Vertical miscentering increases eye lens radiation dose and degrades image quality in pediatric head CT: insights from an in-house heterogeneous phantom.

OBJECTIVE: From a radioprotection perspective, this work aims to quantify how vertical miscentering redistributes tissue-specific dose and alters objective image quality in pediatric head CT, using a novel in-house heterogeneous pediatric head phantom, in order to better characterize avoidable positioning-related dose increases and support optimization of pediatric CT practice. METHODS: A novel pediatric head phantom incorporating validated tissue-equivalent inserts (bone, brain, CSF, eye lens and an air cavity) with machined cavities for OSLD nanodots was scanned on an 80-MDCT system using a pediatric head protocol. The phantom was positioned at isocenter (0 cm) and vertically offset by - 3, -1, + 1 and + 3 cm, each measurement was repeated three times. RESULTS: At 0 cm, mean absorbed doses were 22.93 ± 0.82 mGy (bone), 18.00 ± 0.55 mGy (brain) and 7.97 ± 0.15 mGy (eye lens). Miscentering increased dose non-uniformly and asymmetrically, with the largest sensitivity in the eye lens (10.50 ± 0.20 mGy at - 3 cm, + 31.8%; 9.37 ± 0.15 mGy at + 3 cm, + 17.6%). Noise followed a U-shaped trend, reaching a minimum at isocenter (bone ~ 23 HU at 0 cm against ~ 32 HU at - 3 cm and ~ 29 HU at + 3 cm) with corresponding SNR reduction (air ~ 125 at 0 cm against ~ 90 at - 3 cm). Low-contrast CNR (brain-CSF) peaked at isocenter (~ 5.8) and decreased under miscentering (~ 4.3 at - 3 cm; ~5.0 at + 3 cm). The largest CT-number deviations occurred in air (+ 30 HU at - 3 cm) and bone (+ 15 HU at - 3 cm). CONCLUSION: Miscentering of ± 1 to ± 3 cm measurably increases eye-lens dose and degrades noise-limited image quality and HU fidelity in pediatric head CT. These findings support accurate isocenter alignment as a simple but important radioprotection step in routine pediatric CT workflow. In practice, careful table-height adjustment, verification of head centering using laser or localizer guidance before acquisition, and the use of automated centering tools where available may help reduce avoidable radiation exposure while preserving image quality.