Patent Publication Number: US-10782440-B2

Title: X-ray scanner with partial energy discriminating detector array

Description:
CROSS-REFERENCE 
     The present application is a continuation application of U.S. patent application Ser. No. 15/787,886, entitled “X-Ray Scanner with Partial Energy Discriminating Detector Array” and filed on Oct. 19, 2017, which is a continuation application of U.S. patent application Ser. No. 14/148,452, of the same title, filed on Jan. 6, 2014, and issued as U.S. Pat. No. 9,823,383 on Nov. 21, 2017, which, in turn, relies on U.S. Provisional Patent Application No. 61/749,838, of the same title and filed on Jan. 7, 2013, for priority. All of the aforementioned applications are herein incorporated by reference in their entirety. 
    
    
     FIELD 
     The present specification relates to X-ray inspection systems. More particularly, the present specification relates to a source, detector and object configuration, whereby the energy transmitted through the object being inspected is measured at improved energy resolutions. 
     BACKGROUND 
     Due to persistent security threats and the possibility of terrorist activities, there is a need for deploying high speed, high resolution, and accurate screening devices at places that are likely targets of such activities. In addition, there exists a requirement for screening of baggage and other items for explosives, contraband and other illicit materials. This requires a screening system which is capable of discriminating between different materials based on one or more unique features of each material such as effective atomic number, chemical structure, physical density, among other variables. 
     Cabinet X-ray scanners are capable of performing automated threat detection on articles of baggage and divested items by calculating physical properties of objects from the two-dimensional image generated. For example, material density can be estimated from at least two co-planar projection views. Z-effective can be estimated from at least two overlapping projection views acquired at different energy levels. Thus, increasing the number of views or number of energy levels permits for more precise estimations of physical properties such as material density and Z-effective. 
     Conventional cabinet X-ray systems have a limited number of co-planar views. It is highly desirable for commercial reasons to have as few views as necessary. Typically, a planar X-ray view consists of an X-ray generator and a linear array of X-ray detectors, which constitutes a majority of the component costs in an X-ray scanner. Each additional view increases the component cost of the scanner in an incremental manner. Thus, two views would imply twice the cost of a single view, three views would increase the cost three times, and so forth. Therefore, achieving desired imaging or automated detection performance with the fewest number of views allows the lowest component cost. Increasing performance through other means, such as higher performance detectors, becomes desirable because of the potential lower increase in component cost. 
     Further, increasing energy discrimination also has a detrimental impact on component cost. Commercially available X-ray detectors currently permit up to 128 channels of energy discrimination. Populating a multiple or even a single projection X-ray detector array with energy discriminating detectors is commercially prohibitive. The cost of energy discriminating detectors is significantly higher than conventional dual-energy detector arrays. For example, a conventional single-view X-ray scanner equipped with energy discriminating detectors would incur a three-fold increase in component cost. Reducing the number of energy discriminating detectors employed to increase performance becomes an attractive option, especially if the incremental increase in component cost is less than doubling the cost of the machine, i.e. less than the cost of adding a second view. 
     For an application such as liquids screening, it is desirable to have the full range of energy information permitting for a spectroscopic analysis of the contents in the divested container. Therefore, some applications present the opportunity to restrict the concept of operations such that only a portion of the projection view needs to be populated with multi-energy discriminating detectors in order to obtain improved estimation of the physical properties necessary for effective threat detection. For example, with respect to the divestiture of liquid containers at an aviation security checkpoint for separate threat detection analysis, only a portion of a bin or container need be screened using multi-energy discriminating detectors. 
     Thus, what is needed is an X-ray scanner, having at least a single-view, with limited energy discriminating detector coverage that can meet or exceed the automated detection performance of a dual-view X-ray scanner and still have a lower component cost, thus achieving the trade-off between cost and performance. 
     Accordingly, there is a need for an X-ray system that has an overall improved energy resolution to discriminate and therefore detect certain materials of interest. 
     SUMMARY 
     In one embodiment, the present specification describes a system for screening objects, comprising: a) a receptacle for holding an object; b) a conveyor to move the receptacle through an inspection region; c) first X-ray source for transmitting X-rays through the object for generating a vertical X-ray projection view of the said object; d) a second X-ray source for transmitting X-rays through the object for generating a horizontal X-ray projection view of the said object; e) a first set and a second set of transmission detectors for receiving the X-rays transmitted through the said object; and f) a third and a fourth set of energy discriminating detectors for receiving the X-rays transmitted through the said object, wherein said third and fourth set of energy discriminating detectors are positioned such that they align with the object within the receptacle. 
     In one embodiment, the present specification describes a method for screening objects, comprising: a) providing a receptacle to hold and align an object; b) moving the said receptacle through an inspection region using a conveyor; c) generating an X-ray projection view of the said object using an X-ray source; d) detecting X-rays transmitted through the said object using transmission detectors and at least one energy discriminating detector which is positioned such that it aligns with the object within the receptacle. 
     In one embodiment, the present specification describes a system for screening objects, comprising: a) a receptacle for holding an object; b) a conveyor to move the receptacle through an inspection region; c) an X-ray source for transmitting X-rays through the object for generating an X-ray projection view of the said object; d) a plurality of transmission detectors for receiving the X-rays transmitted through the said object; and e) a plurality of energy discriminating detectors for receiving the X-rays transmitted through the said object, wherein said plurality of energy discriminating detectors are positioned such that they align with the object within the receptacle. 
     In another embodiment, the present specification describes a method for screening objects, comprising: a) providing a receptacle to hold and align an object; b) moving the said receptacle through an inspection region using a conveyor; c) generating an X-ray projection view of the said object using an X-ray source; d) detecting X-rays transmitted through the said object using a plurality of transmission detectors; and e) detecting X-rays transmitted through the said object using a plurality of energy discriminating detectors which are positioned such that they align with the object within the receptacle. 
     In one embodiment, the present specification describes a method for screening objects, comprising: a) providing a receptacle to hold and align an object; b) moving the said receptacle through an inspection region using a conveyor; c) generating a vertical X-ray projection view of the said object using a first X-ray source; d) generating a horizontal X-ray projection view of the said object using a second X-ray source; e) detecting X-rays transmitted through the said object using a first set and second set of transmission detectors; and f) detecting X-rays transmitted through the said object using a third set and fourth set of energy discriminating detectors which are positioned such that they align with the object within the receptacle. 
     In another embodiment, the present specification describes a system for screening objects, comprising: a) a receptacle for holding an object; b) a conveyor to move the said receptacle through an inspection region; c) a first X-ray source for transmitting X-rays through the object for generating a vertical X-ray projection view of the said object; d) a second X-ray source for transmitting X-rays through the object for generating a horizontal X-ray projection view of the said object; e) a first set and a second set of transmission detectors for receiving the X-rays transmitted through the said object; f) a third and a fourth set of energy discriminating detectors for receiving the X-rays transmitted through the said object, wherein said third and fourth set of energy discriminating detectors are positioned such that they align with the object within the receptacle; and g) a processor for receiving output signals from said first, second, third and fourth sets of detectors and overlaying said output signals onto a visual image of the said receptacle and object. 
     In yet another embodiment, the present specification describes a method for screening objects, comprising: a) providing a receptacle to hold and align an object; b) moving the said receptacle through an inspection region using a conveyor; c) generating a vertical X-ray projection view of the said object using a first X-ray source; d) generating a horizontal X-ray projection view of the said object using a second X-ray source; e) detecting X-rays transmitted through the said object using a first and second set of transmission detectors; f) detecting X-rays transmitted through the said object using a third and fourth set of energy discriminating wherein said third and fourth set of energy discriminating detectors are positioned such that they align with the object within the receptacle; and g) processing the output signals from said first, second, third and fourth sets of detectors to form a visual image of the said receptacle and object. 
     In one embodiment, the receptacle is a tray further comprising a foam insert that has at least one channel to align the said object for screening. In one embodiment, the object is a LAG item. 
     In one embodiment, the first and second transmission detectors are dual-energy sensitive stacked detectors while the third and fourth energy discriminating detectors are fabricated from high-Z semiconductor materials including cadmium-telluride (CdTe), cadmium-zinc-telluride (CZT), mercury iodide (HgI 2 ), selenium (Se), lead iodide (PbI 2 ), gallium arsenide (GaAs). 
     The aforementioned and other embodiments of the present specification shall be described in greater depth in the drawings and detailed description provided below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present specification will be further appreciated, as they become better understood by reference to the detailed description when considered in connection with the accompanying drawings: 
         FIG. 1A  is a side perspective view of the inspection system of the present specification showing a source irradiating an object to obtain a vertical projection view of the object; 
         FIG. 1B  is a side perspective view of the inspection system of the present specification showing a source irradiating the object to obtain a horizontal projection view of the object; 
         FIG. 2A  shows a receptacle of the present specification in the form of a tray holding objects to be scanned; and 
         FIG. 2B  is an X-ray image of the tray of  FIG. 2A  obtained using a dual-view embodiment of the inspection system of the present specification. 
     
    
    
     DETAILED DESCRIPTION 
     The present specification is directed towards scanning objects for threat/contraband detection. In one embodiment, the scanning/inspection system of the present specification is configured for screening objects at aviation security checkpoints. However, in alternate embodiments, the scanning/inspection system of the present specification is deployable at any such sites/places that are likely to be targets of terrorist activities—such as, border security checkpoints, entrances to buildings or other vulnerable premises, concert venues, sports venues, and the like. 
     In one embodiment, the scanning/inspection system of the present specification is configured as a single-view system using dual-energy sensitive stacked detectors that are partially populated with multi-energy discriminating detectors for overall enhanced energy resolution and therefore improved discrimination of materials through better estimation of material physical properties such as density and effective atomic number. 
     In one embodiment, the scanning/inspection system of the present specification is configured as a dual-view system using dual-energy sensitive stacked detectors that are partially populated with multi-energy discriminating detectors for overall enhanced energy resolution and therefore improved discrimination of materials through better estimation of material physical properties such as density and effective atomic number. 
     In one embodiment, the transmission detectors are dual-energy sensitive stacked detectors while the energy discriminating detectors are fabricated from high-Z semiconductor materials including cadmium-telluride (CdTe), cadmium-zinc-telluride (CZT), mercury iodide (HgI 2 ), selenium (Se), lead iodide (PbI 2 ), gallium arsenide (GaAs). 
     In accordance with an aspect of the present specification, a “receptacle” can be defined as an open, closed or closable vessel or container for housing objects that need to be scanned. The receptacle ensures that the objects therein are aligned, restricted, constrained or positioned to occupy a predetermined or predefined volumetric space with reference to the source, detector and conveyor configuration of an inspection system. In one embodiment the receptacle is an open tray. In another embodiment, the receptacle is a box with a closable lid. In accordance with one embodiment, the size of the receptacle is on the order of 550 mm wide×685 mm long×140 mm high. 
     In alternate embodiments, the receptacle is a piece of luggage or baggage containing objects that are not necessarily placed, positioned, oriented, or restricted in a predetermined fashion relative to the inspection system. Instead, the objects are placed in a random fashion as would be expected in typical luggage/baggage. 
     In accordance with an aspect of the present specification, an “object to be screened” can be defined as an open, closed or closable vessel, container or housing containing liquid or gel-based items that resemble liquid or gel-based explosives/threats such as liquid, aerosol and gel items (hereinafter referred to as “LAG” items). Categories of LAG items typically found in passenger carry-on baggage include, but are not limited to, alcohol/perfume/deodorants, drinks, foods, household products, medicines, toiletries, and the like. Prior to screening, LAG items are typically divested from baggage, luggage or personal effects and placed in a receptacle for scanning. In accordance with one embodiment, the object is of a size range that allows it to be placed in the receptacle. In one embodiment, volume ranges for a typical vessel to be screened are from 100 mL to 2000 mL. 
     In alternate embodiments, the object comprises any solid, powder or plastic-based threat or contraband items known to persons of ordinary skill in the art and is not limited to LAG items. 
     The present invention is directed towards multiple embodiments. The following disclosure is provided in order to enable a person having ordinary skill in the art to practice the invention. Language used in this specification should not be interpreted as a general disavowal of any one specific embodiment or used to limit the claims beyond the meaning of the terms used therein. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Also, the terminology and phraseology used is for the purpose of describing exemplary embodiments and should not be considered limiting. Thus, the present invention is to be accorded the widest scope encompassing numerous alternatives, modifications and equivalents consistent with the principles and features disclosed. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail so as not to unnecessarily obscure the present invention. 
       FIG. 1A  shows a side perspective view of an embodiment of an energy discriminating partial array inspection system  100  of the present specification in which a single-view vertical projection is provided. System  100  comprises a support frame  105  holding a first X-ray source  110  with focusing means, such as a collimating slit, and a first enhanced array  119  of stacked detectors  120  populated with at least one multi-energy discriminating detector  122  in accordance with an aspect of the present specification. In one embodiment, the detectors are placed in an L-shaped configuration; however, other configurations may be acceptable provided that the detectors are appropriately positioned relative to the inspection region and X-ray source. 
     It should be appreciated that the detector array of the present specification is known as a folded “L” detector configuration, and is made from multiple detector modules  120 ,  122 . Each detector module is positioned at a different angle so as to be perpendicular to the X-ray fan beam. A typical small tunnel X-ray scanner will use approximately 10 detector modules. Not every detector module needs to be replaced with an energy discriminating detector module for the desired performance improvement. The projection of the container onto the image array determines which detector modules need to be of the energy discriminating variety. Referring to  FIG. 1A , two energy discriminating modules  122  are shown to provide the advantages of the present specification. 
       FIG. 1B  is a side perspective view of an embodiment of an energy discriminating partial array inspection system  100  of the present specification in which a single-view horizontal projection is provided. System  100  comprises a support frame  105  holding a second X-ray source  115  with focusing means, such as a collimating slit, and an enhanced array  124  of stacked detectors  125  partially populated with at least one, and preferably two, multi-energy discriminating detectors  127  in accordance with an aspect of the present specification. In one embodiment, the detectors are placed in an L-shaped configuration; however, other configurations may be acceptable provided that the detectors are appropriately positioned relative to the inspection region and X-ray source. The energy discriminating detectors must be positioned to provide the image of the container under inspection. The information contained within the image of the container is used for the analysis and determination of the contents. 
     Referring to  FIGS. 1A and 1B  simultaneously, an inspection area/region in the form of a tunnel is defined between the sources and detectors to allow a receptacle  135  to be transported through using a conveyor  130 . The receptacle  135  holds an object that is to be screened. It should be noted by those of ordinary skill in the art that while the present embodiment of the system  100  is a dual projection view system comprising two sources, in alternate embodiments the system  100  is a single projection view system comprising a single source whereas in still further embodiments the system  100  is a multi-projection view system comprising more than two sources. In one embodiment, the dual projection view system of the present invention employs 10 detector modules within detector array  119  for the vertical view and 9 detector modules within detector array  124  for the horizontal view. 
     Referring to  FIGS. 1A and 1B  simultaneously, in one embodiment, a minimum of two energy discriminating modules  122 ,  127 , respectively, are employed in each detector array for significant enhancement of the physical property measurement of the system of the present specification in order to obtain a complete image of the container. Further, the embodiments shown in  FIGS. 1A and 1B  can be combined to provide a dual-view embodiment, as shown in  FIG. 2B . 
     During screening, the sources project fan beams of X-rays onto the receptacle  135  such that the radiation-fans intersect the conveyor  130  substantially perpendicular relative to the conveyor surface. As shown in  FIG. 1A , in one embodiment, source  110  is positioned to form a vertical projection view  140 . In accordance with another aspect of the present specification, the receptacle  135  ensures that the position of the object to be screened is aligned or restricted relative to the source, detector and conveyor configuration so that portion  142  of the fan beam projection views  140  are sensed by the multi-energy discriminating detectors  122  to obtain improved estimation of the physical properties of the object for effective threat detection. This is described in greater detail below with respect to  FIG. 2B . 
     As shown in  FIG. 1B , in one embodiment, source  115  is positioned to form a horizontal projection view  145 . In accordance with another aspect of the present specification, the receptacle  135  ensures that the position of the object to be screened is aligned or restricted relative to the source, detector and conveyor configuration so that portion  147  of the fan beam projection views  145  are sensed by the multi-energy discriminating detectors  127  to obtain improved estimation of the physical properties of the object for effective threat detection. 
       FIG. 2A  shows an embodiment of an exemplary receptacle of the present specification in the form of a tray  205 . The tray  205  comprises, in one embodiment, a foam insert  210  further comprising at least one channel for aligning at least one object  215  in an optimum position for screening. The insert is, in one embodiment, a mechanical component to ensure the container is in the optimum position for screening, which includes not having contact with other high density objects and mostly lifting the container off the belt for a cleaner view in the image. The insert does not need to be made of foam; it can be made of any lower density material than the container or contents for which screening is desired. In one embodiment, it is possible to make the plastic tray such that it can present the container for analysis, thus obviating the need for an insert. 
       FIG. 2B  shows a dual-view X-ray image of the tray  205  while being transported on a conveyor belt. In accordance with an embodiment of the present specification, dimension B is the width of the conveyor belt  220  (the tunnel walls being very close to the edge of the belt). The tray  205  is wide enough such that is does not permit side-to-side movement. The width of the tray  205  coupled with the size and position of the channel in the foam insert  210  limits the region that can be occupied by the object  215  to a dimension A. Dimension A defines the minimum regions of the imaging arrays (for the respective projection views) that are required to be populated with energy discriminating detectors to enable a more precise physical property measurement (shown as portions  142 ,  147  in  FIGS. 1A and 1B , respectively). In one embodiment, dimension A has a maximum of 125 mm at the belt, and when projected on the detector array this maximum dimension will occupy 20% of the imaging array while dimension B is 575 mm. 
     In one embodiment, the at least one object  215  is a LAG item that is divested from baggage/luggage and put in the tray  205  for scanning. However, in alternate embodiments, object  215  can be any item that is required to be scanned for threat resolution. In one embodiment, the object  215  is a piece of luggage/baggage and is scanned as-is, while being conveyed, without the need for putting the luggage/baggage or objects divested from the luggage/baggage in the tray  205 . 
     Referring back to  FIGS. 1A and 1B , the stacked detectors  120 ,  125  generate dual energy scan data in accordance with one embodiment. As known to persons of ordinary skill in the art, stacked detectors comprise a first detector positioned to detect more of the lower energy, or the softer X-ray photons, and a second detector positioned to detect the balance of the energy, namely the higher energy, or the harder photons. The second detector is typically positioned behind the first detector. The low energy and high energy measurements are combined in a suitable way using a series of calibration measurements derived from dual energy measurements taken of identified organic and metallic materials of known thicknesses and result in the display of images, including organic only or metal only images. The first and second detectors consist of linear arrays of silicon photodiodes covered with scintillation material, which produce light when exposed to X-rays. The light is detected by the photodiodes that produce corresponding photo current signals. The detected data are converted to digital format, corrected for detector gain and offset, and then stored for processing. In another embodiment, detectors  120 ,  125  are conventional single energy arrays as known to persons of ordinary skill in the art. 
     In one embodiment, the multiple-energy discriminating detectors  122 ,  127  are solid state detectors made from semiconductor materials such as cadmium-telluride (CdTe), cadmium-zinc-telluride (CZT), mercury iodide (HgI 2 ), selenium (Se), lead iodide (PbI 2 ), gallium arsenide (GaAs) or any other high-Z material that enables the detectors to be operable at room temperature. These detectors have high energy resolution, as compared to the dual energy stacked detectors and are direct conversion devices (that is, convert radioactive particles, such as photons, directly into electronic signals). 
     In one embodiment, a low-noise, low-power, multi-channel readout application-specific integrated circuit (ASIC) is used for the acquisition of scan data. Each channel of the ASIC has an energy discriminating circuit and a time discriminating circuit. The ASIC also has built-in analog to digital converters (ADCs), or digitizers, to digitize the signal from energy and timing sub-channels. Variation in the digital output of the ASIC is tracked from a reference signal output to generate correction coefficients. The correction coefficients may be then applied to subsequent digital outputs to eliminate or reduce temperature-induced error. 
     System  100  also comprises at least one processor (such as a computer) having access to a memory for storing programmatic instructions in the form of software and/or firmware. The at least one processor may be local to, or remote from, the X-ray source and detectors. Similarly, the memory and programmatic instructions may be local to, or remote from, the X-ray source and detectors. 
     In a single-view configuration, when the programmatic instructions are executed, the at least one processor: a) reconstructs a combined image from scan data generated by the detectors  120 ,  122  wherein each pixel within the image represents an associated mass attenuation coefficient of the object under inspection at a specific point in space and for a specific energy level; b) fits each of the pixels to a function to determine the mass attenuation coefficient of the object under inspection at the point in space; and c) uses the function to automatically determine the identity or threat status of the object under inspection. 
     In a dual-view configuration, when the programmatic instructions are executed, the at least one processor: a) reconstructs a combined image from scan data generated by the detectors  120 ,  122 ,  125  and  127 , wherein each pixel within the image represents an associated mass attenuation coefficient of the object under inspection at a specific point in space and for a specific energy level; b) fits each of the pixels to a function to determine the mass attenuation coefficient of the object under inspection at the point in space; and c) uses the function to automatically determine the identity or threat status of the object under inspection. 
     In one embodiment, the function yields a relationship between mass attenuation coefficients and logarithmic values of energy. The function relates the energy response of the detector arrays at each energy within a range of energies multiplied by a function of the object&#39;s linear attenuation coefficient and density. Determining the identity or threat status of the object under inspection is performed by comparing the object&#39;s linear attenuation coefficient function to data comprising linear attenuation coefficient functions of predefined materials. The comparison yields a fit comparing the relationship between mass attenuation coefficients and logarithmic values of energy obtained from the object under inspection to pre-computed material data for known materials. This allows for improved discrimination of materials through better estimation of material physical properties such as density and effective atomic number. Based on the comparison, pixels which are determined to qualify as potential threat materials are automatically highlighted within the image. 
     Since the multiple-energy discriminating detectors possess higher energy resolution compared to the remaining stacked detectors, persons of ordinary skill in the art would appreciate that the use of multiple energy discriminating detectors enhances the physical property measurement of the system and therefore improves the automated threat detection capabilities. 
     Referring again to  FIGS. 1A and 1B , during operation, when the receptacle containing the object under inspection is moving through the tunnel on conveyor  130  and passing through the X-ray projection fan beams of sources  110 ,  115 , the detector modules  120 ,  122 ,  125  and  127  are sampled repetitively. The projection or scan data pertaining to the dual energy stacked detectors  120 ,  125  as well as the multi-energy discriminating detectors  122 ,  127  are displayed as an integrated image output such that there is no perceivable difference in the region of the image owing to the multi-energy discriminating detectors as opposed to the stacked detectors. 
     The above examples are merely illustrative of the many applications of the system of present invention. Although only a few embodiments of the present invention have been described herein, it should be understood that the present invention might be embodied in many other specific forms without departing from the spirit or scope of the invention. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention may be modified within the scope of the appended claims.