X-ray backscatter detection system

• US 5,181,234 A
• Filed: 05/22/1991
• Issued: 01/19/1993
• Est. Priority Date: 08/06/1990
• Status: Expired due to Term

• Current Assignee: Rapiscan Systems Incorporated (OSI Systems Incorporated)
• Original Assignee: IRT Corporation (William M. Scranton)
• Inventors: Smith, Steven W.
• Primary Examiner(s): Hannaher, Constantine
• Assistant Examiner(s): CHU, KIM KWOK
• Application Number:
  US07/704,292
• Time in Patent Office: 608 Days
• Field of Search: 378/87, 378/62, 378/57, 378/70, 378/86, 378/88, 378/146, 378/901, 378/99, 250/472.1, 382/16, 382/22
• US Class Current: 378/87
• CPC Class Codes: G01V 5/222 measuring scattered radiation
• CPC Class Codes: G21K 4/00 Conversion screens for the conversion of the spatial distribution of X-rays or particle radiation into visible images, e.g. fluoroscopic screens photographic processes using X-ray intensifiers G03C5/17; discharge tubes comprising luminescent screens H01J1/62; cathode ray tubes for X-ray conversion with optical output H01J31/50
• CPC Class Codes: H05G 1/60 Circuit arrangements for obtaining a series of X-ray photographs or for X-ray cinematography
• CPC Class Codes: Y10S 378/901 Computer tomography program or processor

Abstract

A pencil beam of X-rays is scanned over the surface of the body of a person being examined. X-rays that are scattered or reflected from the subject'"'"'s body are detected by a detector. The signal produced by this scattered X-ray detector in then used to modulate an image display device to produce an image of the subject and any concealed objects carried by the subject. The detector assembly is constructed in a configuration to automatically and uniformly enhance the image edges of low atomic number (low Z) concealed objects to facilitate their detection. A storage means is provided by which previously acquired images can be compared with the present image for analyzing variances in similarities with the present image, and provides means for creating a generic representation of the body being examined while suppressing anatomical features of the subject to minimize invasion of the subject'"'"'s privacy.

Claims

1. X-ray imaging apparatus for detecting a low atomic number object carried by or on a human body positioned at a distance from said apparatus comprising:

x-ray source for producing a pencil beam of X-rays directed toward said human body;

scanning means for moving the region of intersection of said pencil beam and said human body over the surface of said human body in a scanning cycle, said scanning cycle being sufficiently short to expose said human body to a low radiation dose;

a detector assembly providing a signal representative of the intensity of the X-rays scattered from said human body as a result of being scanned by said scanning means, said detector assembly being disposed on a same side of said human body as said X-ray source and having an active area with dimensions sufficient to receive a substantial portion of said scattered X-rays to provide a coefficient of variation of less than 10 percent in said signal; and

display means to presenting characteristics of the detector signal to an operator;

wherein said scattered X-rays are distributed across said detector to create an edge effect which enhances edges of said low atomic number object to enable detection.

2. X-ray imaging apparatus as in claim 1 wherein said display means includes means for suppressing anatomical features of said human body.

3. X-ray imaging apparatus of claim 1 wherein at least a portion of said detector assembly is horizontally adjacent said X-ray source.

4. The X-ray imaging apparatus of claim 1 wherein said detector assembly is positioned substantially symmetrically around the pencil beam of X-rays.

5. X-ray imaging apparatus as in claim 1 wherein said detector assembly comprises at least one each of:

a light tight enclosure;

`an X-ray conversion means having an active area for converting the scattered X-rays into light in an interior of said light tight enclosure;

photodetecting means having an active area responsive to said light positioned in the interior of the enclosure wherein the active area of said X-ray conversion means is larger than the active area of said photodetecting means.

6. X-ray imaging apparatus as in claim 5 wherein said photodetecting means comprises a photomultiplier tube.

7. X-ray imaging apparatus of claim 5 wherein said active area of said X-ray conversion means has vertical and horizontal dimensions each greater than one-fifth of said distance between said human body and said apparatus.

8. X-ray imaging apparatus as in claim 5 wherein said light tight enclosure has a generally triangular configuration in side view.

9. X-ray imaging apparatus as in claim 5 wherein said X-ray conversion means comprises a fluorescent chemical screen.

10. X-ray imaging apparatus as in claim 9 wherein said fluorescent chemical screen comprises calcium tungstate and has an areal density in the range of 40 to 140 milligrams per square centimeter.

11. X-ray imaging apparatus as in claim 9 wherein said fluorescent chemical screen comprising lanthanum oxybromide:

thulium activated and has an areal density in the range of 40 to 140 milligrams per square centimeter.

12. Radiant energy imaging apparatus for detecting objects carried on a person'"'"'s body comprising:

radiant energy imaging means providing an image signal representative of said person'"'"'s body and objects carried on said person'"'"'s body by detecting radiant energy modulated by the body;

memory means for storing a library of reference image features derived from previously imaged human bodies;

computational means for comparing features of said image signal to anatomical features of said reference image features to identify anatomical features of said image signal; and

display means for presenting a display signal representative of said image signal wherein its anatomical features have been suppressed.

13. Radiant energy imaging apparatus as in claim 12 wherein said library of reference image features further contains information for comparison to said image signal to permit suppression of benign objects in said display signal.

14. Radiant energy imaging apparatus as in claim 12 wherein the radiant energy comprises X-rays that have been scattered by said person'"'"'s body being examined.

15. Radiant energy imaging apparatus as in claim 12 wherein said display signal includes a generic humanoid outline, and the suppression consists of removing the similar features from said display signal, said generic humanoid outline providing a reference for locating objects on said person'"'"'s body.

16. Apparatus for searching a subject comprising primarily a first low Z material for concealed objects of a second low Z material, comprising:

a source of X-rays;

collimation means for forming a beam of X-rays from said X-ray source, wherein said beam intersects said subject at a region of intersection corresponding to a pixel having a pixel value;

scanning means for moving said region of intersection over the surface of said subject in a scanning cycle so that a plurality of pixels is defined, said scanning cycle being sufficiently short that said subject is exposed to a radiation dose of less than 10 microRem;

a detector adjacent said scanning means for detecting a substantial portion of X-rays backscattered from said subject and providing an image signal comprising a plurality of pixel values representative of X-rays backscattered from said subject as a result of being scanned by said beam wherein said plurality of pixel values has a coefficient of variation resulting from X-ray statistical noise of less than 10 percent; and

display means for visually presenting said image signal.

17. An apparatus for searching a subject as in claim 16 wherein at least 2 percent of X-rays in said beam are detected by said detector.

18. An apparatus for searching a subject as in claim 16 wherein said radiation dose is less than five microRem.

19. An apparatus for searching a subject as in claim 16 wherein X-rays in said beam have an energy in the range of 22 to 37 KeV.

20. Apparatus for searching a person as in claim 16, wherein said source of X-rays comprises an X-ray tube operating at a potential in the approximate range of 35 to 70 Kilovolts.

21. An apparatus for searching a subject as in claim 16, wherein said region of intersection has a diameter in the range of 3.5 to 11 millimeters.

22. An apparatus for searching a person as in claim 16, wherein each pixel comprises an area in the range of 12 to 120 square millimeters.

23. An apparatus for searching a subject as in claim 16, wherein said detector means comprises an effective X-ray sensitive area greater than 150 square inches and is positioned a distance of less than 15 inches from the body of said subject.

24. An apparatus for searching a subject as in claim 16 wherein said display means comprises a computer and a monitor.

25. An apparatus for searching a subject as in claim 24 wherein said computer has a memory in which is stored a library of reference images for comparison to said image signal.

26. An apparatus for searching a subject as in claim 25 wherein said computer uses comparison of said image signal to said library of reference images to suppress certain features within said image signal prior to display on said monitor.

27. In an X-ray backscatter imaging system for searching for a first low Z material on a background having second low Z, said first low Z being lower than said second low Z, a method comprising the steps of:

positioning said background body in an imaging area;

operating an X-ray tube to produce X-rays having a low dose directed toward said imaging area;

collimating said X-rays into a pencil beam of a cross-section measured where said pencil beam intersects said imaging area, said cross-section corresponding to one of a plurality of pixels each pixel having a pixel value;

scanning said pencil beam of X-rays over said image area at a sufficiently high rate to impart less than 10 microRem of radiation dose to said background body;

detecting X-rays that are backscattered from said pencil beam as a result of interacting with matter positioned in said image area; and

displaying a digitally represented image comprising said plurality of pixels wherein each pixel of said plurality is representative of the detected backscattered X-rays produced by matter located at its corresponding cross-section and a coefficient of variation of all pixel values of said plurality of pixels resulting from X-ray statistical noise of less than 10 percent.

28. A method for searching a background body for concealed objects as in claim 27 wherein the step of displaying further comprises comparing said digitally represented image to a library of reference images.

29. A method of searching as in claim 28 wherein the step of displaying further comprises suppressing certain features of said digitally represented image which are common with reference images in said library.

30. A method for searching as in claim 27, wherein the step of collimating produces said cross-section in the range of 12 to 120 square millimeters.

31. A method for searching as in claim 27 wherein the step of operating an X-ray tube comprises producing X-rays having energies in the range of 22 to 37 KeV.

32. A method for searching as in claim 27, wherein the step of operating an X-ray tube comprises setting the X-ray tube potential in the range of 35 to 70 Kilovolts.

33. A method for searching as in claim 27, wherein the step of detecting X-rays comprises positioning an effective X-ray sensitive area greater than 150 square inches positioned a distance less than 15 inches from said subject.

34. A method for searching as in claim 27 further comprising adjusting the time period of said scan cycle and adjusting the area of said cross-sectional area to provide a radiation dose less than 5 microRem.

35. A method for searching as in claim 34 wherein said radiation dose is approximately 3 microRem and said coefficient of variation is approximately 5 percent.

36. In an X-ray backscatter imaging system for searching a person for concealed plastic explosive, a method comprising the steps of:

positioning said person in an imaging area;

operating an X-ray tube to produce X-rays directed toward said imaging area;

collimating said X-rays into a pencil beam of cross-section measured where said pencil beam intersects said imaging area;

scanning said pencil beam over said image area at a sufficiently high rate to impart less than 10 microRem of radiation dose to said person;

detecting a sufficient quantity of X-rays that are backscattered from said pencil beam as a result of interacting with matter positioned in said image area so that modulation of the backscattered X-rays resulting from the presence of said plastic explosive is distinguishable from statistical noise of the detected X-rays; and

displaying an image representative of the modulation resulting from the presence of said plastic explosive in said imaging area.

37. A method for searching a subject for concealed plastic explosive as in claim 36 wherein the step of displaying comprises comparing the modulation of the detected X-rays to a library of image features.

38. A method of searching a subject for concealed plastic explosive as in claim 37 wherein the step of displaying further comprises suppressing a portion of the modulation of the detected X-rays which is in common with said library of image features.

39. A method for searching as in claim 36 wherein said plastic explosive has an effective atomic number in the range of 6 to 8.

40. A method for searching as in claim 36 wherein said radiation dose is less than 5 microRem.

41. A method for searching as in claim 36 wherein the modulation of the backscattered X-rays resulting from the presence of said plastic explosive in said imaging area is surrounded by modulation resulting from the presence of the body of said subject in said imaging area.

42. A method for searching as in claim 36 wherein said plastic explosive has a thickness of less than one-half inch.