Abstract:
X-ray radiation is transmitted through and scattered from an object under inspection to detect weapons, narcotics, explosives or other contraband. Relatively fast scintillators are employed for faster X-ray detection efficiency and significantly improved image resolution. Detector design is improved by the use of optically adiabatic scintillators. Switching between photon-counting and photon integration modes reduces noise and significantly increases overall image quality.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to X-ray screening systems for airport luggage and the like; and, more particularly to screening systems that utilize radiation transmitted through and scattered from an object under inspection to detect weapons, narcotics, explosives or other contraband.  
           [0003]    2. Description of the Prior Art  
           [0004]    X-ray inspection systems that use transmitted radiation have conventionally been employed to detect the shape of high-Z material (Z refers to atomic number) such as steel. The principle objective of these systems is the detection of weapons, such as guns, knives, bombs and the like. A major problem with X-ray inspection systems is the inability thereof to accurately recognize and detect explosives and narcotics made up of low-Z materials. Recently dual energy X-ray systems have been used to improve the detection of low-Z material. Such systems measure the different attenuation that high and low transmitted energy X-rays experience as a result of passage through any material. This principal has allowed the identification of virtually any material so long as the material is not covered by a different Z material. In order to overcome the material overlaying problem, it has been proposed that X-ray transmission be effected from different directions using two X-ray sources, or that the object be scanned from all sides and the results be evaluated with computer tomography.  
           [0005]    Another approach for identifying low-Z material involves detecting the Compton scattered radiation along with the transmitted radiation. Low-Z material such as explosives and narcotics generates more scattered radiation than high-Z material like iron. This scattered radiation differential provides a basis for distinguishing between low-Z and high-Z material in instances where the low-Z material is concealed behind high-Z material.  
           [0006]    Among the more troublesome problems with X-ray transmission and Compton scatter images are their poor resolution and high noise content. The causes of these problems are traced to: a) the relatively poor light collection method used in converting X-ray photons to light photons; and b) photon integration. Poor light collection presents a problem because it requires use of slow (long persistence) phosphor type X-ray detectors. Such detectors oftentimes create blurred images owing to the slow response time of the excited phosphor. The use of photon integration in conventional signal generation produces noisy images, particularly in cases where the transmitted or backscattered X-ray rates are relatively small. For example, U.S. Pat. No. 5,260,982 to Fujii et al. discloses a scattered radiation imaging apparatus. The Fujii et al. apparatus employs long persistence phosphor type X-ray detectors and photon integration yielding relatively low resolution, high noise images.  
           [0007]    Employing relatively fast (very short persistence) phosphors such as Gd 2 SiO 5  or Y 2 O 2 Si, or an organic plastic scintillator (loaded or unloaded with lead or tin), for faster X-ray detection efficiency, would significantly improve image resolution. When coupled with photon-counting electronics to reduce noise, overall image quality would be significantly improved. These improvements would yield a sharper image more capable of recognizing bombs, currency, narcotics and other contraband shapes and accompaniments.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention provides a system and method for X-ray inspection of an object to detect weapons, narcotics, explosives or other contraband.  
           [0009]    When an X-ray photon is absorbed by the scintillator, the scintillator generates photons in the visible part of the spectrum. These photons travel down the scintillator and enter a photon detector, such as a photomultiplier, that is coupled to the scintillator. The photomultiplier effectively converts the photons to electrons that can be processed by electronics for image generation. For the backscatter detector, the X-ray signals will be processed in the counting mode where individual X-rays are counted to generate the Compton backscatter image. For the transmitted beam image, the number of X-ray photons that enter the scintillator can vary over a wide dynamic range that is dependent on the object under inspection. In the case where there is no object or a very weak absorbing object the X-ray rate on the scintillator can be so high that counting individual X-rays is not possible. On the other extreme, for a highly attenuating object the X-ray rate would be very low or even zero. To accommodate this wide range of X-ray rates, the transmission detector system operates in a combined mode comprised of photon counting and photon integrating modes, where the mode is dynamically selected depending on the X-ray rate. This optimized method of collecting X-ray signals yields a superior image, as opposed to using only photon counting or photon integration.  
           [0010]    The spatial resolution in the horizontal plane is accomplished via a pencil beam scanning across the inspection tunnel while a conveyor moves the object through the inspection tunnel. As only one line through the object is excited by the pencil beam at any time, the radiation captured by any scintillation detector is independent from the locus of the scintillation material that is actually hit by an X-ray photon, and must originate from this pencil line. The location of the pencil beam within the object image can be derived from the conveyor moving the object and the rotating disk with apertures that generate the pencil beam. It is possible to generate a direct luminescent image of an object with the transmission detector and an enhanced low Z image from the backscatter detectors and display them on separate monitors.  
           [0011]    Preferably, the images produced by the transmission detector and backscatter detector are displayed as adjacent windows of the same monitor. Signal information from the transmission detector is used to correct for attenuation effects in the backscatter images, thereby avoiding artifacts in the low Z images produced by attenuation due to high Z objects. Conversely, signal information from the backscatter detectors can be used to correct for scatter effects in the transmission image, thereby avoiding artifacts in the high Z image produced by scattering attenuation due to low Z objects. By means of these corrections a greater fraction of the image on the high Z window display is derived from absorption effects of high Z objects, and a greater fraction of the image on the low Z window display is derived from scattering effects of low Z objects. Accordingly, the images displayed by the high Z and low Z windows are more distinct.  
           [0012]    Tomographic information can optionally be obtained by using additional Compton backscatter detectors. Backscattered X-rays originating from elements close to the bottom of the object hit mainly the scintillator next to the entrance slit, while backscatter from elements further up the pencil beam hit all backscatter scintillation detectors nearly equally. Photon collection efficiency is improved and real-time image noise is reduced, when compared to collimation methods that limit angular admittance of photons. The tomographic zones can be displayed in windowed sections on a single monitor or on separate monitors.  
           [0013]    With at least one additional detector overlaying the extant transmission detector, or by dividing the extant transmission detector into low energy and high energy components, dual energy information can be obtained. This information can be displayed as a dual energy image, which is color coded to designate the atomic number of an object under inspection. A single energy image yields only object radiographic density information, as contrasted to a dual energy image, which yields radiographic density and atomic number, Z, of the object under inspection. Combining the information from the backscatter data and dual energy data can further enhance discrimination of different materials and aid in the separation of overlaying materials of different atomic number Z.  
           [0014]    By employing relatively fast scintillators for faster X-ray detection efficiency, the present invention significantly improves image resolution. Detector design is improved by the use of optically adiabatic scintillators. The system switches between photon-counting and photon integration modes to reduce noise and significantly increase overall image quality. In addition, the system automatically adjusts belt speed (i) to allow rapid entrance into the inspection zone, (ii) slow traverse through the inspection zone to prolong residence therein of articles appointed for inspection, and (iii) allow rapid exit from the inspection zone. This automatic belt speed adjustment feature affords increased resolution and reduced noise with minimum speed penalty. Advantageously, the system provides a sharper image that is more capable of recognizing bombs, currency, narcotics and other contraband shapes and accompaniments.  
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0015]    The invention will be more filly understood and further advantages will become apparent when reference is had to the following detailed description and the accompanying drawings, in which:  
         [0016]    [0016]FIG. 1 is a perspective view depicting an overview of an X-ray inspection system using transmitted and Compton backscattered radiation;  
         [0017]    [0017]FIG. 2 is a perspective view showing the arrangement of x-ray generator and pencil beam shaping components of the detector of FIG. 1;  
         [0018]    [0018]FIG. 3 is a perspective view illustrating a backscatter detector;  
         [0019]    [0019]FIG. 4 depicts various views of an alternate embodiment for a backscatter detector;  
         [0020]    [0020]FIG. 5 shows various views for an alternate embodiment for a transmitted radiation detector;  
         [0021]    [0021]FIG. 6 is a block diagram illustrating signal flow for the system of FIG. 1;  
         [0022]    [0022]FIG. 7 is a block diagram depicting the method for X-ray inspection of an object using transmitted and Compton backscattered radiation;  
         [0023]    [0023]FIG. 8 is a perspective view illustrating a dual energy transmitted radiation detector for providing a dual energy image containing radiographic density and atomic number information of an object under inspection; and  
         [0024]    [0024]FIG. 9 is a diagrammatic view depicting a transmission detector and generated transmission signal, which is combined with a scatter correction signal in a summing device to produce and display a transmission signal in a transmission image window of a monitor.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    The invention provides an apparatus and method for X-ray inspection systems that utilize radiation transmitted through and scattered from the object under inspection to detect weapons, narcotics, explosives or other contraband.  
         [0026]    Specifically, as shown in FIGS. 1 and 2, the invention provides a tomographic scanning X-ray inspection system. The system has conveyor  10  for moving object  15  to be scanned though the system. An X-ray generation device  20  generates a pencil beam of X-rays  26 . Pencil beam  26  is repeatedly swept along pencil beam entrance slit  27  across conveyor  10 . The scanning direction of the pencil beam  26  is substantially perpendicular to the object&#39;s movement. In this manner, object  15  is repeatedly scanned as it moves on conveyor  10 . X-ray generation device  20  is known in the art and generally comprises an X-ray tube within or behind rotating wheel  24  having a plurality of slits  25  from which a fan of X-rays  29  are emitted. X-ray shield  28  is provided with slit  23  from which pencil beam  26  emerges.  
         [0027]    Alternatively, the wheel is vertical and provided with holes along its rounded side such that a beam of X-rays is swept along the object. This pencil beam originates from an x-ray source, which emits radiation into cone. A rotating lead covered wheel with four small holes 90 degree apart, makes this conical radiation into a scanning pencil beam. One rotation of the wheel will result in four pencil beam passes through the object under inspection.  
         [0028]    Fast backscatter detectors  12  and  13  generate a backscatter signal when detecting X-rays backscattered by object  15 . The fast backscattered detectors  12  and  13  are positioned on the same side of the moving object  15  as the X-ray generation device  20  and across from each other and proximate to pencil beam entrance slit  27 . Preferably, the fast backscatter detectors  12  and  13  are proximate to (and more preferably, substantially parallel to) pencil beam entrance slit  27  and are comprised of scintillators  40  and  42  and photon detectors  48  and  49  respectively. Scintillators  40  and  42  are comprised of organic plastic. Photon detectors  48  and  49  are preferably photomultipliers. Alternatively, scintillators  40  and  42  are comprised of short persistence phosphor such as Gd 2 SiO 5  or Y 2 O 2 Si. Optionally, only one backscatter detector is used.  
         [0029]    Alternatively, as shown in FIG. 3, backscatter detector  76  comprises two elongated scintillator sections optically linked to at least one photon detector. Each of the scintillator sections is oppositely disposed along pencil beam entrance slit  27 . Scintillator segments  40  and  42  are joined via a simple light pipe block  46 . The light is guided through light pipe block  46  via two 45 degree cuts  80  at the scintillator segment ends touching block  46 . The pickup end of segment  40  has an approximation of a semi-paraboloid  82 . Scintillator  42  has a 45 degree wall  80  for reflection improvement. A perspective view of the detector is shown as  76  in FIG. 3, while a side view is shown as  77 .  
         [0030]    As a further option, two distal backscatter detectors  18  and  19  are employed and positioned to bracket backscatter detectors  12  and  13 , as shown in FIGS. 1 and 2. Preferably, the fast backscatter detectors  18  and  19  are comprised of scintillators  50  and  52  and photon detectors  58  and  59 , respectively. Scintillators  50  and  52  are comprised of organic plastic. Photon detectors  58  and  59  are preferably photomultipliers. Alternatively, scintillators  50  and  52  are comprised of short persistence phosphor such as Gd 2 SiO 5  or Y 2 O 2 Si.  
         [0031]    From the portion of pencil beam  26  which is scattered from object  15 , most of the backscatter will hit one of the backscatter detectors. Scattered radiation which originates from the top of object  15  is picked up rather equally by all backscatter detectors, while scattered radiation which originates closer to the bottom of object  15  shows up mainly in the detectors  40  and  42 .  
         [0032]    A further alternative is illustrated by FIG. 4. Distal backscatter detectors  18  and  19  are combined  70  and comprise two elongated scintillators  50  and  52 . The scintillators are optically linked via light pipe  56  to at least one photon detector  58 . Each of the scintillators is oppositely disposed along pencil beam entrance slit  27 . Preferably, the end of scintillator  52  having photon detector  58  is an approximate semi-paraboloid  82 , while the other ends are provided with 45 degree cuts, shown as  80  in FIG. 5, to improve reflection. The two scintillators  50  and  52  are optically connected with light pipe  56  having cut ends  80  to improve reflection. In FIG. 4, a perspective view of the detector is shown at  70 ; side and top views are shown as  72  and  74 , respectively.  
         [0033]    These scintillators convert the backscatter x-ray energy reaching them into light photons. These light photons are conducted in the scintillators to their respective photomultiplier tubes, which convert the light photons into backscatter signals.  
         [0034]    As shown in FIGS. 1 and 2, transmission detector  17  is used to generate a transmission signal when detecting X-rays  26  which are not absorbed or scattered by the object  15 . The transmission detector  17  is positioned on the opposite side of object  15  as backscatter detectors  12  and  13 . In this manner, object  15  moves between the transmission detector  17  and backscatter detectors  12  and  13 . Preferably, transmission detector  17  is comprised of scintillators  30 ,  31 , and  32  and photon detectors  38  and  39 . Scintillators  30 ,  31 , and  32  are comprised of organic plastic, and are arranged in a U shape, as shown in FIG. 2. Photon detectors  38  and  39  are preferably photomultipliers. Alternatively, scintillators  30 ,  31 , and  32  are comprised of short persistence phosphors such as Gd 2 SiO 5  or Y 2 O 2 Si.  
         [0035]    [0035]FIG. 5 shows a U shaped transmission detector  17  comprised of scintillators  30 ,  31 , and  32 . Detector  17  is provided with rounded corners  83  generating a constant cross section for the whole detector. Preferably, comers  83  may be divided into a plurality of fibers or laminated to minimize light losses. A constant cross section, also called adiabatic, has the least losses for light conduction via total reflection. Photodetector  38  is located at one end of a segment. Preferably, this pickup end has the shape of a semi-paraboloid. That shape focuses more radiation than any other shape into photodetector  38 , mounted atop the focal point of the semi-paraboloid. Alternatively, a multifaceted approximation of a semi-paraboloid is used in place of the difficult to make semi-paraboloid. As a further alternative, there is used a minimum of just two facets at 45 degree in space, as shown in FIG. 6. The opposite end of the segment—the end without the photodetector—has two 45 degree walls which reflect most of the light back into the segment by total reflection.  
         [0036]    Scintillators  30 ,  31 , and  32  convert most of the x-ray energy reaching them into light photons. These light photons are conducted in the scintillators to photomultiplier tubes  38  and  39 , which convert the light photons into transmission signals.  
         [0037]    Referring to FIG. 6, processor  37  processes the backscatter and transmission signals received form their respective detectors into tomographic information for display on display means  36 . Preferably, display means  36  is a CRT or LCD display. Processor  37  is automatically switchable between photon counting and photon integration modes. Optionally, the switching is accomplished manually. Pencil beam location data  54  provides processor  37  with information on the location of the beam. Speed data  55  provides processor  37  with information on the speed of conveyor  10 .  
         [0038]    The reconstructed images of the transmission signal, backscatter signal, and the distal backscatter signal can be displayed separately or as a combined image showing suspicious material as a colored area.  
         [0039]    The method to carryout X-ray inspection of an object using transmitted and Compton backscattered radiation is shown in FIG. 7. A pencil beam of X-rays is generated  60  and scanned  62  across the object to be inspected. X-rays transmitted through the object are detected  64  using a fast transmission detector. A mode of detection  65  is selected from either photon integration or photon counting. X-rays backscattered from the object are detected  74  using a fast backscatter detector, and a mode of detection  75  is selected from either photon integration or photon counting. Optionally, mode selection is omitted for the backscatter image. Next, a transmission image is processed  66  from the detected transmission X-rays and displayed  67 . A backscattered image is processed  76  from the detected backscattered X-rays and displayed  77 . Optionally, the processed transmission and backscatter images are combined  68  and displayed  70  as a composite image.  
         [0040]    Additional scintillation detectors at the top and sides, in close proximity with the scintillation detectors for the transmitted radiation, create additional tomographic layers of resolution and improve the transmission image. The improvement is especially advantageous for high attenuation of the transmitted radiation. The forward scatter is measured and used as a correction for the transmitted radiance detector, which automatically captures forward scatter together with the transmitted radiation. Since the transmission scintillator detector system is divided into sections, those sections which are not currently collecting transmission image information can be used to collect scattered X-rays in the forward direction. The detection of these scattered X-rays can be used to improve the backscatter image or used to create an additional tomographic layer.  
         [0041]    In FIG. 8 there is shown a detector configuration  100  for obtaining dual energy information by properly adjusting the thickness of inner transmission detector  130  and outer transmission detector  120 . Inner detector  130  is U shaped, and comprises scintillators  101 ,  102 , and  103 , as well as photodetectors  108  and  110 . Outer detector  120  comprises scintillators  104 ,  105  and  106 , as well as photodetectors,  107  and  109 . Preferably, corners are divided into a plurality of fibers or laminated to minimize light losses, as described in connection with detector  17  (see FIG. 5). The transmitted pencil X-ray beam first interacts with the inner detector  130 , and by selecting the detector material and thickness it will preferably absorb lower energy. Material and thickness of outer detector  120  would be selected to absorb the higher energy X-rays that exit inner detector  130 . Alternatively, a sheet of filtering material such as copper, steel or the like could be disposed between the inner detector  130  and outer detector  120  to increase discrimination between the high and low energy photons. The signal strengths of the inner and outer detectors are compared to determine the atomic number Z of the object. Further comparison of the dual energy information and scatter information will give a more accurate Z determination and aid in separating overlying materials.  
         [0042]    In FIG. 9 there is shown a transmission detector  201  generating transmission signal  205 . The transmission signal  205  is combined with scatter correction signal  209  in summing device  213 . A corrected transmission signal  215  is displayed in transmission image window  221  on monitor  219 . Backscatter detector  203  generates backscatter signal  211 , which is combined with attenuation correction signal  207  in multiplier  215 . A corrected backscatter signal  217  is displayed in backscatter image window  223 .  
         [0043]    Having thus described the invention in rather full detail, it will be understood that such detail need not be strictly adhered to, but that additional changes and modifications may suggest themselves to one skilled in the art, all falling within the scope of the invention as defined by the subjoined claims.