The effects of multi-echo fMRI combination and rapid T-2*-mapping on offline and real-time BOLD sensitivity

A variety of strategies are used to combine multi-echo functional magnetic resonance imaging (fMRI) data, yet recent literature lacks a systematic comparison of the available options. Here we compare six different approaches derived from multi-echo data and evaluate their influences on BOLD sensitivity for offline and in particular realtime use cases: a single-echo time series (based on Echo 2), the real-time T 2 *-mapped time series ( T 2 * FIT ) and four combined time series ( T 2 *-weighted, tSNR-weighted, TE-weighted, and a new combination scheme termed T 2 * FIT-weighted). We compare the influences of these six multi-echo derived time series on BOLD sensitivity using a healthy participant dataset ( N = 28) with four task-based fMRI runs and two resting state runs. We show that the T 2 * FIT-weighted combination yields the largest increase in temporal signal-to-noise ratio across task and resting state runs. We demonstrate additionally for all tasks that the T 2 * FIT time series consistently yields the largest offline effect size measures and real-time region-of-interest based functional contrasts and temporal contrast-to noise ratios. These improvements show the promising utility of multi-echo fMRI for studies employing real-time paradigms, while further work is advised to mitigate the decreased tSNR of the T 2 * FIT time series. We recommend the use and continued exploration of T 2 * FIT for offline task-based and real-time region-based fMRI analysis. Supporting information includes: a data repository ( https://dataverse.nl/dataverse/rt-me-fmri ), an interactive web-based application to explore the data ( https://rt-me-fmri.herokuapp.com/ ), and further materials and code for reproducibility ( https://github.com/jsheunis/rt- me-fMRI ).