Hi,
On Thu, Oct 16, 2008 at 06:03:14PM +0100, Mark Jenkinson wrote:
> Hi,
>
> I've been having a look at this today and it is a bit weird.
> Firstly, there is a strange masking going on at the edges,
> and I had to take that into account with an eroded mask
> for the weighting volumes (both ref and input). Where
> do these zeros come from and why are they are in a
> simple shape?
Yeah, I noticed them too. Unfortunately, I was not part of this project
when that data was aqcuired, so I cannot quickly provide the true
reason. However, the description of the recording procedure contains
this:
The sequence parameters were as follows: TR = 1300 ms, TI = 650 ms,
TE = 7.4 ms, slice-thickness = 2 mm, slice-gap = 0.4 mm, FOV = 128
mm × 128 mm with an in-plane resolution of 2 mm × 2 mm. Oversampling
in the phase-encoding direction was applied to achieve the desired
in-plane resolution (‘zoom-EPI’) and to remove any fold-in signal
from outside the FOV. The functional part of the session consisted
of three spin-echo EPI scans (TR = 2 seconds, TE = 85 ms, bandwidth
= 1346 Hz/Px, matrix 64 × 64, phaseoversampling).
Maybe it has to do with the removal of the 'fold-in signal'?
If you need more information, I can contact the people who where
involved in the sequence details.
> Once I took that into account I got better results, but I'm
> struggling to get it as good as we typically see full brain
> results, even though it looks like it should be possible.
> It is possible that it is differential distortions between
> the beginning and end of the sequence that causes a
> non 6-dof change which cannot be accounted for, but I
> suspect that 6 dof should still be able to do better than
> I've done so far. I'll have another look at it tomorrow and
> let you know how I get on.
Thanks a lot!
Best,
Michael
--
GPG key: 1024D/3144BE0F Michael Hanke
http://apsy.gse.uni-magdeburg.de/hanke
ICQ: 48230050
|