[Rtk-users] Reconstruction artefact with SART

[vincent]

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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