Wednesday, May 4, 2011

Using GRAPPA for fMRI in the presence of subject motion

I've received a few queries about my opinions on the use of GRAPPA for EPI time series, opinions which have been mentioned in passing in earlier posts. In my user training guide/FAQ are some sections that deal with GRAPPA features and performance, but I didn't include an in-depth illustration of the artifacts or the motion sensitivity. So, to help you make a decision on whether GRAPPA is something you should be using in your fMRI experiments, I'm going to post here a few more images and some movies to highlight the problems that can arise in the presence of significant head motion. I'll focus on R=2 accelerated EPI, but the principles hold for higher acceleration factors as well.

Whether or not you ultimately select GRAPPA for your experiment, it is important to make your determination objectively, taking into account your experimental needs, the benefits of the method, its failure modes and prior studies you can rely on for validation. (See "Beware of physicists bearing gifts!")

** Please note that the following information pertains to the GRAPPA implementation available as product on the Siemens Trio/TIM platform with VB15 software. If you have a different Siemens platform or a different vendor's scanner there may be significant differences in the implementation. **


A brief review of the GRAPPA method

If you don't have even a rudimentary understanding of parallel imaging (PI) generally or the GRAPPA method specifically, I would encourage you to stop reading this post now and go read at least one of these articles: Larkman & Nunes (2007) or Blaimer et al. (2004). Then come back when you're ready to proceed with a speedy review.