Imaging of normal leptomeningeal melanin.

ABSTRACT

PURPOSE: To find CT and MR characteristics of normal leptomeningeal melanin, which can be macroscopically visible at autopsy in decedents with deeply pigmented skin. METHODS: For the normal anatomic study, there were two black subjects who had brain MR and CT within 12 months before autopsy that showed leptomeningeal melanin. For the normal imaging study, brain MR was done on 74 patients (31 black), and CT in 24 of these 74 patients. RESULTS: In the normal anatomic study, the pattern of T2-weighted hypointensities on the surface of the medulla oblongata matched the histopathologic distribution of leptomeningeal melanin. A similar signal was seen in 28 of the 31 black patients in the normal imaging study. Such a signal was subtle, or entirely absent, in the other patients. CT showed no reliable altered attenuation corresponding to the histopathologic distribution of leptomeningeal melanin. CONCLUSION: Normal leptomeningeal melanin is visible on MR as T2-weighted hypointensities along the surface of the medulla oblongata. The signal changes can be profound and might simulate leptomeningeal diseases that contain melanin or iron. This normal pigmentation should be taken into account before diagnosing abnormalities in this region.