Hi,
It seems to be a somewhat common misconception that the fieldmap acquisition has to match the EPI in some way. It doesn't. It is fine to have a different number of slices in each, as the fieldmap is initially registered to the structural scan with BBR. All that is important is that the resolution (and SNR) are sufficient to capture the fieldmap changes, but as the field inhomogeneities vary fairly smoothly, this will by with with 34 slices, and I doubt there will be much difference between the results from the 51 slice or the 34 slice fieldmaps. So you could use the fieldmap, with 34 slices, to set up the slice prescription too if that was more efficient for your scanning.
All the best,
Mark
On 23 Jan 2014, at 14:49, Allison Jack <[log in to unmask]> wrote:
> Greetings FSL experts,
>
> Based on advice I received on here not long ago, I have started acquiring a fieldmap sequence in order to help account for some of the EPI distortions in inferior regions of the brain on my functional images. This is my first time collecting a fieldmap. I am using a conventional Siemens sequence with 51 slices (Can provide more parameters if needed). My functional images are 34 slices.
>
> Over the weekend, some research fellows were running my protocol on my behalf and the MRI tech changed the fieldmap parameters so that the fieldmap had 34 slices. She told them to tell me that it was a problem if the fieldmap and the functional didn't match in terms of the number of slices. My questions are as follows:
>
> 1. I have used the 51-slice fieldmaps to unwarp my 34-slice EPI sequences with what I thought was success on several previous occasions. Is there anything wrong with what I've been doing?
>
> 2. My lab has typically used a T1 weighted axial flash (about 1.5 min) to set up slice prescription for our functionals. If the fieldmap were 34 slices, could it be used to set up functional slice prescription, and if I did reduce the number of slices, would I reduce the quality of my unwarping?
>
> Thanks for any advice!
>
> Best,
> Allison
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