Case report: MR appearance of a retained surgical sponge.

A case of a retained surgical sponge found in the retroperitoneum is presented with findings on magnetic resonance (MR) imaging, computed tomography (CT), ultrasonography (US) and angiography. Among all the performed modalities, a characteristic internal structure of the gauze granuloma was best visualized on MR imaging. If no radio-opaque marker is seen on plain radiography or CT, the folded fabric inner structure visualized on T2-weighted images can be a most important clue to the correct diagnosis of this iatrogenic mass.