[Insight-users] 3D Registration with different spacings

[Wolf, Levin]

Hi everybody,

i´m working on an 3D multisresolution Mutual Information registration, having somestrange effects on the output images. 

using:
itk::MattesMutualInformationImageToImageMetric
itk::VersorRigid3DTransform
itk::VersorRigid3DTransformOptimizer
itk::CenteredTransformInitializer

When i use images with different spacings, the output image is shown in the spacing and resolution of the reference image.
But....my transform parameters seem to be effected by the spacing to.
 

Image A:
CT
resolution: 512x512x181
spacing: 1.17187 ; 1.17187 ; 2


Image B:
PET
resolution: 288x288x181
spacing: 2 ; 2 ; 2
manually shifted by 20mm in X, Y and Z


Registration results in:

Transform Center = [299.058, 300.39, 183.485]
 Versor X = 0.00375238
 Versor Y = 0.00714057
 Versor Z = -0.00055464
 Translation X = -33.267
 Translation Y = -33.5096
 Translation Z = -19.7385
 Iterations    = 60
 Metric value  = -0.223228
 Matrix  = 
0.999897 0.00116283 0.0142765
-0.00105565 0.999971 -0.00751243
-0.0142848 0.00749659 0.99987

Offset = 
[-36.2052, -31.8068, -17.6945]


The Factor between the X- and Y-spacings is (2/1.17187)=1.707

Dividing the Translation in X- and Z-Direction by this factor it fits quite good to my expected results.

 Translation X = -33.267/1.707=19.49
 Translation Y = -33.5096/1.707=19.63
 Translation Z = -19.7385


coincidence ???????  I think there is something wrong with the CenteredTransform...is there is something special to consider?

If anyone have any ideas....


Thanks a lot

Levin