A periureteric venous ring diagnosed by MRI: an unusual cause of flank pain.

We describe a pediatric patient with upper tract collecting system dilatation caused by a periureteric venous ring. The diagnosis was made after MR urography. This rare inferior vena caval anomaly has not been documented in the pediatric literature, nor have the MRI appearances been described. MRI is ideally suited for the assessment of suspected retrocaval ureters in order to identify this variant so that an appropriate surgical approach can be planned.