Abstract:
A system and method for inspecting an object, where both a fan beam and a pencil beam of penetrating radiation are used to illuminating the object concurrently. Both beams may be derived from a single source of penetrating radiation. The pencil beam is noncoplanar with the fan beam and may be scanned with respect to the object. Radiation scattered from the pencil beam within the object is detected, and the scatter signal thus generated is used in conjunction with a transmission signal which characterizes attenuation of the fan beam by the object.

Description:
This application claims priority from U.S. provisional applications Nos. 60/110,223, filed Nov. 30, 1998, and 60/134,413, filed May 17, 1999, both of which applications are herein incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a system and method for inspecting an object with penetrating radiation wherein a source of such radiation provides both a fan beam and a pencil beam that may be scanned across the inspected object. 
     BACKGROUND OF THE INVENTION 
     X-ray systems are commonly employed for such applications as the inspection of materials or containers by illuminating the material or container from the outside by means of fan beams. A fan beam refers to a beam having an opening angle in one dimension substantially larger than the width of the beam in a dimension orthogonal to the first. Additionally, the use of a pencil beam, scanned across the object under inspection is also known and used in the art. 
     The concurrent or alternating application of one or more fan beams and one or more pencil beams is the subject, for example, of copending provisional application No. 60/072,890, filed Jan. 28, 1998, which is herein incorporated by reference. 
     The methods known in the art for concurrently applying multiple beams require multiple sources penetrating radiation. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, in one of its embodiments, there is provided an inspection system for inspecting an object such as a cargo enclosure. The system has a source of penetrating radiation for providing a pencil beam and a fan beam, the pencil beam being noncoplanar with the fan beam. Additionally, the system has a first detector arrangement for detecting penetrating radiation from the fan beam transmitted through the object and generating a transmitted radiation signal. Similarly, the system has a second detector arrangement for detecting penetrating radiation from the pencil beam scattered by the object and generating a scattered radiation signal. Finally, the system has a controller for determining at least one characteristic of the object based at least on the transmitted and scattered radiation signals. 
     In accordance with alternate embodiments of the invention, the source of penetrating radiation may be an x-ray source, including an x-ray tube or a linear accelerator (LINAC), and may be pulsed or continuous. The system may have a collimator for shaping the fan beam and a scanner arrangement for varying the orientation of the pencil beam with respect to the object. Additionally, the object may be in motion with respect to at least one of the pencil and fan beams. 
    
    
     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 the accompanying drawings in which: 
     FIG. 1 provides a top view of an inspection system employing two sources of radiation, one of which provides both a fan beam and a pencil beam in accordance with a preferred embodiment of the present invention; 
     FIG. 2 provides a top view of a chopper wheel embodiment of the present invention for providing both a fan beam and a scanned pencil beam of penetrating radiation; 
     FIG. 3 is a side view of the chopper wheel embodiment of FIG. 2; and 
     FIG. 4 is a cross-sectional view showing typical dimensions for application of an embodiment of the invention to the inspection of train cars. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a plan view of the elements of a rapid x-ray inspection system, designated generally by numeral  10 . A source  12  emits penetrating radiation in a cone-shaped beam  14 . Beam  14  of penetrating radiation, may be, for example, a beam of x-rays such as a polychromatic x-ray beam. Source  12  of penetrating radiation may be a LINAC, for example, or an x-ray tube, for another example. It is preferred that source  12  be a continuous source of penetrating radiation, such as a CW LINAC, for example. For some applications, a pulsed x-ray generator with an appropriate repetition rate may be used. Beam  14  will be referred to in the present description, without limitation, as an x-ray beam. In accordance with a preferred embodiment of the invention, rotating chopper wheel  16  (as described, for example, in copending application U.S. Ser. No. 09/238,686) is used to develop a pencil beam  18  which may be swept in a plane substantially perpendicular to that of the page. The cross section of pencil beam  18  is of comparable extent in each dimension and is typically substantially rectangular. The dimensions of pencil beam  18  typically define the scatter image resolution which may be obtained with the system. A fan beam  20  is also formed from the output of source  12 . Fan beam  20  has an opening angle (out of the page) in a plane substantially perpendicular to that of the page and is restricted to a narrow width within the plane of the page by a collimator as described below. The opening angle of fan beam  20  is a matter of design choice, and fan beam  20  may, indeed, have comparable extent in each dimension within the scope of the invention as described herein and as claimed in any appended claims. 
     Pencil beam  18  is offset by an angle θ from the plane containing fan beam  20 . Angle θ is chosen such that radiation scattered within inspected object  22  (here shown, for example only, as a train car) does not enter any detector  24  of scatter detector array  26  in sufficient intensity to interfere with detection of scatter radiation  28  from pencil beam  18 . Additionally, shield  30  may be provided to further shield scatter detector array  26  from radiation scattered from fan beam  20 . Collimators (not shown) may be used to further increase the isolation of radiation from pencil beam  18  and fan beam  20 . 
     Angle η between pencil beam  18  and normal  32  to surface  34  of inspected object  22  may advantageously be chosen such that scatter detector array  26  receives scattered radiation  28  which is scattered from pencil beam  18  at an angle Θ with respect to pencil beam  18 , where angle Θ is substantially a right angle. The energy of scattered radiation is proportional to the quanitity [1+α(1−cosΘ)] −1 , (where α is proportional to the energy of the beam and equal to unity for an x-ray beam having an energy of 510 keV). Consequently, a photon scattered at an angle Θ of 90° is of higher energy and thus greater penetrating power than a photon scattered directly backward. The energy of the scattered x-ray is generally a substantial fraction of the energy of the incident x-ray and thus the scattered x-ray has considerable penetrating power. 
     The component of fan beam  20  transmitted through inspected object  22  is detected by transmission detector arrangement  38 . In a preferred embodiment of the invention, detector arrangement  38  is an array of x-ray detectors arranged in a row extending out of the page so as to form a slice of an image of attenuation of beam  20  by object  22  at each relative position of the beam and the object. 
     Inspected object or container  22  may be self-propelled through beams  18  and  20  or may be pulled by a mechanized tractor, or by a conveyor of any sort. Object  22  is depicted in FIG. 1, for example, as a cargo car of a train being pulled along track  36 . It is to be recognized that, equivalently, beam  20  may move with respect to object  22  in a direction  40  transverse to the plane of the opening angle of the beam. 
     Within the scope of the invention, any x-ray detection technology known in the art may be employed for transmission detector arrangement  38  as well as for scatter detection arrangement  26 . The detectors may be segmented scintillators or other solid state detectors, for example, or liquid scintillators which may be doped with tin or other metal. Respective output signals from the scatter and transmission detectors  26  and  38  are transmitted to a processor  42 , and processed to obtain images of object  22  and its contents, or to obtain other characteristics such, for example, as mass, mass density, mass distribution, mean atomic number, or likelihood of containing targeted threat material, all as known to persons skilled in the art of x-ray inspection. 
     Additionally shown in FIG. 1 is a second and separate scatter detection arrangement  44  which may be employed in accordance with an alternate embodiment of the invention. A second source  46  emits penetrating radiation which is formed into a scanned pencil beam  48  by rotating chopper wheel  50  so as to scan object  22  from another side from that scanned by scanning pencil beam  18 . Penetrating radiation  52  scattered by object  22  is detected by a second scatter detector arrangement  54  which may be shielded by shield  56  from any radiation scattered from fan beam  20 . 
     Referring now to FIG. 2, a scanning arrangement, designated generally by numeral  58 , is shown for forming pencil beam  18  and fan beam  20 . Pencil beam  18  is formed by a series of tubular collimators  60  distributed as spokes on rotating wheel  62 , shown in this view from the top. Rotating wheel  62  is rotated by means of a rotary actuator such as driving motor  64 . Fan beam  20  is formed by slit collimator  66  offset in angle from the plane of rotating wheel  62  by angle θ. In some cases, such as for low energy x-rays, a slot may be advantageously milled in motor housing  68  to reduce attenuation of x-rays from source  12  prior to reaching collimator  66 , thereby enhancing the flux of x-rays in fan beam  20 . FIG. 3 shows a side view of scanning arrangement  58 . Pencil beam  18  is shown emergent from one of eight radial tubular collimators  60  while fan beam  20  emerges to the far side of rotating wheel  62 . 
     Referring now to FIG. 4, a preferred geometry is shown for the inspection of railway cars  70 , possibly carrying piggy back sea container  72 , in accordance with an embodiment of the invention. Scanning arrangement  16  is displaced a sufficient distance d from center  74  of railway tracks  36  so that opening angle  76  of fan beam  20  covers a substantial portion of car  70 . Transmission detector array  38  intercepts all of fan beam  20  that traverses the inspected car  70 . Portions of fan beam  20  that do not traverse car  70  may be intercepted, as a matter of safety, by stop  78 . Various positions of scanned pencil beam  18  are shown, as is scatter detector array  26 . The geometry of FIG. 4 is shown solely by way of example, and indeed the arrangement is quite flexible and in some applications it may be more advantageous to place the x-ray source and the detectors in vertical or slanted arrangements. 
     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.