Wednesday, March 21, 2012

GRAPPA and multi-band imaging. And motion. Again.

 

It's come to my attention that some of the latest accelerated (aka multiplexed) EPI sequences are now being made available to some sites with vendor/collaborative research agreements, a move that should catalyze their verification, testing and eventual application for neuroscience. The distribution of these pulse sequences to the wider world is great news! The potential is considerable! However, those wanting to conduct neuroscience experiments today with these zippy new tools should bear in mind the not inconsiderable risks. I want to warn you to think very carefully before taking the plunge.

Today's accelerated EPI sequences combine techniques such as multi-band (MB) acquisition with simultaneous echo refocusing (SER) and/or GRAPPA (1,2). In previous posts I've highlighted the increased motion sensitivity of parallel imaging methods such as GRAPPA. The MB family of methods also require "reference scan data" in order to reconstruct the time series images, and as such they are inherently more motion-sensitive than your plain vanilla single-shot EPI. Indeed, similar principles are used to reconstruct MB images as for GRAPPA, and the basic motion sensitivities are the same, i.e. motion during the reference data acquisitions will contaminate all images in a subsequent time series, while motion after the reference data but during the (accelerated) time series will lead to mismatches and spatial artifacts that will degrade temporal stability. In short, using these accelerated sequences is akin to sharpening the motion sensitivity profile of your experiment, and you will need to ensure a high degree of subject compliance to get good data.


Plan, then scan.

Now, I'm not suggesting you dismiss out of hand these sequences for your research. I am suggesting that you apply a lot of forethought, taking the time to consider several important factors. I've written before about evaluating pulse sequences that are new (or new to you). Your first task is to determine whether you even need a fancy, partly validated, highly risky pulse sequence to answer your neuroscience question. If the answer isn't a resounding "yes," why take the risk? Next, you should ask yourself how the pulse sequence should be set up to provide the optimum data. For instance, do you know which slice direction is best for minimizing motion sensitivity and/or receive field bias (g-factor) for the multi-band sequence? And do you know which RF coil to use, and why? If you can't establish your experimental setup based on sound principles that's a suggestion you either don't have the expertise yourself or you aren't collaborating with someone with the requisite expertise. (Me? I could guess, but that's about it! Without doing a validation study of my own I'd be winging it. Which is kinda my point!)

Please don't just go download and use the latest and greatest technique because it's new and cool. I've seen this movie before, and ninety nine times out of a hundred it ends in tears. Please put some justification and logic into your choices before you go and spend hundreds of hours and thousands of dollars finding yet another way that motion can confound an fMRI experiment. Eyes wide open!

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References:

1.  S Moeller, et al. "Multiband multislice GE-EPI at 7 tesla, with 16-fold acceleration using partial parallel imaging with application to high spatial and temporal whole-brain fMRI." Magn. Reson. Med. 63, 1144-53 (2009).

2.  DA Feinberg, et al. "Multiplexed echo planar imaging for sub-second whole brain fMRI and fast diffusion imaging." PLoS ONE 5(12), e15710 (2010).