[Rtk-users] Reconstruction artefact with SART

[vincent]

Hi Simon,

well the step size marginally improved the result.  But using 
CudaVoxelBased as the BP rather than CudaRayCast completely solved it !

Thank you for your time and advice,

best regards,

Vincent

On 27.10.19 21:42, Simon Rit wrote:
> Hi Vincent,
> I'm not sure why the step size would make such a large difference but 
> that's an interesting lead indeed. Feel free to add the step size as 
> an option, I hope this will indeed solve your problem!
> Best regards,
> Simon
>
> On Fri, Oct 25, 2019 at 11:55 AM vincent <vl at xris.eu 
> <mailto:vl at xris.eu>> wrote:
>
>     Hi Simon,
>
>     thank you very much for the suggestions.  I think I have a lead
>     and I was wondering if you could give me your opinion before I
>     roll up my sleeves and dive into the code.
>
>     Since the issue is number-of-voxels-dependent, and because I use
>     CudaRayCast both for forward and backward projections, my guess
>     was that it has to do with  the forward projection operator, and
>     more specifically with the step size along the ray;  while it is
>     an option in rtkforwardprojections, it is not customizable in
>     rtksart. To investigate further, I reprojected the reconstructed
>     FDK volume (700x700x700 voxels, voxel size = 0.23 mm) 3 times,
>     once with Joseph, once with CudaRayCast and the default step (1)
>     and one with a custom step (0.1).  The vertical streaks appear as
>     I expected in the second case but not in the others:
>
>     https://ibb.co/ZX57FPM
>     https://ibb.co/MnMk5Ny
>     https://ibb.co/gzfPQG1
>
>     So I think that adding the step size option in rtksart should get
>     rid of my problem, but I would like your feedback before, as I
>     trust your experience more than my intuition :) !
>
>     Thanks again,
>
>     Vincent
>
>     On 23.10.19 22:30, Simon Rit wrote:
>>     Hi Vincent,
>>     There is a problem indeed! It's hard to guess what's the problem
>>     from the images you sent. I would check the projections to see
>>     what could be wrong. The first thing I would check for example is
>>     whether air is really at 0 (as it should be). You can also look
>>     at the difference between the forward projections of this first
>>     iterate and the measured projections. It's known that SART should
>>     be stopped at a few iterations but 1 should not give such a
>>     result. You can always check if the conjugate gradient provides a
>>     better result, you can add some regularization in it.
>>     Good luck solving this!
>>     Simon
>>
>>
>>     On Wed, Oct 23, 2019 at 3:03 PM vincent <vl at xris.eu
>>     <mailto:vl at xris.eu>> wrote:
>>
>>         Hi everyone,
>>
>>         I am trying to reconstruct a volume representing an aluminum
>>         casted part
>>         and investigate the differences between FDK and SART.
>>
>>         Here are the parameters involved: the images are 700*820
>>         pixels and I
>>         have 900 projections.  The physical pixel size is 0.28 mm and
>>         the sdd
>>         and sid are 558.3 mm and 461.3 mm respectively, which gives me a
>>         magnification factor of 1.2 roughly.  I performed one
>>         iteration of SART
>>         with CudaRayCast as forward and backward projectors and I
>>         wanted to
>>         reconstruct a 700*700*700 volume with a 0.23 mm (= 0.28/1.2)
>>         voxel
>>         size.  The result is shown on the two pictures attached:
>>
>>         https://ibb.co/8sLdJxw
>>         https://ibb.co/N3NLg2D
>>
>>         The result is getting worse as I increase the number of
>>         iterations
>>
>>         I have a hard time understanding where the stripes come from.  A
>>         reconstruction with the standard 256*256*256 volume and 1mm
>>         spacing
>>         gives a "smoother" result, closer to what FDK gives. I
>>         thought it might
>>         be related to the linear system resolution, but unless I am
>>         mistaken, I
>>         am trying to solve a system with 700*700*700 unknown with
>>         700*820*900
>>         equations, which should be ok.
>>
>>         Furthermore, the reconstruction with FDK at the same "full"
>>         resolution
>>         gives satisfactory results.
>>
>>         Any help/explanation will be greatly appreciated !
>>
>>         I thank you very much in advance,
>>
>>         Vincent
>>
>>
>>
>>
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