Patent Number: 048213063
Section: description

FIG. 1 shows a system for slit radiography embodying the ideas underlying the present invention. Reference numeral 1 designates a source emitting X-radiation, which radiation passes through a diaphragm 2 and the schematically shown body of a patient to be incident on an X-ray detector tube 3. Source 1 is mounted for rotation about its axis, while diaphragm 2 and tube 3 are caused to perform a continuous, linear movement during the rotation of source 1 so that a portion of the body of the patient is irradiated by a fan X-ray beam extending normal to the plane of the drawing, with the radiation transmitted being incident on tube 3. Instead of performing a linear movement, the diaphragm may rotate about the axis of rotation of the source. For a more detailed explanation of the operation of the system shown, reference is made to U.S. patent application No. 06/648,707, filed on Sept. 7, 1984. In accordance with the invention, a filter 4 is mounted near diaphragm 2, which filter intercepts a portion of the beam emanating from source 1 to block the relatively soft X-radiation in this beam and pass the harder radiation. In principle, the filter may be of any material known to suit this purpose but materials such as copper or lead are preferred. Tube 3 is evacuated and, during operation, an electrical field is established between cathode 5 and anode 6. In FIG. 1a, cathode 5 is shown on an enlarged scale and includes a cathode support 7 having its rear face coated with a layer 8', 8" of, for example, CsI. In layer 8', 8" the X-radiation is converted into a visible image, which image releases electrons in the photocathode 9 mounted on the rear face of layer 8', 8", which electrons are projected onto the anode and can there be converted in known per se manner into an intensified visible image. The strip-shaped portion 8' of the CsI layer is relatively thin and the strip-shaped portion 8" is of considerably greater thickness, while the system is so dimensioned that the radiation passed by filter 4, i.e. the hardened radiation, is incident on the thicker portion 8" of the CsI layer, so that an image essentially formed by hard X-radiation is formed on the rear face of strip 8". This image causes electrons to emanate from photocathode 9, which electrons are released by the relatively hard, high energy X-radiation. The unfiltered radiation is incident on the thinner strip 8' of the cathode, in which strip predominantly the softer radiation is absorbed, so that in strip 8' an image is formed by the essentially soft X-radiation, whereby electrons are released in the photocathode, of which electrons the percentage caused by soft radiation is considerably higher than the percentage of electrons emanating from the rear face of strip 8" as a result of soft radiation. The above will be elucidated with reference to FIGS. 2 and 3. When determining the graphs of FIGS. 2 and 3, the X-ray source was operated at a constant voltage of 130 kVp and CsI was used as the detection material for layer 8', 8". In FIG. 2, curve A is the product curve of the emission of the X-ray tube, the transmission of a patient (150 mm H.sub.2 O) and the absorption of a CsI layer 8' of 0.05 mm thickness, as a function of the keV value of the X-radiation. Curve B differs from curve A as the radiation is additionally filtered by filter 4, which filter consists of Cu of 2 mm thickness, and as absorption occurs in layer 8" of CsI, which layer has a thickness of 0.3 mm. Both curves are standardized at a peak value 100. The "center of gravity" of curve A is at 57 keV and that of curve B at 85 keV. The light yield of the unfiltered, relatively soft radiation (A) is approximately 2.7 times higher than that of the filtered, relatively hard radiation (B). If this should present a problem when processing the images, the ratio of the widths of the beams and detector strips can be so modified by appropriately locating filter 4 and detector strips 8' and 8" corresponding therewith, on which strips he respective X-ray beams are incident, that the period of time during which strip 8" is exposed to radiation exceeds the period of time during which strip 8' is exposed. Also the curves C and D show the effect of filtering on the keV value of the X-radiation. Also these curves are standardized at a peak value 100. The difference between these curves and those shown in FIG. 2 is that the unfiltered radiation C is absorbed by a CsI layer 8' having a thickness of 0.1 mm instead of 0.05 mm, and that the filtered radiation is filtered by a Cu filter 4 having a thickness of 1 mm instead of 2 mm. The "center of gravity" of curve C is now at 58 keV and that of curve D at 78 keV. The distance between the "centers of gravity" has become smaller, mainly as a result of the lesser filtering. The light yield of the unfiltered radiation (C) is about 2 times higher than that of the filtered radiation (D) and about 1.8 times higher than the light yield obtained when using a CsI detection strip of 0.05 mm thickness, as this is done in the case of curve A. If also a CsI detection strip of 0.3 mm thickness is used for the unfiltered radiation, the "center of gravity" will be at 61 keV and the light yield will be a good 4 times higher than that of the filtered radiation, as shown in curve D. If a CsI detection strip of 1 mm thickness is used for the filtered radiation, the "center of gravity" will be at 88 keV and the light yield will be approximately twice as high as that obtained when using CsI of 0.3 mm thickness. However, for the present the manufacture of a CsI detection layer of 1 mm thickness presents insurmountable technological problems. In the system according to the invention, the processing of the image formed by means of the detection layer 8', 8" should self-evidently be so performed that two images are produced, namely a first image corresponding with the upper part and a second image corresponding with the lower part of the image formed on the anode of the image intensifier tube. This can be realized in different manners. The anode 6 may be composed of two strips 16 and 16' which are each of a different phosphor, so that the electrons emanating from the thicker section of the detector produce light of a color different from that produced by the electrons emanating from the thinner section. It is also possible to use one type of phosphor which is externally coated with two types of filtering material having mutually different spectral transmissivities. Furthermore, it is also possible to employ filters causing the polarization state of the light from the two strips to differ from each other. The difference in color or polarization state renders it possible to split up the light path by means of color- or polarization-sensitive splitter mirrors to two detectors, films or so-called diode arrays. In the case of color information, it is also possible to directly record the information provided by the anode on color film without additional splitting. Furthermore, a conventional anode may be used, in which case the light path can be split up by means of a splitter mirror for application through two parallel objectives to two diode arrays or two films. In the latter case, a shield or knife-edge travelling in front of the film should be employed to ensure that only one strip of the detector is imaged on each film. Finally, the anode may be a glass fiber anode on which diode arrays are provided for scanning the different regions of the anode. This is schematically shown in FIG. 4, where reference numeral 10 designates an X-ray image intensifier tube including a cathode 11, for example of similar structure as cathode 5 of FIG. 1, and an anode 13 mounted on a glass fiber plate 12. Additional glass fiber plate elements 14 and 14' are mounted on glass fiber plate 12, which elements support diode arrays 15 and 15' at their ends remote from plate 12. In this arrangement of the anode, the structure disclosed in Dutch patent application No. 84,01105 may further be used to advantage. Instead of the straight configuration shown, the glass fiber plate elements 14 and 14' may have a slightly curved shape to the effect that, seen in FIG. 4, element 14 is curved in downward direction and element 14' in upward direction, resulting in a less critical mounting of the diode arrays on the ends of elements 14 and 14'. As elements 14 and 14' are composed of glass fibers, the realisation of the curved shape does not present a problem while the light transmission is not impaired thereby. To obtain a proper separation of the two images on the strips, such as 8' and 8", of the cathode, an inactive region can be provided between the two CsI strips, which region is fully non-responsive to X-radiation.