Purpose: To investigate how prone and supine redistribution of a patient's adipose tissue affects the distance from skin to the renal collecting system.
Methods: There were 48 patients who underwent CT intravenous urography with both supine and prone scans. The distance between skin and the posterior lower pole calix was measured in both positions. The difference was calculated using paired t tests. Subgroup analyses were conducted for patients with a body mass index (BMI) ≥ 28 and BMI ≥ 30.
Results: In all patients, the mean distance between skin and the posterior lower pole calix was 9.9 ± 0.3 cm and 8.7 ± 0.3 cm for patients supine and prone, respectively (P < 0.01). Patients with a BMI ≥ 28 had a mean distance of 10.6 ± 0.3 cm and 8.8 ± 0.3 cm in supine and prone positions, respectively (P < 0.01), while patients with BMI ≥ 30 had a mean distance of 11.3 ± 0.3 cm and 9.3 ± 0.3 cm (P < 0.01). Three patients had a BMI > 39 and exhibited differences in skin to the posterior lower pole calix ≥ 3.2 cm between supine and prone positioning. Coefficient of determination analysis for supine minus prone tract length yielded R(2) = 0.70895.
Conclusion: The distance between skin and the renal collecting system is decreased in the prone position when compared with the supine position. This difference increases with the patient's BMI and is further accentuated in morbidly obese patients. In these obese patients, the difference when lying prone can exceed >4 cm.