.com/villekf/OMEGA/blob/master/source/backproject.m) 
% as you like. The example provided uses them the same way as in OSEM, but as 
% they simply implement the operations _y = Ax_ and _x = A^Tb_, respectively, 
% you can implement any algorithm that uses these matrix-vector operations. An 
% example of PSF reconstruction is included as well. Sensitivity image is obtained 
% with backprojection, but can also be omitted by simply having only one output 
% parameter.
% 
% All selected corrections are applied 