We report a patient with biopsy-proven progressive multifocal leukoenc
ephalopathy (PML) who was serially imaged with MRI and magnetization t
ransfer imaging. The magnetization transfer ratio (MTR) was profoundly
and significantly diminished when compared with normal control subjec
ts. The pattern of MTR was distinct from that of MS and periventricula
r ischemic white matter disease. Magnetization transfer imaging techni
ques may aid in the differential diagnosis of PML.