Differential Effect of Iron and Myelin on Susceptibility MRI in the Substantia Nigra.

Background The heterogeneous composition of substantia nigra (SN), including iron, nigrosome-1 substructure, and myelinated white matter, complicates the interpretation of MRI signals. Purpose To investigate R2* and quantitative susceptibility mapping (QSM) in the SN subdivisions of participants with Parkinson disease and healthy control subjects. Materials and Methods In this prospective study conducted from November 2018 to November 2019, participants with Parkinson disease and sex-matched healthy control subjects underwent 3-T MRI. R2* and QSM values were measured and compared in the anterior SN and posterior SN at the rostral (superior) and caudal (inferior) levels. Postmortem MRI and histology correlation of midbrain tissues was evaluated to investigate the effect of myelin and iron in the SN on R2* and QSM values. Results Forty individuals were evaluated: 20 healthy control subjects (mean age, 61 years ± 3 [standard deviation]; 10 men) and 20 participants with Parkinson disease (mean age, 61 years ± 4; 10 men). The R2* values of participants with Parkinson disease were higher in all subdivisions of the SN compared with R2* values in healthy control subjects (all < .05). For QSM, no evidence of a difference was found in the rostral posterior SN (healthy control subjects, 54.1 ppb ± 21.0; Parkinson disease, 62.2 ppb ± 19.8; = .49). The combination of rostral R2* and caudal QSM values resulted in an area under the receiver operating characteristic curve of 0.84. R2* values showed higher correlation with QSM values at the caudal level than at the rostral level within each group (all < .001). Postmortem investigation demonstrated that R2* and QSM values showed weak correlation in the myelin-rich areas ( = 0.22 and = 0.36, < .001) and strong correlation in myelin-scanty areas ( ranged from approximately 0.52 to approximately 0.78, < .001) in the SN. Conclusion Considering the iron and myelin distribution in the substantia nigra subdivisions, the subdivisional analysis of substantia nigra using R2* and quantitative susceptibility mapping might aid in specifically differentiating individuals with Parkinson disease from healthy control subjects. © RSNA, 2021