The presence of contrast agent increases organ radiation dose in contrast-enhanced CT

Routine dosimetry calculations do not account for the presence of iodine in organs and tissues during CT acquisition. This study aims to investigate the impact of contrast agent (CA) on radiation dose. First, relation between absorbed radiation dose and iodine concentrations was investigated using a cylindrical water phantom with iodine-saline dilution insertions. Subsequently, a retrospective study on abdominal dual-energy CT (DECT) patient data was performed to assess the increase of the local absorbed radiation dose compared to a non-contrast scan. Absorbed doses were estimated with Monte Carlo simulations using the individual CT voxel data of phantom and patients. Further, organ segmentations were performed to obtain the dose in liver, liver parenchyma, left kidney, right kidney, aorta, and spleen. In the phantom study, a linear relation was observed between the radiation dose normalized by computed tomography dose index (CTDI) and CA concentrations I${}_{conc}$ (mg/ml) for three tube voltages; $\frac{D_{80kVp}}{CTD{I}_{vol}}$ = 0.14 × I${}_{conc}$ + 1.02, $\frac{D_{120kVp}}{CTD{I}_{vol}}$ = 0.16 × I${}_{conc}$ + 1.21, $\frac{D_{140kVp}}{CTD{I}_{vol}}$ = 0.16 × I${}_{conc}$ + 1.24, and for DECT acquisition; $\frac{D_{DECT}}{CTD{I}_{vol}}$ = 0.15 × I${}_{conc}$ + 1.09. Similarly, a linear relation was observed between the dose increase and the organ iodine contents (R${}^2$ = 0.86 and p${}_{value}$< 0.01) in the patient study. The relative doses increased in the liver (21 ± 5%), liver parenchyma (20 ± 5%), right kidney (37 ± 7%), left kidney (39 ± 7%), aorta (34 ± 6%) and spleen (26 ± 4%). In addition, the local dose distributions changed based on patient's anatomy and physiology. Compared to a non-contrast scan, the organ doses increase by 30% in contrast-enhanced abdominal CT. This study suggests considering CA in dosimetry calculations, epidemiological studies, and organ dose estimations while developing new CT protocols.