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Chloe, Marko, Thank you both for your extremely helpful answers. I hoped this would be the case, but I'm a pessimist by nature. :-) I'll be sure to take your input into consideration for my future field mapping. Once again, thanks, Haakon On 08.06.2009 17:41, Chloe Hutton wrote: > Dear Haakon > >> First: I've yet to see any concrete recommendations as to optimal >> settings for B0 maps. I'm currently gathering field maps using a factory >> optimized sequence which leaves me with a phase and magnitude image that >> the Fieldmap toolbox accepts readily. However, the specific parameters of >> the field map, such as voxel size, and slice number is seldom in >> correspondence with the functional data I collect. Am I right in assuming >> that this is suboptimal, and that we SHOULD acquire field maps that are in >> exact correspondence to the functional images with regard to 1: voxel size >> 2: number of slices and 3: acquisition orientation. Am I forgetting >> something? > > Assuming that you are acquiring gradient echo fieldmaps, the FieldMap > toolbox is designed to accept input for which 1) voxel size, 2) number of > slices and 3) acquisition orientation do NOT have to be the same as the > EPI. We standardly acquire fieldmaps of 3mm^3 voxel size regardless of the > EPI voxel resolution (not all of our EPI sequences have 3mm^3 voxels). > This voxel size is a good trade-off between acquisition time and the > spatial scale of field changes that can be corrected for. > > Importantly, we recommend to acquire 64 transverse slices so that the > whole of the head is covered. This means that field information will be > acquired beyond the EPI slice coverage and avoids any unnecessary > truncation of the EPI data when the distortion correction is applied. > > It is also important that the shim values/currents are the same for both > the EPI and the fieldmap data. On Siemens scanners it is possible to do > this by selecting 'Copy Parameters', from the EPI volume then selecting > 'Adjust Volume' when prescribing the field map acquisition. This step > copies the information about the volume over which the shim is adjusted > for the EPI data into the prescription of the field map, ensuring that the > field was the same over this region. I'm not sure how this can be done > with Philips or GE scanners. > >> Second: Suppose I have a rather large amount of data with fieldmaps >> for in which voxel size and number of slices is not in correspondence with >> the functional data. Is it still possible to perform fieldmap based >> unwarping? > > Ideally, as described above, the field map should be acquired with the > same shim values as is acquired for the EPI data. The actual number of > slices acquired and voxel size is not important as the FieldMap toolbox > can take care of this using the positioning information encoded in the > image headers. > >> Third: Suppose I run multiple experiments during one scanning session, >> in which the acquisition parameters(no. of slices, voxel size, slice gap, >> TR, TE) vary between experiment. Is it necessary to acquire a >> fieldmap matched to each experiment? > > Again, the most important thing is the actual field information that is > acquired for the field map rather than the geometry. > > Best wishes > Chloe > >> I have a sneaking suspicion that the reason why I can't find these >> questions answered anywhere is because the answers are rather obvious to >> those well-versed in these matters, but I hope you will find time to >> answer me in spite of this. >> >> Best wishes, >> Haakon, >> University of Oslo >> >> > >