Phantom evaluation of involuntary mis-centering in CT Scan : consequences for radiation dose and image quality

This study investigates how involuntary patient mis-centering affects dose distribution and image quality in computed tomography (CT), with the goal of reducing radiation exposure while preserving diagnostic performance.

Mis-centering was performed by shifting a Sun Nuclear CTDI phantom vertically and laterally, with dose recorded in specific phantom holes using a 10X6-3CT ionization chamber (Radcal). Measurements were performed on three CT scanners (Philips, FUJIFILM, Hitachi) under identical acquisition parameters. Additionally, a Philips system paired with a Catphan-503 phantom was used to assess image-quality changes. Dose ratios, calculated from absorbed dose measurements in multiple phantom holes, quantified the effect of off-center positioning.

Peripheral doses were highly sensitive to displacement: a vertical offset above the isocenter reduced the dose by up to 35% at the point above the isocenter, while an increase was observed at symmetrical points, while a lateral offset reduced it by up to 18% at points in the direction of displacement. Image-quality metrics were affected to a lesser degree, likely because modern reconstruction algorithms partially compensate for mis-centering.

These findings suggest that deliberate mis-centering may be considered during follow-up CT examinations to spare radiation-sensitive regions without clinically significant loss of image quality.