1. A method for obtaining calibrated image data of a target object with a tomographic apparatus having a radiation source for generating radiation beams, a collimator having a plurality of collimator width settings for collimating the radiation beams, a chamber through which the radiation beams are directed, the chamber having a portion adapted for receipt of an object, a plurality of detectors for detecting radiation emerging from the chamber and for generating response signals responsive to the detected radiation, a data acquisition unit for generating data representative of the response signals, a data processor and a memory, wherein the calibrated image data are used to reconstruct a tomographic image of the target object, said method comprising the steps of: scanning a reference object having physical characteristics similar to the target object, wherein the step of scanning the reference object includes the substeps of: placing the reference object in the portion of the chamber; setting the width of the collimator to a predetermined reference object collimator width; directing radiation beams through the chamber; detecting radiation emerging from the chamber; generating reference object response signals thereto; and generating and storing reference object data for each of the detectors representative of the reference object response signals; scanning the target object, wherein the step of scanning the target object includes the substeps of: placing the target object in the portion of the chamber; setting the width of the collimator to a predetermined target object collimator width; turning on the radiation source to direct radiation beams through the chamber; detecting radiation beams emerging from the chamber and generating target object response signals thereto; and generating and storing target object data for each of the detectors representative of the target object response signals; obtaining processed width correction data for each of the detectors representative of a variation between response signals generated by each of the detectors for radiation directed through the chamber, with the chamber having a selected medium through which said radiation is directed and the collimator being set at the predetermined reference object collimator width, and response signals generated by the detectors for radiation directed through the chamber with the chamber having said selected medium, and the collimator being set at the predetermined target object collimator width; storing the processed width correction data for each of the detectors; differencing the processed width correction data for each of the detectors from the reference object calibration data for each of the detectors to obtain width-corrected calibration data for each of the detectors; and differencing the target object data for each of the detectors from the width-corrected data for each of the detectors to obtain calibrated image data for use in reconstructing a tomographic image of the target object.