The impact of EPI voxel size on SNR and BOLD sensitivity in the anterior medio-temporal lobe: a comparative group study of deactivation of the Default Mode

Abstract

Objectives: To quantify the gain in time-series SNR that can be achieved in the amygdala by reducing EPI voxel size, and to assess the extent to which this advantage is carried through to statistical significance in a group fMRI study, using a cognitive task to trigger task-independent deactivation of anterior medial temporal structures.

Materials and methods: Two groups of seven subjects were posed number-series tasks to induce deactivation of the Default Mode network. This is known from PET work to include the amygdala, which lies in a region of high magnetic field gradient. In 3 T imaging, one group was studied with high resolution EPI with 6 mul voxels, the other with lower resolution EPI with 17 mul voxels. Field maps were acquired to allow field gradients in relevant ROIs to be assessed.

Results: Time-series SNR was 45% higher in the amygdala in the high resolution EPI data than in the low resolution data. In activation results, whilst there was good agreement between other areas, the involvement of the amygdala could only be demonstrated in the high resolution data.

Conclusion: We find that reduction in signal dephasing afforded by high resolution EPI is realized as a substantial increase in SNR and BOLD sensitivity in group fMRI data. This has allowed the first demonstration of the involvement of the amygdala in the Default Mode in fMRI.