Positional variation in the ultrasound appearance of the renal pelvis.

A retrospective analysis of 100 renal sonograms (200 kidneys) was performed to determine how the appearance of the renal pelvis varies in the supine und prone projections. The renal pelvis had no distention in either projection in 100 kidneys. The prone position produced dilatation of a previously normal pelvis with 60 kidneys or increasing dilatation in 6 kidneys. Other changes in the renal pelvis were also identified. We believe this positional change is due to shifting of urine from the normal sized calyces (when supine) to the distensible renal pelvis (when prone). It is possible that elevation of bladder pressure in the prone position may also impede urine flow and produce the observed changes. However, it is important to recognize that the slight dilatation of the renal pelvis does not indicate pathology.