Assessment of brain iron and neuronal integrity in patients with Parkinson's disease using novel MRI contrasts

Postmortem demonstration of increased iron in the substantia nigra (SN) is a well-appreciated finding in Parkinson's disease (PD). Iron facilitates generation of free radicals, which are thought to play a role in dopamine neuronal loss. To date, however, magnetic resonance imaging (MRI) has failed to show significant in vivo differences in SN iron levels in subjects with PD versus control subjects. This finding may be due to the limitations in tissue contrasts achievable with conventional T₁- and T₂-weighted MRI sequences that have been used. With the recent development of novel rotating frame transverse (T_2ρ) and longitudinal (T _1ρ) relaxation MRI methods that appear to be sensitive to iron and neuronal loss, respectively, we embarked on a study of 8 individuals with PD (Hoehn & Yahr, Stage II) and 8 age-matched control subjects. Using these techniques with a 4T MRI magnet, we assessed iron deposits and neuronal integrity in the SN. First, T_2ρ MRI, which is reflective of iron-related dynamic dephasing mechanisms (e.g., chemical exchange and diffusion in the locally different magnetic susceptibilities), demonstrated a statistically significant difference between the PD and control group, while routine T₂ MRI did not. Second, T_1ρ measurements, which appear to reflect upon neuronal count, indicated neuronal loss in the SN in PD. We show here that sub-millimeter resolution T_1ρ and T _2ρ MRI relaxation methods can provide a noninvasive measure of iron content as well as evidence of neuronal loss in the mid-brain of patients with PD.