[Insight-developers] Inverse transforms

[Stephen R. Aylward]

Hi,

1) ITK registration actually returns a transform from the fixed image to 
the moving image
2) ITK resample image filter expects a transform from the fixed image to 
the moving image
3) No model-to-image registration process is easily implemented using a 
fixed-to-moving image transform - the model drives where in the fixed 
image the fit should be calculated.   So, all spatial-object-to-image 
transforms, the landmark-landmark registration stuff we are doing, and 
the ICP method we are implementing apply the transform to the moving 
model/points to quantify their match with the fixed image/points. 
Therefore we have to pass the Inverse transform to the resample image 
filter for them.

We have the landmark initialized MI registration app that must

1) register using landmarks
2) resample the image using the inverse of that transform
3) invert the transform to initialize the MI registration process
4) resmaple the image using the non-inverse of that transform

This is really inconsistent.   Can be documented by saying most 
image-image transforms are fixed to moving transforms and all spatial 
object transforms are moving to fixed.   I say most image-image 
transforms since we are writing one that internally calculates features 
and landmarks from the moving image to drive the registration with the 
fixed image.

I consider this a huge flaw with no easy answer - I understand why the 
resampling is driven by the fixed image.   However, couldn't the 
resampling take the inverse and all of the rest be done in the correct 
way: tranaform maps moving to fixed?

Stephen

-- 
===========================================================
Dr. Stephen R. Aylward
Associate Professor of Radiology
Adjunct Associate Professor of Computer Science and Surgery
http://caddlab . rad . unc . edu
aylward at unc . edu
(919) 966-9695