Hi,
We collect short two short SE-EPI scans (each only ~ 30 sec) with opposing
polarity. Those are used to derive a fieldmap, which is then used to
correct the GE BOLD scans. The collection of pairs of GE BOLD scans with
opposing phase-encode polarity was done so that we wouldn't introduce an
overall L/R asymmetry in terms of distortion/signal dropout, not because
that was necessary for distortion correction.
In your case, you'll probably want to collect your BOLD with either A>>P
or P>>A phase encoding, and then collect the two short SE-EPI scans with
A>>P and P>>A phase encoding. Then use 'topup' to derive the fieldmap
from those.
Note that it is just as effective to correct the BOLD using a traditional
GE fieldmap acquisition, if you want to go with that approach instead.
(It just takes a little longer to collect -- approx 2 min).
cheers,
-MH
--
Michael Harms, Ph.D.
-----------------------------------------------------------
Conte Center for the Neuroscience of Mental Disorders
Washington University School of Medicine
Department of Psychiatry, Box 8134
660 South Euclid Ave. Tel: 314-747-6173
St. Louis, MO 63110 Email: [log in to unmask]
On 5/16/14 1:22 PM, "SUBSCRIBE SPM Lingfei Tang" <[log in to unmask]>
wrote:
>Hi All,
>
>We are trying to acquire data with multiband sequences according to the
>Human Connectom Project
>(http://humanconnectome.org/documentation/data-release/Q2_Release_Appendix
>_I.pdf pg. 38 on) and we would like to correct for the EPI distortions by
>using two dummy EPI sequences (by inverting RO/PE polarity option) and
>the "topup" function suggested by the HCP.
>
>But for our task-related functional data, we can only acquire in one
>direction as the design is event related. In our case, I could generate
>the field map based on the opposing EPI sequences before, but can't use
>applytopup for our real data collected in one direction.
>
>I would like to ask if there is a way still to use Topup to correct the
>distortion or should we collect some other kind of field map to achieve
>this purpose?
>
>Thanks
>LT
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