Optimal trigger threshold with the bolus-tracking technique for the renal CT angiography protocol

Objectives This study aimed to investigate the optimal trigger threshold for the renal CT angiography (CTA) protocol with the bolus-tracking technique. Methods This retrospective study included patients who were suspected of renal diseases or candidate kidney transplant donors who underwent renal CTA from July 2019 to August 2021. Renal arterial (RA) phase scanning was initiated using either one of the following two trigger thresholds: 50 HU (50 HU group; n  = 42) and 100 HU (100 HU group; n  = 63). A radiologist measured CT attenuations of the vasculature and renal parenchyma on RA phase images. Two radiologists classified RA phase images into appropriate RA-, late RA-, or corticomedullary (CM)-timing. Unpaired t-tests and Fisher’s exact tests were conducted to assess differences in the CT attenuations and the proportion of categorical classifications between the two groups. Results Overall, 105 patients (mean age: 59.8 years; 60 men) were included. CT attenuations of the aorta and renal arteries were comparable between the two groups ( P  = .38–.95). CT attenuations of the renal cortex and renal vein were higher in the 100-HU group than in the 50-HU group ( P  = .006–.04). The proportion of the appropriate RA-timing was higher, whereas that of the CM-timing was lower in the 50-HU group than in the 100-HU group (appropriate RA-, late RA-, and CM-timing: 78.6% vs. 50.8%, 19.0% vs. 34.9%, and 2.4% vs. 14.3%; P  = .01 for reviewer 1 and 64.3% vs. 27.0%, 33.3% vs. 63.5%, and 2.4% vs. 9.5%; P  < .001 for reviewer 2, respectively). Conclusion The trigger threshold of 50 HU provided a higher probability of obtaining appropriate RA-timing images than that of 100 HU in the renal CTA protocol with the bolus-tracking technique.