MR urography is more sensitive and specific for noncalculous urinary tract obstruction than is unenhanced CT (51,52). Benign strictures of the ureter may complicate abdominal and pelvic inflammatory processes (eg, appendicitis, Crohn disease, endometriosis), infection (eg, tuberculosis), radiation therapy, surgical or interventional procedures, or stone disease. Benign strictures of the ureter are typically smoothly tapering and not associated with a soft-tissue mass (Fig 14). Cine or excretory MR urography is helpful in gauging the severity of a stricture. In cases of partial obstruction, cine MR urography will demonstrate intermittent distention and collapse of the ureter below the level of narrowing, whereas excretory MR urography will demonstrate contrast enhancement of the ureter distal to the narrowing (see Movie 5 at radiographics.rsnajnls.org/cgi/content/full/28/1/23/DC1). High-grade obstruction will result in delayed excretion of gadolinium-based contrast material on the affected side. Ureteral jets can also be demonstrated with cine urography in the absence of ureteral obstruction.
Extrinsic narrowing of the ureters may occur with entities such as uterine fibroids, fluid collections, retroperitoneal fibrosis, and vascular abnormalities (Fig 15). Benign extrinsic processes may cause deviation of one or both ureters and often result in smooth tapering of the ureter at the site of compression. As with other modalities, retroperitoneal fibrosis can result in medial deviation of the ureters at MR urography. Benign extrinsic compression of the ureter rarely results in complete obstruction.
Neoplastic obstruction of the urinary tract can result from benign or malignant processes. Benign urothelial tumors such as fibroepithelial polyps appear as filling defects at MR urography (53). Malignant processes that can obstruct the ureters include intrinsic urothelial tumors such as TCC of the ureters or bladder, metastatic tumors to the ureters or periureteric tissues, neoplastic lymph nodes, and direct invasion from extraureteral neoplasms (Figs 1, 16). When neoplastic obstruction is suspected, it is important to include sequences to evaluate the soft tissues in addition to MR urographic sequences optimized to visualize the lumen of the urinary tract. Tumor that arises directly from or invades the ureter often results in irregularity of the ureter at the point of obstruction or appears as an irregular filling defect. Unlike calculi, most neoplasms will enhance after intravenous contrast material administration.
The role of MR urography for screening patients at risk for urothelial malignancy has yet to be defined. In one study, static-fluid MR urography performed with a high-resolution technique in two planes through the level of ureteral obstruction was successful in demonstrating eight ureteral and five renal pelvic TCCs in 23 high-risk patients who were poor candidates for other types of imaging examinations (54).
Bladder cancer, cervical cancer, and prostate cancer are relatively common causes of malignant ureteral obstruction (Fig 1). Most malignant tumors of the urinary epithelium are TCCs (Fig 17). TCCs can appear as sessile filling defects or wall thickening (55). Proximal ureteral dilatation may be present. As with other forms of urography, the “goblet” sign can occasionally be seen at MR urography in the setting of TCC of the ureter (3). Urethelial tumors generally have intermediate signal intensity at MR imaging and demonstrate enhancement that is not present with calculi or blood clots (53). It is important to evaluate the entire urinary tract in the setting of TCC, given the propensity of TCC for multifocal involvement
Extrinsic narrowing of the ureters may occur with entities such as uterine fibroids, fluid collections, retroperitoneal fibrosis, and vascular abnormalities (Fig 15). Benign extrinsic processes may cause deviation of one or both ureters and often result in smooth tapering of the ureter at the site of compression. As with other modalities, retroperitoneal fibrosis can result in medial deviation of the ureters at MR urography. Benign extrinsic compression of the ureter rarely results in complete obstruction.
Neoplastic obstruction of the urinary tract can result from benign or malignant processes. Benign urothelial tumors such as fibroepithelial polyps appear as filling defects at MR urography (53). Malignant processes that can obstruct the ureters include intrinsic urothelial tumors such as TCC of the ureters or bladder, metastatic tumors to the ureters or periureteric tissues, neoplastic lymph nodes, and direct invasion from extraureteral neoplasms (Figs 1, 16). When neoplastic obstruction is suspected, it is important to include sequences to evaluate the soft tissues in addition to MR urographic sequences optimized to visualize the lumen of the urinary tract. Tumor that arises directly from or invades the ureter often results in irregularity of the ureter at the point of obstruction or appears as an irregular filling defect. Unlike calculi, most neoplasms will enhance after intravenous contrast material administration.
The role of MR urography for screening patients at risk for urothelial malignancy has yet to be defined. In one study, static-fluid MR urography performed with a high-resolution technique in two planes through the level of ureteral obstruction was successful in demonstrating eight ureteral and five renal pelvic TCCs in 23 high-risk patients who were poor candidates for other types of imaging examinations (54).
Bladder cancer, cervical cancer, and prostate cancer are relatively common causes of malignant ureteral obstruction (Fig 1). Most malignant tumors of the urinary epithelium are TCCs (Fig 17). TCCs can appear as sessile filling defects or wall thickening (55). Proximal ureteral dilatation may be present. As with other forms of urography, the “goblet” sign can occasionally be seen at MR urography in the setting of TCC of the ureter (3). Urethelial tumors generally have intermediate signal intensity at MR imaging and demonstrate enhancement that is not present with calculi or blood clots (53). It is important to evaluate the entire urinary tract in the setting of TCC, given the propensity of TCC for multifocal involvement
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