Hi Vincent, 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! Best regards, Simon
On Fri, Oct 25, 2019 at 11:55 AM vincent <[email protected]> wrote: > 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]> 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 >> >> >> >> >> _______________________________________________ >> Rtk-users mailing list >> [email protected] >> https://public.kitware.com/mailman/listinfo/rtk-users >> >> _______________________________________________ > Rtk-users mailing list > [email protected] > https://public.kitware.com/mailman/listinfo/rtk-users >
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