1. An imaging apparatus, comprising an image acquisition device for generating a plurality of measured projection images of a volume, a digital data processor coupled to the acquisition device for receiving the images, the digital data processor comprising a central processing unit (CPU) in communication with a graphics processing unit (GPU) including a plurality of programmable vertex shaders that are coupled to a plurality of programmable pixel shaders, the CPU transferring at least some of the measured projection images to the GPU, wherein the GPU reconstructs at least a portion of the volume based on comparison of a plurality of forward-projection images corresponding to successive iterations of an estimated density distribution assigned to that volume portion with one or more of the measured projection images without transfer of intermediate texture data to the CPU.