Neuroimaging in Parkinson's disease: from pathology to diagnosis.

Imaging allows a window into the pathology of PD during life, and potentially even prior to the appearance of motor manifestations. Functional imaging using dopaminergic tracers with either PET or SPECT can identify dopamine deficiency but may not reliably differentiate between PD and other akinetic-rigid disorders. On the other hand, dopaminergic tracer imaging can identify pre-motor changes in subjects at high risk of developing PD and may be useful as a biomarker to assess disease progression, with caveats. Glucose or cerebral blood flow imaging can provide complementary information on patterns of cerebral activation and thereby be useful for diagnosis and for the assessment of compensatory strategies. Although traditionally considered to be of limited utility for the study of PD and related disorders, novel magnetic resonance imaging techniques are showing increasing promise for diagnosis and potentially as biomarkers. These applications will be reviewed here, as will the potential use of imaging to assess Braak's hypothesis of caudal to rostral degeneration in vivo.