The pattern of dopamine cell loss in Parkinson's disease is known to be prominent in the ventrolateral and caudal substantia nigra, but less severe in the dorsal and rostral region. Both diffusion tensor imaging and R2* relaxometry of the substantia nigra have been reported as potential markers for Parkinson's disease, but their relative ability to mark disease progression and differences in pathophysiological bases remains unclear.

High resolution T2-weigthed, R2*, and diffusion tensor imaging were obtained from 28 controls and 40 Parkinson's disease subjects [15 early-stage (disease duration≤1 year), 14 mid-stage (duration 2-5 years), and 11 late-stage (duration>5 years)]. Fractional anisotropy and R2* values in both rostral and caudal substantia nigra were obtained for all subjects, and clinical measures (disease duration; levodopa-equivalent daily dosage; “off”-drug Unified Parkinson's Disease Rating Scale motor score) were obtained for Parkinson's subjects.

There was no correlation between fractional anisotropy and clinical measures, whereas R2* was strongly associated with disease progression. Compared to controls, fractional anisotropy in caudal substantia nigra was significantly decreased in Parkinson's disease patients of all stages, whereas in rostral substantia nigra it was decreased significantly only in the late-stage group. R2* in both substantia nigra regions was significantly increased in the mid-stage and late-stage, but not early-stage, of Parkinson's disease subjects.

These findings suggest that fractional anisotropy changes may mark early pathological changes in caudal substantia nigra, whereas the changes in R2* may more closely track Parkinson's disease's clinical progression after symptom onset.