[Insight-users] Questions about apodization in the process of registration

[Jay Li]

Dear Luis,
 
Thank you for your reply. Sorry that I was not able to feed back promptly. I didn't express the question clearly in my previous email.
 
During the registration process, the metric will be calculated based on the overlap of the fixed and moving image for each iteration. Since the area of the overlap may change for each iteration, the number of total pixels counted as the overlap may change, so the value of metric may have discontinuity that might cause the failure in finding the global extreme.
 
To avoid above, different weights could be given to pixels or voxels within the overlap. For example, when calculating the metric, pixels near the boundary are given lower weights, but those ones near the center are given higher weights. I don't know if ITK has done this kind of stuff. Thanks. 
 
Regards,
 
Jay
 
 

Luis Ibanez <luis.ibanez at kitware.com> wrote:

Hi Jay,

By apodization do you refer to the attenuation of
the high frequencies in the spectral domain of the
images ?

I assume that you are concerned about this due to the
resampling of the images during the registration process
as well as after the transform has been extracted.

If that's the case, you don't really need to be too
concerned. It practice this is rarely a problem, unless
your images are actually populated with white noise,
or at least blue noise, which are the type of spectra
having large components in the high frequency range and
therefore requiring attenuation in order to avoid aliasing
during the interpolation used for resampling.

For typical medical images, the high frequency components
are very low, and you are usually safe by using simple
interpolations. That is, the Fourier transform of the
shape functions used by typical interpolation are already
attenuatting high frequencies.

You may want to look at the documentation of the
SincWindowed interpolator where some of this issues
are discussed.


>From the practical point of view your best options
for avoiding local minima are :

- Multi-resolution
- Good initialization



Please let us know if you have further questions.



Regards,



Luis



-----------------
Jay Li wrote:

> Dear all,
> 
> I'm new to ITK. Could anyone give me a hint if ITK has
> taken care of the apodization related problems in the
> process of image registration. I looked through the
> manual and searched the online archives, but couldn't
> find anything related. Will there be any problems to
> find the global extreme without the apodization?
> Thanks in advance.
> 
> Jay
> 
> 
> 




		
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