[MINC-users] minctracc/masking bug?

[Alex Zijdenbos]

Hello all,

I have been chasing some strange behaviour of minctracc, and finally
narrowed it down to something that I would say is a bug in the way
minctracc deals with masks.

I have generated symmetric (about x=0), blurred source- and target images,
and a mask derived from the target image by a simple threshold (and thus
also symmetric). If I run minctracc to register these two without masks, I
end up with a deformation field that is by and large symmetric as well.
However if I add the symmetric mask via the -model_mask option to an
otherwise identical minctracc call, the resulting deformation becomes
highly asymmetric, and drags the source image out. I am using mni_autoreg
0.99.6, libminc 2.1

Summarized in this image:

https://dl.dropboxusercontent.com/u/5709165/test_minctracc.jpg

Row 1: source image
Row 2: target image
Row 3: target mask
Row 4: deformation field magnitude without using mask
Row 5: resampled source image with target mask outline
Row 6/7: as rows 4/5, but after adding the target mask to minctracc

Clearly the addition of the mask has a strong negative (and asymmetric)
impact on the registration, while it seems it shouldn't have much of an
effect at all.

I have actually seen this behaviour before but was never able to pinpoint
it so clearly. What appears to be happening is that masked registrations
have a tendency to 'flow out' in a particular direction (towards the top
right in coronal sections).

Thoughts/suggestions, anyone?

Thanks,

-- Alex