[Rtk-users] Reconstruction artefact with SART

[Simon Rit]

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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