[Rtk-users] FirstReconstruction example

[Simon Rit]

Hi Lars,
The geometry is described in the document geometry.tex, I have added
links at the end of the user's wiki page
http://wiki.openrtk.org/index.php/RTK/Users. Briefly, the point with
coordinates 0,0,0 of your volume is the isocenter, the point with
coordinate 0,0 in projection images is the intersection of your flat
panel with the line going through the x-ray source and the isocenter.
The image origin is defined by ITK as the center of the pixel with
index 0 in memory (first pixel in memory).
I hope this helps,
Simon

On Mon, Oct 15, 2012 at 7:53 PM, Lars Friedrich Lars
<lars-friedrich at gmx.net> wrote:
> Hello,
>
> I have questions concerning the FirstReconstruction example:
> Using the rtk::RayEllipsoidIntersectionImageFilter and rtk::ConstantImageSource artificial projections of an ellipsoid are generated which are packed in the form of an image stack. The geometry of the projections is described by an rtk::ThreeDCircularProjectionGeometry object. Moreover, the images' metrics are described by the image stack itself (spacing, size and ORIGIN).
>
> How should I interpret the origin? As far as I could find out by minor modifications of the example, the origin of the projeciton image stack plays a role (if I change it to 0,0,0, and retain constantImageSource2's origin at -127,-127,-127, just half of the ellipsoid is reconstructed). Why does the origin of the image stack matter? Isn't the origin of a single projection fully defined by the corresponding entry in the rtk::ThreeDCircularProjectionGeometry object (I thought the projOffsetX,projOffsetY args define the offset from the detector central axis crossing point)? Am I right that the effective origin of a single projection image emerges basically from the detector (x/y of origin of image stack) and from consecutive application of the projOffsetX,projOffsetY args? Otherwise, deviating detector positions, e.g. caused by gravity, wouldn't be simulatable.
>
> Morever, which point exactly does the image origin (x/y components) of the projection stack describe? Is it the outer corner of the projection or the center of the first voxel (as in DICOM). The size of a projection in the example is 256.0x256.0 units. However, the stack's origin is at -127.0,-127.0. If the origin should refer to the center of the first projection pixel, it should be essentially -127.5,-127.5, right?
>
> Sorry for the detailed questions, I just would like to understand how RTK is working.
>
> Thanks a lot!
>
> LF
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