Hepatic CT enhancement: effect of the rate and volume of contrast medium injection in an animal model

Background: To evaluate the relative effect of rate of injection and volume of contrast medium on aortic, portal, and hepatic enhancement during computed tomography (CT). Methods: Thirty-eight nonincremental CT examinations were performed in three mini-pigs by using a combination of three different volumes (1.5, 2, and 3 mL/kg) and five different rates (1.5, 3, 4.5, 6, and 7.5 mL/s) of contrast material injection. Time-density enhancement curves of the aorta, portal vein, and liver were plotted over time for each rate of injection, each volume of contrast, and each volume-rate combination. In addition, aortic, portal, and liver peak enhancements, time-to-peak enhancements, optimal scanning intervals, and contrast enhancement indices were calculated for each volume-rate combination. Results: Higher rates of injection increased peak aortic enhancement but had no effect on peak portal or hepatic enhancement. This result may be explained by the dilution of the bolus of contrast medium in the splanchnic circulation. When the results of a 6-mL/s injection of 1.5 mL/kg of contrast material were compared with a 3-mL/s injection of 2 mL/kg, maximum aortic enhancement increased by 32%, whereas maximum liver enhancement decreased by 35%. Conclusion: An increase in the rate of contrast injection results in an increase of peak aortic enhancement even when the total iodine load is decreased. However, an increase of the rate of contrast injection does not increase maximum liver enhancement, which is related to the total iodine dose injected. Therefore, one cannot compensate a decrease in the iodine load by an increase in injection rate in contrast-enhanced CT of the liver.