I obtained the image of the GATE simulation data using the castor program.

I would like to ask the confusing part of applying the attenuation correction to the acquired image. The data is list mode.

if I enter the same image as the phantom size with attenuation coefficient value into the -atn option, will the attenuation factor be calculated within the caster program’s code and corrected for the image?

If the method described above is correct, does the program use the method of Chang’s multiplicative?

If you only recovered true coincidences from the GATE simulation data, and if you are using list-mode data, then with the -atn option you can provide an (interfile) image containing the attenuation coefficient value (in cm-1) to correct for attenuation during reconstruction. Attenuation correction factors will be used as multiplicative correction factors during the computation of the sensitivity image (which is calculated first when you launch the reconstruction), so the sensitivity image will include correction for attenuation.

Thank you for your answer.
But, I didn’t fully understand your answer. So, I have additional questions for you.

Q1. what does mean of ‘only recovered true coincidences for GATE simulation data’?
When converting(GATERootToCastor) data, is it possible to correct only the data obtained by applying the ‘-t’ option?

Q2. If you provide an image filled with only ‘attenuation coefficient value’ after ‘-atn’, is it right for the CASToR program to calculate the ‘damp factor’ according to the distance or position of the phantom?
Or do I have to provide an image that included calculated attenuation factors for each position of the phantom?

Q3. When the simulation data is converted to histogram mode, the Cdh file does not show TOF flags or Histo TOF bin size. What additional options should I use?
I used the commands written below.
castor-GATERootToCastor -m TOF_257_t10_jas2_uniform.mac -s TOF_t10_uniform -i 200116_TOF_t10_uniform_120s.root -o 200120_TOF_t10_uniform_hist_vb_tof -TOF_reso 180 -vb 4 -oh

Thank you for your answer.
Sorry, The damping factor in Q2 is the wrong attenuation factor. So, I wrote the question again below.

Q1. If you provide an image filled with only ‘attenuation coefficient value’ after ‘-atn’, is it right for the CASToR program to calculate the ‘attenuation factor’ according to the distance or position within the phantom?
Or do I have to provide an image that included calculated attenuation factors for each position of the phantom?

You must provide an image containing, for each voxel, the attenuation coefficient value related to your object.

If you used voxelized phantoms in your GATE simulation (source/phantom definition), you must provide a similar image with 511-keV (for PET) attenuation coefficients for each voxels. If you use a geometric phantom/object geometry, you must generate this image.

On a side note, it is not mandatory to use an attenuation image with the same voxel size/number of the reconstruction image matrix though. The attenuation image will be interpolated if the image matrices are different.