This chapter reviews magnetic resonance (MR) techniques from conventional structural MRI to advanced MRI (volumetry, magnetization transfer (MT), neuromelanin imaging, diffusion imaging, and rs-fMRI). It outlines the ways in which these techniques may be used to detect changes in the brain of Parkinson's disease (PD) patients and their relationships with Parkinsonian symptoms. Functional connectivity (FC) methods that take advantage of intrinsic signal fluctuations have demonstrated that the interactions of brain networks are abnormal in PD at the resting state. MRI has proven useful in the differential diagnosis of the various atypical Parkinsonian disorders such as progressive supranuclear palsy (PSP) and the Parkinson variant of multiple system atrophy (MSA-P). In PD, changes in the basal ganglia and brainstem are subtle and restricted to nuclei such as the substantia nigra (SN) and locus coeruleus (LC). PSP patients present extensive changes in the brainstem, basal ganglia, and cortical regions.

The main cause of motor weakness is damage to the primary crossed corticospinal tract. Most patients with stroke (80%-90%) have motor symptoms or signs. Hemiparesis with uniform weakness of the arm and leg associated with hemisensory deficit and speech deficit (dysphasia or dysarthria) usually indicates a large supratentorial lesion that involves the middle cerebral artery (MCA). Such patients have more severe weakness than do those with isolated hemiparesis. Crossed brainstem syndromes, well known with eponyms, are characterized by palsy of one of the 12 cranial nerve pairs associated with a contralateral neurological deficit due to involvement of the neurological long tracts (mainly motor or sensory). The integrity of all motor tracts, with the pyramidal tract as the main descending fiber bundle, but also the corticorubrospinal and corticoreticulospinal systems, appears to account for stroke recovery in a recent in vivo diffusion tensor imaging (DTI) study in chronic stroke patients.