Optimal scan delays for multiphasic renal multidetector row computed tomography performed with fixed injection duration of contrast medium.

OBJECTIVE: The purpose of our study was to prospectively determine optimal scan delays for multiphasic imaging of the kidney performed with multidetector row computed tomography (CT) and fixed injection duration of intravenous contrast medium.

MATERIALS AND METHODS: One hundred ninety-eight patients underwent 3-phase CT of the kidney with 8-slice CT after receiving 2 mL/kg of an intravenous contrast medium of 300 mg I/mL for a fixed duration of 30 seconds. The patients were prospectively randomized into 4 groups according to different scan delays from the start of injection: group 1 (25, 45, 65 seconds), group 2 (30, 50, 70 seconds), group 3 (35, 55, 75 seconds), and group 4 (40, 60, 80 sec). Mean CT values (Hounsfield units [HU]) of the abdominal aorta, renal arteries, veins, cortexes, and medulla were measured. Increases in CT values from precontrast to postcontrast CT (deltaHU) and renal artery-to-vein and renal cortex-to-medulla differential contrasts (deltaHU) were assessed. Qualitative analysis was also performed.

RESULTS: Abdominal aorta and renal artery enhancements peaked at 35 seconds (305 DeltaHU; 253 DeltaHU) after injection start, and renal veins peaked at 45 seconds (196 DeltaHU). Renal cortexes peaked at 40 seconds (197 DeltaHU), and renal medullae peaked at 75 seconds (152 DeltaHU). Renal artery-to-vein differential contrasts were high (95-137 deltaHU) at 25 to 30 seconds, and cortex-to-medulla contrasts were high (79-130 deltaHU) at 30 to 55 seconds and then decreased to less than 10 deltaHU at 75 seconds. Qualitative results were in good agreement with quantitative results.

CONCLUSIONS: With a fixed 30-second injection, estimated optimal scan delays for multiphasic imaging the kidney were, from the start of injection, 25 to 30 seconds for renal arterial CT angiography, 35 to 45 seconds for the corticomedullary, and 75 seconds for the nephrographic phase.