Problems generating reconstructions with CASToR sensitivity image

General Discussions
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1

Dear CASToR users and developers,

I have had difficulties generating the sensitivity images with CASToR. For my work I simulated a fixed DUAL panel Positron Emission Mammography (PEM) system in GATE considering a uniformly distributed 511 keV source within a cuboid shaped phantom filled with air.

image

The geometry and root files from GATE were used to generate the CASToR data/header files. The input data for reconstruction with CASToR are:

03.PEMGeomPixe-CylPET.geom
PEMPixVSAir60s_df.Cdh
PEMPixVSAir60s_df.Cdf

Image reconstruction was performed as follows (using CASToR´s sensitivity image):

castor-recon -vb 2 -df PEMPixVSAir60s_df.Cdh -dout PEMPixVSAir60sCASToRSens -it 4:8 -dim 24,174,174 -fov 60.0,174,174 -off 0.,0.,0. -opti MLEM -conv gaussian,1.5,1.5,1.5::post -proj joseph -th 2 -flip-out Y

However, the reconstructed image seems to have gaps:

image
image

However, when I provide an external sensitivity image generated with Monte Carlo simulation (bypassing the sensitivity image generation step in CASToR) the reconstructed image looks as I expected (without gaps). For that I used the following line of comand.

castor-recon -vb 2 -df PEMPixVSAir60s_df.Cdh -dout PEMPixVSAir60sLIBISens -sens NOrm.hdr -it 4:8 -dim 24,174,174 -fov 60.0,174,174 -off 0.,0.,0. -opti MLEM -conv gaussian,1.5,1.5,1.5::post -proj joseph -th 2 -flip-out Y

Where my sensitivity image is:

NOrm.hdr
NormRwTrans.img

image

image

I would like to know if I am doing anything wrong while generating the sensitivity image with CASToR.
This step is essential for me, as I need to learn how to correct for attenuation with the -atn command, and as far as I understand, this implies that CASToR will use its own sensitivity image.

In the following link are the files used in the reconstruction in case someone needs them to recreate the process.

https://drive.google.com/drive/folders/1UcHIz7PvVxCpsIePLm7v4nDNq8g7_msG?usp=sharing

I thank you in advance for your attention
Dulce Valdivieso

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Dear Dulce,
The default sensitivity image generated by CASToR assumes optimal detector efficiency for each crystal. In your simulation this is probably not the case due to various effects (scattering in detectors, penetration, etc…) and this might be the cause of the artifacts you observe. These effects are included when you generate your sensitivity image from MC simulation data so this results in an image with less artifacts.
In order to correct for these effects during castor reconstruction, one must provide a normalization datafile containing lor-based correction factor (and optionally acf as well). This must be done manually as there is currently no tool to compute this with GATERootToCASToR, however how to handle normalization appears to be a hot topic so we are looking on how to improve this.
In the meantime, a work-around could be to integrate the attenuation image during the generation of your MC sensitivity image. Note that depending on how the image is generated, you could have other visual artifacts due to the mismatch between your custom projector and the projector used during reconstruction (joseph in your case).

Hope this helps,
Thibaut