Introduction
Parkinson’s disease is marked by the loss of neuromelanin (NM)-rich dopaminergic neurons in the substantia nigra pars compacta, particularly in Nigrosome-1 (N1). N1 appears hyperintense on T2*-weighted and susceptibility-weighted imaging (SWI) in healthy individuals. This study employs MRI and quantitative susceptibility mapping (QSM) to assess N1’s structure and susceptibility, evaluating their diagnostic value and correlation with clinical outcomes.

Materials and Methods
Sixty-three PD patients and 31 healthy controls (HC) underwent 3T brain MRI (Biograph mMR, Siemens Healthcare) using a 16-channel coil. Sequences included: 3D T1-weighted MPRAGE (1 mm isotropic voxels); T2-FLAIR (0.5 × 0.5 × 1 mm³); SWI (0.7 × 0.7 × 1.2 mm³) with two acquisitions, one optimized for N1 visualization (TR/TE 29/18 ms).

QSM was estimated using ROMEO for phase unwrapping, FSL BET for brain masking, VSHARP for background field removal, and Tikhonov regularization for susceptibility inversion. Two experts, blinded to diagnosis, manually segmented N1 on optimized SWI using MRIcron; the most representative slice was selected, and volume, area, and susceptibility (χ values) were computed via FSL’s fslstats. Bilateral N1 was visible in all controls, unilaterally in 22 PD, and absent in 26. Two cases were excluded due to motion artifacts.

Results
A Random Forest model classified PD vs. HC using six features. The model showed strong performance, with AUC scores of 0.94 (Volume) and 0.93 (Area), while QSM had an AUC of 0.42, suggesting volume as the most informative biomarker when N1 remains detectable.