Comparing the effect of multi-gradient echo and multi-band fMRI during a semantic task.

The blood oxygenation level dependent (BOLD) signal, as measured using functional magnetic resonance imaging (fMRI), is known to vary in sensitivity across the brain due to magnetic susceptibility artefacts. For example, the ventral anterior temporal lobes have been implicated with semantic cognition using convergent methods (i.e., neuropsychology, PET, MEG, brain stimulation) but less so with fMRI using conventional gradient-echo protocols. Of the methods to alleviate this signal loss, "multi-echo" fMRI has gained popularity. Here, additional volumes are collected with a range of echo times (TEs), subsequent combination of which can improve BOLD contrast-to-noise. However, these additional volumes normally require compromising other MR sequence parameters (e.g., longer repetition times, higher in-plane acceleration). One solution is to combine multi-echo with "multi-band" imaging, in which simultaneous acquisition of multiple slices reduces repetition time again. However, it remains unclear how these two modifications independently or interactively affect fMRI sensitivity across the brain, for univariate or multi-variate analyses. In the current study, we used a factorial design in which the number of echoes and/or bands was manipulated to assess how well semantically related activation/multi-voxel patterns can be detected. When comparing the precision with which activations were detected (i.e., average T-statistics), we found that multi-band protocols were beneficial, with no evidence of signal leakage artefacts. When comparing the magnitude of activations, multi-echo protocols increased activations in regions prone to susceptibility artefacts (particularly in the temporal lobes). Both multi-banding and independent component analysis (ICA)-denoising of multi-echo data tended to improve multi-voxel decoding of conditions. However, multi-echo protocols reduced activation magnitude in more central regions, such as the medial temporal lobes, possibly due to the higher in-plane acceleration entailed. Nonetheless, the multi-echo, multi-band protocol is a promising default option for fMRI on most regions, particularly those that suffer from susceptibility artefacts, as well as offering the potential to apply advanced post-processing methods to take advantage of the increased temporal (or spatial) resolution of multi-band protocols and more principled ICA-denoising based on TE dependence of BOLD signals.