Values of Bolus Tracking Methods for Optimal Hepatic Enhancement

Yang, Dal Mo

Abstract

PURPOSE: Most hepatic metastases are hypovascular, and CT scanning during peak hepatic enhancement is thusimportant for the detection of hepatic lesions. The purpose of this study was to determine whether images obtainedby bolus tracking show greater hepatic enhancement.
MATERIALS AND METHODS
We prospectively evaluated 101 patientswho underwent helical CT of the abdomen, using either a fixed 60-sec delay(n=50) or bolus tracking (n=51). For thelatter, we used a hepatic enhancement threshold of 50HU over baseline on monitor phase to determine a 6-sec delaybetween the monitor and diagnostic scanning phase. For all patients, three region-of-interest measurements wererecorded, one at each of the upper, middle, and lower levels of the liver; the measurements were averaged andsubtracted from the baseline density that measured CT values in three different hepatic segments at the mid-levelof the liver. We compared mean enhancement above the baseline of the liver between fixed 60-sec delay and bolustracking.
RESULTS
A statistically significant difference in the enhancement level of the liver (upper, p=.001;middle, p=.001; lower, p=.003) was noted between fixed 60-sec delay (upper, 65.3+/-16HU; middle, 67.4+/-16.5HU;lower, 68.5(19.4HU) and bolus tracking (upper, 75.2+/-15.5HU; middle, 74.4+/-13.7HU;lower, 75.6+/-13.6HU). With fixeddelay, 86% of patients reached 50HU of enhancement, but with bolus tracking, 98% reached this level. For onenhancement value of 60HU, the corresponding figures were 64% and 86%. Mean delay for the transition betweenmonitoring scans and diagnostic scan initiation was 63.8+/-7.9(range, 48-79)secs. For two patients in whom theenhancement curve did not reach threshold (50HU), the default time was 70 sec; one subsequently failed to reachthis same threshold.
CONCLUSION
Using the same amounts of contrast material, bolus tracking provides a greaterlevel of hepatic enhancement than a fixed 60-ses delay, and is thus helpful for the detection of hypovascularmasses such as metastatic lesions.