The present invention relates to a computed tomograph apparatus used for obtaining tomograms of subjects, for the purpose of medical examination.
FIGS. 1 and 2 schematically illustrate a conventional computed tomograph apparatus. In this apparatus, subject 4 is laid on bed 2. An X-ray tube 6 for emitting X-rays is located above bed 2, for emitting X-rays onto subject 4. The X-rays emitted from tube 6 proceed as a flux of rays along slice plane Y which is perpendicular to the surface of bed 2. More specifically, the radiated X-rays are converted, by collimator 10, to a fan-shaped beam having a sector angle .theta. and a slice thickness W. A principal detector 12, for detecting principal X-rays transmitted linearly through subject 4, and a number of scattered ray detectors 14, for detecting scattered X-rays other than the principal X rays, are arranged opposite to X-ray tube 6, as viewed from subject 4 on bed 2. Principal detector 12 comprises a plurality of channels and is located within the fan-shaped beam. Each of the scattered ray detectors 14 comprises a single channel and is located off the path of the beam and situated symmetrically with respect to slice plane Y. Principal detector 12 and each of scattered ray detectors 14 include a scintillation detector which consists of, for example, a scintillator and photodiodes. As X-ray tube 6, collimator 10, principal detector 12, and scattered ray detector 14 rotate integrally around axis X of rotation, with X-ray tube 6 continuously irradiating subject 4 with X-rays, a tomogram of subject 4 is gradually formed.
When forming a tomogram, preferably only the X-rays emitted from X-ray tube 6 and linearly transmitted through subject 4, and which are eventually detected by principal detector 12, should be used to produce a tomogram image, after processing the data representing the absorbed doses of the X-rays.
However, the X-rays which enter principal detector 12 contain not only those transmitted though subject 4, but also some which are scattered within the subject and others which are not transmitted though the subject at all, arriving at detector 12 simply by bypassing the subject (these rays are hereinafter referred to as scattered rays S). Therefore, it is necessary to include compensatory measures in the computer processing, so as to ensure generation of an accurate tomogram image, taking into account the presence of such scattered rays S. Accordingly, a number of scattered ray detectors 14 are provided in the vicinity of principal detector 12. Since the scattered ray detectors 14 are located off the path of the X-ray beam having a slice thickness W, they do not receive any incident principal X-rays, their incident rays being limited only to scattered rays.
For the above mentioned compensatory measures to be effective, however, it is necessary that rays S scattered in the same area of subject 4 are detected not only by any of scattered ray detectors 14 but also by principal detector 12, because if such rays S are detected only by scattered ray detectors 14, a correct compensatory measures cannot be achieved, thereby permitting artifacts to appear in the resulting tomogram image.
In a computed tomograph apparatus according to the prior art, while a principal detector 12 and scattered ray detectors 14 are provided in the vicinity of each other, their positional relationship is not specifically defined. This is because no definite relationship can be established between the scattered rays incident to principal detector 12 and those incident to scattered ray detectors 14. Consequently, the compensatory measures cannot be correctly achieved.