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 <[email protected]
<mailto:[email protected]>> 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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