Magnetic resonance imaging and extrapyramidal movement disorders.

High-field strength magnetic resonance imaging is an accurate clinical technique for detecting the relative distribution of ferritin in the brain. In normal adults, iron is found in highest concentrations in the globus pallidus, red nucleus, pars reticulata of the substantia nigra, and dentate nucleus of the cerebellum; its distribution is clearly mapped as signal hypointensity (darkness) on a T2-weighted image due to local-field heterogeneities produced by ferritin. Iron is absent at birth and increases in concentration in the putamen in the elderly. Poorly drug-responsive Parkinson's disease (multiple-system atrophy) is characterized by premature signal hypointensity in the putamen and caudate, while Hallervorden-Spatz disease exhibits abnormal hypointensity in the globus pallidus in children. Dyskinetic disorders often have abnormal signal hyperintensity (whiteness) in the putamen related to gliosis.