manually computing the forward and/or backward projections, it 
% is recommended to use the <forward_backward_projections_example.m forward_backward_projections_example.m> 
% and modify it according to your specifications.
% 
% The initial part works very much like <https://github.com/villekf/OMEGA/blob/master/main_PET.m 
% main_PET.m>, however, there are no adjustments for the reconstruction algorithms. 
% You still need to input the necessary information (scanner properties, sinogram/raw 
% data propertie