[Performance evaluation of X-ray computed tomography "TCT-700 S". 2].

UNLABELLED: The performance of the rotate/rotate CT scanner TCT-700 S (Toshiba) was evaluated on the basis of the item entitled "User's initial acceptance tests (phase II)" set forth in "Standard of performance evaluation for x-ray computed tomography (second recommendation)" proposed by Takenaka et al. (1982). This second report in the series describes measurements of exposure and the relation between exposure and such other performance aspects as noise and contrast resolution. The exposure-performance recommendation proposes measurements of average exposure in the scanning field and a radial exposure profile. RESULTS: 1. In the case of multiple scanning, average exposures in the field were uniform because all surfaces of the perpendicular object were oriented toward the incidence of the x-ray beam. At a minimum scanning speed of 1.8 second, however, a discrepancy of 3 times occurred in exposures observed reciprocally from the left and right sides of the object. This phenomenon indicates that only one-half of the object surface is oriented toward the x-ray beam and that the x-ray tube and detectors rotate in mutually opposite directions. In the case of multiple scannings, when slice thicknesses were 10 mm, 5 mm and 2 mm, average exposures were almost uniform. But with a slice thickness of 1 mm, average exposure was twice as great. This measurement indicates inadequate collimation in the case of a 1 mm slice thickness. These average exposures were roughly proportional to the mAs (product of tube current and scan speed) in the range between a minimum of 100 mAs and a maximum of 1200 mAs. 2. Noise increased in proportion to decrease of average exposure.(ABSTRACT TRUNCATED AT 250 WORDS)