A scanning method which is a method of identifying a change in the density of an object, said change representing a difference between the density of said object at a first location and the density of said object at a second location adjacent said first location, the method comprising the steps of: a. arranging a source of ionizing radiation and an array of radiation detectors, where n is an integer from 1 to N, capable of detecting said radiation in such a way that i. radiation is emitted from the source in the direction of the array of radiation detectors, ii. radiation is emitted from the source towards any one of said radiation detectors along a radiation path, said radiation path being defined at each end by the source and the area of each detector facing said source; iii. up to N of said radiation paths may pass through said object; and iv. said source and said detector array are located in fixed positions relative to one another and movable along an arcuate scanning path around said object; wherein said detector array comprises a plurality of N radiation detectors D which are arranged symmetrically opposite said source in such a way that conjugate pairs of detectors (22d and 22e, 22b and 22c) are arranged at equal angles (θ1, θ2) from the source; b. positioning the source and detector array at a known position on said scanning path; c. moving said source and detector array along said scanning path so that the source and detector array rotate through 360 degrees around said object; d. acquiring count data from each detector at a plurality of positions on said scanning path, said count data being related to the number of photons of radiation emitted by the source which have been detected by said detector during the count period; e. converting count data acquired by each detector at each position by means of a fan to parallel beam conversion algorithm to converted count data; f. selecting count data from at least one detector of each said conjugate pair (22d and 22e, 22b and 22c); g. deriving information concerning the density of the object at different positions in the plane of the scanning path from said count data, wherein the method further comprises a step of calibrating the detectors, wherein the calibration step includes obtaining an energy spectrum scan for each detector in the array, characterized in that information from the energy spectrum scan is used to identify a broken or defective detector and, when the broken or defective detector has been identified, data from its paired conjugate detector is selected in step (f). a. arranging a source of ionizing radiation and an array of radiation detectors, where n is an integer from 1 to N, capable of detecting said radiation in such a way that i. radiation is emitted from the source in the direction of the array of radiation detectors, ii. radiation is emitted from the source towards any one of said radiation detectors along a radiation path, said radiation path being defined at each end by the source and the area of each detector facing said source; iii. up to N of said radiation paths may pass through said object; and iv. said source and said detector array are located in fixed positions relative to one another and movable along an arcuate scanning path around said object; wherein said detector array comprises a plurality of N radiation detectors D which are arranged symmetrically opposite said source in such a way that conjugate pairs of detectors (22d and 22e, 22b and 22c) are arranged at equal angles (θ1, θ2) from the source; i. radiation is emitted from the source in the direction of the array of radiation detectors, ii. radiation is emitted from the source towards any one of said radiation detectors along a radiation path, said radiation path being defined at each end by the source and the area of each detector facing said source; iii. up to N of said radiation paths may pass through said object; and iv. said source and said detector array are located in fixed positions relative to one another and movable along an arcuate scanning path around said object; b. positioning the source and detector array at a known position on said scanning path; c. moving said source and detector array along said scanning path so that the source and detector array rotate through 360 degrees around said object; d. acquiring count data from each detector at a plurality of positions on said scanning path, said count data being related to the number of photons of radiation emitted by the source which have been detected by said detector during the count period; e. converting count data acquired by each detector at each position by means of a fan to parallel beam conversion algorithm to converted count data; f. selecting count data from at least one detector of each said conjugate pair (22d and 22e, 22b and 22c); g. deriving information concerning the density of the object at different positions in the plane of the scanning path from said count data,