[Insight-users] Simple problem: assign world coordinates to spatial objects

[Patrik Brynolfsson]

Hello,

I've been trying to figure this out but I can't seem to get it right, but I
assume this is a very simple problem to solve. I am trying to evaluate noise
in CT images using a phantom for which I know the dimensions and positions
of 20 spheres of different sizes. My idea was to construct a group of
spatial objects (EllipseSpatialObject) and connect it to a high dose
reference image which I can use to register other images to, and then apply
the transform to the spatial objects to find their positions i the new
image.

First I create the EllipseSpatialObjects and connect them to a
GroupSpatialObject so that they have the correct positions to each other. I
also know the world coordinates of the spheres in the reference image, but I
wonder how I can place the GroupSpatialObject to match the spheres in the
image. I have a couple of different reference image series, and when I apply
the SpatialObjectToImageFilter and put in the reference image's direction,
spacing and origin I get very different results for different images, which
really confuses me. So, my two questions are:


   1. How do I assign world coordinates to the spatial objects?
   2. What is the difference between SceneSpatialObject and
   GroupSpatialObject? Which one do you recommend for this problem?

Thanks in advance!
---
Patrik Brynolfsson
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