In the present invention, an iterative process is provided for cone-beam tomography (parallel-beam and fan-beam geometries are considered as its special cases), and applied to metal artifact reduction and local reconstruction from truncated data, as well as image noise reduction. In different embodiments, these iterative processes may be based upon the emission computerized tomography (CT) expectation maximization (EM) formula and/or the algebraic reconstruction technique (ART). In one embodiment, generation of a projection mask and computation of a 3D spatially varying relaxation factor are utilized to compensate for beam divergence, data inconsistence and incompleteness.