[MINC-users] minctracc/masking bug?

[Claude LEPAGE]

Hi Alex,

This is a strange behaviour indeed. 

Can you tell me which line of code to change? :-)

One thought: is the resulting grid symmetric with respect to
the field of view? If the grid is asymmetric, the support of
the masked image may be insufficient for some grid points
lying very near the border of the masked image. I dealt with
this in the old N3 where the control points for the splines 
were defined from the bottom-left corner. I made them centered
from version 1.11.0. Maybe we can try the same in minctracc
if it's not already the case. Also, what happens when you
reduce the grid size? Does the error go away? Have you looked
at the masked blurred (or blurred masked) images? 

Claude




> 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
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