Misoperation of CT automatic tube current modulation systems with inappropriate patient centering: phantom studies.

Abstract

Inappropriate patient centering on the gantry changes the size of the localizer radiographs used for CT examinations, influencing the operation of CT automatic tube current modulation because tube current is controlled with information from localizer radiographs. The purpose of this study was to examine the influence of inappropriate patient centering on the gantry isocenter on automatic tube current modulation. An elliptical phantom was scanned with four automatic tube current modulation techniques after acquisition of localizer radiographs in the horizontal and vertical directions with the phantom center shifted from the gantry isocenter in the vertical direction. After scanning, the magnification rate of the frontal localizer radiographs, tube current-time product, and image noise were examined. On phantom studies, the magnification rate of localizer radiographs showed a linear relation to the vertical deviation of the phantom from the gantry isocenter. From 50 mm above to 50 mm below the gantry isocenter, tube current-time products ranged from 75% to 141% compared with those at the gantry isocenter. In addition, increases and decreases in the amount of image noise related to changes in tube current-time product were confirmed. Inappropriate patient centering causes misoperation of automatic tube current modulation systems, in which tube current is controlled with information from localizer radiographs, and thus causes increases in tube current or image noise.