This invention relates to an X-ray CT (computed tomogram) of a subject with X-rays.
Recently, various kinds of apparatuses for observing the tomogrammic structure of various parts of a patient using X-rays have been developed. For example, an X-ray CT apparatus of the third generation is provided with an X-ray source and an X-ray detector. The X-ray source is to irradiate the subject with a flat X-ray fan-shaped beam having a predetermined fan angle. The X-ray detector consists of a plurality of X-ray detection elements which detect the intensity of X-rays generated at the X-ray source and transmitted through the subject. The X-ray source and detector can be rotated in the same direction and at the same time about the subject in a state such that they always face each other on the direct opposite sides of the subject. In this way, X-ray projection data from various directions of an examined section of the subject is collected. After sufficient data has been collected, this data is analyzed by a computer to calculate X-ray absorption factors corresponding to various positions on an examined section of the subject. A graduation corresponding to the X-ray absorption factor is given to a corresponding position of the subject, whereby image data of the subject's examined section can be obtained.
With such a third generation X-ray CT apparatus, a given X-ray detection element of the detector collects X-ray projection data of the subject on a fixed circle at all times. Therefore, the detector of the apparatus has to meet the following requirements.
(1) There should be no sensitivity difference among the detection elements.
(2) The detection elements should be arranged at a constant pitch, and also a plurality of detection elements should be integrated such that there is no seam or gap between adjacent detection elements.
(3) The sensitivity should not vary according to temperature changes.
A detector of ion-chamber type has been used in which xenon (Xe) is sealed under a high pressure and a plurality of metal plates of tungsten, molybdenum, etc. having a high X-ray absorption factor are assembled at a uniform pitch to serve as both collimators and electrodes. However, such a detector of an ion-chamber type has the following drawbacks.
(1) Since a gas is used, the X-ray absorption factor is inferior.
(2) Since the gas is sealed under a high pressure, a pressure vessel is necessary, so that the detector is inevitably large in size, heavy in weight, and high in cost.
(3) During long use, gas leakage occurs resulting in variations in the characteristics.
(4) Microphonic noises are produced because of vibration.
Accordingly, a detector has recently been developed, in which solid-state scintillator elements and photo-semiconductor elements are bonded together. This detector can overcome the above drawbacks in the ion chamber type detector. However, it has the following drawbacks.
(1) Single crystal scintillators are used as the scintillator elements. Therefore, the optical outputs of the elements can not be made uniform, resulting in a sensitivity difference among the elements. In addition, the optical output is reduced with temperature rise, resulting in sensitivity variations. Further, the scintillators are expensive. Furthermore, it is impossible to provide a constant pitch of channels (i.e., elements).
(2) The photo-semiconductor elements are mainly made from a single crystal of silicon. Therefore, it is impossible to provide one having a large enough area to integrate a plurality of channels (e.g., 512 channels).
(3) The photo-semiconductor elements are flat plates. Therefore, if a plurality of photo-semiconductor elements are to be arranged as an arcular row so that they are directed toward a focal point of the X-ray source, a seam or gap is produced between adjacent elements.