1. A method for image generation, the method comprising: providing a plurality of sets of projection data for respective ones of a first set of volumes; acquiring the sets of projection data at respective ones of N imaging positions by generating an image mask for each of a plurality of imaging positions, N being an integer; generating an initial image for a second volume larger than each volume in the first set of volumes; based on at least the initial image and the sets of projection data, reconstructing an image of the second volume using multiple iterations of a single iterative reconstruction process; computing a preconditioner η according to: η=1/(Σ ki H k T W k H k M k ), where k ranges from 1 to N, H k denotes a system matrix associated with the k th imaging position, W denotes a weighting function, and M k denotes the k th image mask; and during the iterative reconstruction process, combining backprojected feedback associated with respective imaging positions using the preconditioner.