Patent Publication Number: US-7593506-B2

Title: Backscatter inspection portal

Description:
The present application is a Continuation Application of U.S. patent application Ser. No. 11/097,092, issued Jul. 15, 2008 as U.S. Pat. No. 7,400,701, and, like that Application, claims priority from U.S. Provisional Application No. 60/561,079, filed Apr. 9, 2004. Both of the foregoing applications are incorporated herein by reference. 

   TECHNICAL FIELD 
   The present invention relates to systems and methods for inspecting objects with penetrating radiation, and, more particularly, the invention relates to inspection systems employing multiple sources of radiation. 
   BACKGROUND ART 
   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 inspected object, while the inspected object is moved past one or more systems that image the contents of the object using penetrating radiation. The determination should be capable of being made while the inspected object is in motion, or, alternatively, while the inspection system is in motion with respect to the inspected person or object. Indeed, since inspection rate, and thus hourly throughput, is at a premium, it is desirable that the vehicle, for example, be driven without requiring the driver or passengers to alight. In case a detection is made, a visual image should be available for verification. 
   The use of images produced by detection and analysis of penetrating radiation scattered from an irradiated object, container, or vehicle is the subject, for example, of U.S. Pat. No. 6,459,764, to Chalmers et al. (the “Chalmers Patent”), issued Oct. 1, 2002, and incorporated herein by reference. The Chalmers Patent teaches backscatter inspection of a moving vehicle by illuminating the vehicle with x-rays from above or beneath the moving vehicle, as well as from the side. 
   The use of an x-ray source and an x-ray detector, both located in a portal, for purposes of screening personnel, is the subject, for example, of U.S. Pat. No. 6,094,472, to Smith, issued Jul. 25, 2000. 
   X-rays are scattered from matter in all directions, therefore, scatter may be detected by an x-ray detector disposed at any angle to the scattering material with respect to the direction of incidence of the illuminating radiation. Therefore, a “flying spot” irradiation system is typically used, whereby a single point on the inspected object is illuminated with penetrating radiation at any given moment, so that the locus of scatter can be determined unambiguously, at least with respect to the plane transverse to the direction of the beam of penetrating radiation. 
   In order to obtain multiple views of an inspected object, multiple backscatter imaging systems may be employed in a single inspection tunnel. This may result in interference, or cross-talk, between respective imaging systems, resulting in image degradation. This is due to the lack of each flying-spot imager&#39;s ability to distinguish the origin of the scattered radiation from each imager&#39;s source. To date, this problem has been addressed by placing the imagers some distance apart to minimize cross talk. This approach causes the size of the overall system to increase. In space-limited applications, this is often undesirable. 
   SUMMARY OF THE INVENTION 
   In one embodiment of the present invention, there is provided an inspection system for inspecting an object. The inspection system has a first source for providing a first beam of penetrating radiation of specified cross-section directed in a first beam direction and 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 temporally interspersed. Additionally, the system has a plurality of scatter detectors for detecting radiation scattered from at least one of the first beam and the other beams by any scattering material within the inspected object 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 creating an image of the scattering material based at least on the scattered radiation signal or for otherwise characterizing the scattering material. 
   In accordance with alternate embodiments of the invention, the first source of penetrating radiation may be an x-ray source, as may the other sources of penetrating radiation. The first beam direction and the direction of any other beam may be substantially coplanar. The various 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. 
   In accordance with yet further embodiments of the invention, emission of penetrating radiation in the first beam may be characterized by a first temporal period and emission of penetrating radiation in the second beam may be characterized by a second temporal period, the first and the second temporal periods offset by fixed phase relationship. The temporal period of each source may be characterized by a duty cycle, and the emission of adjacent sources may be characterized by a phase relationship with respect to an adjacent source, where the phase relationship may equal to 2π times the duty cycle. 
   In accordance with yet further embodiments of the invention, the inspection system may further including a display for displaying a scatter image of material disposed within the inspected object. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing features of the invention will be more readily understood by reference to the following detailed description, taken with reference to the accompanying drawings, in which: 
       FIG. 1  shows a schematic cross sectional view of an x-ray inspection system that uses multiple backscatter imaging systems in accordance with embodiments of the present invention; and 
       FIG. 2  shows a side view of the x-ray inspection system embodiments of  FIG. 1 . 
       FIG. 3  shows a prior art backscatter system employing a rotating chopper wheel of a sort employed in various embodiments of the present invention. 
       FIG. 4  shows a prior art backscatter system employing an electromagnetic scanner of a sort employed in various embodiments of the present invention. 
   

   DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
   In accordance with embodiments of the present invention, beam cross talk is minimized between or among multiple flying-spot backscatter imaging systems configured as a multi-view backscatter inspection system, with no restriction on the distance between the individual imaging systems. In other words, in a multi-view system comprised of individual backscatter imaging systems for each view, the individual imaging systems can be placed as close together as is physically possible, while cross talk is advantageously reduced or eliminated. 
   Methods and advantages of backscatter inspection of a moving vehicle by illuminating the vehicles with x-rays from either above or beneath the moving vehicle are described in U.S. Pat. No. 6,249,567, issued Jun. 19, 2001, which is incorporated herein by reference. In an embodiment of the aforesaid patent, illustrated in  FIG. 3 , rotating chopper wheel  20  is used to develop a pencil beam  14  which may be swept in a plane substantially parallel to that of the page. The formation of pencil beam  14  by a series of tubular collimators  313  distributed as spokes on rotating wheel  20  is known in the art. In accordance with preferred embodiments of the present invention, regions of enhanced backscatter that arise due to materials concealed close to the side walls of a vehicle are revealed without requiring that penetrating radiation traverse the vehicle during the course of inspection. 
     FIG. 1  shows a schematic cross-sectional view of the elements of an inspection system, designated generally by numeral  10 . An object of inspection  18 , which may be animate or inanimate, moves, or is moved, in a direction into, or out of, the page and thus traverses a portal  12 . Portal  12  supports a plurality of sources  13 ,  15 , and  17  of penetrating radiation. Sources  13 ,  15 , and  17  are typically x-ray tubes having beam forming and steering mechanisms known in the art. For example, source  13  emits penetrating radiation in a beam  23  having a cross-section of a specified shape. For scatter imaging applications, a narrow pencil beam is typically employed. Beam  23  of penetrating radiation, may be, for example, a beam of x-rays such as a polychromatic x-ray beam. While source  13  of penetrating radiation is preferably an x-ray tube, for example, however other sources of penetrating radiation, such as a linac (linear accelerator), are within the scope of the present invention, and, indeed, the penetrating radiation is not limited to x-ray radiation and may include gamma ray radiation. 
   A scanning mechanism is provided for scanning beam  23  along a substantially vertical axis, such that, during a portion of a duty cycle, beam  23  is directed in a series of directions such as  24 . Object  18  that is to be inspected moves past beam  23  in a substantially horizontal direction, into the page, in the depiction of  FIG. 1 . In alternate embodiments of the invention, the source and/or other portions of the inspection system may be moved in relation to object  18 , which may be moving itself, or stationary. 
   Source  13  may include a scanning mechanism such as a flying spot rotating chopper wheel  20  (shown in  FIG. 3 ) as known to persons skilled in the art. Alternatively, electromagnetic scanners may be employed, such as scanner  104  (shown in  FIG. 4 ) those described in U.S. Pat. No. 6,421,420, issued Jul. 23, 2002 and entitled “Method and Apparatus for Generating Sequential Beams of Penetrating Radiation,” which is incorporated herein by reference. A source  412  supplies a beam of charged particles  140  that are accelerated to a surface of a target  160 . Electromagnetic beam director  418  can be any electromagnetic beam directing arrangement such as magnetic or electrostatic yokes. Penetrating electromagnetic radiation is emitted by target  160  and pass through a collimator  422  disposed a specified distance from the target, thus producing sequential parallel beams of radiation. 
   Beams of sources  15  and  17  are shown in typical extremal positions of their respective scans, and are labeled  25 ,  26 ,  27 , and  28 . Inspected object  18 , which, as discussed, may refer to a vehicle, a container, or a person, for example, may be self-propelled through beams  23 - 28  or may be conveyed by a mechanized conveyor  29  or pulled by a tractor, etc. In alternate embodiments of the invention, the inspection system, configured, for example, as a portal, may move, or be moved, over an object such as a vehicle that may, itself, be moving or stationary. 
   Beams  23 - 28  will be referred to in the present description, without limitation, as x-ray beams. In accordance with preferred embodiments of the invention, a rotating chopper wheel is used to develop a pencil beam  23 - 28  which may be swept in a plane substantially parallel to that of 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 many shapes. The dimensions of pencil beam  23 - 28  typically define the scatter image resolution which may be obtained with the system. Other shapes of beam cross section may be advantageously employed in particular applications. 
   A detector arrangement, typified by scatter detector  31 , is disposed in a plane parallel to the direction of motion of object  18  during the course of the scan. X-rays  30  scattered by Compton scattering out of beam  24  in an essentially backward 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 supplementarily for detecting x-rays Compton-scattered from beam  24  and similarly, as will presently be described, for each of the other beams incident, in turn, on inspected object  18 . 
   Additionally, transmission detectors disposed distally to the inspected object  18  with respect to the emitting source may be used to augment the scatter image or images with an image of the object as obtained in transmitted x-rays, for example, the detector elements designated  35  and  36  detect the emission of source  13  as transmitted through the inspected object. In another embodiment of the invention, a single separate detector is disposed between the pair of scatter detectors  35  and the pair of scatter detectors  36  and is employed for detection of penetrating radiation transmitted through object  18 . 
   Within the scope of the invention, any x-ray detection technology known in the art may be employed for the detector arrangements  31 - 36 . The detectors may be scintillation materials, either solid or liquid or gaseous, viewed by photo-sensitive detectors such as photomultipliers or solid state detectors. Liquid scintillators may be doped with tin or other element or elements of high atomic number. Respective output signals from the scatter detectors  31 - 36  are transmitted to a processor  40 , and processed to obtain images of feature  42  inside the inspected object  18 . Since incident x-ray photons are scattered by scattering sources within object  18  into all directions, detectors with large areas are used to maximize the collection of the scattered photons. In accordance with certain 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, etc., all as known in the art. 
   In order to allow views of the inspected object from multiple directions, multiple sources  13 - 17  are used to irradiate the inspected object. However, since the photons emitted by each source are scattered in all directions, care must be exercised in order to eliminate cross-talk, i.e., the misidentification of the source of irradiation. In accordance with 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 emitted from the imaging systems is set less-than or equal-to the inverse of the number of imaging systems, or views, in the multi-view system. For example, if the number of views desired is six, each imaging system is set for a duty cycle of ⅙, or less. 
   Next, the phase relationship between each pair of adjacent sources is set to 2π times the duty cycle. This results in sequenced radiation emission from the imagers, eliminating the possibility of concurrent 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-cycles be ⅙, and that their phase relationship be 2π/6, or 60 degrees. 
   In cases where flying-spot systems are realized by mechanical means such as rotating hoops and chopper wheels, these aforesaid criteria may be met by synchronization of the motion of the mechanical chopper elements, biased by phase offsets. Thus, for example, where collimators are rotated to define the path of emergent x-ray beam  23 , close-loop motion controller systems known in the art may be employed to drive the rotation of the collimators. The duty cycle is controlled by setting the fan aperture (the total sweep angle of a beam, i.e., the angle between extremal beams  23  and  24  of a single source), equal to 2π times the duty cycle. In systems where the emitted radiation can be controlled electronically, any desired sequence of irradiation or range of sweep may be set, without limitation, entirely by electronic or software control. 
   By virtue of temporal sequencing which reduces or eliminates cross-talk, sources may be placed in greater proximity than otherwise possible. In particular, sources  13 - 17  may be disposed in a single plane, which advantageously permits virtually simultaneous on/off control of the x-rays regardless of the speed with which the object is passing by the imagers. 
   The system described may advantageously provide for an image to be derived from the perspective of each successive source  13 - 17 .  FIG. 1  shows an exemplary three-view system, with beams  23 ,  25 , etc. each sweeping trajectories that are coplanar. 
   The beams from each imager sweep in sequence, such that no more than one imager is emitting radiation at a time. Thus, source (or ‘imager’)  13  sweeps its beam first. Radiation scattered from the object, as represented by rays  44 , is received by all of the detectors. The signals from each of the detectors are acquired as separate channels by an acquisition system. This process is repeated for each of the three imagers, creating “slices” of the object as it moves by. 
   Referring now to  FIG. 2 , a side view is shown of the arrangement of  FIG. 1 , with elements designated by corresponding numbers. A slot  50  is shown through which the beam of source  13  passes through segments  52  and  54  of detector  31  as object  18  is scanned while moving in a lateral direction  16 . 
   The signals from the detectors can be selectively used to reconstruct an image of the object. Since scattered photons  44  detected by detectors  33  and  34  from source  13  are as useful as scattered photons from source  17 , these same detectors can be shared among all sources, and result in improved scatter collection with efficient use of the detector hardware. 
   Embodiments of this invention, furthermore, may advantageously allow multi-view Flying-Spot X-ray Scatter imaging to be practiced in a smaller operational footprint by eliminating cross talk, and by allowing closer positioning of the individual imagers for each view. The close positioning of these imagers (where an “imager” refers to a source, at least one detector, and associated electronics and signal processing) may also allow sharing of scatter detectors between, or among, imagers, allowing more scatter collection for improved image quality, with efficient use of detector hardware. 
   In applications where scanning of selective regions of the object is desired, co-planar positioning of the imagers allows simultaneous on/off control of the x-rays regardless of the speed with which the object is passing by the imagers. This greatly simplifies the design of the control of x-ray emissions from each imager in the multi-view inspection system, thus individual sequencing of x-ray emissions need not be performed as is typically practiced in systems in which emission is not co-planar. 
   Besides imaging contents of concealing enclosures, in terms of which embodiments of the present invention have been described, other characteristics of inspected objects may be obtained within the scope of the present invention. For example, backscatter techniques may be applied, as known in the art, for deriving mass, mass density, mass distribution, mean atomic number, or likelihood of containing targeted threat material. 
   In accordance with certain embodiments of the invention, x-rays having maximal energies in the range between 160 keV and 300 keV are employed. At this energy, x-rays penetrate into a vehicle, and organic objects inside the vehicle can be detected. Since lower doses of x-ray irradiation are thus possible, automobiles may be scanned using the present invention. For applications where the scanned vehicle may contain personnel, end point energies below 300 keV are preferred. The scope of the present invention, however, is not limited by 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 variations and modifications are intended to be within the scope of the present invention as defined in the appended claims.