Multimodal assessment of nigrosomal degeneration in Parkinson's disease

Background:  Approximately forty percent of all dopaminergic neurons in SNpc are located in five dense neuronal clusters, named nigrosomes. T 2 - or T 2 *-weighted images are used to delineate the largest nigrosome, named nigrosome-1. In these images, nigrosome-1 is a hyperintense region in the caudal and dorsal portion of the T 2 - or T 2 *-weighted substantia nigra. In PD, nigrosome-1 experiences iron accumulation, which leads to a reduction in T 2 -weighted hyperintensity. Here, we examine neuromelanin-depletion and iron deposition in regions of interest (ROIs) derived from quantitative-voxel based morphometry (qVBM) on neuromelanin-sensitive images and compare the ROIs with nigrosome-1 identified in T 2 *-weighted images.

Methods:  Neuromelanin-sensitive and multi-echo gradient echo imaging data were obtained. R 2 * was calculated from multi-echo gradient echo imaging data. qVBM analysis was performed on neuromelanin-sensitive images and restricted to SNpc. Mean neuromelanin-sensitive contrast and R 2 * was measured from the resulting qVBM clusters. Nigrosome-1 was segmented in T 2 *-weighted images of control subjects and its location was compared to the spatial location of the qVBM clusters.

Results:  Two bilateral clusters emerged from the qVBM analysis. These clusters showed reduced neuromelanin-sensitive contrast and increased mean R 2 * in PD as compared to controls. Cluster-1 from the qVBM analysis was in a similar spatial location as nigrosome-1, as seen in T 2 *-weighted images.

Conclusion:  qVBM cluster-1 shows reduced neuromelanin-sensitive contrast and is in a similar spatial position as nigrosome-1. This region likely corresponds to nigrosome-1 while the second cluster may correspond to nigrosome-2.