PET and SPECT in common neuropsychiatric disease.

The most relevant extrapyramidal diseases are Parkinson’s disease (PD) and atypical parkinsonian syndromes such as multiple system atrophy (MSA), progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). In all these, establishment of an early and accurate diagnosis impacts on management, helps to avoid inappropriate treatment and assists in the evaluation of novel drugs. Pathology in extrapyramidal diseases involves the dopaminergic neurotransmitter system, so positron emission tomography (PET) and single-photon emission computed tomography (SPECT) investigations contribute significantly to establishing the correct diagnosis.1–3 Presynaptic nigrostriatal terminal function can be assessed with radioligands suitable for imaging: • aromatic amino acid decarboxylase activity (PET: fluorodopa) • vesicular monoamine transporter type 2 (PET: dihydrotetrabenazine) • plasma membrane dopamine transporter (PET and SPECT: cocaine analogues). Imaging of postsynaptic dopamine receptors has focused on the D2-like receptor system (PET: raclopride, (desmethoxy) fallypride; SPECT: iodobenzamide, iodobenzofuran, epidepride). There are two major clinical indications for these techniques: • confirmation or exclusion of neurodegenerative parkinsonism • the differential diagnosis between PD and atypical parkinsonian syndromes.