[Insight-users] use of transform with itk volume

[J. Van Dalen]

Dear itk users,

First of all, a piece of advice to everybody who uses, or wants to use,
itkMutualInformationImageToImageMetric. It is very interesting first
to read the paper of Paul Viola and William Wells "Alignment by 
Maximization of Mutual Information", Int. J. of Comp. Vis., 24(2), 1997, 
p. 137-154, on which this itk class is based. It makes things much more
comprehensible!

However, there is still a major puzzle in my mutual information program.
I would like to take into account the initial transformation (possibly
containing translation, scaling and rotation) belonging to the image 
volume. Schematically, I do the following when I want to fuse image A onto B: 
A'=I*A and B'=J*B, where I and J are the initial transformations belonging
to the two images. To map A' onto B' I use a "guess" transformation T.
I.e., my real guess to map A onto B is: O = J^-1 * T * I 
However, this procedure fails (the error is often: "All the sampled point 
mapped to outside of the moving image").

To my opinion:
If I only have scaling and translation in my initial transformations I
and J, I might as well make use of SetOrigin and SetSpacing. In fact, 
when I do so, I get good results! 

Does anybody has an idea why my method doesn't work properly? 
Can the filters NormalizeImageFilter and ShrinkImageFilter, that I use, 
have any effect?

Hope someone can help me out here,
Regards,
Jorn.