1. A method of producing a tomographic image of an object from projection data acquired in a helical scan, the data derived from a pair of x-ray fan beams disposed along a z axis and each producing a series of fan beam projections at a plurality of gantry angles .theta. about the z axis and each fan beam projection subtending an angle .phi., the steps comprising: a) acquiring a set of projections over gantry angles .theta. ranging over 180.degree. +.phi. as the imaged object is translated along the z axis, the set of projections forming a first data array indicating the attenuation of x-ray energy in the first fan beam by the object, and forming a second data array indicating the attenuation of x-ray energy in the second fan beam by the object; b) multiplying the data in the first data array by a first set of weighting values; c) multiplying the data in the second data array by a second set of weighting values; d) combining the corresponding weighted data in the first and second data arrays; e) producing the tomographic image from the combined first and second data arrays.