Is R2* a New MRI Biomarker for the Progression of Parkinson's Disease? A Longitudinal Follow-Up

Purpose: To study changes of iron content in basal ganglia in Parkinson's disease (PD) through a three-year longitudinal follow-up of the effective transverse relaxation rate R-2*, a validated MRI marker of brain iron content which can be rapidly measured under clinical conditions. Methods: Twenty-seven PD patients and 26 controls were investigated by a first MRI (t(0)). Longitudinal analysis was conducted among the 18 controls and 14 PD patients who underwent a second MRI (t(1)) 3 years after. The imaging protocol consisted in 6 gradient echo images obtained at different echo-times for mapping R-2*. Quantitative exploration of basal ganglia was performed by measuring the variation of R-2* [R-2*(t(1)) - R-2*(t(0))] in several regions of interest. Results: During the three-year evolution of PD, R-2* increased in Substantia nigra (SN) (by 10.2% in pars compacta, p = 0.001, and 8.1% in pars reticulata, p = 0.013) and in the caudal putamen (11.4%, p = 0.011), without significant change in controls. Furthermore, we showed a positive correlation between the variation of R-2* and the worsening of motor symptoms of PD (p = 0.028). Conclusion: Significant variation of R-2* was longitudinally observed in the SN and caudal putamen of patients with PD evolving over a three-year period, emphasizing its interest as a biomarker of disease progression. Our results suggest that R-2* MRI follow-up could be an interesting tool for individual assessment of neurodegeneration due to PD, and also be useful for testing the efficiency of disease-modifying treatments.