The Impact of Scan Length on the Exposure Levels in 16- and 64-Row Multidetector Computed Tomography: A Phantom Study

Abstract

Higher patient exposure levels have been reported for 64-row multidetector computed tomography (MDCT) compared to 16-row MDCT. The objective of this study was to make a thorough comparison by evaluating the impact of scan length on the exposure levels at 16-row MDCT and 64-row MDCT. Dose-length product (DLP) values were determined to compare exposure levels in 16- and 64-row MDCT. This phantom study does not deal with a possible reduction in image quality induced by an increase in scattered radiation in 64-row MDCT compared to 16-row MDCT. The exposure levels of 64-row MDCT (scan slice thickness, 0.5 mm) are up to 18% lower than those of 16-row MDCT at slice thickness 0.5 mm when scanning an object larger than 12.3 cm. At this value, the plots of the 16- and 64-row DLP values versus scan length cross. The DLP curves of 1- and 2-mm slice thickness 16-row MDCT are in closer resemblance to those of 0.5-mm 64-row MDCT. The respective exposure levels of 1- and 2-mm slice thickness 16-row MDCT exceed those of 0.5-mm 64-row MDCT by up to 4% and 3%, with intersections of 30 and 25 cm, respectively. Lower effective doses are obtained in 64-row MDCT compared to 16-row MDCT (0.5-mm slice thickness) provided that scan length exceeds 12.3, 30, and 25 cm, for 16-row MDCT slice thickness of 0.5, 1, and 2 mm, respectively. Reduced effective dosage in 64-row MDCT compared to 16-row MDCT has not been demonstrated before. Differences in object size may thus explain discrepancies between previous studies with regard to the exposure levels at 64-slice CT compared to 16-slice CT.