[Rtk-users] R: Half Fan dataset

[gabriele.belotti.bergamo at gmail.com]

Dear Simon and RTK users,

I’ve been experimenting on the generation of Half Fan CBCT images successfully from reprojections of CTs starting from Simon’s suggestions.
So far I was able to reconstruct images by displacing the detector in the X direction (+ or -) and completing a single rotation. Results were good and the FOV was of course larger than the one obtained from using the same virtual detector without displacement.

I’ve taken the simulation a step further and I’m currently creating a geometry which is similar to the combination of “rtksimulatedgeometry -n 180 --proj_iso_x <displacement> -o g_1” and “rtksimulatedgeometry -n 180 --proj_iso_x <(-1)*displacement> -o g_2 -f 180” (I’m rotating first between 0° and 180° while displacing by half detector size on +X and then 180° and 360° while displacing by half detector size on -X).
With this single .xml I’m reprojecting a CT into a single .mha using rtkforwardprojections and then I’m using the output as input for rtkfdk.

My results however suffer from a centered artifact, of semi-cylindrical shape, in my opinion caused by the superimposition of rays from the two beams around the isocenter.
This is further supported by the fact that the more I displace the detector the smaller the artefact becomes (of course I can’t displace more than 50% of detector size).
I guess a possible solution would be to have a perfect half-cone x-ray beam by shaping it using a collimator, but I’m not sure how to proceed on this in the simulated environment.
Have you got any suggestions or observation on how to achieve a reconstruction based on this? (two rotations/acquistion given two opposite detector displacements)

Thanks in advance,
Gabriele

 

 

 

Da: Simon Rit <simon.rit at creatis.insa-lyon.fr> 
Inviato: venerdì 11 ottobre 2019 13.10
A: gabriele.belotti.bergamo at gmail.com
Cc: rtk-users <rtk-users at public.kitware.com>
Oggetto: Re: [Rtk-users] Half Fan dataset

 

Hi,

It's easy to generate, you need to offset your detector, either via the RTK geometry or by setting the first coordinate of the origin of your projection to something which makes the projection uncentered. For example, in the geometry :

rtksimulatedgeometry -n 180 --proj_iso_x 100 -o g

rtkprojectshepploganphantom -g g -o proj.mha

rtkfdk -p . -g g -r proj.mha -o fdk.mha

You can simulate from a CT image by following  <http://wiki.openrtk.org/index.php/RTK/Scripts/ForwardProjection> this example.

Simon

 

On Fri, Oct 11, 2019 at 9:58 AM < <mailto:gabriele.belotti.bergamo at gmail.com> gabriele.belotti.bergamo at gmail.com> wrote:

Dear RTK users and developers,

I’m currently experimenting with FDK reconstruction and I’m struggling to find a Half-Fan projection dataset to fiddle around.. Do you know where I can find one? I’ve taken into consideration generating a set of DRRs from an existing phantom. Any help or advice you can give me would be greatly appreciated, thanks!

Gabriele Belotti

 

 

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