IMAGING OF NORMAL LEPTOMENINGEAL MELANIN

PURPOSE: To find the CT and MR characteristics of normal leptomeningea l melanin, which can be macroscopically Visible at autopsy in decedent s with deeply pigmented skin. METHODS: For the normal anatomic study, there were two black subjects who had had brain MR and CT within 12 mo nths 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 p attern of T2-weighted hypointensities on the surface of the medulla ob longata matched the histopathologic distribution of leptomeningeal mel anin. 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 corresp onding to the histopathologic distribution of leptomeningeal melanin. CONCLUSION: Normal leptomeningeal melanin is visible on MR as T2-weigh ted hypointensities along the surface of the medulla oblongata, The si gnal changes can be profound and might simulate leptomeningeal disease s that contain melanin or iron. This normal pigmentation should be tak en into account before diagnosing abnormalities in this region.