dOffset| 
% is an optional value that describes the offsets of each bed position, |subsets| 
% is the optional number of subsets, |implementation| is the used implementation 
% (3, OpenCL, and 4, OpenMP, are supported), |deviceN| is the platform used when 
% using implementation 3.
% 
% The alternative way, is to use user-computed detector coordinates. The constructor 
% is then: |A = forwardBackwardProjectCT(x,y,z, Nx, Ny, Nz, FOVa_x, axial_fov, 
% subsets, implementation, deviceN)|, where |x|, |y| and |z|