A method of performing calibration scan and measurement scan in one and the same scanning operation with a calibration object having the fiducial marks arranged in positions spanning a volume enclosing at least a central portion of the measuring object. This avoids the need for one or more separate calibration scans to be performed in addition to the scanning of the measurement object. Considerable time is thereby saved. The fiducial objects are thus distributed, preferably evenly, around the measuring object, whereby homogeneous calibration is ensured. After having performed a scan of the measuring object together with the calibration object and thereby obtained scan data on the measuring object and corresponding scan data on the calibration object the scan data on the fiducial marks of the calibration object are used to calibrate the CT scanner, and the scan data on the measuring object are used to calculate geometric properties of the measuring object.
