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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 electrons 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 silver 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 photocathode 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 electron 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, phosphor 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.
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