Objective: The purpose of this study was to determine the relative effects of delay time, contrast agent concentration, and contrast agent volume on hepatic enhancement and on aortic and portal vein attenuation for contrast-enhanced spiral CT of the abdomen. The effect of delay time was evaluated by comparing two different delay times for each of three IV contrast media. Three different media were tested to identify whether the effects were different with different iodine concentrations and doses or with different volumes of contrast medium.
Subjects and methods: A total of 169 patients who underwent contrast-enhanced CT of the abdomen with spiral CT imaging were evaluated for enhancement of the liver parenchyma and for postenhancement attenuation of the aorta and portal vein. Total iodine doses of 30 g as 100 ml of iohexol 300 or 125 ml of ioversol 240 and of 40 g as 125 ml of ioversol 320 were evaluated at delay times of 45 and 60 sec and with a low-flow monophasic injection at a rate of 2 ml/sec.
Results: For each contrast medium tested, a 60-sec delay provided greater hepatic enhancement than did a 45-sec delay (43 vs 32 H for iohexol 300, 37 vs 28 H for ioversol 240, and 50 vs 39 H for ioversol 320; all comparisons, p < or = .0001). The aortic attenuation achieved in the abdomen after contrast medium administration was significantly greater with a 45-sec delay than with a 60-sec delay for iohexol 300 (p = .0290) but not for the other contrast media. The portal vein attenuation achieved after contrast medium administration was significantly greater with a 60-sec delay than with a 45-sec delay for all three contrast media (p = .0051).
Conclusion: When IV contrast material was injected with a low-flow-rate monophasic bolus, a delay time of 60 sec, longer than that typically used for dynamic incremental scanning, provided greater hepatic enhancement in spiral CT. This result was significant for the three contrast media tested. Aortic attenuation was significantly greater at shorter delay times when a smaller volume of contrast material was injected. Portal vein attenuation was greater at longer delay times, suggesting that the optimal delay time may be longer than 60 sec. Further work to determine the optimal scanning window for hepatic enhancement in spiral CT by use of time-density curves is indicated.