The effect of contrast agent on delivered dose during diagnostic imaging of thoracic region

This study evaluated the impact of contrast material on the estimation of absorbed dose due to computed tomography pulmonary angiography (CTPA) using the ICRP reference phantoms. To address this issue, we modified the previously developed physiologically based pharmacokinetic (PBPK) model to be conformed to the ICRP reference phantoms. Regarding the standard contrast material injection protocol, we then provided an in-house Maple code to solve the modified PBPK models and obtain the iodine concentration curves for adult male (AM) and adult female (AF) phantoms. The material composition of the phantoms was then adjusted to include the determined iodine mass percentages in different organs and tissues. The dosimetry calculations were performed using Monte Carlo N-Particle extended code (MCNPX) version 2.6.0., and the estimation of absorbed dose was done at 20, 25 and 30 seconds after the injection. The results showed dose increment of 38%-44% in the lungs at 25 s after the injection in comparison with the time before injection, which emphasizes the importance of considering contrast material in the dose estimation. It is known that the image quality of contrast-enhanced CT is related to the amount of iodine concentration in pulmonary vessels. Our calculations showed that the best time for imaging is 25 s after injection (5 s after the end of injection). At this time, the lung absorbed dose is maximized and the absorbed dose to other organs in the scanned region are minimized.