1. A method of estimating the value of a spatially distributed function in regions of interest in an object, from a set of projection values, said projection values comprising the integral sum of a signal detected along a line passing through a region of interest in the object, said signal being related to the value of the function in the region; the method comprising the steps of: measuring the set of projection values at a plurality of loci distributed about the object in a spatial frame of reference, each locus being a projection of one of said lines through the object; specifying a set of regions of interest (ROI's) from a spatial reference dataset, to a spatial geometric sampling size that is smaller than the resolution that is attainable by reconstruction of an image from the set of projection values; mapping the reference data set on the spatial frame of reference; projecting the regions of interest specified previously on to the loci and calculating a value, and an associated confidence measure, of the function for each ROI.