Selective nigrosome vulnerability in Parkinson's disease revealed by histology-anchored ultra-high-field quantitative MRI

Selective vulnerability is a defining characteristic of Parkinson's disease (PD), with post-mortem studies revealing that dopamine-containing cells in the substantia nigra first and most severely affect calbindin-poor compartments, known as nigrosomes. However, testing this phenomenon in living patients has been challenging due to the circular logic inherent in MRI studies that first delineate nigrosomes and then measure them using the same disease-related contrast.

To overcome this limitation, the researchers employed a cohort-specific multimodal 7 Tesla quantitative MRI template. This template was generated by warping histology-derived, calbindin-validated probabilistic masks of nigrosomes 1-5, providing an anatomical scaffold independent of degeneration. The study included 28 patients with PD and 28 age- and sex-matched controls.

In the control group, nigrosomes exhibited distinct magnetic susceptibility and R1 values compared to the surrounding nigral matrix. In PD patients, nigrosomal values shifted upward relative to the matrix in susceptibility, R2*, and R1, with the most pronounced alterations observed in nigrosomes 1, 2, and 4. Whole-nigra analysis, however, did not reveal any group differences.

Within the nigral matrix, disease effects were most pronounced caudally, emerging dorsally from the ventral tier. Susceptibility alterations in nigrosomes 1 and 4 were found to track contralateral motor symptoms asymmetry. The multimodal pattern suggests that the detectable signal in nigrosomes reflects predominantly secondary, iron-associated glial processes rather than neuronal loss itself.

This study successfully recovers the post-mortem spatial pattern of nigral vulnerability in living patients, establishing a degeneration-independent framework for subregional nigral imaging in PD.