Objectives: 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. Materials and Methods: 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. Results: 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. Conclusions: 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.
Klüner et al. (Thu,) studied this question.