Contrast-Enhanced CT Significantly Increased Radiation Dose

The use of contrast media during computed tomography (CT) scans was found to significantly increase the radiation dose present in several organs, a new study has found. The results, published in the June issue of the American Journal of Roentgenology, indicate that both the iodine quantity and the number and timing of contrast-enhanced CT scans should be optimized when setting protocols.

“Currently, contrast enhanced CT is a vastly employed imaging modality,” said Ernesto Amato, PhD, of the department of radiological sciences at the University of Messina, Italy. “Several protocols require multiple acquisitions post-injection of the iodinated contrast medium at different circulation phases.”

Unfortunately, CT dose index or dose-length product evaluations do not account for the increment of radiation dose to the organs and tissues impregnated with iodine, which accumulates at different rates in the acquisition phases.

In a previous study, Amato and colleagues established a new method that would allow for the evaluation of the dose increase when a contrast medium was used in CT scans. The method was based on the increment in Hounsfield units observed in patients’ organs before and after CT scans.

“The increment in radiation dose to an iodine-loaded tissue depends on the quantity of iodine at the time of the scan, and on the imaging parameters (kV, filtration),” Amato said. “Such increment can be calculated from the increment in CT numbers (Hounsfield Units), and from a preliminary calibration procedure with known dilutions of iodine contrast.”

The current study was designed to confirm the findings of their previous study in a larger sample population. The retrospective analysis included 40 consecutive patients who underwent CT with and without contrast. The contrast CT was performed immediately after the unenhanced CT using the same scanning parameters, and the dose increase between the two scans was evaluated in the thyroid, liver, spleen, pancreas and kidneys.  

Results of this confirmation study indicated that the increases seen were in line with the previous study. The researchers found that the average dose increment increases were 19 percent for the liver, 71 percent for the kidneys, 33 percent for the spleen and pancreas, and 41 percent for the thyroid.

“Kidneys exhibited the maximum dose increment, whereas the pancreas showed the

widest variance because of the differences in fibro-fatty involution,” Amato and colleagues wrote. “Finally, thyroids with high attenuation values on unenhanced CT showed a lower Hounsfield unit increase and, thus, a smaller increment in the dose.”

Amato and colleagues concluded by suggesting that this method has great utility to estimate radiation dose to each organ in patients undergoing contrast CT.

“Even if the dose increments remain within the ranges of doses in CT studies, it is important to account for them when evaluating doses and effective doses in CT examinations,” Amato said. “Imaging protocols requiring multiple acquisitions in contrast-enhanced CT examinations should also be optimized in view of these findings.”

They are considering studying other relevant organs, such as the ovaries and tests, or extending the study into the pediatric patient population.