Enhanced Gray Matter Specificity of Integrated Local Correlation Mapping Using Multi-Echo fMRI

Resting-state fMRI is a widely used technique for investigating intrinsic brain connectivity. Recently, the multiecho acquisition approach has been shown to enhance data quality by optimally combining signals from multiple echoes (OC-ME). This study evaluates the comparison between OC-ME and standard single echo (SE) acquisition in measuring local coherence metrics, specifically local integrated correlation (LCOR). SE acquisition is based on a single echo time (TE), which can lead to suboptimal sensitivity in regions where T2* deviates from the chosen TE. In contrast, OC-ME combines information from multiple TEs, improving BOLD sensitivity and reducing physiological and thermal noise. Comparison between SE and OC-ME was performed by acquiring rs-fMRI data from 23 healthy participants at three echo times (TE = 14, 34.63, 55.26 ms). The second echo was used as a representative of the SE data. Both conditions (SE and OC-ME) underwent the same preprocessing steps - except for the combination of echoes in OC-ME - including slice-timing correction, motion correction, regression of motion parameters, white matter and CSF signals, band-pass filtering, and spatial normalization. Beta maps representing LCOR were generated and analysed to assess differences in local brain coherence between the two approaches. The beta maps were then evaluated using tissue masks for gray matter (GM) and white matter (WM). Histograms of LCOR beta values revealed distinct distributions for the two tissue types: WM exhibited a narrow peak at low LCOR values, while GM showed a bimodal distribution, with lower values observed in the voxels adjacent to WM. OC-ME data revealed significantly higher LCOR values compared to SE (voxel-wise p<0.001; cluster-wise FDR-corrected p<0.0005) and improved tissue differentiation. These results highlight potential advantages of OC-ME in improving gray matter specificity of LCOR metrics, providing a more robust tool to study brain connectivity in the resting state.