type 100 glass window 22, as depicted in Fig. 1, tubes. In the preferred embodiments of the without affecting the x-ray image quality. In invention, the spacing between the one embodiment, the window measures 0.1 photocathode screen 42 and the output mm thick, 25 cm by 25 cm and withstood display screen 44 is between 6 mm (at 15 Kv) 40 over 100 pounds per square inch of ana 25 mm (at 60 Kv). Thus, the voltage per 105 pressure. The input window can be square, unit of distance, i.e., the field strength, is at rectangular, or circular in shape, since it is a least 2 Kv/mm. An upper limit to the field high tensile strength material and is under strength is about 5 Kv/mm. In certain prior tension rather than compression. art devices such a high field strength was 45 The x-ray image passing through the not considered feasible for this application 110 window 38 impinges upon a flat scintillator of an image intensifier device because of the screen 40 which converts the image into a field emission problems discussed above light image. This light image is immediately and which are obviated in the embodiment adjacent a flat photocathode screen 42 of the invention described herein by having 50 which converts the light image into a all of the tube elements, save for the 115 pattern of electrons. The scintillator and photocathode-scintillator screen assembly, photocathode screens 40 and 42 comprise a be at a neutral potential with respect to the complete assembly 43. The electron pattern output display screen. on the negatively charged screen 42 is The scintillation screen 40 can be calcium 55 accelerated towards a positively charged tungstate (CaW0 ) or sodium activated, 120 flat phosphor output display screen 44 by 4 cesium iodide (CsI(Na)) or any other type of means of an electrostatic potential supplied suitable scintillator material. Vapor by a high voltage source 46 connected deposited, mosaic grown scintillator layers between the output screen 44 and the are preferred for the highly desired 60 photocathode screen 42. Although the smoothness and cleanliness. Since such 125 display screen 44 is positive with respect to materials and their methods of application the scintillator-photocathode screen are well known to those skilled in the art, assembly 43, it is at a neutral potential with see for example, U. S. Patent No. 3,825,763, respect to the remaining elements of the they will not be described in greater detail. 65 tube, including the metallic envelope 36, to The overall thickness of the scintillator 130 5 1 604 101 7 screen 40 is chosen to be 50 to 600 microns efficiency of Millar's tube is 196 to 200 thick to give a high x-ray photon utilization cdm":mR_1 sec or 196,000 to 200,000 cd- ability, thereby allowing overall low patient sec/m2-R which is obtained with the MCP x-ray dosage levels without a noticeable loss operating at 10,000 gain. Removing the 5 of quality as compared to prior art devices. MCP and its gain would result in a 70 This is because the format of the tube and conversion efficiency around 20 cd-sec/m2- the absence of several sources of R, which is too low. Therefore, Millar's "unsharpness" give an extra margin of article has the effect of leading away from sharpness to the image which can be traded the present invention. 10 off in favor of lower patient dosage levels Referring now more particularly to Figs. 75 with greater x-ray stopping power at the 3, 4 and 5 in enlarged cross-sectional views, scintillator screen 40. the details of the scintillation and Similarly, the photocathode layer 44 is photocathode screen assembly 43 and the also of a material well known to those output display screen assembly 44 are 15 skilled in the art, being cesium and illustrated. The screen assembly 43 80 antimony (Cs3Sb) or multi-alkali metal comprises a scintillator layer 40 of very (combinations of cesium, potassium and smooth calcium tungstate or sodium sodium) and antimony. activated cesium iodide which is vapor The image produced on the phosphor deposited on a smoothly polished nickel 20 screen 44 is the same size as the input x-ray plated aluminum substrate or an anodized 85 image. The output phosphor screen 44 can aluminum substrate 52 which faces the be of the well known zinc-cadmium sulfide input window 38. The techniques of such type (ZnCdS(Ag)) or zinc sulfide type vapor deposition processes are known to (ZnS(Ag)) or a rare earth material like those skilled in the art, see for. example, U. 25 yttrium oxysulfide type (Y202S(Tb)) or any S. Patent No. 3,825,763. For direct viewing 90 other suitable high efficiency blue and/or purposes, the layer 40 is between 200 to 600 green emitting phosphor material. The microns thick. For radiographic purposes, interiorly facing surface of the output the layer 40 could be thinner (50—200p), screen is covered with a metallic aluminum i.e., the image could be less bright. 30 film 48 in the standard manner. The As mentioned above, the purpose of the 95 phosphor layer constituting the screen 44 is scintillator screen 40 is to convert the x-ray deposited on a high Z glass output window image into a light image. On the surface of 50. By high Z is meant that the window glass the scintillation layer 40 which faces away has a high concentration of barium or lead from the substrate 52, a thin, conductive, 35 to reduce x-ray back scatter inside and transparent electrode layer 54 such as a 100 outside the tube and to shield the radiologist vapor deposited metallic foil, i.e., titanium from both primary and scattered radiation. or nickel, is deposited and on top of this is An important factor in determining the deposited the photocathode 42. The usefulness of any x-ray image intensifier photocathode layer 42 converts the light 40 system for medical diagnostic purposes is image from the scintillator layer 40 into an 105 the conversion efficiency of the tube. The electron pattern image and the free conversion efficiency of the image electrons from the photocathode 42 are intensifier tube is measured in terms of accelerated by means of a high voltage output light energy in ergs per square potential from source 46 toward the display 45 centimeter per x-ray input dosage of 1 screen 44, all as mentioned above. The 110 roentgen (erg/cm2-R), which can also be scintillator-photocathode screen 43 in this expressed in terms of candlas-second per invention is suspended from the tube square meter-roentgen (cd-sec/m2-R) if a envelope 36 between the input window 38 green emitting output phosphor like and the output screen 44 by several 50 ZnCdS(Ag) type is used. insulating posts 58. One or more of these 115 Several nine inch diameter working posts may be hollow in center to allow a roximity type image intensifier tubes have high voltage cable 60 from the source 46 to Eeen constructed according to the invention be inserted to provide the scintillator- with a conversion efficiency in the range of photocathode screen 43 at the layer 54, with 55 3,500 to 60,000 erg/cm2-R. The output a negative high potential. The remaining 120 are of the ZnCdS(Ag) type and parts of the intensification tube including thus the conversion efficiency can also be the metallic envelope 36, are all operated at expressed in photometric terms as 500 to ground potential. This concept of 8000 cd-sec/m2-R. This is about equivalent minimizing the surface area which is 60 to a brightness gain of 50 to 800 times over negative with respect to the output screen 125 that of the old-time fluoroscopic screens for results in reduced field emission rate inside example. the tube and allows the tube to be operable at higher voltages and thus higher It is important to compare these results brightness gain. It also minimizes the danger with those reported in the Millar article of electrical shock to the patient or workers 130 65 referred to above. The overall conversion 8 1,605,127 8

if one should somehow come in contact 94% for 120 Kvp x-rays filtered with 23 mm with the exterior envelope of the tube. aluminum, 88% for 80 Kvp x-rays filtered To reduce charges accumulated on the with 23 mm aluminum, and 80% for 60 Kvp insulating posts 58, they are coated with a x-rays filtered with 23 mm aluminum. 5 slightly conductive material such as chrome Referring now more particularly to Fig. 6, 70 oxide which bleeds off the accumulated an x-ray camera 100 incorporating an charge by providing a leakage path of less intensifier tube according to the invention is than 20 Kv/cm. illustrated. The camera 100 includes the The thick, high atomic number (Z) glass proximity type image intensifier tube 34 10 substrate 50 on which the phosphor display described above, a long focal length optical 75 screen 44 is deposited forms one exterior system 138 and a film 140. As mentioned end wall of the vacuum tube envelope 36. above, in prior art, conventional This glass substrate 50 is attached to the radiographic, image intensifier systems the tube envelope 36 by means of a collar 55 optical system magnifies not only the small 15 made of an iron, nickel, chromium alloy, output image but all the minute defects 80 designated to the trade as "Carpenter, No. which may be present in the output screen 456". Since the thermal coefficient of as well, resulting in a need for a more expansion of this alloy matches that of the critical manufacturing process. The image glass and nearly matches that of the tube on the film 140 is shown slightly smaller 20 envelope 36, the collar 55 can be fritted to than that displayed on the output screen of 85 the glass substrate 50 and welded to the tube the intensifier tube 34. In practice, the envelope 36. On the interior surface of the optics 138 reduce the size of the image and, glass wall 50 is deposited the phosphor layer correspondingly reduce the apparent size of 44 which is backed by a protective and defects which may be present in the output 25 electron transparent aluminum thin film 48 screen, resulting in a higher yield, less 90 to prevent light feedback and to provide a expensive and less demanding uniform potential. It also tends to increase manufacturing process. The originally the reflection of the phosphor layer 44 to displayed image at the output screen, give a higher light output gain. however, is much larger than in the 30 The essentially all metallic and rugged conventional tube so that the reduced 95 construction of the tube illustrated image at the film 140 is of better quality minimizes the danger of implosion. The than in conventional systems. small vacuum space enclosed by the tube The large output image size combined represents much smaller stored, potential with the long, in excess of 100 mm, focal 35 energy as compared with a conventional length of the optical system 138 in the 100 tube which further minimizes implosion preferred embodiment makes it less danger. Furthermore, if punctured, the sensitive to thermal expansion than metal behaves differently from glass and the conventional systems. The film 140 is held air simply leaks in without fracturing or- in a film transport 154 which allows the film 40 imploding. to be advanced to take pictures in a serial 105 The photocurrent drawn by the tube from manner. the power supply 46 is dependent, of course, Frequently the films are better viewed on the image surface area of the scintillator- with the emulsion side facing the photocathode screen assembly 43 and the radiologist. In order to obtain the proper 45 output display screen 44. For a tube used orientation, a mirror 170 (or 3 mirrors or 110 for direct viewing, the photocurrent would any odd number of mirrors), shown in be 0.4 to 0.8x10"° amperes/cm2 at an x-ray hidden line fashion in Fig. 6, can be inserted dosage level of 1 mR/sec. into the optical path resulting in a new film The applicant has studied other thin holder position. Mirror 170 can also be 50 metal alloys of the chromium-nickel made of a partially transmissive mirror (a 115 stainless steel type as window materials, and beam splitter) so that two films can be made found that these alloys are also better than with a single x-ray exposure. the well known x^ray window materials like The total system speed of the camera 100 beryllium and aluminum but not as good in in the medical diagnostic region of the x-ray 55 overall performance as the 17—7 PH spectrum, that is 30 to 100 Kev, is in the 120 stainless steel, these other materials are: range of 500 to 5,000 R"1. The system speed precipitation hardened type 15—7 Mo, and is defined as the reciprocal of the x-ray work hardened type 304. radiation dosage incident on the output The applicant has also found that foils of window of the x-ray image intensifier tube 60 above-mentioned alloy windows are 34 in terms of roentgens (R) required to 125 particularly satisfactory for use as x-ray produce a net density of 1.0 on the windows in x-ray image intensifier tubes photographic film 140. The system speed where the foil thickness is under 0.25 mm. can be expressed by the following simplified At 0.125 mm thickness, the x-ray formula S=C T G, where 65 transmission through the 17—7 PH foil is G=conversion efficiency of the image 130 9 1,605,127 9 intensifier in terms of output light energy in quality of accepted film-screen systems and ergs per square centimeter per x-ray input a system speed in the range of 1,000 to 3,000 dosage of 1 roentgen (erg/cm-R), which can R"\ (Kodak is a registered Trade Mark). 60 also be expressed in terms of candelas- It is again important to compare these 5 second per square meter-roentgen (cd- results with those reported in the Millar sed/m2-R). article referred to above. The overall T=fractional light emitted by the output conversion efficiency of Millar's tube is screen collected by the optical system more than 100 times the optimum 65 transferred to the photographic film which requirement for radiography. On the other 10 can be approximated by: hand, removing the MCP and its gain would result in a conversion efficiency which is too t low for radiography purposes. Therefore, T= , Millar's article has the effect of leading 70 4 f1 (l+m)J away from the radiographic camera of the present invention. where transmission of the lens, f=th6 'f The designed system speed is optimized number of lens, m=the magnification of the to take maximum advantage of the amount image. of information provided by the indicent x- 75 15 G=photographic sensitivity of the film in ray quanta, such that the recorded image the spectral region of the emission of the will have balanced image quality and x-ray output phosphor in terms of the reciprocal information. This avoids the problem of a of the incident light energy per square low system speed, i.e., less than the old centimeter in erg/cm2 which is required to photofluorographic camera, where the x- 80 2Q, produce a net density of 1.0. ray information is not fully utilized and Therefore, the same system speed can be unnecessary patient radiation dosage arrived at through many different results. It also avoids the problem of an combinations of the C, T and G. Fig. 7 is an unnecessarily high system speed, as in the illustrative example of the interlinking case of the conventional inverter type of 85 25 nature of these system parameters. Fig. 7 image intensifier tube system, of the Millar, shows the desired operating region of the MCP type proximity tube, where the film is invention, the shaded area, for a exposed with a very small amount of the x- commercial rapid-processable single- ray information so that the recorded photo emulsion x-ray film marketed by Eastman contains an insufficient amount of 90 3© Kodak Company under the brand name of information with a resulting mottled or type 2541 RP/FC film. The key parameter grainy picture. of the optical system, f/V^is plotted against Referring back to Fig. 6, the beam changes in the conversion efficiency of the splitting mirror 170 can be made such that image intensifier tube, C, to achieve the -1 the larger portion of the light beam is 95 35 system speed range of 500 to 5,000 R . The directed to one film while the smaller image magnification of m=0.6 is selected for portion of the light beam is directed to a the purpose of illustration." The system second film. This arrangement has many speed in this case can be approximated by advantages in obtaining radiographs in the formula: cases where wide latitude of x-ray intensity 100 is encountered. For example, the x-ray CT cm2 intensity after passing through a chest normally would exhibit wide differences between the long region and the region behind the heart. In this case, an over- 105 If a green emitting output phosphor like penetrated or over-exposed record can be ZnCdS(Ag) type is used, the conversion made of the region behind the heart on one efficiency in terms of cd-sec/m2-R may also film and the normal lung field can be be used. This scale is also provided in Fig. 7 recorded on the second film; all this done 45 for reference. with a single x-ray exposure. 110 It is important to add here that several nine inch diameter working proximity type WHAT WE CLAIM IS:— image intensifier tubes according to the 1. An x-ray sensitive image intensifier invention have been constructed with a tube comprising an evacuated tube envelope 50 conversion efficiency in the range of 3,000 having an input window comprising at least 2 to 10,000 erg/cm -R. The output phosphors one of iron, chromium and nickel for 115 are of the ZnCdS(Ag) type and thus the receiving an x-ray image; a substantially flat conversion efficiency can also be expressed scintillator screen, as herein defined, in photometric terms as 400 to 1,400 cd- adjacent the input window for converting 2 55 sec/m -R. A prototype system incorporating the x-ray image into a light pattern image, a these tubes, a f/2 optical system with m=0.6, substantially flat photocathode layer 120 and Kodak RP/FC film, achieved image parallel to and adjacent the scintillator 10 1,605,127 10 screen for emitting photoelectrons in a 9. An x-ray sensitive image intensifier pattern corresponding to the light pattern tube according to any preceding claim image, a substantially flat, phosphor display wherein the tube envelope is metallic. screen parallel to and spaced from the 10. An x-ray sensitive image intensifier 65 5 photocathode layer there being no tube tube substantially as described herein with components present in the space between reference to and as illustrated by Figures 2 the photocathode layer and the display screen, to 5 of the accompanying drawings. and means for applying an electrostatic 11. A radiographic camera comprising an voltage to the display screen and the x-ray image intensifier tube according to 70 10 photocathode layer so as to create an any preceding claim, an optical recording electrofield therebetween to accelerate the medium; and an optical lens system adapted pattern of photoelectrons toward the for focussing the image presented on the display screen along substantially parallel, output display screen as a reduced image on straight trajectories to impinge upon the the recording medium. 75 15 output display screen, the paths of such 12. A camera according to Claim 11 parallel straight trajectories being governed wherein the recording medium comprises a solely by the electrostatic voltage applied to photographic film having a sensitivity (G) in the photocathode layer and the display terms of the reciprocal of light energy per screen, the image created at the display square centimeter (erg/cm2) which is 80 20 screen being substantially equal in size to required to produce a net density of 1.0; that of the x-ray image received at the input wherein the image intensifier tube has a window. conversion efficiency (C) in terms of ergs 2. An x-ray image intensifier tube per square centimeter—roentgen (erg/cm2- according to Claim 1 wherein the spacing R); . 85 25 between the photocathode layer and the wherein the optical lens system includes a output display screen is between 6mm to lens and the fractional light (T) emitted by 25mm such that when an electrostatic the output screen which is collected by the potential of 10,000 volts to 60,000 volts is optical lens system and which is transferred applied between them from an external to the photographic film, is defined as 90 30 source, the tube has a linear response with respect to input x-ray dose rates in excess of 0.06 R/Sec. T=- 3. An x-ray image intensifier tube 4P (1+m)2 according to Claim 1 or Claim 2 wherein the 35 tube input window is made of an alloy of where iron, chromium and nickel. transmission of the lens 4. An x-ray sensitive image intensifier F=the f number of the lens, and tube according to Claim 3 wherein the input m=magnification of the image 95 window is made of 17—7 PH type of and wherein the total speed of the camera 40 precipitation hardened, chromium-nickel defined as the product S=CTG, in the stainless steel. medical diagnostic region of the x-ray 5. An x-ray sensitive image intensifier spectrum, 30—100 Kev, is in the range of 1 tube according to any preceding claim 500 to 5,000 R- for the film to achieve a net 100 including a source of high voltage for density of 1.0. 45 applying a field potential between the 13. A camera according to Claim 12 photocathode layer and the display screen wherein the conversion efficiency C of the of between 2Kv/mm and 5Kv/mm. intensifier tube is in the range of 3,000 to 2 6. An x-ray sensitive image intensifier 14,000 erg/cm -R. 105 tube according to any preceding claim 14. An x-ray sensitive radiographic 50 wherein the scintillator screen comprises a camera substantially as described herein vapour deposited layer of sodium activated, with reference to and as illustrated by cesium iodide (CsI(Na)). Figures 6 and 7 of the accompanying 7. A x-ray sensitive image intensifier tube drawings. 110 according to any of Claims 1 to 4 wherein 55 the scintillator screen comprises a vapour For the Applicants. deposited layer of calcium tungstate LLOYD WISE, TREGEAR & CO., (CaW04). Chartered Patent Agents, 8. An x-ray sensitive image intensifier Norman House, tube according to Claim 6 or Claim 7 105—109 Strand, 60 wherein the scintillator screen is between London, WC2R 0AE. 200 and 600 microns thick.

Printed for Her Majesty's Stationery Office, by the Courier Press, Leamington Spa, 1981 Published by The Patent Office, 25 Southampton Buildings, London, WC2A 1AY. from which copies may be obtained. 1605127 COMPLETE SPECIFICATION

4 SHFFT? drawinging is a rreproduction of the Originalnol on a reduced scale Sheet 1

20s 22, /6-, 24j 26 -10

X RAY SOURCE n u \ A r

CAMERA

FIG—I.

FIG—2. 1604086 COMPLETE SPECIFICATION

4 SHEETS Growing is a reproduction of the Original on a reduced scale Sheet 2 r5

l5

FIG.—3.

FIG—4. 1604086 COMPLETE SPECIFICATION

1 CUPPTS drawinging isis aa reproductionr of * bHbtli the Qrlginalnai oonn aa re(juce(j scaje Sheet 3

FIG.-. 5. 1605127 COMPLETE SPECIFICATION

4 SHEETS drnwine 's 11 rcPr°dur-'ion of the Original on a reduced scale Sheet 4

FIG 6.

i i i i 100 300 1,000 3,000 z C, Cd-sec/m -R FIG 7.