[Insight-users] Versor3DRegistration and the masks of meanSquareMetric

[Martin Urschler]

hello all,

I'm using a Versor3DRegistration with a MeanSquaresMetric for an 
iterative registration procedure.
I'm using the output of one registration iteration to calculate a 
misregistration region which is used in the next registration step as a 
mask for the metric.

There are two issues that I have in mind which kind of confuse me.

First, the MeanSquaresMetric allows you to specify a fixedImageMask and 
a movingImageMask. I understand the fixed mask. During metric 
calculation one iterates over the fixed image voxels. If the physical 
location of the fixed image voxel is inside the fixedImageMask, then it 
will be used for the metric calculation.
But, I don't understand the moving image mask (which I've been using up 
to now). If you specify a moving image mask, then in the course of the 
optimization it may well be possible that you start your optimization 
with parameters that won't map you into the moving image mask area, 
which would lead to an abort of the optimizer although optimization may 
easily be possible. And even if there are a few voxels that map into the 
movingImageMask with the initial parameters the metrics derivative will 
be quite useless and could point you in the entirely wrong direction ...

So my point is in short: Is it a good thing to specify a moving image 
mask in a registration??? Or do I misunderstand something here?

If I'm right then this should somehow be documented to not use the 
movingImageMask in a registration - since I guess it would be overkill 
to remove the movingImageMask from the ImageToImageMetric... :-)


Second, I (still) have some problems understanding the 
Versor3DTransform. I looked through the itk software guide and think I 
understood the CenteredRigid2DTransform. Now my question is: Is the 
VersorRigid3DTransform also a centered transform? I.e. if I specify an 
initial versor, an initial translation and an initial rotation center, 
will an optimization take into consideration the rotation center as 
well. Or will the center of rotation stay the same during optimization? 
The Software guide isn't clear about that (at least version 2.0.2 isn't).

In my application I have some registration behaviour that I don't 
understand but it might also come from the issue mentioned above, 
therefore I'd like to eliminate things one by one...

thanks,
Martin