Dear Wen Wei,
> 1. When we are getting the fieldmap, do we use exactly the same parameters
> as we did with the previous EPI acquisition?
I'd recommend keeping the same voxel sizes, FOV, slice thickness, orientation etc.
but it is not critical. However, I also recommend you use a spin-echo sequence,
not an EPI sequence.
> 2. You used 2.5ms offset on your 3T scanner. Ours is a 1.5 T GE scanner.
> Should we use the same offset or else please?
As the phase accrued is proportional to the time taken, and all of your
frequencies will be half of that on a 3T scanner, it is probably best to
double this value to keep the phase differences of the same order. However,
the longer you leave it the more signal decay occurs, but for 5ms I think
you should be OK. Again, it isn't critical. The smaller the value the
less phase wraps occur and the less t2 decay, but the SNR is reduced.
Using higher values induces more phase wraps but with better SNR as long as
the t2 decay isn't excessive. The phase unwrapping should be able to cope
with quite a lot of phase wraps, so that isn't too much of a constraint.
In short, try 5ms, as I think it should be OK.
If the phase images look bad then try a different value.
Best wishes,
Mark
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