Single photon-emission computed tomography imaging in early Parkinson's disease.

Current diagnostic criteria for Parkinson's disease (PD) rely on the presence of motor signs, but it is now evident that they require months or years to become clinically manifest. By contrast, initial manifestations of PD may involve several nonmotor domains, such as cognition, mood, smell or sleep, and their recognition is critical if one is aiming for early diagnosis to provide neuroprotection. In this context, imaging tracers are gaining relevance as they are increasingly recognized as biological markers of disease progression. In this review, we will discuss the role of single photon-emission computed tomography imaging in the early phases of PD and in the differential diagnosis of essential tremor and atypical parkinsonism. We will also discuss the role of imaging of monoaminergic nondopamine systems to assess nonmotor features.