Magnetic resonance imaging
Structural magnetic resonance imaging (MRI) is used to assess regional tissue abnormalities and atrophy [42]. Neuromelanin‐sensitive MRI of the substantia nigra (SN) and locus coeruleus has shown promising diagnostic accuracy in differentiating both idiopathic and monogenic forms of PD from healthy controls [43, 44]. Likewise, longitudinal changes in MRI index (R2*) of midbrain iron content are associated with declining motor function in early-stage PD, and thus, may serve as a biomarker of disease progression [45].
Diffusion tensor MRI (diffusion tensor imaging, DTI) measures the magnitude and direction of water molecule flow in the brain tissue allowing indirect assessment of microstructural integrity and white matter tract injury in the brain [46]. High-resolution DTI seems to be useful in distinguishing PD patients from healthy controls based on characteristic readouts in the SN and olfactory structures, but in early-stage PD results have been less conclusive and need further validation. DTI readouts of the corpus callosum, putamen, midbrain and cerebellum, however, may be useful to distinguish atypical parkinsonism from PD.
Functional MRI (fMRI) monitors brain activity by measuring cerebral oxygen-rich blood flow. Significantly reduced functional connectivity in resting-state fMRI have been reported within the basal ganglia circuits of patients with early-stage PD [43]. Indeed, it is a promising indicator of early dysfunctions and may help to identify patients at risk of developing PD [47, 48].