[Castor-users] Difference in reconstructed image in histogram vs list-mode data when using multiSiddon projector

[Etienne Auger]

Hello fellow CASToR users,

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.

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 :

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"

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"

[cid:f7922b70-f2fc-4938-9e82-bc3f6750a2a2]

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 :

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"

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"

[cid:e4e2fdd6-ff58-48a1-a2e1-2f0c491fceda]
In addition to the white spots appearing in list-mode at axial extremities, the histogram version looks "smoother".

Thank you to provide me an explanation if you understand what is going on!

Étienne
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.castor-project.org/pipermail/castor-users/attachments/20200526/9e4ca268/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: siddon.png
Type: image/png
Size: 313083 bytes
Desc: siddon.png
URL: <http://lists.castor-project.org/pipermail/castor-users/attachments/20200526/9e4ca268/attachment-0002.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: multiSiddon.png
Type: image/png
Size: 296591 bytes
Desc: multiSiddon.png
URL: <http://lists.castor-project.org/pipermail/castor-users/attachments/20200526/9e4ca268/attachment-0003.png>