Effect of iodinated contrast media enhancement on size-specific dose estimates in emergency thoracic CT

Accurate estimation of patient radiation dose is crucial in emergency computed tomography (CT), where rapid multiphase imaging is routine. The Size-Specific Dose Estimate (SSDE) incorporates patient size using the water-equivalent diameter (Dw), but standard calculations assume patient size remains constant across scan phases. Contrast administration, however, modifies tissue attenuation and may alter both Dw and SSDE.

A retrospective analysis of 100 consecutive emergency thoracic CT examinations was conducted, each including non-contrast (C-) and contrast-enhanced (C+) phases. Automatic tube-current modulation was applied for all acquisitions. From 22,000 slice-level observations, the scanner-reported volume CT dose index (CTDIvol), Dw, size-specific conversion factor (f), and SSDE were calculated using AAPM-recommended equations. Phase-wise comparisons were performed using paired t-tests.

Mean SSDE increased from 11.64 ± 1.11 mGy (C-) to 11.96 ± 1.17 mGy (C+), a statistically significant 2.7 % rise (p < 0.001). The apparent cause was a 3.4 % decrease in Dw (21.98 mm → 21.24 mm, p < 0.001), reflecting higher post-contrast attenuation interpreted as smaller patient size. This led to a 2.8 % increase in f, further amplified by the scanner's compensatory increase in tube current.

SSDE is a dynamic rather than static parameter, increasing systematically during contrast-enhanced thoracic CT because of both algorithmic artefacts and real modulation of scanner output. Reliance on a single averaged SSDE across multiphase protocols may underestimate patient dose.

Phase-specific SSDE reporting provides a more accurate and transparent assessment of patient radiation exposure in emergency CT. Integrating automated, phase-resolved SSDE computation into dose-management systems would enhance clinical decision-making, support protocol optimization, and improve adherence to the ALARA principle in daily radiology practice.