Assessment of a bolus-tracking technique in helical renal CT to optimize nephrographic phase imaging

To evaluate a bolus-tracking technique in helical computed tomography (CT) for identifying the onset of the nephrographic phase and to determine the effect of varying the volume and injection rate of contrast material on nephrographic phase onset.

Seventy-five patients underwent bolus tracking of contrast material followed by helical renal CT. In 50 patients, 150 mL of 60% iodinated contrast material (iohexol or iothalamate meglumine) was injected at either 2 mL/sec (25 patients [group 1]) or 3 mL/sec (25 patients [group 2]). In 25 patients who had previously undergone nephrectomy, 100 mL of 60% iodinated contrast material was injected at 3 mL/sec (group 3). Nephrographic phase onset was determined by visually assessing the transition to a homogeneous nephrogram during a monitoring scan series starting 40 seconds after injection.

Nephrographic phase onset ranged from 60 to 136 seconds (mean, 89 seconds +/- 17 [+/- SD]). Statistically significant differences in mean onset times were observed among groups 1 (103 seconds +/- 12), 2 (91 seconds +/- 16), and 3 (75 seconds +/- 9) (P < .001). Multiple regression analysis showed patient age, contrast material volume, and injection rate to be independent predictors of nephrographic phase onset. Contrast material volume, patient age, and patient weight were independent predictors of the degree of renal enhancement.

Nephrographic phase onset is highly dependent on methods of contrast material administration and patient characteristics.