PURPOSE: To optimize the transition time between the triggering point in monitoring scanning and theinitia-tion of diagnostic hepatic arterial phase (HAP) scanning in hepatic spiral CT, using a bolus triggeringtechnique. MATERIALS AND METHODS: One hundred consecutive patients with focal hepatic lesion were included inthis study. Patients were randomized into two groups. Transition times of 7 and 11 seconds were used in group 1and 2, respectively. In all patients, bolus triggered HAP spiral CT was obtained using a semi-automatic bolustracking program after the injection of 120 mL of non-ionic contrast media at a rate of 3 mL/sec. When aorticenhancement reached 90 HU, diagnostic HAP scanning began after a given transition time. From images of group 1 andgroup 2, the degree of parenchymal enhancement of the liver and tumor-to-liver attenuation dif-ference weremeasured. Also, for qualitative analysis, conspicuity of the hepatic artery and hypervascular tumor was scored andanalyzed. RESULTS: Hepatic parenchymal enhancement on HAP was 12.07+/-6.44 HU in group 1 and 16.03+/-5.80 HU ingroup 2 (p<.05). Hypervascular tumors were detected in seven patients in group 1 and 13 patients in group 2.Tumor-to-liver contrast was 20.43+/-9.47 HU in group 1 and 28.77+/-12.75 HU in group 2 (p> .05). In the evaluationof conspicuity of hepatic artery, there was no statistically significant difference between the two groups(p>.05). The conspicuity of hypervascular tumors in group 2 was higher than in group 1 (p<.05). CONCLUSION: HAPspiral CT using a bolus triggering technique with a transition time of 11 seconds provides better HAP images thanwhen the transition time is 7 seconds.