[Insight-users] Registration - Mutual Information Metric - Sampling from Moving Image

[Luis Ibanez]

HI Sharath,

1) What value of region are you passing to

 registration->SetFixedImageRegion( region )    ?

   The Metric only uses the pixels inside that region.

   So, depending on the region that you are passing,
   you may not be using the 2000x2000 pixels of
   the full image.

2)   Why do you have to use a big region in the Fixed image ?

3) What Transform are you using ?
    A scaling of 0.5 is unusual, and may indicate that
    the spacing declared by your images is wrong.

4)  What is the content of the images ?
      are these medical images ?


5) Sure, you can sample from the Moving image,
    You simply need to swap  the Fixed and Moving
    images and you should be ready to rerun the
    registration.

6) What "functions", in what "respective" files are
    you referring to ?

   If you want to sample from the Moving Image, then
   In principle you just need to replace:

        registration->SetFixedImage( imageA );
        registration->SetMovingImage( imageB );
        registration->SetFixedImageRegion( regionA );

with:

        registration->SetFixedImage( imageB );
        registration->SetMovingImage( imageA );
        registration->SetFixedImageRegion( regionB );



   Regards,


        Luis


----------------------------------------------------------------------------
On Sat, Jul 11, 2009 at 12:48 PM, Sharath Venkatesha <sharath20284 at yahoo.com
> wrote:

> Hi Luis,
>
> I am already using the
>
>  registration->SetFixedImageRegion( region )
>
> My scaling parameters are around 0.6 and rotation of 6-7 degrees, I have to
> use a big region  in the fixed image (I am initialiazing with scale =0.5,
> and hence using a region of size 1280*960).  The performance has not
> improved much.
>
> Hence I want to *sample pixels from the moving image,* and do a *backward
> mapping to pixels in the fixed image*( by the inverse transform
> parameters, that are estimated in each iteration of the optimizer). In this
> case, I think that almost all the points will be mapped, and hence the
> metric will be better. Also, it will take care of the scale and translation,
> as in the previous case, a wrong initial scale will make me miss out the the
> pixel values in the fixed image.
>
>
> Do I need to change the functions in the respective files, or is there a
> simpler way to do this, or is already implemented?
>
> Thanks,
>
> Sharath Venkatesha
>
> ------------------------------
> *From:* Luis Ibanez <luis.ibanez at kitware.com>
> *To:* Sharath Venkatesha <sharath20284 at yahoo.com>
> *Cc:* Insight users <insight-users at itk.org>
> *Sent:* Saturday, July 11, 2009 8:49:51 AM
> *Subject:* Re: [Insight-users] Registration - Mutual Information Metric -
> Sampling from Moving Image
>
>
> Hi Sharath,
>
> You are right in that it is not a good idea to take samples
> from the entire 2000 x 2000 pixels image if we expect that
> only a regions of 640 x 480 is going to match the Moving
> image.
>
> The solution is to simply call
>
>             registration->SetFixedImageRegion( region )
>
>  and set "region" to the region of interest from the
> 2000 x 2000 image, where you anticipate to find matches
> for the moving image.
>
>
> You will find examples on the use of
>
>        SetFixedImageRegion()
>
> in the ITK software Guide
>
>        http://www.itk.org/ItkSoftwareGuide.pdf
>
> and most (if not all) the corresponding Image Registration
> examples in
>
>             Insight/Examples/Registration
>
>
>
>
>    Regards,
>
>
>          Luis
>
>
>
> ---------------------------------------------------------------
> On Fri, Jul 10, 2009 at 9:15 PM, Sharath Venkatesha <
> sharath20284 at yahoo.com> wrote:
>
>>
>> Hi,
>>
>> I am working with images where the Fixed image is 2000*2000 pixels and the
>> moving image is 640*480 (sub region of the fixed image). In all the Mutual
>> Information metrics, the sampling is done on the Fixed image, and the
>> transformed points are mapped on the moving image.
>>
>> I figured out that it is incorrect to sample the points randomly in the
>> full fixed image ( as in my case, the moving image is a sub-part of the
>> fixed image, and only 30-40% points were mapped), I used an approximate
>> region of the fixed image from which sampling should be done. This has not
>> improved the performance/accuracy of registration.
>>
>> I am thinking of sampling pixels from the moving image, and do a backward
>> mapping to pixels in the fixed image. In this case, I think that almost all
>> the points will be mapped, and hence the metric will be better.
>>
>> Do I need to change the functions in the respective files, or is there a
>> simpler way to do this, or is already implemented?
>>
>> Thanks,
>>
>> Sharath Venkatesha
>>
>>
>>
>>
>>
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>
>
>
>
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>
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