[Castor-users] artifacts in the reconstruction
tmerlin
Thibaut.Merlin at univ-brest.fr
Tue Dec 22 19:20:10 CET 2020
Dear Alessandro,
Thank you for the feedback.
In histogram mode, the converter fills an histogram with all the events
recorded in the simulation. The histogram does not depend on any
geometry and blindly assumes all crystal combinations are available, so
that is why you end up with a very large file with a lot of nil events.
This could actually be improved by checking first if the line is
available depending on the transaxial angle restriction in the geom
file, but this is not implemented.
This does not usually lead to such artifacts as in your first
reconstruction, I believe this could be due to the octagonal geometry. I
will have a closer look at that.
I would however expect that removing all the zero-events from the
histogram could lead to artifacts, as in histogram mode the lines of
response in the histogram are used to generate the sensitivity image.
This doesn't appear in your second reconstructed image, but that might
be object/system dependent.
Normalization artifacts are also more or less visible depending on the
object. I believe they would be more visible with the simulation of a
large object with uniform activity.
Best regards,
Thibaut
On 18/12/2020 10:08, Alessandro Pilleri wrote:
> Dear All,
> with the intent to include the normalization coefficients I was
> looking at the data file created by castor-GATERootToCastor and I saw
> that It contains an odd number of events (LORs), in which an event is
> identified by a pair crystal1-crystal2. The scanner (with a
> coincidence scheme of 1vs3) has ~9.7 million LORs but the entries in
> the data file are more than 25 million. This event includes LOR
> identified for example by crystals belonging to the same module, even
> adjacent crystals, that are clearly not in coincidence. The "amount"
> value for these events is equal to zero. If I remove all the
> zero-amount events from the data file and create a new histogram, the
> reconstruction shows no artefacts, even without the application of the
> normalization correction. I attach the images, left with all the
> events, right after removing the zero-amount LORs.
>
> Is this normal behaviour?
>
> Best
> Alessandro
>
>
> image.png
>
>
>
>
>
>
> On Wed, Dec 16, 2020 at 1:31 PM Alessandro Pilleri
> <a.pilleri at gmail.com <mailto:a.pilleri at gmail.com>> wrote:
>
>
> Dear Simon,
> thank you for your answer.
>
> I used the GateRoot converter to create the data file in histogram
> mode and to automatically generate the geometry (-geo parameter).
> It automatically added the 135 deg min angle difference in the
> .geom file.
> I thought the min angle difference was applied in the loop over
> the data, so I tried to change it in the .geom file. By the way,
> now I created two different data files changing the digitizer
> option in the gate macfile to change the coincidence scheme and
> the sensitivity images do look different.
>
> For the application of the normalization coefficients, in
> histogram mode, do I have to manually modify the datafile to
> change the normalization coefficients of the events from 1 to the
> appropriate coefficient?
>
> Alessandro
>
> On Wed, Dec 16, 2020 at 11:42 AM STUTE Simon
> <simon.stute at chu-nantes.fr <mailto:simon.stute at chu-nantes.fr>> wrote:
>
> Hi Alessandro,
>
>
> The artifacts are totally normal if you do not apply a
> normalization correction, especially with low number of
> detector panels.
>
>
> As for the min angle difference, did you let CASToR compute
> the sensitivity image in both cases (135 and 180) ? Are they
> different ?
>
> The min angle difference is not apply in the loop over the
> data, so it should be applied by yourself when building the
> datafile.
>
> Also, how did you build the geometry of your scanner, from
> your own LUT file or using the GATE converter ?
>
>
> By the way, your emails successfully reached the mailing list.
>
>
> Best
>
> Simon
>
>
> ------------------------------------------------------------------------
> *De :* Castor-users
> <castor-users-bounces at lists.castor-project.org
> <mailto:castor-users-bounces at lists.castor-project.org>> de la
> part de Alessandro Pilleri <a.pilleri at gmail.com
> <mailto:a.pilleri at gmail.com>>
> *Envoyé :* mardi 15 décembre 2020 16:37
> *À :* castor-users at lists.castor-project.org
> <mailto:castor-users at lists.castor-project.org>
> *Objet :* [Castor-users] artifacts in the reconstruction
> Dear castor-users,
> I give an update to my previous email.
> I tried to change the min angle difference in the geometry
> file from 135 (meaning a coincidence schema 1vs3) to 180
> (meaning a coincidence schema 1vs1) and the two
> reconstructions provided exactly the same results.
>
> Could the artifact be related to some kind of bug (or a misuse
> by my side) related to the "min angle difference"?
>
> Thank you
> Alessandro
>
>
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--
Thibaut MERLIN -- PhD
Docteur en Imagerie Médicale au Laboratoire de Traitement de l'Information Medicale (LaTIM - INSERM UMR 1101)
Institut Brestois de recherche en Bio-Santé (IBRBS)
12 Avenue Foch, 29200 Brest, FRANCE
Tel: 06.75.12.24.90
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