1. A method of pre-processing beam attenuation data for reconstructing a three dimensional (3D) image of a user selectable portion of an object using an inverse Radon transformation comprising the steps of: providing a mutually spaced irradiating beam source and a surface array radiation detector in fixed relationship to one another; providing said object therebetween; irradiating at least said portion of said object within a field of view of said source; rotationally scanning said portion with respect to said fixed source and detector along a trajectory enclosing upper and lower extents of said portion by respective upper and lower scan paths joined therebetween by a connecting scan path; identifying a select region on said array detector surface as a beam projection of said object bounded between similar said upper and lower scan path projections; acquiring beam attenuation data at said array detector surface for a plurality of source positions along said scan trajectory; retaining only beam attenuation data acquired within said select region; and processing only said retained data to reconstruct a 3D image of said object by said inverse Radon transformation.