Abstract:
A system and method of density detection in a target object involve irradiating the target object, detecting a first and second discrete number of photons penetrating the target object through respective first and second prescribed volumes and entering respective first and second radiation detectors. First and second numbers of photons detected by the first and second radiation detectors are counted, and a display output signal is generated in response to the first and second numbers. A graphical representation of the densities within the first and second volumes of the target object is displayed.

Description:
[0001]    This is a Continuation application of U.S. Ser. No. 09/398,980 for DENSITY DETECTION USING DISCRETE PHOTON COUNTING filed Sep. 17, 1999 (File No. 65603), by Verbinski, et al., which is a Continuation application of U.S. Ser. No. 08/921,854, for DENSITY DETECTION USING DISCRETE PHOTON COUNTING, filed Sep. 2, 1997 (File No. 57871), by Verbinski, et al., now abandoned, which is a Continuation application of U.S. Ser. No. 08/546,999, for DENSITY DETECTION USING DISCRETE PHOTON COUNTING, filed Oct. 23, 1995 (File No. 57510), by Verbinski, et al., now abandoned, all of which are incorporated herein by reference. 
     
    
     
         [0002]    A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of this patent document or the patent disclosure, as it appears in the United States Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever.  
         BACKGROUND OF THE INVENTION  
         [0003]    The present invention relates to density detection using discrete photon counting, and more particularly to using discrete photon counting to generate an image indicative of the densities in a target object. Even more particularly, the present invention relates to using discrete photon counting to perform density measurements in a target object and generating an image of the contents of such target object in response thereto.  
           [0004]    There are many instances in the security or customs field when it is necessary to examine or inspect, in a non-destructive way, the contents of a target object, such as a closed package, box, suitcase, cargo container, automobile semi-trailer, tanker truck, railroad car, e.g., box car or tanker car, or the like. For example, customs departments are routinely charged with the responsibility of inspecting vehicles coming into a country to make sure such packages do not contain drugs or other contraband. Similarly, drug smugglers frequently carry out their criminal acts by hiding illegal drugs in vehicles such tanker trucks, and then sending the trucks through a border checkpoint. When security personnel encounter suspicious vehicles or other containers being transported over international boundaries, they must perform a careful inspection of such vehicles to ascertain their contents.  
           [0005]    When suspicious vehicles are discovered, they generally must be examined or inspected on location in what is referred to as a “secondary inspection area.” If secondary inspection reveals the presence of contraband (e.g., drugs), then the vehicle may be impounded, the driver arrested, and the contraband disposed of. If, on the other hand, the examination reveals the absence of contraband, then the vehicle may be allowed to proceed in a normal manner.  
           [0006]    The process used to examine or inspect a suspicious vehicle should be quick, simple, and as unintrusive as possible. Unfortunately, conventional inspection mechanisms require either visual inspection by others and/or scent inspection by dogs. These conventional methods require that the vehicle stop and wait for the inspection to be completed, which can take a half hour or more. This is both inconvenient and time consuming for both customs officials and the vehicle drivers and occupants. Furthermore, such inspection may put officers at personal risk if a vehicle has been booby trapped or if the vehicle&#39;s driver or other occupants become nervous and decide to attack the customs officer inspecting their vehicle. What is needed, therefore, is a non-invasive technique for inspecting the contents of a suspicious vehicle without requiring that the vehicle be stopped and manually inspected.  
           [0007]    One attempt to satisfy this need involves the use of high levels of radiation to determine the densities of the vehicle and/or the contents of such vehicle. Unfortunately, this approach requires that the vehicle be stopped and evacuated prior to inspection, because such high levels of radiation can be physically harmful to the vehicle&#39;s occupants if they remain in the vehicle during inspection. Disadvantageously, inspection using high levels of radiation not only requires that the vehicle be stopped, and therefore delayed, but poses a risk to stowaways that may be aboard the vehicle, and unwilling to voluntarily evacuate, when the vehicle is stopped for inspection. Therefore, what is needed is a non-invasive technique for inspecting the contents of a suspicious vehicle without requiring the use of high levels of radiation.  
           [0008]    A further problem posed by manual inspection techniques arises when tanker trucks or railroad cars, after having been emptied, seek to cross a border in order to refill. Because such tankers cannot be completely emptied without releasing the pressure in such tankers and venting noxious gases into the atmosphere, the tankers typically are kept nominally under pressure. The venting of noxious gases would be hazardous and ecologically unacceptable. The contents of such tankers typically go uninspected by customs agents in order to avoid the venting of such gases. Unfortunately, drugs smugglers are well aware of this fact, and therefore utilize tanker trucks and railroad cars to import illegal drugs, knowing that they will not be inspected at the border. This provides just one of numerous additional examples of cases where invasive or intrusive inspection into vehicles, or other containers, is not feasible or desirable. Thus, further emphasizing the need for a non-intrusive approach to vehicle inspection.  
           [0009]    The present invention advantageously addresses the above and other needs.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention advantageously addresses the needs above as well as other needs by providing a system and method employing discrete photon counting to perform density measurements in a target object and generate to an image of the contents of such target object in response thereto.  
           [0011]    In one embodiment, the present invention can be characterized as a system for detecting densities within an target object. The system employs a radiation source; a detector array comprising a plurality of radiation detectors; an electrical subsystem coupled to the detector array; and a display device for displaying a graphical representation of the densities within the target object in response to a display output signal. The electrical subsystem discretely counts the number of photons detected by each radiation detector; and generates the display output signal in response thereto.  
           [0012]    In another embodiment, the present invention can be characterized as a method of detecting densities within a target object. The method has the steps of irradiating the target object; detecting a first discrete number of photons penetrating the target object through a first prescribed volume and entering a first radiation detector; and detecting a second discrete number of photons penetrating the target object through a second prescribed volume and entering a second radiation detector. Next, the method involves discretely counting a first number of photons detected by the first radiation detector; discretely counting a second number of photons detected by the second radiation detector; and generating a display output signal in response to the first number of photons and the second number of photons. The display output signal is used to display a graphical representation of the densities within the first volume of the target object and the second volume of the target object. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein:  
         [0014]    [0014]FIG. 1 is a schematic diagram of a system made in accordance with one embodiment of the present invention and of a tanker truck containing contraband material, wherein discrete photon counting is used to perform density measurements in a tanker truck and wherein an image is generated of the contents of such tanker truck in response thereto;  
         [0015]    [0015]FIG. 2 is a block diagram of the system of FIG. 1 showing gamma/x-ray detectors coupled through 16-channel processing units, RS-485 line drivers, and an RS-485 interface card to a computer, wherein the computer processes discrete photon count information received from the detectors and causes a display device to display an image of the contents of a target object, such as the tanker truck of FIG. 1, in response thereto;  
         [0016]    [0016]FIG. 3 is a block diagram showing the detectors of FIG. 2 coupled through preamplifiers, amplifiers, discriminators, pulse generators and an RS-485 line driver that make up one embodiment of the 16-channel processing units of FIG. 2;  
         [0017]    [0017]FIGS. 4A, 4B,  4 C,  4 D,  4 E,  4 F,  4 G,  4 H and  4 I are a schematic diagram showing one variation of an analog portion the 16-channel processing units of FIGS. 2 and 3;  
         [0018]    [0018]FIGS. 5A, 5B and  5 C are a schematic diagram showing one variation of a digital portion of the 16-channel processing units of FIGS. 2 and 3;  
         [0019]    [0019]FIG. 6 is a block diagram of functional components that make up one embodiment of a software system with which the computer of FIG. 2 is controlled;  
         [0020]    [0020]FIG. 7 is a flow chart showing the steps traversed by the computer of FIG. 2 in response to the software system of FIG. 6 when an image generation program is executed; and  
         [0021]    [0021]FIG. 8 is a diagram illustrating a preferred screen layout for the image displayed on the display device of FIG. 2. 
     
    
       [0022]    APPENDIX A is a source code listing of a firmware operating system including steps traversed by the 16-channel processing units of FIG. 2 in order to quickly relay the photon count information received from the detectors to the computer.  
         [0023]    Corresponding reference characters indicate corresponding components throughout the several views of the drawings.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    The following description of the presently contemplated best mode of practicing the invention is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims.  
         [0025]    Referring first to FIG. 1, a schematic diagram is shown of a system made in accordance with one embodiment of the present invention and of a tanker truck containing contraband material, wherein discrete photon counting is used to perform density measurements in the tanker truck and wherein an image is generated of the contents of such tanker truck in response thereto.  
         [0026]    Shown are the tanker truck  10 , concealed contraband  12 , a detector array  14 , a detector array truck  16 , a radiation source  18 , a radiation source truck  20 , processing electronics  22  and a graphical display device  24 .  
         [0027]    The detector array  14  employs a plurality of “oversized” high efficiency gamma-ray detectors  26 , e.g., between twenty and sixty, e.g., forty-eight, detectors arranged in a vertical column. The detectors  26  make it possible to scan the tanker truck  10  with a very low intensity gamma-ray field. In order to facilitate the use of very low intensity gamma-radiation, oversized detectors  26  are used, such as are available as Part No. 1.5M1.5M1.5, NaI (Tl) (sodium iodide crystal, thallium activated) (with R2060 photomultiplier tube) from BICRON of Ohio. Such gamma-ray detectors are scintillation counter-type detectors and are 3.8 cm in diameter, 3.8 cm high and mounted on a 3.8 cm photo-multiplier tube (PMT).  
         [0028]    The very low intensity gamma-ray field useable with gamma-ray detectors  26  is low enough in intensity to allow operating personnel to work within it. For example, the very low intensity gamma-ray field may use 662 keV gamma-ray energy. However, a stronger gamma-ray or x-ray source than this can be used in the interest of faster density measurements, while still allowing operating personnel to work within the very low intensity field. Such can be used for example when the vehicle under inspection is travelling at high speeds.  
         [0029]    The radiation source  18  is preferably a 3.7×10 10  Bq shuttered source of Cs-137 gamma-rays, i.e., 662 keV gamma-ray energy. Alternatively, a Co-60 source may be used. A suitable source is readily available as Model No. SH-F-2 from Ohmart Corporation of Ohio. The radiation source includes a collimator that provides a 45° vertical opening, measured from horizontal upwards, and a 10° lateral opening. The shutter is electrically actuated.  
         [0030]    The radiation source  18  provides gamma-rays that are only moderately attenuated by steel walls typically found in tanker trucks or railroad cars. Yet such rays are sufficiently attenuated by contraband packages to make them easily detectable by measuring the penetration of the gamma-rays emitted from the source and deriving relative material densities therefrom. In addition, there is negligible backscattering of the gamma-ray energy from the tanker walls, much less than would occur if a high-powered x-ray source was utilized, although an x-ray source may be desirable for high speed inspection applications or for inspection of unmanned vehicles.  
         [0031]    The detector array truck  16  and the radiation source truck  20  are designed to travel synchronously along parallel tracks. The trucks  16 ,  20 , the tracks, a synchronous drive motor and a variable frequency generator for controlling the speed of the synchronous drive motors are available as Model No. SA0100 from Becker Equipment (Maakh Becker P.E.) of Vista, Calif., however, numerous known substitutes can be employed.  
         [0032]    In operation, the trucks  16 ,  20  are moved synchronously along parallel paths spanning the entire length of the tanker truck or the railroad car to be inspected by the synchronous drive motors. Alternatively, and preferably, the truck or railroad car being inspected can be driven between the detector array  14  and the radiation source  18 , which in such alternative are both stationarily mounted.  
         [0033]    The vertical “linear array” configuration of the detectors  26  is made to provide a resolution of grid points spaced about every 5 cm along the length of the target vehicle, and about 4.3 cm, on average, along the height of the target vehicle, as projected on the target vehicle vertical center plane, running lengthwise. This resolution is adequate to achieve a detectability limit of less than half a kilogram of contraband per 4.3 cm by 5 cm gridpoint, or pixel. The pixel size can easily be varied by appropriately selecting the location of the radiation source and the detectors  26  within the detector array  14 , and by varying the distance between inspection points longitudinally via choice of counting interval and scan speed along the length of the target vehicle.  
         [0034]    Preferably, the entire length of the tanker truck or railroad car is scanned automatically, in a single sweep. In order to accomplish automatic scanning in the truck-mounted embodiment shown, the detector array truck  16  and the radiation source truck  20  are moved in a parallel fashion along the tracks at a constant speed, with a counting interval selected to effect a longitudinal pixel length (i.e., grid-spacing interval) of 1 to 2 inches for a typical tanker truck inspection. This grid size, as mentioned above, can easily be selected by one skilled in the art based on the disclosure provided herein and dependent upon an optimum tradeoff between minimum contraband content detectability, throughput (i.e., inspection time per tanker truck), gamma-ray field-strength (and other safety considerations), and overall system cost.  
         [0035]    As mentioned above, if a sufficiently high field-strength is utilized, the detector array  14  and the radiation source  18  may be fixed or stationary rather than mounted on the radiation source truck  20  and the detector array truck  16 . In such an arrangement, the tanker truck  10  or railroad car can be driven past the detector array  14  and radiation source  18  with the determination as to the densities within the tanker truck  10  being made automatically as the tanker truck  10  passes between the radiation source  14  and the detector array  18 .  
         [0036]    Thus, in the embodiment shown in FIG. 1 and in the preferred embodiment, a truly non-invasive inspection technique is provided in which there is no need to evacuate the vehicle to be inspected, or, in the preferred embodiment, to even stop or slow down the vehicle in order to effect inspection of such vehicle.  
         [0037]    Referring next to FIG. 2, a block diagram is shown of the system of FIG. 1 showing gamma/x-ray detectors coupled through 16-channel processing units, RS-485 line drivers, and an RS-485 interface card to a computer, wherein the computer processes discrete photon count information received from the detectors and causes a display device to display an image of the contents of a target object, such as the tanker truck of FIG. 1, in response thereto.  
         [0038]    The detector array  14  is depicted, as are the electronics  22  and the graphical display device  24 . The detector array  14  employs the plurality of gamma/x-ray detectors  26 . The gamma/x-ray detectors  26  are coupled in groups of 16 to 16-channel data processing circuits  30 . Preferably, three groups of gamma/x-ray detectors  26  are used. In practice, the number of gamma/x-ray detectors  26  used depends on the height of the vehicles to be inspected and the resolution, i.e., number of pixels, in the image desired. In a preferred embodiment, 48 gamma/x-ray detectors are used.  
         [0039]    The data processing circuits  30 , of which there are preferably three, are each coupled to an RS-485 line driver  32 , which is coupled to an RS-485 interface  34 . The RS-485 interface  34  is embodied on a circuit card located within a computer system  36 . A suitable RS-485 interface is available as Model No. 516-485, Part No. 3054 from Seal Level Systems, Inc., and from numerous other vendors under respective model/part number designations.  
         [0040]    The computer system  36 , which is preferably a Pentium-90 based personal computer system, operates programmatically under the control of a software system. The computer system  36  receives detector pulses from each of the 16-channel data processors  30 , in response to the detection of individual photons by the gamma/x-ray detectors  26 . As explained in further detail hereinbelow, the software system processes the incoming detector pulses, evaluating their relative amplitudes, i.e., energies, and generates a radio graphic image-like display output signal in response thereto. The radio graphic, image-like display output signal is coupled to the graphical display device  38 , which is preferably a Super-VGA monitor, and is used by the graphical display device  38  to generate a graphical representation of the densities within the vehicle under inspection.  
         [0041]    Unlike prior art systems, which do not generate a graphical representation, i.e., a “picture”, of the densities of the contents of the vehicle under inspection, the present embodiment generates such a picture. Advantageously, this allows for easy visual interpretation of the results of the scanning of the vehicle under inspection, as opposed to interpreting more subtle indications of the densities within the vehicle under inspection as may be required in prior art systems.  
         [0042]    Advantageously, the preferred software system also causes the display of a reference image simultaneous with the image generated in response to the vehicle under inspection, so that an operator of the present embodiment can easily make a visual comparison between what a vehicle of the type being inspected should “look like”, and what the vehicle under inspection actually “looks like”. Such “side-by-side” inspection further simplifies the detection of contraband using the present embodiment.  
         [0043]    As a result of the very low intensity gamma-ray or X-ray radiation used by the present embodiment, photon penetration, as opposed to backscatter, can be used to generate a side, as opposed to a bottom/top, image of the vehicle under inspection. This represents a significant improvement over prior art systems wherein a bottom/top presentation of the radiation source is required to avoid the need for excessive radiation shielding, but dictates that the vehicle&#39;s frame, drive train, etc., interface with the density measurements taken based on radiation penetration. Backscatter-type density measurement systems are less accurate due to the non-uniform backscattered radiation on which they rely for density measurement.  
         [0044]    Referring next to FIG. 3, a block diagram is shown of the detectors of FIG. 2 coupled through preamplifiers, amplifiers, discriminators, pulse generators and an RS-485 line driver that make up one embodiment of the 16-channel processing units of FIG. 2.  
         [0045]    Each of the radiation detectors  26  is coupled to preamplifier  14  within the 16-channel data processing nit  30 . Each preamplifier  14  is coupled to an amplifier  42 , which is in turn coupled to a discriminator  44 . Each discriminator  44  is coupled to a pulse generator  46 , which generates an electrical pulse for each photon received into the radiation detector  26  coupled thereto. Because very low intensity gamma-ray or x-ray radiation is used with the present embodiment, pulse pileup is generally not of significant concern.  
         [0046]    The pulse generators  46  within each of the 16-channel data processing units  30  are coupled to a line driver  32 . Each of the 16-channel data processing units  30  includes its own line driver  32 . The line drivers  32  operate under the programmatic control of a firmware operating system, such as shown in APPENDIX A.  
         [0047]    In operation, the preamplifiers  40 , and amplifiers  42  function in a conventional manner to amplify signals generated by the detectors  26 . Outputs of the amplifiers  42  are passed along to the discriminators  44 , which impose a noise threshold on the amplified signal. Waveforms within the amplified signal that exceed the noise threshold are passed along to the pulse generator, which generates a pulse for each waveform within the amplified signal corresponding to a received gamma-ray or x-ray photon.  
         [0048]    The line driver  32  passes the pulses generated by each of the pulse generators  46  within a particular 16-channel data processing unit  30  along to the computer via the RS-485 interface  34 .  
         [0049]    Referring next to FIGS. 4A, 4B,  4 C,  4 D,  4 E,  4 F,  4 G,  4 H and  4 I, a schematic diagram is shown of one variation of an analog portion the 16-channel processing units of FIGS. 2 and 3. The schematics of FIGS. 4A, 4B,  4 C,  4 D,  4 E,  4 F,  4 G,  4 H and  4 I are self-explanatory to one of skill in the art and therefore further explanation of these figures is not made herein.  
         [0050]    Referring next to FIGS. 5A, 5B and  5 C, a schematic diagram is shown one variation of a digital portion of the 16-channel processing units of FIGS. 2 and 3. The schematics of FIGS. 5A, 5B and  5 C are self-explanatory to one of skill in the art and therefore further explanation of these figures is not made herein.  
         [0051]    Referring next to FIG. 6, a block diagram is shown of functional components that make up one embodiment of a software system with which the computer of FIG. 2 is controlled.  
         [0052]    Upon initialization (Block  100 ), the computer, under control of the software system, initializes (Block  102 ), and loads a default color map display (Block  104 ), which maps detected densities within the vehicle under inspection, i.e., pulse counts, to specific colors to be produced on the display device  38 . Next, the user is presented with a main menu (Block  106 ), and the computer is instructed to wait until an operator instructs the software system as to what step to take next.  
         [0053]    One of the options available to the operator is a help function (Block  108 ). The help function displays tutorial and/or reference information to the operator via the display device, as is common in the computing arts.  
         [0054]    Another option presented to the operator is the “Display Image from Disk in Upper Window” option (Block  110 ). When selected this option allows the operator to load a saved display image from a hard or floppy disk drive within the computer, and to automatically display the image in the upper display window on the display drive. (See FIG. 8) Generally, the upper display window, in accordance with the present embodiment, is used to display a reference image, i.e., an image of the truck under inspection, but while empty, i.e., containing no contraband.  
         [0055]    A further option that can be selected by the operator is a “Display Image from Disk in Lower Window” option (Block  112 ). When selected this option allows the operator to load a saved image from a hard or floppy disk drive within the computer, and automatically displays the image in the lower display window on the display drive. (See FIG. 8) Generally, the lower display window, in accordance with the present embodiment, is used to display an inspection image, i.e., an image of the vehicle under inspection. A useful function of this option is for reinspection of a vehicle at a later time by a supervisor in order to maintain quality control. Because the image is stored on disk, it is not necessary that the vehicle be present when this re-inspection takes place. The saved image of the vehicle, after being loaded, can easily be visually compared with the reference image loaded into the upper display window.  
         [0056]    The next option available to the operator is the “Save Image from Lower Window to Disk” option. This option can be used to save an image of a vehicle under inspection for later reinspection, or can be used to save a reference image after a known empty vehicle has been inspected, i.e., scanned using the present embodiment.  
         [0057]    Using a “Load Color Lookup Table from Disk” option (Block  116 ), the operator is able to load a previously saved color lookup table from disk. This allows the user to retrieve a color map, different from the default color map, so that a different set of colors can be mapped to various density measurements, i.e., pulse counts within the vehicle.  
         [0058]    The next option is the “Acquire Image from Counters and Display to Screen” option (Block  118 ). This option initiates an image generation program, as described below in reference to FIG. 7, which causes the detector array  14  and the radiation source  18  to perform density measurements and causes the display of an image indicative of the various densities within the vehicle under inspection in the lower display window on the screen display. Advantageously, the present embodiment allows the operator to display a reference image in the upper display window while the inspection is being conducted, so that he or she can visually compare what the vehicle under inspection should look like empty with what the vehicle under inspection in fact looks like. In this way, the operator is able to determine whether or not the vehicle under inspection may contain contraband.  
         [0059]    The next two options (Blocks  120  and  122 ) allow the operator to set values for what is referred to herein as the “K” constraint and the “L” constraint. These two “constraints” function in a manner similar to the well known functioning of the brightness and contrast controls on commonly available cathode ray tube-type displays. These values affect the mapping of colors to the various pulse counts, which is performed as follows:  
         [0060]    (1) a “white” level, i.e., a number of counts corresponding to zero density, is determined for each sensor during detector calibration, which is a step in image acquisition as described below in reference to FIG. 7;  
         [0061]    (2) the variable “T” is then set equal to this white level times the reciprocal of the number of photons counted by a particular detector at a particular horizontal position on the vehicle;  
         [0062]    (3) If “T” is less than one, i.e., more photons are counted than the white level, then T is set equal to one;  
         [0063]    (4) the variable “D” is then computed as follows: D=254/(1+L)ln(T*K),  
         [0064]    where L and K are the constraints mentioned above, which are initially set to one, and where T is defined above; and  
         [0065]    (5) If “D” is less than 1 or greater than 254, then D is set to 1 or 254, respectively.  
         [0066]    The significance of the number 254 in the above computations is that there are 256 possible colors displayable on the preferred Super-VGA display device, however this number could be adjusted up or down to yield an appropriate color mapping where more or fewer colors are displayable.  
         [0067]    Other options available to the user are options to “Redisplay Both Windows” (Block  124 ), “Redisplay the Upper Window” (Block  126 ) and “Redisplay the Lower Window” (Block  128 ). Redisplay options such as these are useful to the operator if the images displayed on the display device  38  become corrupted in some way, as for example may occur if text is sent to the display device  38  while it is displaying a graphical image.  
         [0068]    The user may also “Reset a Default Color Map Array” (Block  130 ), “Load a Next Color Map Array” (Block  132 ) and “Load a Previous Color Map Array” (Block  134 ). These options are used to step through various preconfigured color maps, and to reestablish the default color map, so that the operator can utilize a color map that best emphasizes the features of the vehicle under inspection that he or she is inspecting.  
         [0069]    Other options available to the user are to “Reset Modified Color Table” (Block  136 ), “Increase Color Table Gain” (Block  138 ), “Decrease Color Table Gain” (Block  140 ), “Increase Color Table Offset” (Block  142 ), and “Decrease Color Table Offset” (Block  144 ). These options affect the “mx+b” relationship between the photon counts determined by the gamma/x-ray detectors and the colors displayed on the display device. The “gain” (m) is initially set, or can be reset, to one, and the “offset” (b) is initially set, or can be reset, to zero. These two parameters allows the operator to “zoom” in on a particular range of densities for mapping within the color table by increasing or decreasing the offset in order to establish a minimum density of interest (with every density below this density being mapped to zero density (or “white”), and by increasing or decreasing the gain in order to establish a maximum density of interest (with every density above this density being mapped to maximum density (or “black”).  
         [0070]    A final operator-selectable option depicted in FIG. 6 is an “End” option (Block  146 ). This option is used by the operator to exit the software system and to return control to an operating system, such as is known in the art of computer technology.  
         [0071]    As can be seen, various color maps, gains and offsets can be used to produce a variety of images representing densities detected within a vehicle during an inspection using the present embodiment.  
         [0072]    Referring next to FIG. 7, a flow chart is shown of the steps traversed by the computer of FIG. 2 in response to the software system of FIG. 6 when an image generation program is executed.  
         [0073]    Upon being initiated (Block  200 ), the image generation is initialized (Block  202 ), and the movement of the radiation source truck  20 , and the detector array truck  16 , if used, is initiated (Block  204 ). Next, the detectors  26  are calibrated (Block  206 ) by irradiating the detectors with the radiation source  18  at a point along the track before the radiation source truck  20  and the detector array truck  16  reach the vehicle to be inspected, i.e., before the vehicle under inspection is interposed between the detector array  16  and the radiation source  18 . Such irradiation of the detectors  26  establishes a baseline of radiation (or “lwhite” photon count level) corresponding to a density in the vehicle being inspected of approximately zero and corresponding to a maximum photon count. Three photon counts are made in this manner for each detector  26 . Such counts are then arranged for each detector  26  and then stored in an array having a white level element for each detector  26 .  
         [0074]    A horizontal position is then set to zero (Block  208 ). The horizontal position corresponds to a position along the track arbitrarily selected to be a first position at which density measurements are taken. This horizontal position should be at a point before the vehicle is interposed between the detector array  14  and the radiation source  18 .  
         [0075]    Next, a detector count is set to zero (Block  210 ), which corresponds to a first of the detectors  26  in the detector array  14  to be queried for a photon count. Next, this detector is queried for a photon count and is instructed to restart counting photons (Block  212 ). In response to this instruction, the detector queried restarts counting photons (Block  214 ) and the previously determined number of photon counts is passed along to the computer (Block  216 ). This number of photon counts is stored into an array within a memory in the computer (Block  218 ) and is then converted into a pixel value (Block  220 ). Conversion into the pixel value includes mapping the number of photon counts to a color to be displayed on the display device. Such mapping is described more completely above in reference to FIG. 6. Next, the detector number queried is converted into a vertical position on the screen display (Block  202 ) and the horizontal position of the radiation source truck  20  and the detector array truck  16  along the tracks is converted to a horizontal position on the screen display. Next, the pixel at the determined horizontal and vertical positions is illuminated using the color corresponding to the number of photon counts, as previously converted (Block  224 ).  
         [0076]    Next, a determination is made as to whether all of the detectors  26  in the detector array  14  have been queried for a number of photon counts for the current horizontal position (Block  226 ). If all the detectors have not been queried (Block  226 ), the detector number to be queried is incremented (Block  227 ) and execution of the image generation program continues by querying the next detector in the detector array  14  for the number of photon counts, and by instructing such detector to restart counting (Block  212 ). Execution continues from this point as described above (Block  214  et seq.) If all the detectors within the detector array  14  have been queried for the current horizontal position (Block  226 ), the horizontal position is incremented (Block  228 ) and a determination is made as to whether or not the radiation source truck  20  and the detector array truck  16  are still moving (Block  230 ). If the radiation source truck  20  and the detector array truck  16  are still moving (Block  230 ), the detector to be queried is reset to zero (Block  210 ) and execution of the image generation program continues as described above (Block  212  et seq.).  
         [0077]    If the radiation source truck  20  and the detector array truck  16  have stopped moving (because they have reached the farthest point of travel down the tracks (Block  230 )), execution of the image generation program is terminated (Block  232 ).  
         [0078]    Referring next to FIG. 8, a diagram is shown illustrating a preferred screen layout for the images displayed on the display device of FIG. 2.  
         [0079]    As shown, the screen display  300  is divided into an upper display  302 , a lower display  304  and a color bar  306 . In accordance with the present embodiment, the upper display  302  can be used, as mentioned above, to display images stored on disk. These images will generally be reference images used for visual comparison with an image representative of a vehicle under inspection. The lower display  304 , in addition to being able to display images stored on disk, is used to display images, as they are generated, indicative of the various densities within the vehicle under inspection. Both the upper and lower displays  302 ,  304  are color mapped using the current color map, gain and offset, so that they can be visually compared to one another.  
         [0080]    Any differences in a reference image, and an image generated during inspection of a vehicle may indicate the presence of contraband within the vehicle under inspection.  
         [0081]    The color bar  306  indicates the colors that are mapped to the various densities detectable by the present embodiment, serving as a reference to the operator as to which colors indicate higher densities than others. As suggested in FIG. 8, colors nearer to the top of the color bar  306  are indicative of more density, i.e., fewer photons counted as penetrating the vehicle under inspection, and colors nearer to the bottom of the color bar  306  are indicative of less density, i.e., more photons counted as penetrating the vehicle under inspection.  
         [0082]    Thus, a system and associated methods are provided in the present embodiment for determining the densities within a vehicle under inspection based on discrete photon counting, and for generating an image indicative of such densities. Advantageously, such determination is made based on discrete photon counting, thereby eliminating the need for high levels of gamma-ray or x-ray radiation.  
         [0083]    The present embodiment, thus, eliminates the need to stop and manually inspect vehicles at border crossings, and other inspection points. In addition, the present embodiment advantageously eliminates the need to stop and evacuate the vehicle before it is subjected to very high strength gamma-ray or x-ray radiation. Advantageously, one variation the present embodiment provides for the determination of densities within the vehicle without the need even to stop the vehicle. Slightly higher radiation levels may, in accordance with this variation, be used to reduce or even eliminate the slowing needed to determine the densities within a vehicle, and to generate an image indicative thereof.  
         [0084]    While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.