[Insight-users] Multiresolution registration shrink factors

[David Wikler]

Dear ITK Users,

I have been trying some registration with the MultiResMIRegistration
example and it did give good results. I choosed parameters according
to the provided example PracPETtoPD.txt file. However, I don't understand the
way multiresolution is handled in this parameter file.
Fixed image is high resolution
  256 256 26
  1.25 1.25 4.0
Moving image is low resolution
128 128 15
2.590723 2.590723 8.0

and multires levels and shrinking numbers are
4
2 2 1   (fixed image)
4 4 1   (moving image)

So, I understood from the docs that

first pass will register 
the moving image shrinked to 32x32x14 
to
the fixed image shrinked to 128x128x26

second pass will register 
the moving image shrinked to 64x64x14 
to
the fixed image shrinked to 256x256x26 (not shrinked anymore)

third pass will register 
the moving image shrinked to 128x128x14 (not shrinked anymore)
to
the fixed image shrinked to 256x256x26 (not shrinked anymore)

fourth pass will register 
the moving image shrinked to 128x64x14 (not shrinked anymore)
to
the fixed image shrinked to 256x256x26 (not shrinked anymore)

If I understood it right, I don't understand this scheme, I would
have choosed something like

4
8 8 1
4 4 1

which would have shrinked high and low resolution images to similar
resolution and would have ended by only one full resolution pass.

In the PracCTtoT1.txt, it is the other way around, so I am happier
but I still don't understand why pass 4 and 5 are at the same
resolution levels.

I could have missed something in the documentation though I read 
about it both in the Doxygen file, MultiResMIReg and SoftwareGuide 
PDF files

Could somebody help me ?

Thank you very much.

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