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Hi Vikash -<br>
<br>
<div class="moz-cite-prefix">On 09/13/2012 01:17 PM, Vikash Gupta
wrote:<br>
</div>
<blockquote
cite="mid:CALSm4EkC+hOq-Sb9rbmiD+Gb-8Dfv907VPiDxpSs9ZoRmXimyg@mail.gmail.com"
type="cite"><br>
<div>Suppose I have a point in the Physical Space of the moving
image and I want to find the corresponding point in the fixed
image, given that I have the transformation, is it so that if I
apply the transformation to this point in the moving image I
will get the corresponding point in the fixed image, and the
corresponding voxel ?</div>
</blockquote>
<br>
No. <br>
<br>
You apply the transformation to a point in the fixed image and get
the corresponding point in the moving image.<br>
<br>
Now, whether those points are in physical space (thus respecting
image origin and direction vectors) or some kind of normalized image
space (with zero origin and identity matrix for orientation) is
something I am not sure about, so I am going to leave that for
someone else to answer. My suspicion is that it is indeed physical
space coordinates.<br>
<br>
<blockquote
cite="mid:CALSm4EkC+hOq-Sb9rbmiD+Gb-8Dfv907VPiDxpSs9ZoRmXimyg@mail.gmail.com"
type="cite">
<div><br>
</div>
<div>Here is an example,</div>
<div><br>
</div>
<div>F_origin=[-120.0000 78.6926 -15.5144] (output from
fixedImage->GetOrigin())</div>
<div>M_Origin=[-67.9766, 80.7186, -128.159]
movingImage->GetOrigin()</div>
<div><br>
</div>
<div>Now I believe these two points in the physical space also
correspond to the respective voxels with Index =(0 0 0)</div>
</blockquote>
<br>
I would agree with that.<br>
<br>
<blockquote
cite="mid:CALSm4EkC+hOq-Sb9rbmiD+Gb-8Dfv907VPiDxpSs9ZoRmXimyg@mail.gmail.com"
type="cite">
<div><br>
</div>
<div>And on registration this is the transformation matrix I get</div>
<div><br>
</div>
<div>#Insight Transform File V1.0</div>
<div># Transform 0</div>
<div>Transform: AffineTransform_double_3_3</div>
<div>Parameters: 0.999985 0.00438959 -0.00335042 -0.00439659
0.999988 -0.0020834 0.00334123 0.0020981 0.999992 0.187256
0.905001 -0.172994</div>
<div>FixedParameters: 0 0 0</div>
<div><br>
</div>
<div>So, if you notice there is a huge gap in the origins of the
two images which are not reflected in the translation (last 3
terms of parameters) parameters of the transformation matrix. <br>
</div>
</blockquote>
<br>
Well, the first question here is - how do you initialize the affine
registration? If you simply use the identity transformation, then
you start with a zero translation, and it is possible that that gets
you stuck in an unreasonable local optimum of the registration cost
function. You could try using something like a <a class="el"
href="http://www.itk.org/Doxygen42/html/classitk_1_1CenteredTransformInitializer.html"
title="CenteredTransformInitializer is a helper class intended to
initialize the center of rotation and the ...">CenteredTransformInitializer</a>
object to initialize your transformation.<br>
<br>
Ultimately, though, you want to use ITK to reformat the moving image
to the fixed image grid according to your transformation and see
whether that gives you a reformatted moving image matching the fixed
image. If yes, your transformation is okay and then you can try to
interpret it. If no, you have a problem computing the transformation
in the first place, and you need to fix that first.<br>
<br>
<blockquote
cite="mid:CALSm4EkC+hOq-Sb9rbmiD+Gb-8Dfv907VPiDxpSs9ZoRmXimyg@mail.gmail.com"
type="cite">
<div><br>
</div>
<div>So, how do i find the voxel correspondences between the two
images. Could it be as simple as apply the transformation to the
points in individual voxels of the moving image and calculate
the correspong points in the physical space of the Fixed Image.
??</div>
</blockquote>
<br>
Again - transformation is applied to points in the FIXED image,
resulting in points in the MOVING image. I know this is
counter-intuitive, but because you "pull back" the image information
into the fixed grid when you reformat, images transform in the
opposite direction as do, for example, point sets or other
geometries.<br>
<br>
Hope this helps.<br>
<br>
TR<br>
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