<div dir="ltr"><div>Thanks for sharing the result. We should definitely try to work on a more efficient and matched projector / backprojection pair. In the meantime, my feeling is that you could also use some spatial regularization to reduce the effect of unmatched projectors.</div><div>Best regards,</div><div>Simon<br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Oct 29, 2019 at 6:19 PM 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,</p>
<p>well the step size marginally improved the result. But using
CudaVoxelBased as the BP rather than CudaRayCast completely solved
it !</p>
<p>Thank you for your time and advice,</p>
<p>best regards,</p>
<p>Vincent<br>
</p>
<div>On 27.10.19 21:42, Simon Rit wrote:<br>
</div>
<blockquote type="cite">
<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" 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">
<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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