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Tuesday, April 1, 2014

i-fMRI: A virtual whiteboard discussion on multi-echo, simultaneous multi-slice EPI

Disclaimer: This isn't an April Fool!

I'd like to use the collective wisdom of the Internet to discuss the pros and cons of a general approach to simultaneous multislice (SMS) EPI that I've been thinking about recently, before anyone wastes time doing any actual programming or data acquisition.

Multi-echo EPI for de-noising fMRI data

These methods rest on one critical aspect: they use in-plane parallel imaging (GRAPPA or SENSE, usually depending on the scanner vendor) to render the per slice acquisition time reasonable. For example, with R=2 acceleration it's possible to get three echo planar images per slice at TEs of around 15, 40 and 60 ms. The multiple echoes can then be used to characterize BOLD from non-BOLD signal variations, etc.
The immediate problem with this scheme is that the per slice acquisition time is still a lot longer than for normal EPI, meaning less brain coverage. The suggestion has been to use MB/SMS to regain speed in the slice dimension. This results in the combination of MB/SMS in the slice dimension and GRAPPA/SENSE in-plane, thereby complicating the reconstruction, possibly (probably) amplifying artifacts, enhancing motion sensitivity, etc. If we could eliminate the in-plane parallel imaging and do all the acceleration through MB/SMS then that would possibly reduce some of the artifact amplification, might simplify (slightly) the necessary reference data, etc.

A different approach?