Magnetic resonance imaging
Structural magnetic resonance imaging (MRI) is used to assess regional
tissue abnormalities and atrophy [42]. Neuromelaninâsensitive 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].