Spiral CT angiography of the renal arteries: Should a scan delay based on a test bolus injection or a fixed scan delay be used to obtain maximum enhancement of the vessels?

Purpose: The purpose of this work was to assess the optimal scan delay for spiral CT angiography (SCTA) of the renal arteries in achieving optimal vascular contrast enhancement and to compare the utility of a delay based on test bolus injection versus that of a fixed scan delay. Method: Seventy patients underwent renal artery SCTA with a 140 mi bolus of contrast agent injected at 3 ml/s. In 35 patients (Group A), a fixed scan delay of 27 s was used; in the other 35 (Group B), the scan delay was based on the transit time (T-Test) of a test bolus injection. The scan delays in this group were set at T-Test + 5 s (n = 5), T-Test + 10 s (n = 8), T-Test + 15 s (n = 4), or T-Test + 20 s (n = 18). For all 70 patients, the time intervals between T-RA (time to scanning the renal arteries) and T-Max (time to maximum aortic enhancement after 140 mi bolus injection) were calculated, after which it was determined in which group of patients T-RA occurred closest to T-Max. Linear regression and mean squared error (MSE) were used for statistical analysis. Results: For Group A, mean T-RA and T-Max were 38 and 50 s, respectively. Mean (T-RA - T-Max) was -12 s with MSE of 185,76. For Group B, mean T-RA and T-Max were 45 and 52 s. Mean (T-RA - T-Max) values were -15, -12, -11, and -1 s for scan delays Of T-Test + 5 s, T-Test + 10 s, T-Test + 15 s, and T-Test + 20 s, respectively, with MSEs of 253.80, 158.00, 137.50, and 30.00. Conclusion: SCTA of the renal arteries was best performed with a scan delay of T-Test + 20 s. However, analysis of our data showed that similar results could be expected with a delay of 44 s.