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Hi Etienne,<br>
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Hope you are doing good !<br>
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With listmode data, the sensitivity image is computed once at the beginning, before iterating. So this image is always the same during iterations. If you use a stochastic projector (which is the case of multi-siddon), the system matrix is different at each
iteration and also for the computation of that sensitivity image. Thus, in the image update step, there is a mismatch between the system matrices used for the sensitivity and for the loop over events to compute the correction image.<br>
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With histogram data, in CASToR, the sensitivity image is computed simultaneously while processing histogram bins for each update. So even with stochastic projectors, there is a perfect match between the system matrices used for the sensitivity and the correction
image.<br>
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In short, with a stochastic projector, you will never be able to have a perfect match between listmode and histogram reconstructions.<br>
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Also, for listmode reconstruction, you should theoritically use an infinite number of lines to generate the sensitivity image. To do so with the current CASToR code (also with your version I think), you should launch a reconstruction with -sens-only option
and an as-big-as-possible number of lines to compute the sensitivity. Then input that sensitivity image to the reconstruction with -sens option. I would say, empirically, that you should have two orders of magnitude between numbers of lines for the sensitivity
and for iterations.<br>
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Hope this helps<br>
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Simon</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> Castor-users <castor-users-bounces@lists.castor-project.org> on behalf of Etienne Auger <Etienne.Auger@USherbrooke.ca><br>
<b>Sent:</b> Tuesday, May 26, 2020 9:44:49 PM<br>
<b>To:</b> castor-users@lists.castor-project.org <castor-users@lists.castor-project.org><br>
<b>Subject:</b> [Castor-users] Difference in reconstructed image in histogram vs list-mode data when using multiSiddon projector</font>
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Hello fellow CASToR users,</div>
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Have you ever experienced visible differences in reconstructed images depending on the type of data format you used? Indeed, for the same simulation of a Derenzo type phantom made in GATE, I obtain different reconstructed images whether I converted the data
in histogram or list-mode format using the "castor-GATERootToCastor" utility. This seems to happen only when I use the multiSiddon projector though. Since the sampling with this projector is random, I set the seed with "-rng-seed 0" to ensure the differences
do not come from a different LOR sub-sampling. I would expect the images to be, within numerical errors, identical since they come from the same information, just formatted differently... I am currently using CASToR v2.0.2.</div>
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Here is an example using the Siddon projector, that gives almost identical images, as expected. The top image is list-mode, the middle one is histogram mode, and the bottom one is the subtraction. The command lines used to reconstruct both images are :</div>
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List-mode : "castor-recon -df data.lm_CstrProj.Cdh -dout image.lm.siddon.15mmAx -it 32:1 -dim 100,100,50 -vox 0.3,0.3,0.3 -vb 1 -th 4"<br>
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Histogram mode : "castor-recon -df data.hm_CstrProj.Cdh -dout image.hm.siddon.15mmAx -it 32:1 -dim 100,100,50 -vox 0.3,0.3,0.3 -vb 1 -th 4"<br>
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<img size="313083" tabindex="-1" data-outlook-trace="F:1|T:1" src="cid:f7922b70-f2fc-4938-9e82-bc3f6750a2a2" style="max-width:100%"><br>
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Here is the same example using the mutliSiddon projector, that gives different images. Again, the top image is list-mode, the middle one is histogram mode, and the bottom one is the subtraction. The command lines used to reconstruct both images are :<br>
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List-mode : "castor-recon -df data.lm_CstrProj.Cdh -dout image.lm.multiSiddon_25.15mmAx -it 32:1 -dim 100,100,50 -vox 0.3,0.3,0.3 -vb 1 -th 4 -proj multiSiddon,25 -rng-seed 0 -oit -1"<br>
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Histogram mode : "castor-recon -df data.hm_CstrProj.Cdh -dout image.hm.multiSiddon_25.15mmAx -it 32:1 -dim 100,100,50 -vox 0.3,0.3,0.3 -vb 1 -th 4 -proj multiSiddon,25 -rng-seed 0 -oit -1"<br>
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<img size="296591" tabindex="-1" data-outlook-trace="F:1|T:1" src="cid:e4e2fdd6-ff58-48a1-a2e1-2f0c491fceda" style="max-width:100%"><br>
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In addition to the white spots appearing in list-mode at axial extremities, the histogram version looks "smoother".</div>
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Thank you to provide me an explanation if you understand what is going on!</div>
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Étienne</div>
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