Neuroimaging and transcranial ultrasonography in Parkinson's disease

Parkinson's disease is a progressive, widespread, neurodegenerative disease in which the involvement of the dopaminergic neurons of the substantia nigra results in significant dopamine depletion in the striatum. Newer imaging modalities reviewed here, using various radioligands, positron emission tomography, and single-photon emission computed tomography, have made it possible to assess the in vivo presynaptic and postsynaptic dopaminergic function. This is not only important from a diagnostic standpoint; these tests are being increasingly studied as surrogate markers to assess disease progression and responses to various interventions, including drugs. A brief comment on their role as a putative biomarker of the disease is also included. Because Parkinson's disease involves multiple neurotransmitter systems, neuroimaging of neurotransmitter systems other than dopamine is also discussed. Lastly, the evidence supporting the use of transcranial ultrasonography and substantia nigra hyperechogenicity in the diagnosis of Parkinson's disease is presented, along with some controversies that surround this technique.