[Insight-users] Error in Registration results

[Emma Ryan]

Hi Luis,

  Thank you for your reply. Well, the algorithm takes very long. It stops only because it reaches the 500th iteration (the limit). Yes, both are CT images.

I guess the lungs collapse because I do the registration in one step, instead of using Rigid registration as a precursor.

My dataset is as such :

Set 1 : 512 x 512 x 30 (resolution 0.68 x 0.68 x 1.0)
Set 2: 512 x 512 x 30 (resolution 1.0 x 1.0 x 1.0)

The ITK example file has a default spacing of  (1, 1, 1) I guess and I had used this earlier, hence the result I guess. 

I have since changed it. But the results are not good. The output image does not have default values in the middle, but the difference images are highly speckled. I am not sure why this has happened. I have yet to run enough tests to make sur ethat I do not get any default values in the center slices.

Also, doesn't ITK automatically get the spacing information from the dicom files ? 
I wrote a program to read dicom series, generate a .vtk file and rewrite teh dicom series.

While the output images of this dicom conversion match the input, the .vtk file has wrong spacing information (1, 1, 1) instead of (0.68, 0.68, 1). Hence I am also not able to view it in Volview.

What happens to the spacing information, is a mystery.

Any tips how I can solve this ?

Emma


----- Original Message ----
From: Luis Ibanez <luis.ibanez at kitware.com>
To: Emma Ryan <eryanvtk at yahoo.com>
Cc: insight-users at itk.org
Sent: Thursday, July 5, 2007 1:41:13 PM
Subject: Re: [Insight-users] Error in Registration results


Hi Emma,



        mm... it is hard to explain indeed.


Your intuition is correct regarding the fact that the
default pixel value "shouldn't" appear in the middle slices.

That being said, it is not "impossible" for the value to
appear there.  The fact that it is appearing, is an indication
that one of the nodes in the BSplineGrid has a deformation so
large that it sends the point outside of the image physical
extent. In other words, the registration failed largely.

The extreme deformation that the two sliced display are also
a bad indication of the results of the registration. For
example, the first slice in your email has the lungs collapsed
towards the medial plane.

You should probably rerun the registration by using different
parameters for the optimizer. In particular you probably should
set the Bound values for the parameters of the Transform.


BTW:

1) How many iterations did the algorithm *actually* ran ?

2) Where the two images of the same modality ?

    The metric used in this example is MeanSquares,
    so it pretty much assumes that the images are of
    the same modality and with a very close distribution
    of intensity values.



  Regards,



      Luis



-----------------
Emma Ryan wrote:
> Hi,
> 
>   I am trying to register 2 volumes.  I use ITK example - 
> DeformableRegistration7.cxx to do so. The default pixel values is 100. 
> So any empty space is filled with this grey value.  So it is 
> understandable if the first  slice shows this grey region since they are 
> tyring to align along the 'Z' direction. Also, if this grey value 
> appears along the boundary of the images, it makes sense.
> 
>   But how does one explain the appearance of the default grey value 
> (100) in the center slices.  I have attached some images and labelled 
> them with thier corresponding slice number.
> 
>   I thought the problem was with resampling, but that part of the code 
> seems correct, besides I have used the example cpp file as is.
> 
>  I see some of these 'artifacts' in the center slices as well.
> Any clue ?
> 
> Emma
> 
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