Dual phase hepatic CT: Influence of scanning direction on liver attenuation

OBJECTIVE. We measured changes in hepatic attenuation during arterial and p ortal phase acquisition of hepatic CT in the craniocaudal and caudocranial directions. SUBJECTS AND METHODS. In 10 of 20 patients undergoing dual phase helical CT during staging for colorectal cancer, images in both phases were obtained in the craniocaudal direction. Ten patients underwent imaging in the caudoc ranial direction. Attenuation values in the aorta and in the peripheral and central liver regions of interest were measured on each slice. Central and peripheral liver attenuation was also measured in 10 additional patients u ndergoing unenhanced CT. RESULTS. Both peripheral and central regions of interest revealed progressi vely increasing attenuation during the arterial phase, irrespective of scan ning direction. During the portal phase, hepatic attenuation was stable in the craniocaudal direction but decreased in the caudocranial direction (p < 0.05, Wilcoxon's signed rank sum test). Central hepatic attenuation was lo wer than peripheral attenuation in unenhanced livers and in enhanced livers during both phases of caudocranial acquisition. We determined no significa nt difference during the arterial phase of enhancement in the craniocaudal direction. CONCLUSION. The direction of acquisition does not influence sequential live r enhancement during the arterial phase. Craniocaudal acquisition produces more stable enhancement during the portal phase. Differences in attenuation between the central and peripheral areas of the liver are probably unrelat ed to contrast administration.