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Fig. 1. —57-year-old woman who underwent colectomy for colorectal cancer. MR urogram shows dilatation of left renal pelvis and ureter, caused by extrinsic compression of ureter by postoperative fibrosis. Histogram was obtained by tracing on MR urogram circular region of interest (size, 1000 mm²) to include urinary tract (B) and surrounding tissue (A). Among image pixels belonging to urinary tract, virtual endoscopy threshold is identified as cluster of pixels with lower signal intensity.