Multi-row detector CT angiography of the abdominal aorta: Bolus tracking versus fixed start delay

Purpose: Prospective comparison between bolus tracking and fixed start delay in multi-row detector CT (MDCT) angiography of the abdominal aorta, to determine which method yields the highest mean contrast. Materials and Methods: In a randomized trial, 71 patients underwent multi-row detector MDCT angiography of the abdominal aorta using either a fixed delay of 25 s (n = 36, group F) or an automatic bolus tracking with a 125-HU threshold in the descending aorta (n = 35, group B). All patients received 150 ml of Iopamidol with 300 mg/ml iodine content, power-injected at 4 ml/s. Attenuation values after contrast administration were assessed in aorta, iliac and femoral arteries. Continuous data were compared using two-tailed t-tests and categorical data using Fisher's test. Results: The trigger threshold was reached in all cases. Group B and F did not differ significantly in terms of age (mean 71 +/- 10 years, p = 0.59), weight (mean 81 +/- 16 kg, p = 0.28) and size (mean 173 +/- 8 cm, p = 0.65). An abdominal aneurysm was found in 33 patients of each group. Effective triggered start delay varied between 20 and 44 s in group B (mean 30.3 +/- 5.2 s). Mean global arterial-phase vessel attenuation was 372 HU in group B and 349 HU in group F (no significant difference, p = 0.143). Values tended to be lower in the aorta than in the pelvic arteries (group B: 353 HU aorta, 387 HU iliac arteries; group F: 328 HU aorta, 361 HU iliac arteries). Thus, attenuation tended to be higher further downstream, suggesting the preferred use of a start delay longer than 25 s. Conclusion: Bolus tracking in MDCT angiography did not significantly improve the mean contrast in the aorta and pelvic vessel as compared to using a fixed delay. It appears to be useful to extend the start delay to approximately 30-35 s in MDCT angiography.