Flattened red nucleus in progressive supranuclear palsy detected by quantitative susceptibility mapping

Introduction: Progressive supranuclear palsy (PSP) involves midbrain structures, including the red nucleus (RN), an iron-rich region that appears as a high-contrast area on quantitative susceptibility mapping (QSM). RN may serve as a promising biomarker for differentiating parkinsonism. However, RN deformation in PSP remains elusive. This study aimed to evaluate RN deformation in PSP using coronal QSM images and compare them with those of Parkinson's disease (PD) and healthy controls (HC). Methods: We evaluated the QSM images of 22 patients with PSP, 37 patients with PD, and 43 HC. We developed a grading system to assess RN deformation on coronal QSM images and classified them into three grades. The midbrain and RN volumes were extracted using distinct approaches, and their relationship with grading was investigated. For validation, coronal QSM images of 16 PSP patients from a different institution were assessed. Results: In PSP, 59 % of the patients displayed a flattened RN of grade 3, which we termed a Rice-Grain Appearance. The volume reductions in midbrain and RN were associated with deformation. Differentiation based on the presence of this appearance yielded a specificity of 1.000 (CI: 1.000–1.000) and sensitivity of 0.591 (0.385–0.796) for distinguishing PSP from others. Secondary dataset also showed that 56 % of patients with PSP were classified as grade 3. Conclusion: In coronal QSM images, the flattened RN shape appears to be specific to PSP compared to PD and HC and may serve as a marker to help differentiate PSP in future clinical settings.