Patent Number: 048213063
Section: summary

The invention relates to a system comprising an X-ray source, an elongate detector tube and a slit diaphragm between the X-ray source and the detector tube, the slit-shaped aperture of the diaphragm extending parallel with the longitudinal axis of the detector tube and the detector tube including at least one cathode extending in the longitudinal direction of the tube and at least one anode located opposite to the cathode and likewise extending in the longitudinal direction of the tube, the tube being evacuated and, during operation, an electrical field being established between the cathode and the anode. Such an elongate X-ray detector tube is disclosed in Dutch patent application 79,00878 and is particularly suited for use in slit radiography. It is known from the article "Computerized dual-energy imaging: a technical description" by J. Coumans et al in Medicamundi, Vol. 27, No. 3, 1982, to produce so-called dual-energy X-ray images by alternately operating an X-ray source at two different high voltage levels, for example 70 and 120 kVp. The alternate application of the two voltages to the X-ray source results in the generation of X-ray beams having mutually different "energetic centers of gravity", in other words, mutually different hardnesses. By successively irradiating the object to be examined, such as the body of a patient, with X-radiation having a first energetic center of gravity, and X-radiation having a second energetic center of gravity, it is possible to so process, for example by means of a computer, the resultant X-ray images that, for example, only tissue and no bones are imaged, permitting the imaging of tissue located behind, for example, ribs. This is the result of the fact that different materials in, for example, the human body exhibit different absorption to X-radiation of different hardnesses. The use of different levels for the high voltage supply of an X-ray source entails the drawback that the irradiation has to be performed at two successive points of time, while the interval between these points of time must not be too long as otherwise movements of the object under examination can result in errors when processing the images obtained by means of the different X-ray beams. In slit radiography, as described in, for example, U.S. patent application No. 06/648,707, filed on Sept. 7, 1984, mechanical scanning is required for obtaining a complete image of a patient, while the scanning time for a total image height of 0.4 m is approximately 1 second. This scanning time is so long that a subsequent second scan, at a different X-ray source voltage, inevitably leads to a changed geometry of the image of the patient, which is unacceptable. It is an object of the invention to provide facilities permitting the application of the so-called dual energy image processing techniques to slit radiography, without the need for switching the anode voltage of the X-ray source. To this end, in accordance with the invention in a system of the above type a filter is mounted near the slit diaphragm in the path between the X-ray source and the detector tube, which filter intercepts a portion of the X-ray beam emitted by the source over the entire length of the slit-shaped aperture and blocks relatively low energy X-radiation in this beam portion, and the cathode is provided with an X-ray detection layer consisting of two essentially parallel strips extending in the longitudinal direction of the tube, one of these strips receiving the radiation passed by the filter and the other of the strips receiving the unfiltered radiation. The one strip preferably is of considerably greater thickness than the other strip. The invention is based on the insight that, in slit radiography, the local exposure time is considerably less than the canning time for the complete image. The local exposure time is the time required by a flat fan X-ray beam for passing a point of the patient. This beam is obtained by means of the slit diaphragm between the X-ray source and the patient, which diaphragm only allows X-radiation to be incident, after passing through the patient, on the detector tube within a spatial angle defined by the strip-like detector seen from the focal point of the X-ray source. When this slit diaphragm is thought to be divided into two narrower, superimposed slits of equal length, which slits need, in general, not be equally narrow, the original X-ray beam may be regarded to be composed of two superimposed, even flatter fan subbeams each incident on an associated narrow strip of the detector. The mechanical scanning thus results in two images, with a time difference of less than 0.1 second between the instants at which the same points of the patient are recorded. It is known that the radiation is hardened by placing a plate of suitable thickness of, for example, Pb or Cu in an X-ray beam, which means that lower energy radiation, i.e. radiation of lower frequencies, is attenuated to a higher degree than higher energy radiation, i.e. radiation of higher frequencies. Matter acts as a high pass filter upon X-radiation passing therethrough. Low pass filters for X-radiation cannot be realized in actual practice. In accordance with the invention, such a high pass filter is so mounted at the slit diaphragm in the X-ray beam path that it intercepts a portion of the flat fan X-ray beam, which portion actually constitutes one of the aforesaid subbeams. The passage of this subbeam through the filter results in a shift of the "energetic center of gravity" to a higher energy relative to that of the subbeam which passes through the slit diaphragm without being filtered. Furthermore, in accordance with a preferred embodiment of the invention the detector is so arranged that the strip on which the unfiltered beam is incident, predominantly absorbs soft radiation and optimally passes hard radiation while the strip on which the filtered beam is incident, optimally absorbs the radiation hardened by the filter. In this manner, the spacing between the "energetic centers of gravity" with which the beams act on the detector, can be increased even further. To achieve the detector characteristics desired, the strip of the detector on which the filtered beam is incident, is of considerably greater thickness than the other strip. If the detector includes an X-ray screen, besides the thickness of the screen also the screen material of one strip can be selected to differ from that of the other strip. For example, screen material consisting of atoms of low atomic number will predominantly absorb soft radiation.