For all practical purposes, MRI is not routinely used in the urgent evaluation of upper tract obstruction. In comparison to the other imaging modalities, MRI is less available, more costly, and frequently less sensitive in the accurate diagnosis of upper tract obstruction, especially related to stone disease. For example, urinary calcifications are poorly visualized with MRI, making this a less valuable test in instances of obstruction related to stones. For select patients including pregnant females in whom more anatomic detail is required than that available with US, however, MRI may be considered as a confirmatory imaging test. The use of MRI may also have a role in the evaluation of patients with upper urinary tract obstruction and compromised renal function (serum creatinine >2.5 mg/dl). In a study by Shokeir and colleagues, 149 patients with upper tract obstruction were evaluated with not only KUB and US but also MRI urography (MRU) and noncon-trast CT (Shokeir et al. 2004). While noncontrast CT was superior for detecting obstructing stones (100% sensitivity), MRU was superior in detecting the 113 cases in which obstruction unrelated to stone disease (sensitivity of 89 %) compared to noncontrast CT (sensitivity of 40%) or combined KUB and US (sensitivity of 18%).
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