[Castor-users] Normalization

[Sadek Nehmeh]

Many thanks Simon for your detailed response. Please see below an example transaxial slice and the summed coronal image for a 2.5 min of simulation. The images are artifact-free event though no normalization correction was applied. I had to go with 30min of data to see the ring artifacts, even though very mild especially when compared to I am used to from STIR.

Regards,
Sadek




[cid:image002.png at 01D68A9B.07DF5890]




From: STUTE Simon <simon.stute at chu-nantes.fr>
Sent: Monday, September 14, 2020 5:26 AM
To: Sadek Nehmeh <san2028 at med.cornell.edu>; castor-users at lists.castor-project.org
Subject: [EXTERNAL] RE: Normalization


Dear Sadek,



The PET sensitivity image is computed using one of the three following method :

 - if a histogram datafile is provided, then the sensitivity is computed by backward-projecting the product of attenuation and normalization corrections of each LOR of the datafile, one-the-fly for each subset

 - if a listmode datafile is provided, without normalization datafile, a double loop is performed over all crystals of the scanner so that for each possible crystal pair, the corresponding line joining the two crystals is used to forward-project the attenuation image (if provided) and backward-project the corresponding attenuation coefficient factor in the sensitivity image. In that case, no scanner normalization is taken into account so that it is implicitly assumed that all crystal pairs have the same efficiency

 - if a listmode datafile is provided, a normalization datafile can be provided so that the sensitivity image will be computed from it. This normalization datafile contains all LORs that may detect events, with associated attenuation and normalization coefficients. A loop is done over all these LORs and the coefficients are backward projected.



Could you send an image that you reconstructed ?



If you did not proceed to the simulation of a normalization scan (using a rotating source or a "donnut" source) in order to compute normalization coefficients and build a normalization datafile, then it is very likely that you have artifacts in your image.



Did you try to sum up all slices of the reconstructed image ? Do you have sufficient statistics in your dataset ?



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 Sadek Nehmeh <san2028 at med.cornell.edu<mailto:san2028 at med.cornell.edu>>
Envoyé : vendredi 11 septembre 2020 18:26
À : castor-users at lists.castor-project.org<mailto:castor-users at lists.castor-project.org>
Objet : [Castor-users] Normalization

Dear Castor Developers and Users,

I have successfully reconstructed the GATE images for a Biograph Vision. The images look great and artifact-free. The interesting thing is that even though I did not apply any normalization correction, the images do not show any rings artifacts! Could someone please clarify some details about how the sensitivity image is produced and how the algorithm compensates for the expected ring artifacts?

Thanks,
Sadek
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