[Insight-users] Registration and Spacing : Bug

[Luis Ibanez]

Hi Stephan,

According to the tests we ran with Carolyn, using unit spacing
will  be the easiest way to get around the problem for now.

However, the registration framework is intended to work with
non-unit spacing. Most medical images have anisotropic spacing,
it is then quite important to take the spacing into consideration.

In other words, this is something that we have to track down
and fix.

Having images with equal spacing also set you in an easiest
case. However, most multi-modality registration problems will
bring different spacings in both modalities. That's part of the
reason for performing the registration in world coordinates.

It would also be nice to setup the registration parameters in
a way that is independent from the image characteristics.
This is relatively easy when pure Translation transforms are
used, but is less trivial with transforms whose parameters
mix angles, translations and scales.


   Luis


-------------------------
Stephan Moser wrote:
> Hi Luis
> 
> Thanks for your effort. Michael and I have studied the code today as
> well and we are not sure whether the GradientFilter has its influence on
> the problem, since at the moment it does not take into consideration the
> spacing. We assume that the derivative is therefore calculated for a
> unit spacing.
> 
> Using 40*40*40 sized images in one case with spacing 1.0
> and in a second case with 2.0, we have not been able to scale the
> registration process correctly so that it would yield the same result;
> we would like to do this to obtain 'generally applicable' parameters for
> registration of bone images.  Do you think it might be a good idea to
> set the spacing of the images to 1.0 and always perform the registration
> that way? We have the impression that registration can be done without
> considering world coordinates, as long as the images are spaced the
> same, or is there a fundamental mistake in our thought?
> 
> Thanks, Stephan
> 
> 
> On Tue, 22 Jul 2003, Luis Ibanez wrote:
> 
> 
>>Hi Michael, Carolyn
>>
>>We were finally able to duplicate the problem you both
>>reported concerning the registration of image data with
>>non-unit spacing.
>>
>>http://www . itk . org/pipermail/insight-users/2003-July/004215 . html
>>http://www . itk . org/pipermail/insight-users/2003-July/004314 . html
>>
>>Using a simple registration case with translation transform,
>>we found that a very small spacing causes the optimizer to
>>become erratic.
>>
>>We are tracking this problem in order to find a fix.
>>
>>Our guess is that the spacing is not correctly considered
>>when computing the metric derivatives in the registration
>>method. A bug report has been filed in GNATS.
>>
>>
>>  Thanks for pointing this out.
>>
>>
>>     Luis
>>
>>
>>
>>
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>>
> 
>