[Two-phase helical hepatic CT. Contrast-injection protocol, optimal timing and its usefulness in clinical cases]

The usefulness of two-phase helical computed tomography (CT) of the liver was evaluated in clinical cases. First, an optimal scanning protocol was determined using time-attenuation analysis. Monophasic dynamic CT was performed with 100 ml of contrast media (iodine 300 mg/ml) injected either at 3 ml/s or at 2 ml/s. Aortic, hepatic and portal time-attenuation curves were made for each protocol. The results showed that these two different injection protocols produce equivalent enhancements and that the injection rate of 2 ml/s is satisfactorily applicable to clinical cases. The first scanning (arterial phase) must be started 40-45 s after the beginning of the injection of contrast media and the second scanning (delayed phase) 80-120 s after the beginning of injection. Using these CT protocols, 327 cases were examined. In this study 83 hepatic lesions (hepatocellular carcinoma : HCC, n = 29; suspected HCC, n = 30; hemangioma, n = 24) were evaluated. There were 15 HCCs smaller than 30 mm in diameter (71.4%) detected by either arterial phase or delayed phase alone. This result indicates that two-phase helical hepatic CT is very useful in the detection of small HCC. Particularly, four of 5 HCCs of less than 10 mm in size (80%) showed a hyperattenuation area in the arterial phase alone. HCCs which have sufficient vascularity were also easily demonstrated. However, this two-phase helical hepatic CT could not demonstrate 11 lesions (13.3%) with almost normal blood supply. In such cases the complementary role of ultrasound (US) seems to be important. Hence as a screening of hepatic mass lesions both CT and US are necessary. Most HCC could be differentiated from hemangioma by an enhancement pattern using this protocol. But the small liver lesions of less than 15 mm in size with homogeneous hyperattenuation in the arterial phase and isoattenuation in the delayed phase included HCC, hemangioma and metastatic tumors.