[Insight-users] Inversion of transformation fields

[Floca, Ralf]

Hello everyone.

I am working on different registration approaches. Some of them consists
of more then one registration steps (e.g. rigid preregistration and non
rigid registration). All of them are data driven, so working directly on
the image. But in some cases, there are some additional information like
reference points (e.g. fiducals) attached. To keep them up to date
through out the whole registration process, they also must be
transformed.
Now the problem. ITK uses back transform to ensure dense images. This is
appropriate for the registration of the image itself, but to transform
meta information (like reference points, to have them up to date in
interim registration steps) a transform from moving to result image is
needed. In cases of a rigid registrations I could use the back transform
function for every single point to achieve this. But the fem, for
example, only produces a transformation field and has no back transform
or inversion capabilities.
Are there any suitable methods to invert this field to achieve a
transform of points from the moving image space to the result image
space?
The easy way: inverting all vectors of the field would only be a
solution, if the spacing was infinitesimal (whish it was so). So this
would be no solution.

I am thankful for every reply and comments.

Thanks and best regards,
Ralf o Floca

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Ralf Floca
University of Heidelberg                           
Institute for Medical Biometry and Informatics
Department of Medical Informatics
Im Neuenheimer Feld 400            
D-69120 Heidelberg, Germany             Tel. : 06221/56-7484  
http://www.med.uni-heidelberg.de/mi     Fax  :         -4997    
ralf_floca at med.uni-heidelberg.de        Sekr.:         -7483
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