[Insight-developers] Problems with affine MI registration

[Lydia Ng]

Paul, Bill:

How many iterations are you running?

In our experiments we have found that translation converges
quite quickly but a large number of iterations (10-20k) is
required for the rotation and scaling to converge.

A multi-resolution approach is needed to increase the 
translation capture range.
We found that multi-resolution made little difference
to the capture range for rotation but the downsampling of the
images helps with robustness.

Conservatively, I would say the capture range for rotation
is about 5-10 degrees. Higher if the images are the same
modality.

As discussed at various times at the tcon, 
there are also several parameters that can be tweaked:

- Parzen window width
Setting it to approx 0.3 seems to work if you normalized
the image intensity to mean of 0 and variance of 1

- Scaling between translation and affine matrix/rotation parameters
Values of order of 50-100 should work for 256x256 images.
Since small changes in rotation/scaling can have a large effect
we can only make small incremental changes at each iteration
Hence the large number of iterations required.

- Rotation/Scaling origin
Where the origin is set makes a big difference to the convergence.
Setting it to the center of mass or of the image should help.

Currently this is achieved by setting the origin of the PhysicalImage.
Alternatively, we could set this in the Transformation object?
Comments?

If none of the above helps, I could have a try if put your
data on a web/ftp site somewhere.

On the multiresolution front, most of the base
classes for the multiresolution framework is in the
repository. But there are still a couple of issues to sort
out.

Lydia



> -----Original Message-----
> From: insight-developers-admin@public.kitware.com
> [mailto:insight-developers-admin@public.kitware.com]On Behalf Of
> Lorensen, William E (CRD)
> Sent: Thursday, June 21, 2001 4:56 AM
> To: 'Paul Hughett'; insight-developers@public.kitware.com
> Subject: RE: [Insight-developers] Problems with affine MI registration
> 
> 
> I am also having some problems with the registration. I'm 
> using the Rigid/mi/gradient descent. I can
> successfully register translated volumes, but cannot if they 
> require a rotation.
> 
> Bill
> 
> -----Original Message-----
> From: Paul Hughett [mailto:hughett@mercur.uphs.upenn.edu]
> Sent: Wednesday, June 20, 2001 3:44 PM
> To: insight-developers@public.kitware.com
> Subject: [Insight-developers] Problems with affine MI registration
> 
> 
> 
> I'm gearing up to try the ITK image registration algorithms on real
> data.  I took the test program for affine/mutual information/gradient
> descent registration from the repository and modified it to try some
> variations in the actual transformation and optimization parameters.
> Discovered that the algorithm seems to work reasonably well for
> translations, as long as they aren't too large, but I can't get it to
> work at all for either isotropic or anisotropic scaling; an expansion
> of even a few percent causes the algorithm to fail.  Neither
> increasing the learning rate nor increasing the number of samples up
> to 500 seems to help.
> 
> Any suggestions?
> 
> Paul Hughett
> 
> 
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