Case report. The iron rim sign.

A chronic cystic cerebral hematoma occurred in the frontal lobe and mimicked both clinically and radiographically a frontal sinus mucocele. A rim seen on multidirectional tomography and unenhanced computed tomography was presumed to be composed to calcium. However, electron microscopic analysis revealed the rim to be composed almost entirely of iron from hemosiderin breakdown products. This is the first such report of an "iron rim" that we could find in the literature.