Echo planar imaging (EPI) is the MRI technique that is most widely used for blood oxygen level-dependent (BOLD) functional MRI (fMRI). Recent advances in EPI speed have been made possible with simultaneous multi-slice (SMS) methods which combine acceleration factors M from multiband (MB) radiofrequency pulses and S from simultaneous image refocusing (SIR) to acquire a total of N = S×. M images in one echo train, providing up to N times speed-up in total acquisition time over conventional EPI. We evaluated accelerations as high as N = 48 using different combinations of S and M which allow for whole brain imaging in as little as 100. ms at 3. T with a 32 channel head coil. The various combinations of acceleration parameters were evaluated by tSNR as well as BOLD contrast-to-noise ratio (CNR) and information content from checkerboard and movie clips in fMRI experiments. We found that at low acceleration factors (N. ≤. 6), setting S=. 1 and varying M alone yielded the best results in all evaluation metrics, while at acceleration N = 8 the results were mixed using both S=. 1 and S=. 2 sequences. At higher acceleration factors (N > 8), using S=. 2 yielded maximal BOLD CNR and information content as measured by classification of movie clip frames. Importantly, we found significantly greater BOLD information content using relatively fast TRs in the range of 300. ms-600. ms compared to a TR of 2. s, suggesting that faster TRs capture more information per unit time in task based fMRI.
Bibliographical noteFunding Information:
This work was supported in part by the National Institutes of Health grants: NIH-Human Connectome Project U54MH091657 , R44 NS073417 .
- Functional imaging
- Multiplexed EPI
- Simultaneous multi-slice