Case report: MR findings of a retained surgical sponge.

We describe here a case in which a retained surgical sponge progressed into the perirenal space and posterior pararenal space from the pelvic cavity. Sonography, CT, and T1-weighted MR image analyses could not show clearly the characteristic structure of the surgical sponge, but T2-weighted MR imaging showed a folded fabric appearance within the cystic mass. Surgical findings confirmed a lobulated cystic mass containing yellowish fluid and a surgical sponge.