Implementation of multi‐contrast, multi‐echo SAGE‐fMRI in aging and Alzheimer's disease

Abstract

AbstractBackgroundAn advanced spin‐ and gradient‐echo (SAGE) functional MRI (fMRI) sequence was implemented in healthy aging control (HC) and cognitively impaired (CI) groups to improve functional assessment of AD‐relevant regions susceptible to signal dropout in standard fMRI.MethodFive HC and four CI were included in this preliminary study. SAGE‐fMRI data were acquired at 3T (Ingenia, Philips) with 2 gradient‐echoes (GRE), 2 asymmetric spin‐echoes (SE), and 1 spin‐echo (TE1‐5 = 5.97/18.76/36.048/49.27/62.06 ms, acquisition matrix: 64×64; voxel size: 3.75x3.75 mm; slice thickness: 5.0 mm; TR=3000 ms). SAGE‐fMRI data were processed using single‐echo and multi‐echo combinations and weighted (w) summations (GRE (macrovascular): TE2, T2*, wT2*; SE (microvascular): TE5, T2, wT2). Specifically, TE2 (similar to standard fMRI) and TE5 were processed individually. Dynamic T2 *and T2 were fit using a piece‐wise function, while weighted summations for wT2 *and wT2 were calculated as previously described. Subsequently, all SAGE‐fMRI images were processed using standard procedures with FSL and AFNI. Face‐name memory encoding and recall tasks conditions were organized in alternating block designs and consisted of novel (unfamiliar faces) and repeated (one of two recurring faces) phases, with brief rest periods between conditions. AFNI was used to calculate the statistical parametric maps of response to stimuli from all fMRI data (3dDeconvolve) and to run a two‐sample t‐test between HC and CI groups (3dttest++).ResultHC showed increased activation compared to CI for memory encoding and recall tasks across each SAGE‐fMRI analysis method. Weighted summation SAGE contrasts showed robust differences between groups in the cingulate gyrus and frontal pole in the memory encoding task. Moreover, the frontal pole had sizeable differences between groups in memory recall, particularly in the microvascular contrasts; these weighting factors revealed more distinct differences between groups across regions of interest for the memory recall task than the macrovascular contrasts. Notably, in the memory recall task, T2* and T2 distinguished between groups in AD‐relevant regions such as the hippocampus and parahippocampal gyrus where single‐echo fMRI did not.ConclusionPreliminary results reveal widespread decrease in task‐related activation from HC to CI groups, with further distinction between the two groups in AD‐relevant regions such as the hippocampus and parahippocampal gyrus via SAGE‐fMRI.