Scanning method

1. 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 a linear 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 are arranged at equal angles from the source;

b. calibrating the detectors, wherein the calibration step includes obtaining an energy spectrum scan for each detector in the array;

c. positioning the source and detector array at a known position on said scanning path;

d. moving said source and detector array along said scanning path so that the source and detector array rotate through 360 degrees around said object;

e. 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;

f. converting count data acquired by each detector at each position by means of a fan to parallel beam conversion algorithm to converted count data;

g. selecting count data from at least one detector of each said conjugate pair;

h. deriving information concerning the density of the object at different positions in the plane of the scanning path from said count data wherein information from the energy spectrum scan in step (b) is used to identify a broken or defective detector and, when the broken or defective detector has been identified, data from its conjugate pair is selected in step (g).

2. A method according to claim 1 , wherein count data is selected from at least one detector of each said conjugate pair on the basis of the count data counted by each said detector.

3. A method according to claim 2 , wherein the count data acquired by each detector of a conjugate pair is compared to determine whether either of said detectors is defective.

4. A method according to claim 2 , wherein count data from a detector is compared with a predetermined range of value of counts which would be expected from the detector in order to determine if the detector is defective.

5. A method according to claim 1 , wherein the count data acquired by each detector of a conjugate pair is compared to determine whether either of said detectors is defective.

6. A method according to claim 1 , wherein count data from a detector is compared with a predetermined range of value of counts which would be expected from the detector in order to determine if the detector is defective.

7. A method according to claim 1 , wherein said calibration is carried out in the absence of the object to be scanned.

8. A method according to claim 7 , wherein information from the energy spectrum scan is used to identify suitable energy windows within which to count photons detected by the detector.

9. A method according to claim 1 , wherein the calibration step is carried out in the same or substantially the same environmental conditions as will be used for the scan.

10. A method according to claim 9 , wherein information from the energy spectrum scan is used to identify suitable energy windows within which to count photons detected by the detector.

11. A method according to claim 1 , wherein information from the energy spectrum scan is used to identify suitable energy windows within which to count photons detected by the detector.

12. A method according to claim 1 , wherein information from the calibration is used to identify which detector of a conjugate pair of detectors offers the higher resolution of the pair.

13. A method according to claim 12 , wherein count data from the detector within a conjugate pair which offers the higher resolution is selected in step (g).

14. A scanning apparatus according to the invention comprises: a source of ionizing radiation and an array of radiation detectors, capable of detecting said radiation, said source and detector array being arranged 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 an object which is to be scanned; 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 symmetrically arranged opposite said source in such a way that conjugate pairs of detectors are arranged at equal angles from the source;

means to move said source and detector array along said scanning path so that the source and detector array rotate through 360 degrees around said object;

data processing means which is programmed to

a. acquire 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;

b. convert count data acquired by each detector at each position by means of a fan to parallel beam conversion algorithm to converted count data;

c. select count data from at least one detector of each said conjugate pair wherein the data processing means is programmed to use information from an energy spectrum scan from a calibration to identify a broken or defective detector and, when the broken or defective detector has been identified, select data from its paired conjugate detector; and

d. derive information concerning the density of the object at different positions in the plane of the scanning path from said count data.