[Rtk-users] Artefacts with GPU-based iterative reconstruction with RTK

[Simon Rit]

Hi Mathé,
Nice to hear that RTK is useful. I have CCed the RTK user mailing list,
you'll probably have faster answers with it in the future. Luckily this
time, I'm back from my annual leave today!
I bet the problem is the hard coded step length in the GPU ray casting.
There's been an update since v2.7.0 to fix this issue
https://github.com/RTKConsortium/RTK/commit/990ae96992529ca8f54abe4c1ab48e40950a8092
See
https://docs.openrtk.org/en/latest/documentation/docs/rtk_3_migration_guide.html#cuda-forward-ray-cast-step-size-default
.
Can you try setting the --step option to 0.08 to see if it helps? This
would be the new default of the new version.
I hope it helps,
Simon

PS: sorry about the --config, it's already been merged
https://github.com/RTKConsortium/RTK/commit/d0f15386cbc1c89376753469eea23debcdf78a3f.
The goal was to uniformize the Python and C++ applications. Will that be a
big issue for you? If yes, please open an issue to trigger a conversation.

On Mon, Aug 17, 2026 at 1:51 PM Mathé Zeegers <M.Zeegers at milabs.com> wrote:

> Dear Simon Rit and/or other RTK developers,
>
> I am writing you to ask for your suggestions as I am stuck with an
> RTK-related problem for a long time now. I am not sure if I have used the
> right e-mail address, so my apologies if I have not.
>
> Since more than a year, I am working for MILabs as the X-ray CT developer.
> For a long time, we have used RTK for GPU-based FDK reconstructions, and
> we have only used an SART in its CPU-based version. We are now looking into
> integrating (more) GPU-based iterative methods.
>
> I have implemented a GPU-based version of CGLS using the templates from
> the RTK github repository (version 2.7.0, commit eb3fd5b). I use this in
> combination with ITK 5.7.0 and ITKCudaCommon 2.1.0. The problem that keeps
> coming up is that my reconstructions do not look at all like what they are
> supposed to look like with the GPU-based reconstruction. I have attached an
> image to this mail to clarify this.
>
> In my reconstruction, I use 720 images with the default settings from the
> rtkprojectors_section.ggo file. The number of CGLS iterations is set to 30
> and the voxel size is set to 0.08.
> When I run CGLS with forward projector CudaRayCast and backprojector
> CudaRayCast, the result is very grainy and unsharp.
> When I change the backprojector to CudaVoxelBased, the result initially
> looks better with a very low number of iterations, but with more iterations
> the reconstruction volume rapidly becomes very stripy and the details
> disappear as well. As a reference, I also ran the CPU reconstruction and
> the results look totally fine in this case.
>
> I have tried to switch to the RTK implementation of SART/OSSART/SIRT, but
> I encounter the same problem there as well.
>
> To double check, I have also used the ASTRA and TIGRE GPU-reconstructions
> and these give correct results. Still, I would like to get this
> GPU-reconstruction to run with RTK since in my opinion it is a more
> comprehensive toolbox, and the GPU reconstruction is much faster than the
> CPU reconstruction.
>
> Do you know what the problem is here and how this can be resolved?
>
> Please let me know if something is unclear or if additional information is
> required.
>
> Thank you in advance.
>
> Kind regards,
> Mathé Zeegers
>
> PS: In the RTK v3 migration guide it is stated that the -config
> command-line option will be removed because it was not known to be used in
> practice. I would like to let you know that we actually use that feature
> very frequently.
>
>
>
> *Mathé Zeegers*
>
> Preclinical X-ray CT Specialist
>
> *m.zeegers at milabs.com* <m.zeegers at milabs.com>
>
>
>
> This message may contain confidential information and is intended
> exclusively for the addressee. If you receive this message unintentionally,
> please do not use the contents but notify the sender immediately by return
> e-mail. MILabs B.V. is a legal person by public law and is registered at
> the Chamber of Commerce for Midden-Nederland under no. 302.075.15.
>
>
>
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