Perirenal MR high signal - A new and sensitive indicator of acute ureteric obstruction

Purpose: This study was carried out to determine the incidence of perirenal magnetic resonance (MR) high signal in acute ureteric obstruction as demonstrated by half-Fourier acquisition single shot turbo spin-echo (HASTE) MR. In addition, we evaluated the sensitivity of this perirenal MR high signal as a predictor of acute ureteric obstruction. Materials and Methods: A prospective evaluation of 55 consecutive patients with suspected ureteric obstruction was carried out using the HASTE MR sequence. Images were compared to concurrent IV urography (IVU) or to computed tomography (CT) where these were available. Acute and chronic ureteric obstruction were differentiated by clinical evaluation. Results: Forty-one patients had obstructed kidneys. HASTE MR accurately predicted the presence of acute ureteric obstruction in 20/23 (87%) based on presence of perirenal MR high signal. None of these showed evidence of contrast medium extravasation on the concurrent IV urogram. 15/18 (83%) chronically obstructed kidneys demonstrated no perirenal high signal on HASTE MR, and the remaining three showed only a trace of perirenal high signal. CT showed perirenal stranding in only 2/8 patients with acutely obstructed kidneys. Conclusion: In acute ureteric obstruction, HASTE MR shows perirenal high signal intensity much more commonly than IVU shows extravasation or CT showing perirenal stranding. The origin of this MR signal is uncertain but may represent oedema, lymphatic distension or free fluid from forniceal rupture. HASTE MR can accurately distinguish between acute and chronic ureteric obstruction based on the degree of perirenal high signal.