Imaging neurodegeneration in Parkinson's disease.

Neuroimaging techniques have evolved over the past several years giving us unprecedented information about the degenerative process in Parkinson's disease (PD) and other movement disorders. Functional imaging approaches such as positron emission tomography (PET) and single photon emission computerised tomography (SPECT) have been successfully employed to detect dopaminergic dysfunction in PD, even while at a preclinical stage, and to demonstrate the effects of therapies on function of intact dopaminergic neurons within the affected striatum. PET and SPECT can also monitor PD progression as reflected by changes in brain levodopa and glucose metabolism and dopamine transporter binding. Structural imaging approaches include magnetic resonance imaging (MRI) and transcranial sonography (TCS). Recent advances in voxel-based morphometry and diffusion-weighted MRI have provided exciting potential applications for the differential diagnosis of parkinsonian syndromes. Substantia nigra hyperechogenicity, detected with TCS, may provide a marker of susceptibility to PD, probably reflecting disturbances of iron metabolism, but does not appear to correlate well with disease severity or change with disease progression. In the future novel radiotracers may help us assess the involvement of non-dopaminergic brain pathways in the pathology of both motor and non-motor complications in PD.