抄録
Computed Tomography (CT), a new method of medical X-ray imaging has become the center of wide interest in clinical medicine and it is about to achieve general use in Japan. In this review, the technical background, the principle, present status and future prospect of this technique are described with some explanations of reconstruction algorithms.
The technical background of CT consists of X-ray transverse tomography, RI imaging devices with on-line data acquisition system and algorithms of 3-D reconstruction from projections. A CT-Scanner is a kind of apparatus which make the transverse tomograph, but is different from conventional ones in principle. The features of this apparatus are as follows. 1) It uses a narrow beam of X-ray. 2) The radiation detector is a scintillation counter. 3) It reconstructs distribution of absorption coefficients in the cross section from intensities of transmitted X-ray. The CT-Scanner can show variations of absorption coefficients in the soft tissues. EMI's brain-scanner is described as an example of the first generation scanners. New types of CT-Scanners, which are more rapid in scanning speeds and finer in resolutions, are also described. Among them, Dynamic Scanner produced by JEOL is promising because it scans X-ray not mechanically but electro-optically. In future, one may develop a dynamic CT-Scanner by the use of an electro-optical scan.
Algorithms of 3-D reconstruction from projections are explained in relation to CT. Iterative methods, Fourier transform methods and convolution methods are mentioned. For the parallel beam geometry, convolution methods are best of all algorithms. But, for the fan beam geometry, many methods are being studied in order to obtain the optimum one.
