Figure 12a. UPJO in a 71-year-old woman. (a) Coronal volume-rendered CT image shows the main renal artery (long black arrow) and a variant lower segmental renal artery (short black arrow). The lower segmental artery crosses posteriorly to the UPJO. An early-branching retroaortic left renal vein is noted, with the lower-pole branch crossing anteriorly to the dilated pelvis in the region of the UPJO (white arrow). (b) Oblique sagittal hilar clock-face view shows a renal vein branch and the main renal artery above the dilated renal pelvis (short arrow). The early-branch lower-pole renal vein is noted below the dilated renal pelvis (long arrow). The distal normal ureter is seen (white arrowhead). Note how the dilated extrarenal pelvis balloons anteriorly (black arrowhead) in the potential space created between the vasculature above and below.