PROBLEM TO BE SOLVED: To restrain the occurrence of a noise on projected data after correction, in relation to an unsuitable size, and improve image quality by finding the size of the photographing part of a specimen with a size data operation means, and allowing the selection of water-correction data or calibration data of such a size as suitable for the size of the photographing part. ;SOLUTION: The photographing part of a patient P is scanned with an X-ray CT scanner, and air-correction projection data from the surrounding area of the patient P is saved in the memory 9 of a correction data operation part 10. In this case, aperture width data having a maximum value is obtained from aperture width data so far obtained, via a specimen size measurement part 7, when a scanning angle reaches 180 degrees, and the maximum width of the patient P (specimen width) is obtained on the basis of the aperture data having a maximum value. Also, water-correction data for a matrix size larger than but closest to a circular size with patient width as a diameter is read out from the memory 9 and, then, the air-correction projection data is corrected with the water-correction data in the correction data operation part 10.;COPYRIGHT: (C)1997,JPO
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