[Rtk-users] Reconstruction artefact with SART

[Simon Rit]

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> 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> 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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