The Importance of Patient Centering to Limit Radiation Dose in CT

The use of various tools that influence radiation dose and image noise, such as X-ray intensity shaping filters (bowtie filters), often requires accurate patient centering in the scan field of view. With the use of these bowtie filters, patient centering is critical to achieve the optimal balance of reduced radiation dose with reduced noise. To assess the influence of patient centering on image quality and dose requirements, researchers conducted imaging scans with bowtie filters using phantom centers that were positioned at 0, 3, and 6 cm below the center of rotation, or isocenter. These models were then retrospectively applied to scans from 273 adult body patients. The researchers reported that miscentering by 3 to 6 cm resulted in an increase in surface dose of 18% and 41%, respectively, and an increase in image noise of 6% and 22%, respectively. In addition, the retrospective analysis of adult body scout scan projection radiograph scans found that 46% of patients were miscentered in elevation by 20 to 60 mm, with a mean of 23 mm below the isocenter. Overall, the study concluded that patient miscentering resulted in a dose penalty of up to 140%, with a mean penalty of 33%, assuming that increases in tube current were necessary to compensate for increased image noise. The authors suggested automatically providing patient-specific centering and scan parameter selection information during CT studies so that radiologic technologists could improve workflow, achieve consistent image quality, and reduce radiation dose.¹⁹

Another study focused on the impact of patient miscentering on image noise and surface radiation dose using 3 different commercial CT scanners. Phantoms were positioned at 0, 2, 4, and 6 cm below the isocenter of the scanner's field of view. The resulting measurements were applied to imaging studies from 480 patients. The researchers reported that for a 64-slice CT scanner, the maximum increase in surface dose was 13.5%, 33.3%, and 51.1% with miscenterings of 2, 4, and 6 cm, respectively. An analysis of the available patient scout scans determined that patients were miscentered by an average of 2.2 cm below the isocenter. This resulted in increases in patient dose and image noise by 23% and 7%, respectively.²⁰These studies demonstrate the importance of patient centering to limit radiation dose while optimizing image quality.