HK1104181B - Eliminating cross-talk in a backscatter inspection portal comprising multiples sources by ensuring that only one source is emitting radiation at a time - Google Patents

Description

Cancellation of crosstalk in a backscatter inspection portal comprising multiple sources by ensuring that only one source is emitting radiation at a time

Technical Field

The present invention relates to systems and methods for inspecting objects with penetrating radiation, and more particularly, the present invention relates to inspection systems that employ multiple radiation sources.

Background

It is desirable to determine the presence of objects, such as contraband, weapons, or explosives, that have been concealed, for example, in a moving vehicle, or on a person, or in any object under inspection, as the object under inspection moves past one or more systems that use penetrating radiation to image the contents of the object. It should be possible to make a determination while the object being inspected is in motion, or alternatively, while the inspection system is in motion relative to the person or object being inspected. In fact, because inspection speed and, thus, hourly throughput is very important, it is desirable that, for example, the vehicle be driven without requiring the driver or passenger to alight. In the case where a test is made, the visual image should be available for verification.

The use of images produced by detecting and analyzing penetrating radiation scattered from an irradiated object, container, or vehicle is the subject of, for example, U.S. patent No. 6,459,764 issued to Chalmers et al ("Chalmers patent") published on 10.1.2002. The Chalmers patent discloses backscatter inspection of a moving vehicle by illuminating the vehicle with x-rays from above or below the moving vehicle and from the side.

The use of an x-ray source and an x-ray detector, both located at the entrance, for photographing (screening) persons is the subject of us patent No. 6,094,072 to Smith, published, for example, 7, 25/2000.

X-rays scatter from the object in all directions, and thus scatter may be detected by an X-ray detector arranged at any angle to the scattering material relative to the direction of incidence of the illuminating radiation. Therefore, it is common to use a "flying spot" irradiation system, whereby a single point on the object under examination is irradiated with penetrating radiation at any given moment, so that the trajectory of the scatter can be unambiguously determined, at least with respect to a plane transverse to the beam direction of the penetrating radiation.

To obtain multiple views of the inspected object, multiple backscatter imaging systems may be employed in a single inspection tunnel. This may lead to interference or cross-talk between the imaging systems, causing image degradation. This is due to the lack of ability of the flying spot imagers to discern the source of the scattered radiation from the source of each imager. Heretofore, this problem has been addressed by placing the imagers some distance apart to minimize crosstalk. This approach results in an increase in the size of the overall system. In space-constrained applications, this is often undesirable.

Disclosure of Invention

In one embodiment of the present invention, an inspection system for inspecting an object having features that move in particular directions relative to the inspection system by virtue of movement relative to a local frame of reference of the object, the inspection system, or both, is provided. The inspection system has a first source for providing a first beam of penetrating radiation of a particular cross-section directed in a first beam direction generally transverse to the direction of motion of the object. It also has a second source for providing a second beam of penetrating radiation in a second beam direction and may have additional sources of additional beams. The beams of penetrating radiation are interspersed in time (temporallyInterpersed). In addition, the system has a plurality of scatter detectors for detecting radiation scattered from at least one of the first beam and the other beam by any scattering material within the object under examination and for generating a scattered radiation signal. The system may also have one or more transmission detectors for detecting penetrating radiation transmitted through the object. Finally, the system has a controller for generating an image of the scattering material based at least on the scattered radiation signal, or for otherwise characterizing the scattering material.

According to an alternative embodiment of the invention, the first source of penetrating radiation may be an x-ray source, but may also be another source of penetrating radiation. The first beam direction and the direction of any other beam may be substantially coplanar. The multiple sources may include a beam scanning mechanism, such as a rotating chopper wheel or an electromagnetic scanner, and one or more of the beams may be pencil beams.

According to another embodiment of the invention, the emission of penetrating radiation of the first beam is characterized by a first time period and the emission of penetrating radiation of the second beam is characterized by a second time period, the first and second time periods being offset by a fixed phase relationship. The time period of each source is characterized by a duty cycle and the emissions of adjacent sources are characterized by a phase relationship with respect to the adjacent sources, where the phase relationship may be equal to 2 pi times the duty cycle.

According to a further embodiment of the invention, the examination system may further comprise a display for displaying a scatter image of a material arranged within the object under examination.

Drawings

The above features of the present invention will be more readily understood by reference to the following detailed description made with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic cross-sectional view of an x-ray inspection system using multiple backscatter imaging systems in accordance with an embodiment of the invention; and is

FIG. 2 shows a side view of an embodiment of the x-ray inspection system of FIG. 1.

Detailed Description

In accordance with embodiments of the present invention, beam cross-talk is minimized between multiple flying spot backscatter imaging systems configured as a multi-view backscatter inspection system, while there is no limit to the distance between the individual imaging systems. In other words, in a multi-view system consisting of a single backscatter imaging system for each view, the individual imaging systems can be placed close together to a physically feasible degree of closeness, while advantageously reducing or eliminating crosstalk.

The method and advantages of backscatter inspection of a moving vehicle by illuminating the vehicle with x-rays from above or below the moving vehicle are described in U.S. patent No. 6,249,567, published 6-19-2001, which is incorporated herein by reference in its entirety. According to a preferred embodiment of the present invention, the backscatter enhancement area that occurs due to the concealed material being close to the side wall of the vehicle can be revealed without the need for penetrating radiation that traverses the vehicle during the inspection process.

FIG. 1 shows a schematic cross-sectional view of elements of an inspection system generally designated by reference numeral 10. An inspection target 18, which may be animate or inanimate, moves or is moved in a direction into or out of the page to traverse the portal 12. The portal 12 supports a plurality of sources 13, 15 and 17 of penetrating radiation. Sources 13, 15 and 17 are typically x-ray tubes with beam forming and steering mechanisms as known in the art. For example, the source 13 emits penetrating radiation in a beam 23 having a cross-section of a particular shape. For scatter imaging applications, a narrow pencil beam is typically employed. The beam 23 of penetrating radiation may be, for example, an x-ray beam such as a polychromatic x-ray beam. Although the source of penetrating radiation 13 is preferably an x-ray tube, for example, other sources of penetrating radiation, such as a linear accelerator (linac), are within the scope of the present invention, and indeed penetrating radiation is not limited to x-ray radiation and may include gamma ray radiation.

A scanning mechanism is provided for scanning the beam 23 along a generally vertical axis such that the beam 23 is directed in a series of directions such as 24 during portions of the duty cycle. In the depiction of fig. 1, the object 18 to be inspected moves in a generally horizontal direction through the beam 23, into the page. In alternative embodiments of the present invention, the source and/or other portions of the inspection system may be movable relative to object 18, and object 18 may itself be movable or stationary.

The source 13 may include a scanning mechanism such as a flying spot rotating chopper wheel as known to those skilled in the art. Alternatively, an electromagnetic scanner may be employed, such as those described in U.S. patent No. 6,421,420 entitled "Method and Apparatus for Generating Sequential Beams of networking Radiation" published on 7/23 2002, which is hereby incorporated by reference in its entirety.

The beams of sources 15 and 17 are shown in their typical extreme positions of scanning and are labeled 25, 26, 27 and 28. The object 18 under inspection as discussed above may refer to, for example, a vehicle, container, or person that may be automatically passed through the beams 23-28 or may be conveyed by a motorized conveyor or towed by a tractor or the like. In an alternative embodiment of the invention, the inspection system, for example configured as a portal, may be moved or moved over an object, such as a vehicle, wherein the object itself may be moving or stationary.

The beams 23-28 in this description will be referred to as x-ray beams and are not limited thereto. According to a preferred embodiment of the invention a rotating chopper wheel is used to generate pencil beams 23-28 which can be scanned in a plane substantially parallel to the page. The cross-section of pencil beam 23 is of comparable extent in each dimension and is typically substantially circular, although it may be of many shapes. The size of pencil beams 23-28 generally defines the scatter image resolution achievable with this system. Other shapes of beam cross-sections may be advantageously employed in particular applications.

The detector arrangement, represented by scatter detector 31, is arranged in a plane parallel to the direction of movement of object 18 during the scanning process. X-rays 30 scattered by compton scattering from beam 24 in a substantially opposite direction are detected by one or more backscatter detectors 31 disposed between source 13 and object 18. Additional detector arrangements 32, 33, 34, 35 and 36 may be used to assist in detecting compton scattered x-rays from beam 24 and, similarly, each of the other beams incident on object under examination 18 in turn, as will be described below.

In addition, a transmission detector disposed at a distal end of the inspected object 18 relative to the emission source may be used to augment the scatter image with the object image obtained in the transmitted x-rays, e.g., detector elements designated 35 and 36 detect radiation transmitted through the inspected object of source 13. In another embodiment of the invention, a single separate detector is disposed between a pair of scatter detectors 35 and a pair of scatter detectors 36 and is used to detect penetrating radiation transmitted through object 16.

Within the scope of the invention, x-ray detection techniques known in the art may be employed for the detector arrangements 31-36. The detector may be a solid, liquid or gaseous scintillation material that is viewed by a light sensitive detector such as a photomultiplier or solid state detector. The liquid scintillator may be doped with tin or other elements or elements of high atomic number. The output signals from scatter detectors 31-36 are transmitted to processor 40 and processed to obtain an image of a feature 42 within inspected object 18. Because incident x-ray photons are scattered in all directions by scattering sources within object 18, detectors with large areas are used to maximize the collection of scattered photons. According to some embodiments of the invention, processor 40 (otherwise referred to herein as a "controller") may also be employed to derive other characteristics of the scattering object, such as its mass, mass density, effective atomic number, and the like, as is known in the art.

To allow views of the object under examination from multiple directions, the object under examination is irradiated using a plurality of sources 13-17. However, since the photons emitted by each source are scattered in all directions, care must be taken to eliminate cross-talk, i.e., misidentification of the irradiation source. According to embodiments of the present invention, cross talk is advantageously reduced or eliminated by ensuring that only one imager is emitting radiation at a time. First, the duty cycle of the beams transmitted from the imaging system is set to be less than or equal to the inverse of the number of imaging systems or the number of views in a multi-view system. For example, if the desired number of views is 6, each imaging system is set to a duty cycle of 1/6, or less.

Next, the phase relationship between each pair of adjacent sources is set to 2 π times the duty cycle. This results in the radiation being emitted sequentially from the imagers, eliminating the possibility of simultaneous emission from more than one imager. For example, a multi-view inspection system with 6 sources would require that they run at the same frequency, that their duty cycle be 1/6, and that their phase relationship be 2 π/6, or 60 degrees.

In the case of flying-spot systems implemented by mechanical means such as rotating rings (rotating hooks) and chopper wheels, the above criteria can be met by synchronizing the movement of mechanical chopper elements biased by phase shifts. Thus, for example, where the collimator is rotated to define the path of the emerging x-ray beam 23, a closed loop motion controller system known in the art may be employed to drive the rotation of the collimator. The duty cycle is controlled by setting the fan aperture (fan aperture) (the total scan angle of the beams, i.e. the angle between the extreme beams 23 and 24 of a single source) equal to 2 pi times the duty cycle. In systems where the emitted radiation can be electronically controlled, any desired irradiation sequence or scan range can be set, without limitation, entirely by electronic or software control.

Due to the temporal ordering of the reduction or elimination of crosstalk, the sources can be placed closer together than would otherwise be possible. In particular, the sources 13-17 may be arranged in a single plane, which advantageously allows for virtually simultaneous on/off control of x-rays regardless of the speed at which the object passes through the imager.

The described system may advantageously provide images from the perspective of each successive source 13-17. Fig. 1 shows an exemplary three-view system, with beams 23, 25, etc. each scanning a coplanar track.

The beams from each imager are scanned sequentially so that only one imager is emitting radiation at a time. Thus, the source (or 'imager') 13 first scans its beam. Radiation scattered from the object, represented by rays 44, is received by all detectors. The signal from each detector is acquired by an acquisition system as a separate channel. This process is repeated for each of the three imagers, generating a "slice" of the object as it passes by.

Referring now to fig. 2, there is shown a side view of the arrangement of fig. 1, with elements assigned corresponding reference numerals. A slit 50 is shown through which the beam of source 13 passes through portions 52 and 54 of detector 31 as object 18 is scanned while the object is moving in lateral direction 16.

The signals from the detectors may optionally be used to reconstruct an image of the object. Because scattered photons 44 from source 13 detected by detectors 33 and 34 are as useful as scattered photons from source 17, these same detectors can be shared among all sources and result in efficient use of detection hardware, thereby improving scatter collection.

Furthermore, embodiments of the present invention may advantageously allow multi-view flying spot x-ray scatter imaging to be implemented in a smaller operating footprint by eliminating cross-talk and by allowing the individual imagers for each view to be placed closer together. The close proximity of these imagers (where "imager" refers to the source, at least one detector, and associated electronics and signal processing) may also allow sharing of scatter detectors between the imagers, allowing for efficient use of detector hardware for better scatter collection for improved image quality.

In applications where it is desired to scan a selected area of an object, the co-planar placement of the imagers allows for simultaneous on/off control of the x-rays regardless of the speed of the object past the imagers. This greatly simplifies the design of the control of x-ray emissions from each imager in a multi-view inspection system, eliminating the need to perform individual sequencing of x-ray emissions as is commonly implemented in systems with non-coplanar emissions.

In addition to imaging the contents of the hidden enclosure, embodiments of the invention have been described in this regard, and other features of the inspected object may be obtained within the scope of the invention. For example, backscatter techniques known in the art may be applied for deriving mass, mass density, mass distribution, mean atomic number, or likelihood of containing the targeted threat material.

According to some embodiments of the invention, x-rays with a maximum energy in the range between 160keV and 300keV are used. At this energy, the x-rays penetrate into the vehicle and can detect organisms inside the vehicle. Since a low dose of x-ray radiation is thus possible, the invention can be used to scan automobiles. For applications where the scanned vehicle may contain personnel, endpoint energies below 300keV are preferred. However, the scope of the present invention is not limited to the range of penetrating photons employed.

The described embodiments of the invention are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art. All such modifications and variations are intended to be included herein within the scope of this disclosure as defined by the following claims.

Claims (15)

1. An inspection system for inspecting an object having features that move in one direction relative to the inspection system, the system comprising:

a. a first source for providing a first beam of penetrating radiation having a particular cross-section;

b. a first beam scanning mechanism for scanning a first beam in a first beam direction substantially transverse to the direction of motion of the object;

c. a second source for providing a second beam of penetrating radiation having a particular cross-section;

d. a second beam scanning mechanism for scanning a second beam in a second beam direction substantially coplanar with and substantially perpendicular to the first beam direction, the second beam being interspersed in time with the first beam of penetrating radiation;

e. a plurality of scatter detectors, each scatter detector being positioned to detect radiation scattered from the first and second beams by any scattering material within the inspected object and for generating a scattered radiation signal; and

f. a controller for generating an image of the scattering material based at least on the scattered radiation signal.

2. The inspection system of claim 1, wherein the inspection system is fixed relative to a local frame of reference.

3. The inspection system of claim 1, wherein the inspection system is in motion relative to a local frame of reference during an inspection process.

4. The inspection system of claim 1 wherein the first source of penetrating radiation is an x-ray source.

5. The inspection system of claim 1 wherein said first and second beam scanning mechanisms are rotating chopper wheels.

6. The inspection system of claim 1 wherein said first and second beam scanning mechanisms comprise electromagnetic scanners.

7. The inspection system of claim 1 wherein the first beam of penetrating radiation is a pencil beam.

8. The inspection system of claim 1 wherein the emission of penetrating radiation of the first beam is characterized by a first time period and the emission of penetrating radiation of the second beam is characterized by a second time period, the first and second time periods being offset by a fixed phase relationship.

9. The inspection system of claim 8 wherein the time period for each source is characterized by a duty cycle.

10. The inspection system of claim 9 wherein the time period for each source is characterized by a phase relationship with respect to adjacent sources equal to 2 pi times the duty cycle.

11. The inspection system of claim 1, further comprising a display for displaying a scatter image of material disposed within the object.

12. The inspection system of claim 1 further comprising at least one transmission detector for detecting at least one of the first and second beams transmitted through the inspected object and for generating a transmitted radiation signal.

13. A method for inspecting an object, the method comprising:

a. illuminating the object with penetrating radiation forming a first beam, the first beam being scanned in a first beam direction substantially transverse to a direction of motion of the object;

b. illuminating an object with penetrating radiation forming a second beam scanned in a second beam direction substantially transverse to a direction of motion of the object, the second beam direction being substantially coplanar with the first beam direction, the second beam direction being in a fixed and substantially perpendicular direction relative to the first beam direction, the second beam direction being temporally interspersed with respect to the first beam direction;

c. detecting radiation scattered from the first and second beams by the object and at least one detector for detecting scatter from the first and second beams to generate a scattered radiation signal; and

d. characterizing the object based on the scattered radiation signal.

14. The method of claim 13, further comprising:

e. displaying a scatter image of the scattered radiation signal.

15. The method of claim 13, wherein the step of characterizing the material disposed within the vehicle comprises combining a scattered radiation signal obtained during illumination with the first spectral component with a scattered radiation signal obtained during illumination with the second spectral component.