Dear Justin and others,
>
> The equation you posted matched what we measured for dwell time:
>
> calculated_dwell = (1000 * 13.243)/(434.215*(43 +1)) = 0.6932 ms
> measured_dwell = ~0.713 ms
>
> So that's good news. For the readout time:
>
> calculated_epi_readout = (0.6932*(43-1)/1000) = 0.0291 seconds
>
> It's possible thats the "full readout" without partial fourier, since:
>
> 0.0291 * 0.755102 = 0.022 seconds
>
> which approximately matches what we actually measured from the scanner.
>
> So this would mean the epi factor takes SENSE (but not partial fourier) into account.
the SENSE factor should be taken into account, but NOT partial fourier. Since partial fourier essentially just mirrors the echoes already acquired the echo-to-echo phase evolution will be the same in the “non-acquired” as in the acquired echoes.
Importantly, if you work with reversed PE blips and you intend to subsequently feed the topup results into eddy, it doesn’t really matter what value you put in. As long as you make sure the same values (ideally the same acqparams.txt) are used for both topup and eddy. It only becomes important if you want to take the topup fieldmap and use it in some other software (like for example fugue). For the standard recommend diffusion pre-processing with topup+eddy you can just put any plausible value (I always use 0.05 when I don’t know the truth).
Jesper
>
> I'm going to try topup with a readout of 0.0291 (as per the instructions for topup to take partial fourier into account), look at the output fieldmap from topup and then compare it to a fieldmap we acquired. I've read that multiband does not affect the readout time, but I'm not so sure. Hopefully another expert can chime in.
>
> Thanks,
>
> -Justin
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