Parkinson disease (PD) is a neurodegenerative disorder with the major pathological feature of dopaminergic neuron loss in the substantia nigra (SN) and, subsequently, decreased axons projected to the striatum.¹ Currently, diagnosis of PD is mainly based on clinical criteria characterized by motor symptoms including bradykinesia, rigidity, resting tremor, and postural instability.^2,3 Developing reliable in vivo biomarkers to monitor disease severity and reflect the underlying dopaminergic degeneration is important for future disease-modifying therapy in PD.