Generalize a 2D BSplines transform to 3D

[Patrik Brynolfsson]

Hello, 

I am trying to generalize 2D transforms to the 3D domain. The linear transforms are quite straight forward except for the similarity transform, which has one scale factor for all dimensions. (For generalizing a 2D similarity transform to 3D I would like the scale factor to be 1 in the 3rd dimension, but that's beside the point)

I am trying to create a 3D BSplines transform from a 2D BSplines registration, and I cannot get it to work. My idea was to add a third dimension in the BSplines array, and let it be zeros. However, when I apply this transform to a 3D version of the 2D image, it returns an unaltered version of the moving image. 

Is there anyone who has an insight in what I should do? 

Thanks in advance!

//Patrik Brynolfsson

[m.st...@lumc.nl]

Dear Patrick,

Three comments come to mind:

- the parameters are ordered as follows: first all x displacements, then all y, then all z

- You would need to copy the above a number of times, to impose this on all the slices of the 3D volume. Otherwise only this first "slice" is deformed which is just outside the image itself, and you would indeed see no deformations. The number of times is equal to the number of slices not in the image but in the B-spline grid

- you need to define the 3D B-spline grid as well, for which I recommend looking into the code how we normally do it: this grid extends in domain just outside the fixed image domain (it is a bit larger), I believe one control point to the left and two to the right.

HTH, Marius

---

From: elastix-imag...@googlegroups.com <elastix-imag...@googlegroups.com> on behalf of Patrik Brynolfsson <patrik.br...@gmail.com>
Sent: 12 August 2022 1:54 PM
To: elastix-imageregistration
Subject: [elastix-imageregistration] Generalize a 2D BSplines transform to 3D

 

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