[Insight-users] MI Registration example translation parameters

[David Wikler]

Dear ITK users,

I have been using MultiResMIRegistration example 
with success to register PET to MR datasets but 
I have difficulties to interpret the resulting
transformation as it is written on standard 
output.
I used the algorithm on a dataset where I knew
I would only have translation, so rotation matrix
will similar to 
[1 0 0
 0 1 0
 0 0 1]

My PET data is 
128x128x63 volume with
2.57x2.57x2.4 voxel size
My MR data is
256x256x50 volume with
1.05x1.05x1.3 voxel size

1. Expected results

With standard visualization tools, I can evaluate the
Z translation to 17 PET slices towards patient feet,
meaning 40.8 mm
 
2. The registration result I get with datasets oriented
Feet to Head along the z axis is

0.999897 0.00950767 -0.0107511 
-0.00960494 0.999913 -0.00903231 
0.0106643 0.00913464 0.999901 

[as expected]

Overall transform offset: 
34.2316  26.2821  -2.60786  

[where I think units are MRI voxels as I used MRI
as fixed and PET as moving image. yhat would mean 
3 mm z translation which is not what I expected,
even if I am wrong about units and we are in PET
voxels units, it is still far from the reality]

3. The registration result I get with datasets oriented
Head to Feet along the z axis is

Overall transform matrix: 
0.999868 -0.0144369 0.00750628 
0.0144862 0.999874 -0.00655476 
-0.0074107 0.00666263 0.99995 

Overall transform offset: 
36.6828  31.1387  86.6143 

[which is not better]

4 Question:

I am apparently confused by spatial references and units
of the output matrix because resamples output images are 
well registered.
I feel it is probably related to the preconditioning of
the images, resampling, centering and to the fact that both
volumes have different heights but I was unable to find the
solution. 
Could somebody help me with that problem ?

Thanks you very much

David


---------------------------------------------- 
David Wikler, Ir
ULB - Erasme Hospital - PET Scan
808 route de Lennik - B1070 Brussels - Belgium
Phone: 32 2 5556603 - Fax: 32 25556631
Email: dwikler@ulb.ac.be