<div dir="ltr"><div>Hi Vincent,</div><div>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!</div><div>Best regards,</div><div>Simon<br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, Oct 25, 2019 at 11:55 AM vincent <<a href="mailto:vl@xris.eu">vl@xris.eu</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF">
<p>Hi Simon, <br>
</p>
<p>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.</p>
<p>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:</p>
<p><a href="https://ibb.co/ZX57FPM" target="_blank">https://ibb.co/ZX57FPM</a><br>
<a href="https://ibb.co/MnMk5Ny" target="_blank">https://ibb.co/MnMk5Ny</a><br>
<a href="https://ibb.co/gzfPQG1" target="_blank">https://ibb.co/gzfPQG1</a><br>
</p>
<p>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 :) !</p>
<p>Thanks again,</p>
<p>Vincent<br>
</p>
<div>On 23.10.19 22:30, Simon Rit wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">
<div>Hi Vincent,</div>
<div>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.<br>
</div>
<div>Good luck solving this!</div>
<div>Simon<br>
</div>
<div><br>
</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Wed, Oct 23, 2019 at 3:03
PM vincent <<a href="mailto:vl@xris.eu" target="_blank">vl@xris.eu</a>> wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi
everyone,<br>
<br>
I am trying to reconstruct a volume representing an aluminum
casted part <br>
and investigate the differences between FDK and SART.<br>
<br>
Here are the parameters involved: the images are 700*820
pixels and I <br>
have 900 projections. The physical pixel size is 0.28 mm and
the sdd <br>
and sid are 558.3 mm and 461.3 mm respectively, which gives me
a <br>
magnification factor of 1.2 roughly. I performed one
iteration of SART <br>
with CudaRayCast as forward and backward projectors and I
wanted to <br>
reconstruct a 700*700*700 volume with a 0.23 mm (= 0.28/1.2)
voxel <br>
size. The result is shown on the two pictures attached:<br>
<br>
<a href="https://ibb.co/8sLdJxw" rel="noreferrer" target="_blank">https://ibb.co/8sLdJxw</a><br>
<a href="https://ibb.co/N3NLg2D" rel="noreferrer" target="_blank">https://ibb.co/N3NLg2D</a><br>
<br>
The result is getting worse as I increase the number of
iterations<br>
<br>
I have a hard time understanding where the stripes come from.
A <br>
reconstruction with the standard 256*256*256 volume and 1mm
spacing <br>
gives a "smoother" result, closer to what FDK gives. I
thought it might <br>
be related to the linear system resolution, but unless I am
mistaken, I <br>
am trying to solve a system with 700*700*700 unknown with
700*820*900 <br>
equations, which should be ok.<br>
<br>
Furthermore, the reconstruction with FDK at the same "full"
resolution <br>
gives satisfactory results.<br>
<br>
Any help/explanation will be greatly appreciated !<br>
<br>
I thank you very much in advance,<br>
<br>
Vincent<br>
<br>
<br>
<br>
<br>
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