[Insight-users] Image Registration

[Torsten Rohlfing]

On 09/13/2012 03:27 PM, Vikash Gupta wrote:
> Hi Torsten,
> Thanks once again for your patient email. Yes so I resampled the 
> moving image to the fixed image space, with the output transformation 
> matrix (my output transformation matrix is inverted) and I get the 
> image in the expected co-ordinates of the fixed image.
> So, I will apply this transformation (or the inverse) to the points in 
> physical spaces of the images in order to take one to other or the way 
> around, as you suggested.
>
> One last thing that startles me as i mention before is the gap between 
> the translation parameters, which intutively should be more, but I 
> didn't write the registration routine so probably that is an issue I 
> should find out by looking at the registration routine, how things are 
> implemented there.

Well, one explanation for the near-zero translation despite very 
different origins could be if the images had very different fields of view.

If you simply add a lot of background around one of the images without 
changing the physical coordinates of the pixels that are already there, 
then you will need to adjust the image origin accordingly, but the 
translation vector would not change at all.

The point here is, that the first pixel in your image (which is the 
origin pixel) rarely has any meaningful anatomy information, so where it 
maps matters little to nothing. It is usually more interesting to see 
where the CENTER pixel maps; I suspect in your images, the center of 
each is near the zero physical coordinate. So if you have your object of 
interest (e.g., patient) centered in your images, and the center is also 
the zero coordinate, then there would be little translation needed to 
align them. Yet, if the images have different fields of view, your 
origins would be very different.

And you're very welcome :)

TR
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