Efficient Use of Automatic Exposure Control Systems in Computed Tomography Requires Correct Patient Positioning

Background: Image quality and radiation dose to the patient are important factors in computed tomography (CT). To provide constant image quality, tube current modulation (TCM) performed by automatic exposure control (AEC) adjusts the tube current to the patient's size and shape. Purpose: To evaluate the effects of patient centering on tube current-time product (mAs) and image noise. Material and Methods: An oval-shaped acrylic phantom was scanned in various off-center positions, at 30-mm intervals within a 500-mm field of view, using three different CT scanners. Acquisition parameters were similar to routine abdomen examinations at each site. The mAs was recorded and noise measured in the images. The correlation of mAs and noise with position was calculated using Pearson correlation. Results: In all three scanners, the mAs delivered by the AEC changed with y-position of the phantom (P