1. A method of reconstructing a tomographic image using the parallel beam filtered backprojection algorithm from data, q(.alpha.,.beta.) acquired with a fan beam configuration with constant or spatially variant focal lengths, such method comprising the steps of: performing a fast fourier transform on the fan beam data, q(.alpha.,.beta.), with respect to a set of projection angles, .beta.; forming a linear combination of complementary data elements of the transformed data, lying at projection bin .alpha. and -.alpha.; linearly interpolating the linear combination to obtain a regularly spaced set of data, P.sub.k.sup.(w) (.xi.), in fourier space; performing an inverse fourier transform on the linearly interpolated data; and reconstructing an image from the inverse transformed data using the filtered back projection algorithm.