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Abstract
Accelerated data acquisition with simultaneous multi-slice (SMS) imaging for functional MRI studies leads to interacting and opposing effects that influence the sensitivity to blood oxygen level-dependent (BOLD) signal changes. Image signal to noise ratio (SNR) is decreased with higher SMS acceleration factors and shorter repetition times (TR) due to g-factor noise penalties and saturation of longitudinal magnetization. However, the lower image SNR is counteracted by greater statistical power from more samples per unit time and a higher temporal Nyquist frequency that allows for better removal of spurious non-BOLD high frequency signal content. This study investigated the dependence of the BOLD sensitivity on these main driving factors and their interaction, and provides a framework for evaluating optimal acceleration of SMS-EPI sequences. functional magnetic resonance imaging (fMRI) data from a scenes/objects visualization task was acquired in 10 healthy volunteers at a standard neuroscience resolution of 3 mm on a 3T MRI scanner. SMS factors 1, 2, 4, and 8 were used, spanning TRs of 2800 ms to 350 ms. Two data processing methods were used to equalize the number of samples over the SMS factors. BOLD sensitivity was assessed using g-factors maps, temporal SNR (tSNR), and t-score metrics. tSNR results show a dependence on SMS factor that is highly non-uniform over the brain, with outcomes driven by g-factor noise amplification and the presence of high frequency noise. The t-score metrics also show a high degree of spatial dependence: the lower g-factor noise area of V1 shows significant improvements at higher SMS factors; the moderate-level g-factor noise area of the parahippocampal place area shows only a trend of improvement; and the high g-factor noise area of the ventral-medial pre-frontal cortex shows a trend of declining t-scores at higher SMS factors. This spatial variability suggests that the optimal SMS factor for fMRI studies is region dependent. For task fMRI studies done with similar parameters as were used here (3T scanner, 32-channel RF head coil, whole brain coverage at 3 mm isotropic resolution), we recommend SMS accelerations of 4x (conservative) to 8x (aggressive) for most studies and a more conservative acceleration of 2x for studies interested in anterior midline regions.
Highlights
Two dimensional simultaneous multi-slice (SMS) echo planar imaging (EPI) is a technique for accelerating MRI data acquisition that is well suited for improving the sensitivity of task-based and resting state functional magnetic resonance imaging studies
The three orthogonal slices shown are the same as those displayed in Figure 1, and the white circles indicate the V1, parahippocampal place area (PPA), and ventromedial pre-frontal cortex (vmPFC) regions of interest (ROIs)
Results from the tSNR measurements confirm the expected trends for the two isolated effects: relative image signal-tonoise ratio (SNR) decreased with increasing SMS factor and blood oxygen level-dependent (BOLD) sensitivity improved as a function of how much high frequency noise was removed
Summary
Two dimensional simultaneous multi-slice (SMS) echo planar imaging (EPI) is a technique for accelerating MRI data acquisition that is well suited for improving the sensitivity of task-based and resting state functional magnetic resonance imaging (fMRI) studies. Several key developments advanced the acquisition method from its original form to an SMS EPI implementation that provided large acceleration factors and high image quality These included the CAIPIRINHA method (Breuer et al, 2005) which improved the reconstruction conditioning by imposing relative in-plane shifts between the slices, adaptation of CAIPIRINHA to be compatible with EPI read-outs (Nunes et al, 2006), and the introduction of blipped-controlled aliasing (blipped-CAIPI) (Setsompop et al, 2012) which mitigated the problems of voxel-tilting and through-slice dephasing that affected the original EPI CAIPIRINHA implementation. Two studies demonstrate benefits for task fMRI in visual (Chen et al, 2015) and auditory (De Martino et al, 2015) areas, but regions in the mid-brain where the SMS reconstruction problem is less well conditioned remain unexplored
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