Multi-detector row CT of the kidney: Optimizing scan delays for bolus tracking techniques of arterial, corticomedullary, and nephrographic phases

Abstract

Purpose To determine optimal scan delays for renal arterial-, corticomedullary-, and nephrographic-phase imaging with multi-detector row computed tomography (MDCT) of the kidney using a bolus-tracking technique. Methods and materials One hundred and twenty-eight patients underwent three-phase CT scan of the kidney with eight-row MDCT after receiving 2 mL/kg of 300 mg I/mL contrast medium at 4 mL/s. Patients were prospectively randomized into three groups with different scan delays for the three scan phases (arterial, corticomedullary, and nephrographic) after bolus-tracking triggered at 50 HU of aortic contrast enhancement: group 1 (5, 20, 45 s); group 2 (10, 25, 50 s); and group 3 (15, 30, 55 s). Mean CT values (HU) of the abdominal aorta, renal artery, renal vein, renal cortex, and renal medulla were measured; increases in CT values pre- to post-contrast were assessed as contrast enhancement. Renal artery-to-vein and renal cortex-to-medulla contrast differences were also assessed. Qualitative analysis was also performed. Results Mean renal artery enhancement was 240–288 HU at 5–15 s after the trigger and peaked at 10 s ( P < .001). Mean renal cortical enhancement was 195–217 HU at 10–30 s and peaked at 25 s ( P < .01). Contrast enhancement in the renal medulla increased gradually and reached mean 145 HU at 55 s. Cortex-to-medulla contrast difference was high (110–140 HU) at 5–30 s and decreased below 30 HU at 45 s after the trigger. Renal artery-to-vein contrast difference was high (121–125 HU) at 5–10 s. Qualitative results correlated well with quantitative results. Conclusion For the injection protocol used in this study, optimal scan delays after the bolus-tracking trigger were 5–10 s for renal arterial, 15–25 s for corticomedullary, and 50–55 s for nephrographic phases.