Abstract:
A method for performing security screening at a checkpoint is provided. The method includes providing an X-ray imaging system having a scanning area and providing a supporting device for supporting articles to be scanned in the scanning area, wherein the supporting device has at least two reference areas manifesting respective X-ray signatures when exposed to X-rays, the X-ray signatures being distinguishable from one another. The method further includes placing an article to be scanned on the supporting device, introducing the article to be scanned in the scanning area while the article is supported by the supporting device and using the X-ray imaging system for deriving the X-ray signatures of the reference areas and for obtaining an X-ray image of the article while the supporting device is in the scanning area. The method further includes using the X-ray signatures to derive X-ray attenuation information from the X-ray image and using the X-ray attenuation information in determining if the article is a security threat.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 USC §120 of:
         U.S. provisional patent application Ser. No. 61/094,743 filed on Sep. 5, 2008 by Michel Roux et al.; and   U.S. provisional patent application Ser. No. 61/097,060 filed on Sep. 15, 2008 by Michel Roux et al.       

    
    
     This application also claims the benefit of priority under 35 USC §119 based on international PCT patent application no.: PCT/CA2007/001749 filed in the Canadian Receiving Office on Oct. 1, 2007 by Aidan Doyle et al. and presently pending. 
     The contents of the above-referenced patent documents are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to technologies for assessing the threat status of materials by means of penetrating radiation such as X-rays. The invention has numerous applications, in particular it can be used for scanning hand carried baggage at airport security check points. 
     BACKGROUND 
     Some liquid or combinations of liquid and other compounds may cause enough damage to bring down an aircraft. As no reliable technology-based solution currently exists to adequately address this threat, authorities have implemented a ban of most liquid, gels and aerosols in cabin baggage. 
     As a result, there have been disruptions in operations (e.g., a longer screening process; additional line-ups), major inconveniences for passengers (as well as potential health hazards for some) and economic concerns (e.g., increased screening costs; lost revenues for airlines and duty free shops; large quantities of confiscated—including hazardous—merchandise to dispose of), and so on. 
     Clearly, there is a need to provide a technology-based solution to address the threat of fluids that are flammable, explosive or commonly used as ingredients in explosive or incendiary devices. 
     SUMMARY 
     As embodied and broadly described herein the invention provides a method for performing security screening at a checkpoint. The method includes providing an X-ray imaging system having a scanning area and providing a supporting device for supporting articles to be scanned in the scanning area, wherein the supporting device has at least two reference areas manifesting respective X-ray signatures when exposed to X-rays, the X-ray signatures being distinguishable from one another. The method further includes placing an article to be scanned on the supporting device, introducing the article to be scanned in the scanning area while the article is supported by the supporting device and using the X-ray imaging system for deriving the X-ray signatures of the reference areas and for obtaining an X-ray image of the article while the supporting device is in the scanning area. Yet, the method includes using the X-ray signatures to derive X-ray attenuation information from the X-ray image and using the X-ray attenuation information in determining if the article is a security threat. 
     As embodied and broadly described herein the invention also includes a X-ray inspection station for performing security screening on articles, the X-ray inspection station having an X-ray scanning area where one or more articles are exposed to X-rays and a supporting device for supporting one or more articles while the articles are exposed to X-rays in the scanning area, wherein the supporting device has at least two reference areas manifesting respective X-ray signatures when exposed to X-rays, the X-ray signatures being distinguishable from one another. A computer based processing unit is provided for:
         i) deriving the X-ray signatures of the reference areas and collecting X-ray image data of the article while the supporting device is in the scanning area;   ii) using the X-ray signatures to derive X-ray attenuation information from the X-ray image data;   iii) using the X-ray attenuation information in determining if the article is a security threat.       

     As embodied and broadly described herein the invention also includes a tray for supporting an article while the article is subjected to an X-ray inspection in an X-ray imaging apparatus, the X-ray imaging apparatus including an array of X-ray detectors, the tray including at least two reference areas manifesting respective X-ray signatures that are distinguishable from one another, at least one of the reference areas having an extent such that X-rays passing through the reference area are received by a majority of the X-ray detectors of the array of X-ray detectors. 
     As embodied and broadly described herein the invention also includes a belt for carrying an article to be subjected to an X-ray inspection in and out of the scanning area of an X-ray imaging apparatus, the belt including at least two reference areas manifesting respective X-ray signatures when exposed to X-rays, the X-ray signatures being distinguishable from one another. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying Figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A detailed description of examples of implementation of the present invention is provided herein below with reference to the following drawings, in which: 
         FIG. 1  is a block diagram of an apparatus using X-rays to scan hand carried baggage at a security check point, according to a non-limiting example of implementation of the invention; 
         FIG. 2  is a plan view of a tray for carrying materials as they undergo security screening, according to a non-limiting example of implementation of the invention; 
         FIG. 3  is a cross-sectional view taken along lines  3 - 3  in  FIG. 2 ; 
         FIG. 4  is a block diagram of the processing module of the apparatus shown in  FIG. 1 ; 
         FIG. 5  is a plan view of the tray according to a variant. 
         FIG. 6  is a flow chart of a process according to a non-limiting example of the invention for performing threat assessment; 
         FIG. 7  is a flow chart of a process according to a non-limiting example of the invention for performing self-calibration of the X-ray imaging apparatus of  FIG. 1 ; 
         FIG. 8  is a graph illustrating the relationship between grey scale values in an X-ray image and corresponding attenuation levels; 
         FIG. 9  is another graph illustrating the relationship between grey scale values in an X-ray image and corresponding attenuation levels; 
         FIG. 10  is a top plan view of a belt of an X-ray imaging apparatus of the type shown in  FIG. 1 , the X-ray imaging apparatus being omitted for clarity; 
         FIG. 11  is a side elevation view of the belt shown in  FIG. 10 ; 
         FIG. 12  is a front elevation view of the belt of  FIGS. 10 and 11 , also showing the detectors array of the X-ray imaging apparatus; 
         FIG. 13  is an example of an X-ray image, showing various image segments and how they relate to respective detectors of the detectors array; 
         FIG. 14  is a top plan view of the tray according to a second variant; 
         FIG. 15  is a graph showing the relationship between the grey scale level in X-ray image data and attenuation values. 
     
    
    
     In the drawings, embodiments of the invention are illustrated by way of example. It is to be expressly understood that the description and drawings are only for purposes of illustration and as an aid to understanding, and are not intended to be a definition of the limits of the invention. 
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , there is shown a specific non-limiting example of a system  10  for use in screening containers with liquids, in accordance with a non-limiting embodiment of the present invention. The system  10  comprises an X-ray imaging apparatus  100  that applies an X-ray screening process to a material such as a liquid  104  contained in a container  102  that is located within a screening area of the X-ray imaging apparatus  100 . In an airport setting, a passenger may place the container  102  in a tray which is then placed onto a conveyor  114  that causes the container  102  to enter the screening area of the X-ray imaging apparatus  100 . The X-ray imaging apparatus  100  outputs an image signal  116  to a processing module  500 . The processing module then processes the X-ray image data conveyed by the signal  116 . 
     The processing module  500  may be co-located with the X-ray imaging apparatus  100  or it may be remote from the X-ray imaging apparatus  100  and connected thereto by a communication link, which may be wireless, wired, optical, etc. The processing module  500  processes the image data and executes a method to produce a threat assessment  118 . The processing module  500  is computer based and its functionality is provided by suitable software executing on a computing platform. 
     The threat assessment  118  is provided to a console  128  and/or to a security station  132 , where the threat assessment  118  can be conveyed to an operator  130  or other security personnel. The console  128  can be embodied as a piece of equipment that is in proximity to the X-ray imaging apparatus  100 , while the security station  132  can be embodied as a piece of equipment that is remote from the X-ray imaging apparatus  100 . The console  128  may be connected to the security station  132  via a communication link  124  that may traverse a data network (not shown). 
     The console  128  and/or the security station  132  may comprise suitable software and/or hardware and/or control logic to implement a graphical user interface (GUI) for permitting interaction with the operator  130 . Consequently, the console  128  and/or the security station  132  may provide a control link  122  to the X-ray imaging apparatus  100 , thereby allowing the operator  130  to control motion (e.g., forward/backward and speed) of the conveyor  114  and, as a result, to control the position of the container  102  within the screening area of the X-ray imaging apparatus  100 . 
     In accordance with a specific non-limiting embodiment the X-ray imaging apparatus  100  is a dual-energy X-ray imaging apparatus  100 . However, persons skilled in the art will appreciate that the present invention is not limited to such an embodiment. Such dual-energy X-ray imaging apparatus  100  has a source which emits X-rays at two distinct photon energy levels, either simultaneously or in sequence. Example energy levels include 50 keV (50 thousand electron-volts) and 150 keV, although persons skilled in the art will appreciate that other energy levels are possible. 
     The processing module  500  receives the image signal  116  and processes the signal to determine if the liquid  104  in the container  102  poses a security threat. The determination can include an explicit assessment as to weather the liquid  104  is a threat or not a threat. Alternatively, the determination can be an identification of the liquid  104  or the class of materials to which the liquid  104  belongs, without explicitly saying whether the liquid  104  is threatening or not threatening. For example, the processing module can determine that the liquid  104  is “water” hence the operator  130  would conclude that it is safe. In a different example, the processing module  500  determines that the liquid  104  belongs to a class of flammable materials, in which case the operator  130  would conclude that it would be a security threat. Also, the determination can be such as to provide an explicit threat assessment and at the same time also provide an identification of the liquid  104  in terms of general class of materials or in terms of a specific material. The results of the determination are conveyed in the threat assessment signal  118  which is communicated to the console  128  and/or the security station  132  where it is conveyed to the operator  130 . 
       FIG. 4  is a high level block diagram of the processing module  500 . The processing module  500  has a Central Processing Unit (CPU)  508  that communicates with a memory  502  over a data bus  504 . The memory  502  stores the software that is executed by the CPU  508  and which defines the functionality of the processing module  500 . The CPU  120  exchanges data with external devices through an Input/Output (I/O) interface  506 . Specifically, the image signal  116  is received at the I/O interface  506  and the data contained in the signal is processed by the CPU  508 . The threat assessment signal  118  that is generated by the CPU  508  is output to the console  128  and/or the security station  132  via the I/O interface  506 . 
     In a specific example of implementation, the system  10  is used in conjunction with a tray  200  shown in  FIG. 2  to perform security screening of liquid products. The tray  200  is used as a receptacle in which objects to be screened, such as liquid products or other materials or articles, are placed and put on the conveyor belt of the X-ray imaging system  100 . The tray  200  is provided with one or more distinct areas that have X-ray signatures which can be used as references against which the X-ray imaging apparatus  100  can self-calibrate. 
     The tray  200  defines a surface  202  which is generally flat and on which the liquid product that is being screened rests. In the example shown in the drawings, the surface is shaped as a rectangle with rounded corners. Evidently, different shapes or configurations can be used without departing from the spirit of the invention. 
     The surface  202  is provided with raised edges or rim  204  that extend in a continuous fashion around the periphery of the surface  202 . The raised edges  204  prevent articles placed in the tray  200  to fall outside during the screening operation. The height of the raised edges  204  can vary without departing from the spirit of the invention. 
     The surface  202  defines five distinct areas. The first area  206  is the base material from which the tray  200  is made. That material may be any synthetic material that has the required strength and durability characteristics for the intended application. The four additional distinct areas  208 ,  210 ,  212  and  214  are in the form of inserts that are placed in respective receptacles in the base material  206 . The areas  208 ,  210 ,  212  and  214  are in the shape of rectangles placed near respective corners of the tray  200 . It is to be expressly noted that the shape, placement in the tray  200  and the number of the areas  208 ,  210 ,  212  and  214  can vary without departing from the spirit of the invention. 
     The areas  206 ,  208 ,  210 ,  212  and  214  are distinct in that they have different X-ray signatures. Accordingly, when an X-ray image is taken of the tray  200  alone, the areas  206 ,  208 ,  210 ,  212  and  214  will show up differently in the image. Preferably, the area  206  is made of material that is selected to provide a weak X-ray signature such as to limit its effect in the image and thus make the other articles that are put on the tray  200  more visible. In that sense, the area  206  attenuates the X-ray beam little or not at all. In contrast, the areas  208 ,  210 ,  212  and  214  are designed to provide different levels of X-ray attenuation, as it will be discussed later. 
     More specifically, each area  208 ,  210 ,  212  and  214  can be made from a material providing the desired degree of X-ray attenuation. This solution can be implemented by providing an insert made from the selected material that is placed in the base material  206  of the tray  200 . This feature is best shown in  FIG. 3  which is a cross-sectional view of the tray  200  taken at the level of the area  214 . Specifically, the base material of the tray is provided with a receptacle  300  in which is placed an insert  302  defining the area  214 . To ensure a snug fit the insert  302  is manufactured to be of about the same size as the receptacle  300 . In this fashion, the insert  302  is held in the receptacle  300  as a result of friction fit. Evidently, other mounting methods can be provided without departing from the spirit of the invention. One possible variant is to use a fastening mechanism that would allow the insert  302  to be removed. In this fashion, the insert  302  can be replaced with another insert, if the original insert is damaged or if it is deemed appropriate to change the X-ray signature of the area  214 . 
     In a specific and non-limiting example of implementation the X-ray signature of anyone of the reference areas  206 ,  208 ,  210 ,  212  and  214  can be expressed as the gray scale level intensity of the pixels in the portion of the X-ray image that depicts respective reference area. Generally, the gray scale level intensity represents the degree of attenuation of the X-rays as they pass through the object. The grey scale level can be relatively uniform across the reference area  206 ,  208 ,  210 ,  212  and  214 . This is the case when the reference area  206 ,  208 ,  210 ,  212  and  214  is made of material that is homogenous and thus attenuates X-rays uniformly. Another example of an X-ray signature is a situation when the area  206 ,  208 ,  210 ,  212  and  214  is not homogeneous and thus creates a certain gray scale level profile or pattern. The pattern may be regular or irregular. 
     Generally speaking, the X-ray signature of a reference area  206 ,  208 ,  210 ,  212  and  214  is the response produced by the reference area  206 ,  208 ,  210 ,  212  and  214  when the reference area  206 ,  208 ,  210 ,  212  and  214  interacts with X-rays. There are a number of interactions possible, such as:
         The Rayleigh scattering (coherent scattering)   The photoelectric absorption   The Compton scattering (incoherent scattering)   The pair production   Diffraction       

     The photoelectric absorption of X-rays occurs when an X-ray photon is absorbed, resulting in the ejection of electrons from the shells of the atom, and hence the ionization of the atom. Subsequently, the ionized atom returns to the neutral state with the emission of whether an Auger electron or an X-ray characteristic of the atom. This subsequent X-ray emission of lower energy photons is however generally absorbed and does not contribute to (or hinder) the X-ray image making process. This type of X-ray interaction is dependent on the effective atomic number of the material or atom and is dominant for atoms of high atomic numbers. Photoelectron absorption is the dominant process for X-ray absorption up to energies of about 25 keV. Nevertheless, in the energy range of interest for security applications (for today&#39;s state-of-the-art security screening systems, the energy levels commonly utilized lie between 50 keV and 150 keV), the photoelectric effect plays a smaller role for low Zeff values with respect to the Compton scattering, which becomes dominant. 
     Compton scattering occurs when the incident X-ray photon is deflected from its original path by an interaction with an electron. The electron gains energy and is ejected from its orbital position. The X-ray photon looses energy due to the interaction but continues to travel through the material along an altered path. Since the scattered X-ray photon has less energy, consequently it has a longer wavelength than the incident photon. The event is also known as incoherent scattering because the photon energy change resulting from an interaction is not always orderly and consistent. The energy shift depends on the angle of scattering and not on the nature of the scattering medium. Compton scattering is proportional to material density and the probability of it occurring increases as the incident photon energy increases. 
     The diffraction phenomenon of the X-rays by a material with which they interact is related to the scattering effect described earlier. When the X-rays are scattered by the individual atoms of the material, the scattered X-rays may then interact and produce diffraction patterns that depend upon the internal structure of the material that is being examined. 
     As to the pair production interaction, it refers to the creation of an elementary particle and its antiparticle from an X-ray photon. 
     That response produced by a material as it interacts with X-rays can be expressed in terms of gray level value, gray level patterns seen in the X-ray image or other physical manifestation. 
     The selection of the proper material for making the inserts  302  for the various reference areas  208 ,  210 ,  212  and  214  can be made by in a number of ways. The insert may or may not be made from a homogenous material. An example of a non-homogeneous structure is an assembly of layers made from different materials that in combination would provide the desired X-ray signature. Another example is a mixture of different materials intended to create a pattern in the X-ray image. The person skilled in the art will recognize that an almost infinite number of different X-ray signatures can be developed by selecting the proper material or materials and by mixing or assembling them in the appropriate manner. 
     Examples of materials that can be used include plastics such as polyethylene, polypropylene or others. Their density or composition can be varied to obtain the desired X-ray signature. 
     An advantage of performing a comparison between X-ray signatures extracted from the same image data is the elimination or at least the reduction of X-ray induced variations in the system response. In this fashion, the system is self-referencing. 
     The example of implementation shown in  FIG. 2  depicts the areas  208 ,  210 ,  212  and  214  placed in the respective corners of the tray  200 . This is done in order to reduce the likelihood of obscuring anyone of those areas  208 ,  210 ,  212  and  214  by an article that is placed in the tray. For instance, if an article is put in the tray immediately above anyone of those areas  208 ,  210 ,  212  and  214 , the X-ray signature of that area may not be correctly read since the X-ray image will be the result of a composite response (the area  208 ,  210 ,  212  and  214  and the article on top of it). In order to further reduce the possibility of obscuring the areas  208 ,  210 ,  212  and  214  it is possible to place the areas  208 ,  210 ,  212  and  214  at a location that is outside the zone in the tray where the articles to be screened are located. An example of such embodiment is shown in  FIG. 5 . The tray  700  defines a central article receiving area  702  in which are placed the articles to be screened. The article receiving area  702  is surrounded by a rim portion  704  that extends peripherally and fully encircles the article receiving area  702 . The rim portion  704  has a top area  706  that is flat and that is sufficiently wide such as to accept the reference areas  208 ,  210 ,  212  and  214 . In this fashion, articles to be screened are unlikely obscure anyone of the areas  208 ,  210 ,  212  and  214  that remain outside the central article receiving area. 
     The tray  200  provides a material reference during the X-ray scanning process which can be used to limit or avoid altogether machine induced variations in the results by performing a self-calibration operation. Since in practice different X-ray imaging apparatuses are never identical and manifest some variations that can be either at the level of the X-ray detectors elsewhere in the machine, those variations can impact the detection results. 
     More specifically, the tray  200  can be used as a known reference for the X-ray imaging apparatus  10 . Accordingly, when the X-ray scanning process is performed the X-ray imaging apparatus  100  can use the X-ray signature of the tray  200  to self-calibrate. 
     Since in the course of an X-ray scanning operation the tray  200  will be used repeatedly, the self-calibration operation occurs with regularity, thus enhancing the performance of the X-ray imaging apparatus in terms accuracy in identifying security threats. 
     A general view of the threat assessment and self-calibration process is shown at  FIG. 6 . At step  800  the process starts. At step  802  a passenger at a security checkpoint, such as at an airport, removes articles from his/her hand carried luggage. Examples of articles include containers holding liquids or other articles such as electronic equipment. 
     At step  804  the removed articles are placed in a tray that includes reference areas, say tray  700 . In addition to the articles that are removed from the hand carried luggage, additional articles can also be included such as shoes (in the instance the individual is being requested by security personnel to have his/her shoes scanned), a belt and a jacket among others. 
     At step  806  the tray  700  is placed on the belt  114  of the X-ray imaging apparatus  100 , which carries the tray  700  with the articles therein inside the X-ray imaging apparatus  100 . 
     At step  808  an x-ray image of the tray  700  and of the articles therein is taken. The X-ray image data is then processed at step  810  to perform a self-calibration of the X-ray imaging apparatus  100 . Once the X-ray imaging apparatus  100  is self-calibrated, the x-ray image data is processed to perform a threat assessment of the articles in the tray  700 . 
     In this example, the self-calibration of the X-ray imaging apparatus  100  and the threat status assessment are performed during the same X-ray scanning cycle. This self-calibration can be repeated at every scanning cycle, thus reducing as much as possible machine induced variations over time. If machine induced variations drift over time, such as the result of temperature, humidity of other environmental factors, the repeated self-calibration will track those drifts and thus enhance the detection results. 
     Once the scanning operation is completed the person takes away his/her belongings from the tray. The empty tray is then brought back and placed near the entry side of the X-ray imaging apparatus such that it can be used by another person. If every tray in the set of trays provided with the X-ray imaging apparatus  100  use reference areas, every time a tray is used to perform a scan of articles, a self-calibration operation occurs. 
     Note that it is not essential to perform self-calibration every time the X-ray imaging apparatus  100  scans articles to detect their threat status. One possibility is to perform the self-calibration operation at every other scanning cycle or at any other frequency deemed appropriate for the intended application. The selection of the frequency at which the self-calibration will occur can be done in a number of possible ways, namely:
         1. The set of trays that are used to scan articles in the X-ray imaging apparatus  100  is provided with a sub-set that enable the self-calibration (trays with reference areas) and a sub-set that cannot be used to perform self-calibration (trays without any reference areas). Since those trays are used repeatedly, the self-calibration process will occur only when a tray with reference areas is being scanned. The frequency at which the self-calibration occurs can be set by determining the mix of trays that enable self-calibration and those that do not, as desired.   2. The X-ray imaging apparatus  100  can be programmed such as to run the self-calibration process for one tray in a sequence of trays that are being scanned, such as every second, third or fourth tray, for instance.   3. The X-ray imaging apparatus can be manually controlled to run the self-calibration. The X-ray imaging apparatus has on its console  300  controls that are actuated by the operator to run the self-calibration. The operator thus takes the decision at which frequency the self-calibration occurs.       

     The self-calibration operation, in terms of X-ray image data processing will be described in greater detail in connection with  FIGS. 7 ,  8  and  9 . 
     With reference to  FIG. 7 , the process starts at step  900 . At step  902  the X-ray image data is processed by the processing module  200  to locate the reference areas in the tray. At step  904  the X-ray signature of each reference area is acquired. In the example shown in the drawing, the X-ray signature is conveyed by the grey scale level or value of the reference area in the image. Accordingly, the grey scale value associated with each reference area is measured to acquire the X-ray signature. 
     As can be seen in the graph at  FIG. 8 , each grey scale value can be mapped to a certain X-ray attenuation level. At step  906  the nominal X-ray signatures of the respective reference areas are obtained. In this specific example, the nominal X-ray signatures are associated with specific attenuation information. The nominal X-ray signatures can be obtained from different sources, as generally described earlier. For instance, the nominal X-ray signatures are stored in a database that is accessible to the processing module  200 . By performing the analysis of the X-ray image data, the processing module  200  can extract the identities of the reference areas and on the basis of the identity information extract the signature information from the database. Alternatively, the nominal X-ray signatures can be encoded directly in the tray such that they can be read by the processing module in the X-ray image data. One example is a bar-code encoding that is machine readable. 
     At step  910  nominal X-ray signatures are used by the processing module  200  to create a relationship between the X-ray image data and corresponding attenuation information. This is best illustrated in the graph of  FIG. 8  which maps grey scale values to attenuation levels. The process is performed by software executed by the processing module  200 . 
     Assume that the tray has three different reference areas, namely reference area A, a reference area C and an intermediate reference area B. Reference areas A, B and C are associated with progressively decreasing attenuation levels. For the sake of this example, consider that reference area A is associated with a 10% attenuation level, reference area B with 5% attenuation level and reference area C with 2% attenuation level. 
     The grey scale value associated with the reference area A is plotted against an attenuation values axis (at the known 10% attenuation level) to create a data point  1000 . 
     The grey scale value  1006  associated with the reference C is also plotted against the attenuation values axis at the known value of 5%, which creates a second data point  1010 . 
     The data points  1000  and  1010  can be used to establishing a linear relationship between the X-ray image data and corresponding attenuation levels, where the relationship is corrected with respect to known references (A and C). Note that in this example, the X-ray image data is expressed in terms of grey scale values and the relationship is therefore established between the grey scale values read from the image and the attenuation levels. However, in situations where the X-ray image data is conveyed in a way other than grey scale values, the data conveying the image information can equally well be mapped to attenuation levels. 
     The two data points  1000  and  1010  allow creating a linear relationship between the grey scale values and the attenuation levels. In this fashion, any grey scale values residing between the grey scale values of data points  1000  and  1010  will be mapped to attenuation levels according to a linear relation. In practice, this linear relationship may not accurately reflect the reality of the physics involved, in which case additional data points can be used to create a more accurate map. In the example shown in  FIG. 10 , the intermediate reference B provides a data point  1012  that corresponds to 5% attenuation. An algorithm can be used to create a best fit curve over the three data points which will define the relationship between the grey scale values and the attenuation levels. 
     While the above example illustrates a situation where the reference areas A, B and C are all located in the lower end of the attenuation level scale, in the range between 2% and 10%, the references can be selected in a different area of the scale. More particularly, the references can be selected such as to cover a wider range of attenuations, a range of attenuations located near the upper end of the attenuation level scale (close to 100% attenuation), or anywhere else between the upper end and the lower end. For instance, in a variant, the references can be selected such as to span the entire scale. 
     From the point of view of calibration results, the location of the reference areas in the lower end of the attenuation scale (low attenuation) allows calibrating the X-ray imaging apparatus  10  in the operating range where the apparatus usually manifests the most drift. Accordingly, performing a calibration in this area is likely to improve in a tangible way the accuracy of detection. 
     In addition, the lower end of the attenuation scale (low attenuation) tends to be non-linear; accordingly the placement of reference areas in that region is likely to produce a more accurate map between the grey scale levels and the corresponding attenuation values.  FIG. 15  illustrates this point.  FIG. 15  shows a graph of the relationship between the grey scale levels and attenuation over the entire attenuation range, namely 0% to 100%. The region  2200 , in the lower end of the attenuation scale (low attenuation) is non-linear. In a scenario where two reference points are used for the calibration and they are placed such as to locate one (point  2202 ) in the region  2200  and one (point  2204 ) outside the region  2200 , the resulting map  2206  may not track well the curve. In a different scenario, where the reference points  2208  and  2210  are all placed in the region  2200 , the resulting map  2212  better follows the curve. 
       FIG. 9  provides an example of a different situation where the tray has 5 reference areas spread over the entire attenuation range, namely reference areas D, E, F, G and H corresponding to 100%, 75%, 50% and 25% attenuation levels, respectively. By using a best fit algorithm a curve can be laid over the data points that establish the relationship between the grey scale values and the attenuation levels. 
     Referring back to  FIG. 7 , the process terminates at step  912  by analyzing the X-ray image data to determine if the articles in the tray present a security threat. To perform the threat assessment the processing module  200  determines the levels of X-ray attenuation associated to pixels or groups of pixels in the X-ray image. The level of attenuation is obtained on the basis of the relationship established earlier where grey-scale values are mapped to attenuation levels, and the resulting map is stored in the memory of the processing module  200 . The processing module  200  uses the grey scale level as an input to the attenuation map and derives an attenuation values, accordingly. The attenuation values are then processed to determine the threat status of the articles. 
     In a possible variant, the reference areas used to perform the self-calibration operation are associated with the belt  114  that is used to carry the tray and the articles to be scanned through the X-ray imaging apparatus  100 . This example is best shown in  FIGS. 10 ,  11  and  12 . 
       FIG. 10  is a plan view of the belt  1200 . The belt is an endless sheet mounted on two rollers  1202  and  1204 . One or both of the rollers can be used to drive the belt such that it advances articles through the scanning station of the X-ray imaging apparatus  100 ′. The belt  1200  is provided with a plurality of reference areas. More specifically, four reference areas are used, namely reference areas  1206 ,  1208 ,  1210  and  1212 . Each reference area is shaped as a strip of material that extends across the direction of movement of the belt  1200 . In a specific example, the strips are oriented generally transversally with relation to the direction of movement of the belt  1200 . The length of each reference area  1206 ,  1208 ,  1210  and  1212  is somewhat less than the transverse dimension of the belt  1200 . 
     The materials that constitute the reference areas  1206 ,  1208 ,  1210  and  1212  can be mounted on top of the belt  1200  surface and secured thereto in any suitable fashion. Alternatively, the materials can be mounted on the bottom of the surface or imbedded in the belt  1200  such that they are not visible to the eye. 
     In operation the articles to be scanned (with or without tray) should not be placed directly over the reference areas  1206 ,  1208 ,  1210  and  1212  to avoid obscuring them. It is better if the articles to be scanned are immediately adjacent the reference areas  1206 ,  1208 ,  1210  and  1212  such that they do not overlap while at the same time the X-ray image data encompasses them both. To avoid an overlap an arrangement can be provided to indicate to the user that articles should not be placed over the reference areas  1206 ,  1208 ,  1210  and  1212 . The arrangement can include a physical barrier, such projections (not shown extending outwardly of the belt  1200  surface which intuitively indicate that no articles can be placed at location on the belt. Alternatively, the arrangement includes markings to indicate to a user that no articles should be placed in that area. The markings my include text, pictograms or a combination of both. 
     Note that the markings can be used in conjunction with the physical barrier. 
     Yet the arrangement can also include markings to indicate where articles, including a tray, can be placed such as not to overlap with the reference areas. Those markings may be lines that delineate a boundary in which articles are to be placed. For instance, the marking may be in the shape of a rectangle  1214  indicating where articles to be scanned should be placed. 
     One advantage in using elongated strips of material to form the reference areas which are fed transversally to the direction of movement of the belt  1200  (the direction of movement is depicted by the arrow  1216 ) is to allow a majority of the X-ray detectors and preferably all of the X-ray detectors of the X-ray imaging apparatus  100  to sense the reference areas. 
     With specific reference to  FIG. 12 , which is a front view of the X-ray imaging apparatus  100 , specifically showing the belt  1200  and a detector array  1400 . The detector array  1400  has a plurality of detectors  1402  that are arranged to form an L-shape, including a horizontal arm and a vertical arm. The elongated strip of material can therefore be “seen” by each detector, which allows constructing for each detector an individual attenuation map of the type described earlier. 
     This is best shown in  FIG. 13  which is a representation of an X-ray image obtained from the detector array  1400 . The image  1500  shows articles  1502  that are being scanned and also the reference areas. The X-ray image is assembled from individual image strips  1504 , where each image strip is derived from the output of a detector  1402 . Since the reference materials have geometry such that a portion of each of them appears in each image strip, that image strip can be processed independently to create a detector specific attenuation map. 
     Accordingly, the processing module  500  would therefore store in its memory a series of attenuation maps, one for each detector. In such case, when the X-ray image data is processed, the portions of the image data that are derived from a given detector are processed against the attenuation map of that detector to determine the attenuation values. For instance, grey scale levels of pixels in any one of the strips  1504  are used as inputs to the attenuation map of the detector associated with the image strip to determine the attenuation levels. 
     Note that similar reference material geometries can be used on trays as well.  FIG. 14  is a top plan view of a tray  1600  having an area  1602  to receive articles to be scanned and a rim portion  1604  on which are placed reference materials to form reference areas  1606  and  1608 . In the example shown the tray  1600  is provided with two reference areas  1606  and  1608  but more can be provided if desired. 
     The tray has an imaginary longitudinal axis and an imaginary transverse axis. In use, the tray should be placed such that the reference areas  1606  and  1608  extend across the longitudinal axis, which coincides with the direction of movement of the tray in the scanning area of the X-ray imaging apparatus  10 . In a specific example, the reference areas  1606  and  1608  are perpendicular to the direction of movement of the belt (arrow  1608 ). Markings can be placed on the tray such that users can place the tray in the proper orientation. The markings can include pictograms of text. An arrow placed on the bottom of the area  1602  is an example of a marking. 
     Another example is to make the tray  1600  sufficiently long such that if it is placed transversely on the tray it will not fit in the entry of the X-ray machine. In this fashion, the tray can only be used in a single orientation. 
     Although various embodiments have been illustrated, this was for the purpose of describing, but not limiting, the invention. Various modifications will become apparent to those skilled in the art and are within the scope of this invention, which is defined more particularly by the attached claims.