Objective: Scan delays for CT angiography of the abdomen are currently individualized by matching a scan delay to the transit time of a test bolus of IV contrast material to the abdominal aorta. We sought to determine whether this procedure improves the extent and uniformity of aortic enhancement when compared with the use of a standard scan delay that is determined empirically.
Subjects and methods: CT angiography of the upper abdomen (either the entire abdominal aorta or the aorta spanning the length of both kidneys for dedicated renal studies) was obtained in 48 patients who were considered to have normal cardiac function as determined by their referring physicians. Patients were randomized to undergo CT angiography that used either a standard scan delay of 20 or 22 sec (group 1, n = 26) or a tailored scan delay (group 2, n = 22), all other imaging parameters being identical. Tailored delays were determined by measuring contrast transit time to the abdominal aorta with software from the SmartPrep system. For each patient, aortic attenuation was measured at three different anatomic levels, and a lower threshold of 160 H was considered optimal. The number of attenuation values above this threshold, the mean attenuation, and the attenuation changes for each unit length along the abdominal aorta were then compared for the two groups of patients.
Results: Fifty-eight (74%) of 78 aortic attenuation values in group 1 and 55 (88%) of 69 in group 2 were above 160 H. Mean attenuation values were 192.2 H +/- 8.1 (+/-SE) in group 1 and 199.2 H +/- 10.1 in group 2. These values and the attenuation changes for each unit length along the abdominal aorta-3.7 H/cm +/- 1.7 for group 1 and 2.2 H/cm +/- 2.0 for group 2- were not significantly different.
Conclusion: Tailoring the scan delay for CT angiography of the abdomen in patients considered to have normal cardiac function requires additional IV contrast agent, time, and patient imaging and does not improve the extent or uniformity of aortic enhancement over that obtained with a standard, empiric scan delay.