I am trying to implement MLEM algorithm for SPECT image reconstruction in 3D. But I am confused with probability matrix. How can I calculate probability matrix?
Analytical calculation+ PSF, Ref: Accurate image reconstruction based on standard-Gaussian-model fitted system matrix in multipinhole small animal SPECT imaging
You can also adapt the method described in the paper below to build a device-dedicated matrix (if your working with a real camera) :
"A practical way to improve contrast-to-noise ratio and quantitation for statistical-based iterative reconstruction in whole-body PET imaging., Med Phys 2009"
Another and simple solution is to use Siddon or Bresenham tracing. These are geometrical solutions easy to compute but the reconstruction will be inaccurate.