Computed tomography apparatus and method for classifying objects

• US 6,317,509 B1
• Filed: 02/11/1998
• Issued: 11/13/2001
• Est. Priority Date: 02/11/1998
• Status: Expired due to Term

• Current Assignee: Analogic Corporation
• Original Assignee: Analogic Corporation
• Inventors: Crawford, Carl R., Simanovsky, Sergey, Hiraoglu, Muzaffer, Bechwati, Ibrahim M.
• Primary Examiner(s): Au, Amelia M.
• Assistant Examiner(s): Dastouri, Mehrdad
• Application Number:
  US09/022,059
• Time in Patent Office: 1,371 Days
• Field of Search: 382/131, 382/128, 382/132, 382/141, 382/143, 382/154, 382/149, 382/168, 382/205, 382/219, 382/256, 382/257, 382/260-265, 382/286, 382/308, 378/4, 378/21, 378/57, 378/901, 345/419, 345/420, 345/421, 345/423, 345/424, 250/363.04, 250/367, 250/390.04
• US Class Current: 382/131
• CPC Class Codes: G01N 33/22 Fuels, explosives liquid hydrocarbons G01N33/28
• CPC Class Codes: G01V 5/226 using tomography
• CPC Class Codes: G06V 20/52 Surveillance or monitoring of activities, e.g. for recognising suspicious objects recognising microscopic objects G06V20/69
• CPC Class Codes: G06V 2201/05 Recognition of patterns representing particular kinds of hidden objects, e.g. weapons, explosives, drugs

Abstract

A method and apparatus detects sheet explosives in computed tomography (CT) data. In particular, sheet-shaped objects such as sheet explosives can be discriminated from other object shapes and detected. The detection includes analyzing a neighborhood of voxels surrounding a test voxel. If the density of the test voxel is sufficiently different from the mean density of the neighboring voxels, then it is concluded that the test voxel is associated with a sheet object. Sheet objects can also be detected by eroding the CT data so as to eliminate voxels associated with thin objects. Remaining objects are then subtracted from the original data, leaving only thin sheet-shaped objects. Erosion of the data can be performed by identifying a neighborhood of voxels surrounding a voxel of interest. If the number of voxels having densities below a predetermined threshold exceeds a predetermined number, then it is assumed that the test voxel is a surface voxel and is removed from the object. A connectivity process can be applied to voxels to combine them into objects after sheets are detected to prevent sheets from being inadvertently removed from the data by erosion. A dilation function can then be performed on the eroded object to replace surface voxels removed by erosion. A corrected mass using the mean eroded density of the object can be computed and compared to mass thresholds to classify the object as to whether it poses a threat. Multiple mass thresholds can be used, each of which is associated with a particular density range based on the density of an expected threat object.

Claims

1. A method of detecting sheet objects represented in reconstructed computed tomography (CT) data for a region comprising:

identifying a plurality of volume elements in the reconstructed CT data for the region, each volume element being associated with a density value;

performing a sheet voxel identification process on the reconstructed CT data for the region to identify those volume elements associated with any sheet objects represented by the reconstructed CT data for the region and to discriminate those volume elements associated with any sheet objects from volume elements associated with other object shapes; and

after performing the sheet voxel identification process, connecting the volume elements identified by the sheet voxel identification process as associated with any sheet objects into sheet objects,

wherein performing a sheet voxel identification detection process comprises;

performing a predetermined number N of erosion steps during which volume elements are removed from the reconstructed CT image data for the region, said erosion steps producing eroded CT image data;

identifying an eroded object in the eroded CT image data;

removing the volume elements of the eroded object from the reconstructed CT image data; and

labeling an object in the reconstructed CT image data after removal of the eroded object as a sheet object.

2. The method of claim wherein the number N is based on an expected thickness of a sheet object.

3. A method of detecting sheet objects represented in computed tomography (CT) data for a region comprising:

identifying a plurality of volume elements in the CT data for the region, each volume element being associated with a density value;

performing a sheet voxel identification process on the CT data for the region to identify those volume elements associated with any sheet objects represented by the CT data for the region and to discriminate those volume elements associated with any sheet objects from volume elements associated with other object shapes; and

after performing the sheet voxel identification process, connecting the volume elements identified by the sheet voxel identification process as associated with any sheet objects into sheet objects;

further comprising, for each volume element of interest;

identifying a plurality of neighboring volume elements of the region;

comparing the density value associated with each of the neighboring volume elements with a predetermined object density range;

counting a number of neighboring volume elements whose density values are not within the predetermined object density range;

comparing the number of neighboring volume elements whose density values are not within the predetermined object density range with a predetermined threshold; and

if the number of neighboring volume elements whose density values are not within the predetermined object density range exceeds the predetermined threshold, removing the volume element from the object.

4. The method of claim 3 wherein the predetermined object density range is selected such that volume elements at a surface of the object are removed from the object.

5. The method of claim 3 wherein the predetermined object density range is selected such that volume elements at a surface of an interior void region of the object are not removed from the object.

6. A method of detecting sheet objects represented in computed tomography (CT) data for a region comprising:

identifying a plurality of volume elements in the CT data for the region, each volume clement being associated with a density value;

performing a sheet voxel identification process on the CT data for the region to identify those volume elements associated with any sheet objects represented by the CT data for the region and to discriminate those volume elements associated with any sheet objects from volume elements associated with other object shapes; and

after performing the sheet voxel identification process, connecting the volume elements identified by the sheet voxel identification process as associated with any sheet objects into sheet objects;

further comprising;

computing a distance between a first object and a second object represented in the CT data;

computing a first density associated with the first object and a second density associated with a second object;

computing a difference between the first density and the second density; and

combining the first object and the second object into a single combined object if the difference between the first density and the second density is below a predetermined density difference threshold and the distance between the first object and the second object is below a predetermined distance threshold.

7. The method of claim 6 further comprising:

computing a first mass of the first object and a second mass of the second object;

combining the mass of the first and second objects into a combined mass; and

classifying the single object according to the combined mass.

8. The method of claim 7 wherein the classifying step comprises:

comparing the combined mass to a predetermined mass threshold; and

if the combined mass exceeds the predetermined mass threshold, labeling the single combined object as a threat object.

9. An apparatus for detecting sheet objects represented in reconstructed computed tomography (CT) data for a region, comprising:

a volume element identifier arranged so as to identify a plurality of volume elements in the reconstructed CT data for the region, each volume clement being associated with a density value;

voxel identification processor arranged so as to perform a sheet voxel identification process on the reconstructed CT data for the region to identify those volume elements associated with any sheet objects represented by the reconstructed CT data for the region and to discriminate those volume elements associated with any sheet objects from volume elements associated with other object shapes; and

a volume element connector arranged so as to connect the volume elements identified by the voxel identification process as associated with any sheet objects into sheet objects;

wherein the voxel identification processor comprises;

an erosion processor arranged so as to perform a predetermined number N of erosion steps during which volume elements are removed from the reconstructed CT image data for the region, said erosion steps producing eroded CT image data;

an erosion object identifier arranged so as to identify an eroded object in the eroded CT image data;

a volume element remover arranged so as to remove the volume elements of the eroded object from the reconstructed CT image data; and

an object labeler arranged so as to label an object in the reconstruced CT image data after removal of the eroded object as a sheet object.

10. The apparatus of claim 9 wherein the number N is based on an expected thickness of a sheet object.

11. An apparatus for detecting sheet objects represented in computed tomography (CT) data for a region, comprising:

a volume clement identifier arranged so as to identify a plurality of volume elements in the CT data for the region each volume element being associated with a density value;

voxel identification processor arranged so as to perform a sheet voxel identification process on the CT data for the region to identify those volume elements associated with any sheet objects represented by the CT data for the region and to discriminate those volume elements associated with any sheet objects from volume elements associated with other object shapes; and

a volume element connector arranged so as to connect the volume elements identified by the sheet voxel identification process as associated with any sheet objects into sheet objects;

a neighboring volume element identifier arranged so as to identify, for each volume element of interest, a plurality of neighboring volume elements of the region;

a density comparator arranged so as to compare the density value associated with each of the neighboring volume elements with a predetermined object density range;

a counter arranged so as to count a number of neighboring volume elements whose density values are not within the predetermined object density range;

a number comparator arranged so as to compare the number of neighboring volume elements whose density values are not within the predetermined object density range with a predetermined threshold; and

a volume element remover arranged so as to remove the volume element from the object if the number of neighboring volume elements whose density values are not within the predetermined object density range exceeds the predetermined threshold.

12. The apparatus of claim 11 wherein the predetermined object density range is selected such that volume elements at a surface of the object are removed from the object.

13. The apparatus of claim 11 wherein the predetermined object density range is selected such that volume elements at a surface of an interior void region of the object are not removed from the object.

14. An apparatus for detecting sheet objects represented in computed tomography (CT) data for a region, comprising:

a volume element identifier arranged so as to identify a plurality of volume elements in the CT data for the region, each volume element being associated with a density value;

voxel identification processor arranged so as to perform a sheet voxel identification process on the CT data for the region to identify those volume elements associated with any sheet objects represented by the CT data for the region and to discriminate those volume elements associated with any sheet objects from volume elements associated with other object shapes; and

a volume element connector arranged so as to connect the volume elements identified by the sheet voxel identification process as associated with any sheet objects into sheet objects;

a distance computing component arranged so as to compute a distance between a first object and a second object represented in the CT data;

a density computing component arranged so as to compute a first density associated with the first object and a second density associated with a second object;

a difference computing component arranged so as to compute a difference between the first density and the second density; and

an object combiner arranged so as to combine the first object and the second object into a single object if the difference between the first density and the second density is below a predetermined density difference threshold and the distance between the first object and the second object is below a predetermined distance threshold.

15. The apparatus of claim 14 further comprising:

a mass computing component arranged so as to compute a first mass of the first object and a second mass of the second object;

a mass combiner arranged so as to combine the mass of the first and second objects into a combined mass; and

an object classifier arranged so as to classify the single object according to the combined mass.

16. The apparatus of claim 15 wherein the means for classifying the single object according to the combined mass comprises:

a mass comparator arranged so as to compare the combined mass to a predetermined mass threshold; and

an object labeler arranged so as to label the single combined object as a threat object if the combined mass exceeds the predetermined mass threshold.

17. A computed tomography (CT) scanning system for detecting sheet objects in a region comprising:

a volume element identifier arranged so as to identify a plurality of volume elements in reconstructed CT data for the region, each volume element being associated with a density value;

voxel identification processor arranged so as to perform a sheet voxel identification process on the reconstructed CT data for the region to identify those volume elements associated with any sheet objects represented by the reconstructed CT data for the region and to discriminate those volume elements associated with any sheet objects from volume elements associated with other object shapes; and

a volume element connector arranged so as to connect the volume elements identified by the sheet voxel identification process as associated with any sheet objects into sheet objects;

wherein the voxel identification processor comprises;

an erosion processor arranged so as to perform a predetermined number N of erosion steps during which volume elements are removed from the reconstructed CT image data for the region, said erosion steps producing eroded CT image data;

an erosion object identifier arranged so as to identify an eroded object in the eroded CT image data;

a volume element remover arranged so as to remove the volume elements of the eroded object from the reconstructed CT image data; and

an object labeler arranged so as to label an object in the reconstructed CT image data after removal of the eroded object as a sheet object.

18. The CT scanning system of claim 17 wherein the number N is based on an expected thickness of a sheet object.

19. A computed tomography (CT) scanning system for detecting sheet objects in a region comprising:

a volume element identifier arranged so as to identify a plurality of volume elements in the CT data for the region, each volume element being associated with a density value;

voxel identification processor arranged so as to perform a sheet voxel identification process on the CT data for the region to identify those volume elements associated with any sheet objects represented by the CT data for the region and to discriminate those volume elements associated with any sheet objects from volume elements associated with other object shapes; and

a volume element connector arranged so as to connect the volume elements identified by the sheet voxel identification process as associated with any sheet objects into sheet objects;

a neighboring volume element identifier arranged so as to identify, for each volume element of interest, a plurality of neighboring volume elements of the region;

a density comparator arranged so as to compare the density value associated with each of the neighboring volume elements with a predetermined object density range;

a counter arranged so as to count a number of neighboring volume elements whose density values are not within the predetermined object density range;

a number comparator arranged so as to compare the number of neighboring volume elements whose density values are not within the predetermined object density range with a predetermined threshold; and

a volume element remover arranged so as to remove the volume element from the object if the number of neighboring volume elements whose density values are not within the predetermined object density range exceeds the predetermined threshold.

20. The CT scanning system of claim 19 wherein the predetermined object density range is selected such that volume elements at a surface of the object are removed from the object.

21. The CT rscanning system of claim 19 wherein the predetermined object density range is selected such that volume elements at a surface of an interior void region of the object are not removed from the object.

22. A computed tomography (CT) scanning system for detecting sheet objects in a region comprising:

a volume element identifier arranged so as to identify a plurality of volume elements in the CT data for the regions each volume element being associated with a density value;

voxel identification processor arranged so as to perform a sheet voxel identification process on the CT data for the region to identify those volume elements associated with any sheet objects represented by the CT data for the region and to discriminate those volume elements associated with any sheet objects from volume elements associated with other object shapes; and

a volume element connector arranged so as to connect the volume elements identified by the sheet voxel identification process as associated with any sheet objects into sheet objects

a distance computing component arranged so as to compute a distance between a first object and a second object represented in the CT data;

a density computing component arranged so as to compute a first density associated with the first object and a second density associated with a second object;

a difference computing component arranged so as to compute a difference between the first density and the second density; and

an object combiner arranged so as to combine the first object and the second object into a single object if the difference between the first density and the second density is below a predetermined density difference threshold and the distance between the first object and the second object is below a predetermined distance threshold.

23. The CT scanning system of claim 22 further comprising:

a mass computing component arranged so as to compute a first mass of the first object and a second mass of the second object;

a mass combiner arranged so as to combine the mass of the first and second objects into a combined mass; and

an object classifier arranged so as to classify the single object according to the combined mass.

24. The CT scanning system of claim 23 wherein the means for classifying the single object according to the combined mass comprises:

a mass comparator arranged so as to compare the combined mass to a predetermined mass threshold; and

an object labeler arranged so as to label the single combined object as a threat object if the combined mass exceeds the predetermined mass threshold.