 are the detector 
% coordinates in the transaxial and axial directions.
% 
% In each of the three cases, the class object can simply be used as |A * x| 
% to compute forward projection and |A' * y| to compute backprojection. Subsets 
% can be set with |A.subset = subset_number|. Alternatively, you can use the class 
% object in function: |forwardProject(A, f, subset_number)| and |backwardProject(A, 
% y, subset_number)|.
% 
% *Extracting the system matrix*
% 
% The class object can also be used to form the