Leveraging multi-echo EPI to enhance BOLD sensitivity in task-based olfactory fMRI

Functional magnetic resonance imaging (fMRI) usingblood-oxygenation-level-dependent (BOLD) contrast relies on gradient echoecho-planar imaging (GE-EPI) to quantify dynamic susceptibility changesassociated with the hemodynamic response to neural activity. However, acquiringBOLD fMRI in human olfactory regions is particularly challenging due to theirproximity to the sinuses where large susceptibility gradients induce magneticfield distortions. BOLD fMRI of the human olfactory system is furthercomplicated by respiratory artifacts that are highly correlated with eventonsets in olfactory tasks. Multi-echo EPI (ME-EPI) acquires gradient echo dataat multiple echo times (TEs) during a single acquisition and can leverage signalevolution over the multiple echo times to enhance BOLD sensitivity and reduceartifactual signal contributions. In the current study, we developed an ME-EPIacquisition protocol for olfactory task-based fMRI and demonstrated significantimprovement in BOLD signal sensitivity over conventional single-echo EPI(1E-EPI). The observed improvement arose from both an increase in BOLD signalchanges through aT₂ ^* -weightedecho combination and a reduction in non-BOLD artifacts through the applicationof the Multi-Echo Independent Components Analysis (ME-ICA) denoising method.This study represents one of the first direct comparisons between 1E-EPI andME-EPI in high-susceptibility regions and provides compelling evidence in favorof using ME-EPI for future task-based fMRI studies.