to the forward and/or backward projections. 
% If you wish to use different projectors for the forward or backward projections, 
% you need to manually set options.projector_type between them. E.g. compute forward 
% projection when |options.projector_type = 2| and then set |options.projector_type 
% = 1| and compute backprojection. This, however, is untested and may especially 
% not work when using implementation 1. If you wish to use different projectors, 
% it is recommended to use implementation 3 and 