[MRI diagnosis of neurodegenerative disorders].

Routine use of high field MRI has greatly contributed to the clinical diagnosis of neurodegenerative disorders, because MRI enables to visualize degenerative process showing either atrophy of the specific areas or degeneration of specific structures. Among many specific MRI signs which have been hitherto proposed to be diagnostic for certain neurodegenerative disorders, the author discussed here some clinically useful ones with neuropathological interpretations. "Humming-bird sign" is highly diagnostic for progressive supranuclear palsy (PSP) because it represents focal atrophy of the rostro-dorsal portion of mid-brain tegmentum where the neural centers for vertical gaze specifically affected in PSP are located. Un-treated Wilson disease patients show frequently "face of giant panda sign" of mid-brain which disappears after successful treatment. Although the sign is pathognomonic for Wilson disease, neuropathological entity of this MRI abnormality has not been known yet. MRI enables to discriminate two types of cerebellar atrophies; cerebellipetal atrophy in which ponto-, spino- and olivo-cerebellar fibers are lost, and cerebellofugal atrophy in which loss of Purkinje cells is the main pathological process. In cerebellipetal atrophy, cerebellar white matter shows T2 high signal due to the degeneration of nerve fibers in it but T2 low signal of dentate nucleus is usually well preserved. This combination of degenerative process realized" black teeth sign" of dentate nucleus in MRI. On the other hand, cerbellofugal atrophy shows "white teeth sign" of dentate nucleus, because the loss of Purkinje cell axons causes signal change of dentate nucleus where the axons of Purkinje cells are concentrating. "White teeth sign" could also be observed in case of the degeneration of dentate nucleus itself, like in DRPLA, but differential diagnosis between Purkinje cell loss and dentate degeneration is not so difficult, because the atrophy of the superior cerebellar peduncle is detectable in the latter but never seen in the former condition.