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

PURPOSE: To evaluate a bolus-tracking technique in helical computed tomogra phy (CT) for identifying the onset of the nephrographic phase and to determ ine the effect of varying the volume and injection rate of contrast materia l on nephrographic phase onset. MATERIALS AND METHODS: Seventy-five patients underwent bolus tracking of co ntrast material followed by helical renal CT. In 50 patients, 150 mL of 60% iodinated contrast material (iohexol or iothalamate meglumine) was injecte d at either 2 mL/sec (25 patients [group 1]) or 3 mL/sec (25 patients [grou p 2]). In 25 patients who had previously undergone nephrectomy, 100 mL of 6 0% iodinated contrast material was injected at 3 mL/sec (group 3). Nephrogr aphic phase onset was determined by visually assessing the transition to a homogeneous nephrogram during a monitoring scan series starting 40 seconds after injection. RESULTS: Nephrographic phase onset ranged from 60 to 136 seconds (mean, 89 seconds +/- 17 [+/- SD]). Statistically significant differences in mean ons et times were observed among groups 1 (103 seconds +/- 12), 2 (91 seconds /- 16), and 3 (75 seconds +/- 9) (P < .001). Multiple regression analysis s howed patient age, contrast material volume, and injection rate to be indep endent predictors of nephrographic phase onset. Contrast material volume, p atient age, and patient weight were independent predictors of the degree of renal enhancement. CONCLUSION: Nephrographic phase onset is highly dependent on methods of con trast material administration and patient characteristics.