We present an efficient scheme for the forward and backward projector implementation for helical cone-beam x-ray CT reconstruction using a pre-calculated and stored system matrix approach. Because of the symmetry of a helical source trajectory, it is sufficient to calculate and store the system matrix entries for one image slice only and for all source positions illuminating it. The system matrix entries for other image slices are copies of those stored values. In implementing an iterative image reconstruction method, the internal 3D image volume can be based on a non-Cartesian grid so that no system matrix interpolation is needed for the repeated forward and backward projection calculation. Using the proposed scheme, the memory requirement for the reconstruction of a full field-of-view of clinical scanners is manageable on current computing platforms. The same storage principle can be generalized and applied to iterative volume-of-interest image reconstruction for helical cone-beam CT. We demonstrate by both computer simulations and clinical patient data the speed and image quality of VOI image reconstruction using the proposed stored system matrix approach. We believe the proposed method may contribute to bringing iterative reconstruction to the clinical practice.