A new sensitive imaging biomarker for Parkinson disease?

Over the past 25 years, considerable effort has been expended in developing neuroimaging methods capable of differentiating patients with Parkinson disease (PD) from healthy controls. These techniques have been developed with the hope that they could be used as biomarkers for both trait (presence of the disease) and state (severity of the disease), capable of convincingly establishing the diagnosis, defining the presence of the disease at its earliest stages, and serving as a surrogate marker for progression of the underlying disease. The commonest of these evaluate various components of the presynaptic dopaminergic nigrostriatal pathway.¹ Others have used metabolic PET scans to identify disease-related functional brain networks.² These techniques have relatively high sensitivity and specificity for distinguishing even patients with mild PD from healthy controls; however, there remains some overlap and they are expensive, often not widely available, and involve exposure to ionizing radiation.

Over 20 years ago, MRI of the midbrain was first reported to be a promising method of demonstrating the degeneration of the substantia nigra pars compacta (SNc) in PD. Unfortunately, the original claim that narrowing of the MR signal from the SNc differentiated patients from controls on routine MRI³ was never realized. MR diffusion …