X-ray examination apparatus

Abstract: An exit section (15) of an X-ray image intensifier tube (5), in this case comprising the exit phosphor screen, the exit window (18) and the basic optical system (6), is optimized so as to achieve a high light yield, low optical aberration and a high resolution. The exit window (18) notably has a pre-compensating geometry for curvature of the image plane of the basic optical system (6), or the exit window (18) comprises an interference filter, or the exit phosphor layer contains a layer of a comparatively slow phosphor in addition to a layer of a customary phosphor in order to achieve noise-suppressing image integration.

Description: The invention relates to an x-ray examination apparatus comprising an x-ray image intensifier with an entrance screen with a photocathode, an electron-optical system, an exit section with a phosphor screen on an exit window, an image pick-up device to derive an electronic image signal from a light-optical image on the exit window, and a light-optical image transfer system to image the light-optical image on a light-sensitive face of the image pick-up device. An X-ray examination apparatus of this kind is known from US-A-4,809,309. In a system described therein, the image transfer in the exit section often gives rise to the loss of a comparatively large part of the luminescent light to be generated in an exit screen. As a result, the brightness of an image-carrying light beam at the area of a subsequent image recording system, for example a television pick-up tube, a CCD camera, a film foil etc. is usually too low for optimum imaging. This situation can usually be improved only by way of an undesirable increase of the radiation dose in the imaging X-ray beam. From the French patent application FR-A-1 442 118 an image intensifier for infrared light ect. is known comprising an exit window having the form of a single component with a concave inner face and a concave outer face that are shaped to high precision to permit image transfer without distortion on the exit window. An object of the invention is to provide an x-ray examination apparatus with an x-ray image intensifier having an exit section that is operative to form an intense, high resolution image-carrying light beam with few optical abberations. This object is achieved by an x-ray examination according to the invention which is characterized in that the exit window comprises an inner element and an outer element, the inner element having a concave inner face facing the photocathode and supporting the phosphor screen, the inner element having a flat outer face facing away from the photocathode, the outer element serving as an input lens for the image transfer system and the outer element having a flat inner face facing said flat outer face of the inner element and a curved outer face which faces the light-optical transfer system, the outer element being cemented to the inner element so as to form a rigid connection between the exit window and the image transfer system. A preferred embodiment of an x-ray examination apparatus according to the invention is characterized in that the outer element is a flat-concave or a flat-convex lens. An object plane of the exit window of the x-ray image intensifier tube exhibits a precompensating image plane curvature for a subsequent optical image transfer system. Any resultant optical aberrations are then compensated for by adaptation in the optical system itself, or the exit window is constructed as a fibre-optical plate, so that an exit side thereof acts as an object plane for, for example the basic optical system, its curvature not contributing to the lens effect of the primary imaging by the optical system. In a fibre-optical window of this kind, a curvature can be imparted to the entrance side, being a carrier for the exit phosphor screen, which curvature is adapted to the curvature, if any, of the object plane of the electron-optical system of the X-ray image intensifier tube. The shape of the inner surface and the properties of the electron-optical system can then be optimized in an integrated manner. The exit window can be constructed so as to be comparatively thick in order to reduce disturbing halo effects. The use of a halo-reducing thick window, specifically as an exit window, is known per se from US 4,353,005 and it is known per se from the international application WO 85/00465 to provide the exit window with a fibre-optical plate. As has already been stated, a curvature can be imparted to the exit side of the window so to precompensate for image field curvature of the optical system. A precompensating curvature can also be applied by providing a glass plate, exhibiting the correct curvature, as a replica of the optical system on the outer side of the exit window. When this plate is provided on the exit window by way of optical cement, loss of light due to additional reflection or refractive index gradients is avoided. In a further preferred embodiment, an interference filter is provided on the outer side of the exit window. Because light incident at excessive angles is then reflected, improvement of the MTF is possible without giving rise to a loss in response of light yield, because the light incident at an excessively oblique angle is reflected again so that it partly contributes to the imaging again. Such an interference filter may consist of very many layers and is capable of reflecting light emanating from the window at an angle exceeding a given value. Due to subsequent reflection in the phosphor layer, this light will at least partly contribute to imaging again. Thus, a higher light intensity can be achieved (within the numerical aperture of the basic lens system) without incurring a substantial loss of resolution. Using an interference filter mounted between the output phosphor and the exit window, a similar effect can be achieved because light emerging from the phosphor layer at an excessive angle is reflected so that it can be used again. The exit window is formed notably as a fibre-optical plate, on the outer side of which there is provided an interference filter. When the interference filter is arranged on the outer side of the window, its inner side remains free for adaptation to the electro-optical system, etc. The effective light yield in an ultimate image can also be increased by means of an interference filter provided on the inner side of the exit window, i.e. between the exit phosphor layer and the window. It is to be noted that an interference filter for selection of light which is incident within a given angular range is known for the purpose of imaging from US 4,634,926. Further technical details of such an interference filter are disclosed therein. The light yield of the exit screen can also be increased by using an optically suitably dense metal backing layer. To this end, the metal backing layer is customarily constructed so as to be comparatively thick. However, such a thickness has the drawback that more photoelectrons do not contribute to imaging due to absorption in the layer. In order to avoid such a loss, in a preferred embodiment the usually aluminium metal backing layer is not provided by vapour-deposition, but by a deposition technique resulting in a layer of denser packing, for example by sputtering or CVD. In addition to an optically dense metal backing layer for optimum reflection of light generated in the phosphor layer, a dense layer also has an attractive function as a chemical shielding layer for shielding the phosphor layer against notably alkalis from the entrance screen of the tube. Such chemical shielding can also be realised by means of a layer of material especially adapted for this purpose. Because such a layer need not necessarily be reflective, a high degree of freedom exists as regards the choice of the material, which benefits the optimization in respect of density and electron transparency. A suitable material in this respect is, for example aluminium oxide which is preferably deposited again by sputtering or CVD so as to achieve a dense packing. Use can also be made of the optically transparent layers which are described in US 4,831,249 and which can be provided, for example also between the phosphor layer and the metal backing layer. Thus, a flatter substrate layer can be realized for the metal backing layer and the metal backing layer itself can be constructed so as to be thinner again. A thickness variation in, for example the radial direction can be imparted to such a shielding layer or metal backing layer so as to optimize the local light intensity homogeneity in the emanating image-carrying light beam and to compensate for, for example electron-optical deviations occurring therein. In a further preferred embodiment, in which the exit phosphor screen whether or not provided on a substrate is optically coupled to an inner surface of the exit window, the exit window constitutes an optical component of the light-optical image transfer system. The number of gradients in the refractive index, and hence the loss of light, can thus be reduced. The exit window notably forms a concave-flat lens, in the concave part of which there is provided the exit phosphor screen, an input lens of a subsequent image transfer system, i.e. a basic lens system, being cemented to its flat side. In addition to optimum light transfer, a rigid connection is thus obtained between the exit screen and the optical transfer system, so that optical aberrations due to non-exact optical positioning, for example relative to the optical axis of the assembly, are avoided. The otherwise necessary postfocusing of the optical transfer system onto the phosphor exit screen is thus also avoided. Moreover, defocusing due to atmospheric pressure variations, temperature fluctuations and the like is also precluded. In another preferred embodiment, the exit phosphor screen comprises two sub-layers, a first sub-layer thereof which is situated near the exit window exhibiting a comparatively long afterglow. When for a second phosphor sub-layer, being situated further from the exit window and composed of a phosphor having a customary or comparatively short afterglow, a thickness is chosen which is adapted to a high voltage to be applied, a choice can be made between an exit image having a comparatively short (or customary) afterglow and an exit image which is desired for noise integration and which has a comparatively long afterglow, said choice being made possible by high-voltage variation. A noise integration desired because of the nature or the processing of the diagnostic imaging can thus be realised merely by high-voltage variation in the X-ray image intensifier tube itself. Such noise integration is not at the expense of a loss of light. Activation of an exit phosphor layer having a long afterglow is notably coupled to a read-out via a non-integrating read-out system such as a CCD camera. Some preferred embodiments in accordance with the invention will be described in detail hereinafter with reference to the drawing. Therein: Fig. 1 shows an X-ray examination apparatus in accordance with the invention, Fig. 2 shows an inner element of the exit window for said X-ray examination apparatus, and Fig. 3 shows an embodiment of an exit window, which is optically coupled to a first lens of a relevant basic optical system. The drawing shows an X-ray source 1 with a power supply 2, a patient support 3 for a patient 4 to be examined, an X-ray image intensifier tube 5, a basic objective system 6, a semi-transparent mirror 7, a film camera 8, a television pick-up tube 9, and a television monitor 10 of an X-ray examination system. The X-ray image intensifier tube 5 comprises an entrance window 11, an entrance screen 12 with a luminescent layer 13, preferably made of CsI, and a photocathode 14 and an exit screen 15. The X-ray image intensifier tube also comprises an electron-optical system 16 which includes, in addition to the entrance screen 12 and the exit screen 15 which is preferably provided on an inner side of an exit window 18, an electron-optical system 19 which comprises one or more intermediate electrodes which are not separately shown. An incident X-ray beam 20 irradiates a part of a patient to be examined. An image carrying X-ray beam 21 transmitted thereby is incident on the entrance screen 12. The X-ray beam 21 incident on the entrance screen is converted in the entrance screen into an image-carrying beam of photoelectrons 22 which is accelerated to, for example 25 kV so as to be imaged on the exit screen 15. An image-carrying light beam 24, formed in the exit screen 15, leaves the image intensifier tube via the exit window 18. This light beam is used to expose, as desired, a photographic plate 26 in the camera 8 or a target 28 of the television pick-up tube 9. According to the present Patent Application, an output section 30 comprises the exit screen 15, the exit window 18 and the basic optical system 6. Fig. 2 shows an inner element 32 of the exit window which acts as a support for the exit screen 15. The inner element 32 is preferably made of glass and its side 34 which supports the exit screen 15 is concave. Such a concave shape enables the realisation of an optimum image plane for the imaging of the photoelectron beam; it can also serve to compensate for image plane deviations of the basic optical system. If the concave shape leads to a shape of the exit window which is less suitable from a lens-optical point of view, compensation can be achieved by way of the exit surface 36 of the inner element which is shown to be flat in the drawing. Thus, an optimum concave shape for the focusing of the photoelectrons can always be combined with an optimum object plane for the basic optical system. The exit window may comprise a fibre-optical window but may also consist of a homogeneous glass, an optical replica being cemented to an outer side of said window in order to compensate for known aberrations in the imaging properties of the basic optical system. The advantage of a separate replica consists in that the X-ray image intensifier tube and the basic optical system remain universally usable and that adapted precompensation can be achieved by means of the replica and the known imaging properties of the basic optical system. Fig. 3 shows an exit window 80, comprising a phosphor layer 15, deposited in a concave part of an inner surface 82 and a lens 86 which is provided on a flat outer side 84 and which constitutes a first lens of the basic optical system 6. A curvature of a supporting face 88 as desired for the photoelectron optical system can thus be combined, also when use is made of a homogeneous glass window, with an optimum object plane for the basic optical system. Because no clearance is present between the exit window and a first lens of the basic optical system, optimum focusing and correct rigid mounting are ensured. The exit window 80 also constitutes, in conjunction with the lens 86, a comparatively thick exit window so that the occurrence of halo phenomena in the exit image is also avoided. Such a construction combines optimum image transfer with high light yield. The latter is achieved because gradients are avoided in the coupling of the exit window to a first lens of the basic optical system and also because use is made of homogeneous glass in which the loss of light is substantially smaller than in a fibre-optical system. A second lens 88 completes the basic optical system 6 whereby the desired images can be formed in a customary manner.

Claims: An x-ray examination apparatus comprising an x-ray image intensifier (5) with an entrance screen (12) with a photocathode, an electron-optical system (22), an exit section (30) with a phosphor screen (15) on an exit window (18), an image pick-up device (9) to derive an electronic image signal from a light-optical image on the exit window (18), a light-optical image transfer system (6) to image the light-optical image on a light-sensitive face (28) of the image pick-up device (9), characterized in that the exit window (18) comprises an inner element (15,32,80) and an outer element (86). the inner element having a concave inner face (34, 82) facing the photocathode and supporting the phosphor screen (15), the inner element having a flat outer face (84) facing away from the photocathode, the outer element serving as an input lens for the image transfer system and the outer element having a flat inner face facing said flat outer face of the inner element and a curved outer face which faces the light-optical transfer system, the outer element being cemented to the inner element so as to form a rigid connection between the exit window (18) and the image transfer system (6). An x-ray examination apparatus as claimed in Claim 1, characterized in that the outer element (86) is a flat-concave or a flat-convex lens. An x-ray examination apparatus as claimed in Claim 1 or 2, characterized in that the inner element comprises a fibre-optic window. An x-ray examination apparatus as claimed in Claim 3, characterized in that the fibre-optical window is provided with an interference filter. An x-ray examination apparatus as claimed in any one of the preceding claims, characterized in that the phosphor screen comprises a first sub-layer consisting of a phosphor material having a first afterglow time and a second sub-layer consisting of a phosphor material having a second afterglow time.

Inventors: COLDITZ JOHANNES K E, DE VRIEZE HENRICUS M, DIEMER WILLEM H, JONK LAURENS H J, RONGEN ENGELBERTUS, SCHEUERMANN JOHANNES W J M, SIMONS AUGUST L H, COLDITZ, JOHANNES K.E., DE VRIEZE, HENRICUS M., DIEMER, WILLEM H., JONK, LAURENS H.J., RONGEN, ENGELBERTUS, SCHEUERMANN, JOHANNES W.J.M.,, SIMONS, AUGUST L.H., Colditz, Johannes K.E., c/o INT. OCTROOIBUREAU B.V, De Vrieze, Henricus M., c/o INT. OCTROOIBUREAU B.V, Diemer, Willem H., c/o INT. OCTROOIBUREAU B.V., Jonk, Laurens H.J., c/o INT. OCTROOIBUREAU B.V., Rongen, Engelbertus, c/o INT. OCTROOIBUREAU B.V., Simons, August L.H., c/o INT. OCTROOIBUREAU B.V.
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1992
Main classification: H01J31
IPC codes: H01J29, H01J31