## Fan beam and parallel beam projection and back-projection operators

Expressions for the fan beam and parallel beam projection and back-projection operators are given along with an evaluation of the point source response for the back-projection operators. The back-projection operator for the fan beam geometry requires the superposition of projection data measured over 360/sup 0/. Both the fan beam and parallel beam geometries have back-projection operators with point source responses which are proportional to 1/mod (r-r/sub 0/), and thus two-dimensional Fourier filter techniques can be used to reconstruct transverse sections from fan beam and parallel beam projection data. The two-dimensional Fourier filter techniques may have the speed over other methodsmore »