Most of these iterative algorithms are based on an implementation of the Radon transform that acts as forward projector.  We show that this implementation of the Radon transform significantly outperforms in efficiency other state-of-the-art operators with O(N2log2N) complexity. 
the Radon transform
This operator and its adjoint, the backprojector, are typically called few times per iteration and represent the computational bottleneck of the reconstruction process. 
the backprojector
Here, we present a Fourier-based forward projector, founded on the regridding method with minimal oversampling. 
the regridding method with minimal oversampling