[Magnetic resonance tomography of xanthogranulomatous pyelonephritis. Epidemiology, pathogenesis and symptoms].

Xanthogranulomatous pyelonephritis (XGP) is a rare chronic infection of the kidney. In this overview article we reevaluated the magnetic resonance imaging (MRI) findings of our four patients with histologically proven XGP and correlated them with the CT and ultrasound findings described in the literature. Additionally, we give an overview of the pathogenesis and epidemiology. The following criteria were found in MRI: hydronephrosis and/or urethral stricture, vesicoureteral reflux or neurogenic bladder depletion disorders, and calculus as the origin of urinary obstruction. Moreover, in contrast to normal pyelonephritis, XGP is accompanied by fatty deposits in the peripelvic parenchyma, which demonstrate contrast enhancement and form the typical so-called bear paw sign. As XGP is often accompanied by severe kidney failure, contrast-enhanced CT with nephrotoxic contrast agents should be avoided and MRI should be performed instead. Because of proven reduced nephrotoxicity of gadolinium chelates, MRI seems to be superior to CT in patients with suspected XGP. Essential for the correct diagnosis is consideration of the complete medical history including recurrent pyelonephritis to avoid malpractice.