1. An imaging system comprising: a data memory into which an acquired sinogram of projection data of an entire object is loaded; and an object reconstruction processor which reconstructs a volumetric image representation of the entire object in successive volumetric image portions, which reconstruction processor is programmed to: extract sinogram subsets each corresponding to a prespecified axial length from the data memory one at a time, the prespecified axial lengths partially overlapping, sequentially reconstruct sinogram subsets into the successive volume image portions with an iterative reconstruction algorithm using a trailing portion of a preceding image portion as an initial estimate for a leading edge portion of a currently reconstructed image portion of the image and reconstructs the trailing edge portion of the currently reconstructed image portion to be used as an initial estimate for a leading edge portion of a next subsequent image portion.