Not infrequently, the renal pelvis projects outside the kidney, medial to the renal sinus. This is best seen in a transverse section through the renal hilum. It is frequently ‘baggy’, containing anechoic urine, which is prominently demonstrated on the ultrasound scan (Fig. below). The importance of recognizing the extrarenal pelvis lies in not confusing it with dilatation of the PCS, or with a parapelvic cyst or collection.
Ultrasound images TS through the RK demonstrating a baggy extrarenal pelvis. The PCS remains undilated, and this should not be confused with hydronephrosis.
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