Hi,
These are numerical errors. How did you create the projections? It seems
that you have used the voxelized phantom to simulate the projections. This
enhances strongly these artefacts. This is why most people compute
simulated projections analytically with simple analytical shapes (as the
Shepp Logan phantom).
If you share your projections meta information, I can illustrate how to do
the simulation differently. If you already used analytical simulations, a
way to improve the quality is to increase the sampling (use more and finer
pixels in the projections).
If you want to reduce the artefacts from these projections without
modifying the projections sampling, you need to remove some high
frequencies with a proper windowing. You can try --hann 1 for example on
the rtkfdk command line.
I hope this helps,
Simon

On Wed, Feb 14, 2018 at 1:47 PM, <anais.capouil...@student.uliege.be> wrote:

> Hi,
>
> When I use FDK to reconstruct my volume, there are some parts inside the
> volume where the error is up to 10%, even in the middle of my volume. When
> I compute the difference between the phantom and the reconstructed object I
> can see some patterns of small lines where the error is way bigger than in
> the rest of the volume.
> I need to reconstruct my volume with at least one big area with no error
> bigger than few percents of error for each voxel of the area. The patterns
> of errors occur too often to select an area sufficiently big.
> Unfortunately, I cannot change the number of projections I use because
> this is imposed to me.
>
> Therefore, I want to know if it is possible to obtain better results with
> FDK or if I have to use an iterative algorithm.
>
>
>
> For the parameters of the geometry, I used 180 projections, sdd=978.5, and
> sid=478.5. For the projections I used a spacing of 0.8 and a dimension of
> 1024. And for the reconstruction, I used a spacing of 0.5, and a dimension
> of 204*404*204.
>
> I joined two screenshots of the absolute difference between the phantom
> and the reconstruction (one zoomed on some of the small lines of error, and
> another one not zoomed).
>
>
> If you want the images of my phantom, the reconstruction, and the absolute
> difference between the two (with the actual values and with relative
> values), I uploaded them here: https://drive.google.com/drive/folders/
> 194k2CDomeLlmVxybTllSKWYhpCZyPvVx?usp=sharing
>
>
> Thank you.
>
> Anaïs
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>
>
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