Is cortical grey matter diffusivity an early biomarker for neuronal damage?

Background: There is growing evidence that pathological changes in Alzheimer's disease (AD) begin several years before the symptom onset. Neuroimaging approaches can be used to detect microstructural alterations in the early stages of neurodegeneration using diffusion‐weighted MRI. Cortical grey matter mean diffusivity (cMD) is thought to be increased in AD following pathological alterations resulting in disrupted neuronal function, neuronal death and cellular atrophy. We hypothesised that changes in MD could be detected early in the disease progression and that they would be correlated with structural imaging biomarkers. Method: 72 patients (mean age=69.1, SD=9.2) cognitively normal (CN, n=21) and with mild cognitive impairment (MCI, n=51) were included in this study. All participants underwent T1‐weighted MPRAGE MRI, diffusion tensor imaging (DTI) and PET amyloid ([18F]flutemetamol). Regional cortical thickness was measured on T1‐MPRAGE using FreeSurfer v.6.0. DTI data were processed with FSL v.6.0 to compute MD maps. Standard uptake ratio (SUVR) values were calculated in SPM12 to identify the amyloid deposition within cortical regions. Amyloid status (Aβ‐positive/Aβ‐negative) was determined from processed data using a cerebellar gray matter reference to create target‐to‐cerebellar ratio. Subjects were considered positive with an uptake value of greater than mean+2SD of the control subjects. Student's t‐test were used to compare imaging modalities between CN and MCI participants. Pearson's correlation coefficients were calculated to assess the relationship between imaging modalities. Result: There was a significantly increased MD in MCI compared to CN participants in hippocampus (p=0.0213) and in medial temporal lobe (p=0.033). There were no significant group differences neither in whole brain mean cortical thickness (p=0.6984) nor in temporal cortical thickness (p=0.0923). Interestingly, when splitting subjects based on their amyloid status, we found that whole brain MD was significantly negatively correlated with whole brain cortical thickness in amyloid negative (CN+MCI amyloid negative, p<0.001) but not in amyloid positive (CN+MCI amyloid positive, p=0.99). Conclusion: We found that changes in gray matter diffusivity could be detected in the early stages of AD pathology. We also found that there is a direct relationship between MD and structural changes (i.e. cortical thickness) only before amyloid deposition suggesting that MD provides information about early microstructural changes before macrostructural changes. [ABSTRACT FROM AUTHOR]