Iterative reconstruction methods, such as the expectation maximization (EM) algorithm and its extended approaches, have played a prominent role in emission computed tomography due to their remarkable advantages over the conventional filtered backprojection method. However, since iterative reconstructions typically are comprised of repeatedly projecting and backprojecting the data, the computational load required for reconstructing an image highly depends
on the performance of the projector-backprojector pair used in the algorithm. In this work we compare quantitative performance of representative methods for implementing projector-backprojector pairs-ray-tracing methods, rotation-based methods, and pixel-driven methods. To reduce the overall cost for the projection-backprojection
operations for each method, we investigate how previously computed results can be reused so that the number of redundant calculations can be minimized. Our experimental results demonstrate that, while the rotation based methods can be useful for simplifying the correction of important physical factors, the computational cost to achieve good accuracy is considerably higher than that of the ray-tracing methods.