Impacts of simultaneous multislice acquisition on sensitivity and specificity in fMRI

Simultaneous multislice (SMS) acquisition can be used to decrease the time between acquisition of fMRI volumes, which can increase sensitivity by facilitating the removal of higher-frequency artifacts and boosting effective sample size. The technique requires an additional processing step in which the slices are separated, or unaliased, to recover the whole brain volume. However, this may result in signal “leakage” between aliased locations, i.e., slice “leakage,” and lead to spurious activation (decreased specificity). SMS can also lead to noise amplification, which can reduce the benefits of decreased repetition time. In this study, we evaluate the original slice-GRAPPA (no leak block) reconstruction algorithm and acquisition scenarios used in the young adult Human Connectome Project (HCP), as well as split slice-GRAPPA (leak block). In addition to slice leakage, signal leakage can result from spatial smoothing, i.e., smoothing leakage, which leads to inflated regions of activation. Previous studies have generally found that SMS acquisition results in higher test statistics and/or a greater number of activated voxels. Here, we use simulations to disentangle this phenomenon into true positives (sensitivity) and false positives (decreased specificity). Slice leakage was greatly decreased by split slice-GRAPPA. Noise amplification was decreased by using moderate acceleration factors (AF = 4). We examined slice leakage in unprocessed fMRI motor task data from the HCP, which used the original slice-GRAPPA. When data were smoothed, we found evidence of slice leakage in some, but not all, subjects. We also found evidence of SMS noise amplification in unprocessed task and processed resting-state HCP data.