Assessment of gray matter microstructure and synaptic density in Alzheimer’s Disease: a multimodal imaging study with DWI and SV2A PET

Abstract

AbstractBackgroundDiffusion weighted imaging (DWI) is proposed to show alterations to microscopic barriers (O'Donnell, et al. 2011) associated with neurodegeneration (Jacobs, et al. 2013). Positron emission tomography (PET) imaging has also been used to measure synaptic density. Previously, we have shown that synaptic density is lower in Alzheimer’s Disease (AD) participants (Mecca, et al. 2020) and is correlated with cognitive performance in patients with AD. Here, we aim to investigate the relationship between DWI metrics and synaptic density in gray matter in the hope of better understanding of AD neurobiology.MethodThirty‐four amyloid positive participants with clinical diagnosis of AD and 18 amyloid negative cognitively normal (CN) participants were included in this study. Early AD‐affected regions (entorhinal cortex, hippocampus, posterior cingulate, and precuneus) were chosen as the primary region of interest (ROI). Synaptic density was measured using [11C]UCB‐J PET where the distribution volume ratio in ROIs was computed using SRTM2 with cerebellum as reference region. Average axial diffusivity (AxD), fractional anisotropy (FA), mean diffusivity (MD), and radial diffusivity (RD) within each ROIs was computed using Freesurfer 6. Between group comparisons were performed using an unpaired t‐test. Association between gray matter DWI metrics and synaptic density was assessed using univariate regression analyses.ResultsAxD was significantly higher in AD participants in entorhinal cortex and hippocampus bilaterally as well as right posterior cingulate and left precuneus. FA was significantly lower in left hippocampus, and significantly higher in right hippocampus and right posterior cingulate in AD participants. MD was significantly higher in AD participants in bilateral entorhinal cortex, left hippocampus and left precuneus. RD was significantly higher in AD participants in left precuneus, bilateral entorhinal cortex and hippocampus. Within AD participants, synaptic density was inversely correlated with AxD, MD, and RD in the entorhinal cortex and hippocampus bilaterally and positively correlated with FA in the left precuneus. These associations were absent in the CN group.ConclusionWe found that DWI metrics had significant correlation with synaptic density in Early AD‐affected regions in participants with AD, but not in CN participants. We hypothesize that DWI outcome alterations is reflective of gray matter microstructural changes related to synaptic loss.