Impact of Vertical Off-centering on Radiation Dose and Image Noise in Photon-counting Computed Tomography: A Phantom...

Photon-counting CT (PCCT) represents a newer CT technology with reduced electronic noise and potentially better dose efficiency than conventional CT. However, it remains unclear how vertical off-center positioning affects dose and image quality across a spectrum of patient sizes. The aim was to quantify the effects of vertical off-centering on radiation dose and image noise in PCCT using anthropomorphic phantoms representing both adult and pediatric body sizes.

Three anthropomorphic phantoms (adult male, 10-year-old, and 5-year-old) were scanned on a commercially available PCCT system at multiple vertical offsets using a posteroanterior localizer with the x-ray tube positioned below the phantom. Chest and abdomen protocols were used, with radiation doses, Monte Carlo-simulated organ doses, and image noise recorded at each offset.

Off-centering markedly affected radiation dose, whereas image noise differed primarily between the predefined image quality levels. A strong linear relationship was observed between vertical offset and CTDI vol [median R 2 (IQL) = 0.85 (0.78-0.98)]. Downward off-centering (-4 cm) increased radiation dose by up to 16% in adults and 17% in pediatric phantoms for both chest and abdominal scans, with the largest effects in chest scans without tin filtration. Upward off-centering (+4 cm) reduced dose by up to 11% in adults and 8% in pediatric phantoms. Larger phantoms showed steeper regression slopes, indicating stronger dose dependence on positioning. In contrast, no consistent dependence of image noise on vertical off-centering was observed within a given image quality level [median R 2 = 0.23 (0.03-0.52)]. Across all offsets, the overall variation reached +72%/-47% in chest PCCT and +66%/-13% in abdominal PCCT.

Vertical off-center positioning substantially affects radiation dose in PCCT, whereas image noise appears largely independent of vertical positioning within a given image quality level. Meticulous isocenter alignment remains crucial for both adult and pediatric imaging to avoid unnecessary radiation and sustain diagnostic image quality.