[Insight-users] Direction of the transform in the registration framework

[Anja Ende]

Hello,

I think I now see how this would work. Had missed that the resampler
would iterate through all the voxels in the fixed image grid and
assign the transformed them intensity values interpolated from the
moving image. Now, I can see how this would result in a translation in
the opposite direction after the resampling.

However, one last question in the same vein. The guide says:
"If we have used the Transform that maps coordinates from the moving
image physical space into the fixed image physical space, then the
resampling process could not guarantee that every pixel in the grid of
the fixed image was going to receive one and only one value. In other
words, the resampling will have resulted in an image with holes and
with redundant or overlapped pixel values."

Is this true for even simple transforms as I mentioned before. I am
having difficulty seeing how this would be the case for affine or
rigid transforms. Here every voxel sould map to a unique position in
space, right? Can someone give an example or explain when this might
occur?

Thanks,

Anja

On 24 March 2010 23:39, Anja Ende <anja.ende at googlemail.com> wrote:
> Hello,
>
> This is discussed in the software guide but I wam having a bit of
> trouble understanding why the transform that is optimized is the one
> that takes you from points fixed image to points in moving imge.
>
> The guide says:
>
> "What tends to create confusion is the fact that when the Transform
> shifts a point on the positive X direction, the visual effect of this
> mapping, once the moving image is resampled, is equivalent to manually
> shifting the moving image along the negative X direction".
>
> I am struggling to see how this is. Say as an example,the optimized
> transformation from fixed to moving image is a simple translation by 5
> mm. So, my deformation field is where every point is shifted by 5 mm.
> How does the resampler use this information and how is equivalant to
> shifting the moving image by -5 mm? If someone could help me
> understand what happens say at a chosen point..it would help me a lot.
> I think I am not understanding how the resampler would effect the
> moving image given that the transformation is:
>
> 1 0 0 5
> 0 1 0 0
> 0 0 1 0
> 0 0 0 1
>
> I would be really grateful if someone could illustrate this to me...
>
> Many thanks!
>
> Cheers,
>
> Anja
>



-- 
Cheers,

Anja