CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is also a continuation-in-part of United States Patent Application Number 12/787,930, filed on May 26, 2010, and which relies on U.S. Provisional Patent Application No. 61/181,068 filed on May 26, 2009, for priority.

The present application is also a continuation-in-part of United States Patent Application Number 12/788,083, filed on May 26, 2010, and which relies on U.S. Provisional Patent Application No. 61/181,070 filed on May 26, 2009, for priority.

The present application is also a continuation-in-part of United States Patent Application

Number 12/485,900, filed on June 16, 2009.

The present application is also a continuation-in-part of United States Patent Application Number 12/792,931, filed on June 3, 2010, and which relies on U.S. Provisional Patent Application No. 61/183,591 filed on June 3, 2009, for priority.

The present application is also a continuation-in-part of United States Patent Application

Number 12/835,682, filed on July 13, 2010, and which relies on U.S. Provisional Patent Application No. 61/225,257 filed on July 14, 2009, for priority.

The present application is also a continuation-in-part of United States Patent Application Number 12/787,878, filed on May 26, 2010, and which relies on U.S. Provisional Patent Application No. 61/181,077 filed on May 26, 2009, for priority, which is:

1. A continuation-in-part of U.S. Patent Application No. 12/485,897, filed on June 16, 2009, which is a continuation of U.S. Patent Application No. 10/554,656, filed on October 25, 2005, and now issued U.S. Patent Number 7,564,939, which is a 371 national stage application of PCT/GB04/01729, filed on April 23, 2004 and which, in turn, relies on Great Britain Application No. 0309387.9, filed on April 25, 2003, for priority;

2. A continuation-in-part of U.S. Patent Application No. 12/371,853, filed on February 16, 2009, which is a continuation of U.S. Patent Application No. 10/554,975, filed on October 25, 2005, and now issued U.S. Patent Number 7,512,215, which is a 371 national stage application of PCT/GB2004/01741 , filed on April 23 , 2004 and which, in turn, relies on Great Britain Application Number 0309383.8, filed on April 25, 2003, for priority;

A continuation-in-part of U.S. Patent Application No. 12/651,479, filed on January 3, 2010, which is a continuation of U.S. Patent Application No. 10/554,654, filed on October 25, 2005, and now issued U.S. Patent Number 7,664,230, which is a 371 national stage application of PCT/GB2004/001731, filed on April 23, 2004 and which, in turn, relies on Great Britain Patent Application Number 0309371.3, filed on April 25, 2003, for priority;

A continuation-in-part of U.S. Patent Application No. 12/364,067, filed on February 2, 2009, which is a continuation of U.S. Patent Application No. 12/033,035, filed on February 19, 2008, and now issued U.S. Patent Number 7,505,563, which is a continuation of U.S. Patent Application No. 10/554,569, filed on October 25, 2005, and now issued U.S. Patent Number 7,349,525, which is a 371 national stage filing of PCT/GB04/001732, filed on April 23, 2004 and which, in turn, relies on Great Britain Patent Application Number 0309374.7, filed on April 25, 2003, for priority;

A continuation-in-part of U.S. Patent Application Number 12/758,764, filed on April 12, 2010, which is a continuation of U.S. Patent Application No. 12/211,219, filed on September 16, 2008, and now issued U.S. Patent Number 7,724,868, which is a continuation of U.S. Patent No. 10/554,655, filed on October 25, 2005, and now issued U.S. Patent Number 7,440,543, which is a 371 national stage application of PCT/GB2004/001751, filed on April 23, 2004, and which, in turn, relies on Great Britain Patent Application Number 0309385.3, filed on April 25, 2003, for priority; A continuation-in-part of U.S. Patent Application Number 12/697,073, filed on January 29, 2010, which is a continuation of U.S. Patent Application No. 10/554,570, filed on October 25, 2005, and now issued U.S. Patent Number 7,684,538, which is a 371 national stage application of PCT/GB2004/001747, filed on April 23, 2004, and which, in turn, relies on Great Britain Patent Application Number 0309379.6, filed on April 25, 2003, for priority;

A continuation-in-part of United States Patent Number 7,876,879, issued on January 25, 2011 and U.S. Patent Application No. 12/142,005, filed on June 19, 2008, both of which are 371 national stage applications of PCT/GB2006/004684, filed on December 15, 2006, which, in turn, relies on Great Britain Patent Application Number 0525593.0, filed on December 16, 2005, for priority;

8. A continuation-in-part of U.S. Patent Application No. 12/478,757, filed on June 4, 2009, which is a continuation of U.S. Patent Application No. 12/364,067, filed on February 2, 2009, which is a continuation of U.S. Patent Application No. 12/033,035, filed on February 19, 2008, and now issued U.S. Patent Number 7,505,563, which is a continuation of U.S. Patent Application No. 10/554,569, filed on October 25, 2005, and now issued U.S. Patent Number 7,349,525, which is a 371 national stage filing of PCT/GB04/001732, filed on April 23, 2004 and which, in turn, relies on Great Britain Patent Application Number 0309374.7, filed on April 25, 2003, for priority. In addition, U.S. Patent Application number relies on Great Britain Patent Application Number 0812864.7, filed on July 15, 2008, for priority; and

9. A continuation-in part of U.S. Patent Application No. 12/712,476, filed on February 25, 2010, which relies on U.S. Provisional Patent Application No. 61/155,572 filed on February 26, 2009 and Great Britain Patent Application No. 0903198.0 filed on

February 25, 2009, for priority.

Each of the aforementioned PCT, foreign, and U.S. applications, and any applications related thereto, is herein incorporated by reference in their entirety. FIELD OF THE INVENTION

The present invention relates to X-ray inspection systems. More particularly, the present invention relates to a source-detector configuration, whereby the energy transmitted through an object being inspected is measured at a wide range of substantially mono -energetic energies. BACKGROUND OF THE INVENTION

Due to persistent security threats and the possibility of terrorist activities, there is a need for deploying high speed, high resolution, and more accurate screening devices at places that are most likely targets of such activities. In addition, there exists a requirement for screening of baggage, cargo and other items for explosives 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.

The use of X-ray computed tomography for the screening of baggage has become fairly common in recent times, since the cross-sectional image data that such imaging systems produce is generally of high quality and of reasonable quantitative accuracy. Known tomographic imaging systems tend to be based on a rotating gantry which carries, as a minimum, an X-ray source with a large stationary array of detectors and more commonly an X-ray source and an opposing array of X-ray detectors which rotate together around the object under inspection. The information collected is reconstructed using known algorithms, such as a filtered backprojection algorithm or an iterative algorithm to produce a two-dimensional image.

In more recent systems, the object is able to move continuously through the imaging plane during data acquisition and, through the use of a cone-shaped X-ray beam with a two dimensional array of detectors, a three-dimensional reconstructed image is produced using filtered backprojection or iterative reconstruction methods. In a further scanning embodiment, a stationary gantry system may be envisaged with a complete ring of rapidly switchable X-ray sources and a sensor array comprising one or more rings of X-ray detectors which may be used to form a three-dimensional image as the item under inspection passes through the imaging plane.

Such images, however produced, are capable of being reconstructed into an image that is substantially determined by the mass attenuation coefficient of the material under inspection. The mass attenuation coefficient is determined through the combination or probabilities of X-ray interaction in the object through the photoelectric effect (absorption), Compton effect (inelastic scattering), Rayleigh effect (elastic scattering) and the density of the material. The individual detector thus sees an intensity of radiation at any point in time which is due to both those primary X-rays which have passed through the object unimpeded (i.e. without being absorbed and without being scattered) and those which have arrived at the detector due to one or more scattering interactions.

The mass attenuation coefficient is equal to the linear attenuation coefficient divided by density. The linear attenuation coefficient is a quantity that characterizes how easily a material or medium can be penetrated by a beam of light, sound particles, or other energy or matter. A large attenuation coefficient means that the beam is quickly "attenuated" or weakened as is passes through the medium, and a small attenuation coefficient means that the medium is relatively transparent to the beam. Therefore, the atomic number of the material under inspection plays a dominant role in determining the effective linear attenuation coefficient through its impact on the probability of photoelectric effect interactions while the density of the material plays a significant role in determining the mass attenuation coefficient of the material.

Thus, there is a need for an improved X-ray inspection system and method that detects the presence of predefined materials based on the mass attenuation coefficients of the materials.

SUMMARY OF THE INVENTION

The present specification discloses a method for processing X-ray data to determine an identity of an object under inspection, comprising: transmitting a plurality of X-rays, wherein said X-rays have a range of energies, through the object; detecting said transmitted X-rays at a detector array, wherein each detector in said detector array outputs a signal proportional to an amount of energy deposited at said detector by a detected X-ray; reconstructing an image from said signal, 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; fitting each of said pixels to a function to determine the mass attenuation coefficient of the object under inspection at the point in space; and using said function to determine the identity of the object under inspection.

Optionally, the function yields a relationship between mass attenuation coefficients and logarithmic values of energy. The function relates the energy response of the detector array at each energy within the range of energies multiplied by a function of the object's linear attenuation coefficient and density. Determining the identity of the object under inspection is performed by comparing the object'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. Based on said comparison, pixels which are determined to qualify as potential threat materials are highlighted within said image. The energies are substantially mono -energetic energies. The mono -energetic energies are within a range of 20 keV to 250 keV. The detectors output the signal proportional to the amount of energy deposited at said detector by the detected X-ray to an amplifier. The amplifier amplifies said signal and outputs said amplified signal to a multi -channel analyzer. The reconstruction is performed by processing said signal in accordance with at least one of a filtered back projection algorithm or an iterative reconstruction method. At least one image is reconstructed for each energy level detected by the detector array. The detector array has a resolution of lOkeV and wherein said energy source has a peak electron energy of at least 200 keV. A set of at least twenty images are reconstructed. Twenty images are derived from signals corresponding to X-rays having energies of at least lOkeV, 20 keV, 30keV, 40 keV, 50keV, 60 keV, 70keV, 80 keV, 90keV, 100 keV, HOkeV, 120 keV, 130keV, 140keV, 150keV, 160 keV, 170keV, 180keV, 190keV, and 200 keV. At least one image is reconstructed for each energy bin defined by the detector array. Energy bins are defined by 50 keV increments. Based on said energy bins, four images are reconstructed.

In another embodiment, the present specification discloses an X-ray system for processing X-ray data to determine an identity of an object under inspection, comprising: an X- ray source for transmitting a plurality of X-rays, wherein said X-rays have a range of energies, through the object; a detector array for detecting said transmitted X-rays, wherein each detector in said detector array outputs a signal proportional to an amount of energy deposited at said detector by a detected X-ray; and at least one processor having access to a memory for storing programmatic instructions, wherein when said programmatic instructions are executed, said processor: a) reconstructs an image from said signal, 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 said pixels to a function to determine the mass attenuation coefficient of the object under inspection at the point in space; and c) uses said function to determine the identity of the object under inspection.

Optionally, the function yields a relationship between mass attenuation coefficients and logarithmic values of energy. The function relates the energy response of the detector array at each energy within the range of energies multiplied by a function of the object's linear attenuation coefficient and density. Determining the identity of the object under inspection is performed by comparing the object'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. Based on said comparison, pixels which are determined to qualify as potential threat materials are highlighted within said image. The energies are substantially mono -energetic energies. The mono -energetic energies are within a range of 20 keV to 250 keV.

The X-ray system further comprises an amplifier for receiving and amplifying the signal proportional to the amount of energy deposited at said detector. The amplifier outputs said amplified signal to a multi-channel analyzer. At least one image is reconstructed for each energy bin defined by the detector array. The detector array has a resolution of lOkeV and wherein said energy source has a peak electron energy of at least 200 keV. A set of at least twenty images are reconstructed. Twenty images are derived from signals corresponding to X-rays having energies of at least lOkeV, 20 keV, 30keV, 40 keV, 50keV, 60 keV, 70keV, 80 keV, 90keV, 100 keV, HOkeV, 120 keV, 130keV, 140keV, 150keV, 160 keV, 170keV, 180keV, 190keV, and 200 keV. At least one image is reconstructed for each energy bin defined by the detector array. The energy bins are defined by 50 keV increments. Based on said energy bins, four images are reconstructed.

In another embodiment, the present specification discloses an X-ray system for processing X-ray data to determine an identity of an object under inspection, comprising an X- ray source for transmitting a plurality of X-rays, wherein said X-rays have a range of energies, through the object; and a detector array for detecting said transmitted X-rays, wherein said detector array comprises a first detector disposed in front of a second detector, and wherein said detector array is configured relative to the transmitted X-rays such that the transmitted X-rays pass through the first detector before passing through the second detector.

Optionally, the first detector is thinner than the second detector. The first detector has a scintillator thickness in a range of 0.2mm to 1.0mm. The second detector has a scintillator thickness in a range of 2.0 mm to 4.0mm.

In another embodiment, the present specification discloses an X-ray system for processing X-ray data to determine an identity of an object under inspection, comprising an X- ray source for transmitting a plurality of X-rays, wherein said X-rays have a range of energies, through the object; and a detector array for detecting said transmitted X-rays, wherein said detector array comprises a first set of detectors and a second set of detectors, wherein each of said first set of detectors comprise scintillator material and a filter configured relative to the scintillator material such that the transmitted X-rays pass through the filter before passing through the scintillator material, and wherein each of said second set of detectors comprise scintillator material and do not comprise a filter.

Optionally, 25% to 50% of the detector array comprises the first set of detectors and wherein 75% to 50% of the detector array comprises the second set of detectors. The filter comprises copper with a thickness ranging from 0.2 to 0.5mm. The first set of detectors detect a higher net energy X-ray spectrum than the second set of detectors. The first set of detectors and the second set of detectors are arranged in a tessellated pattern in the detector array. The first set of detectors and the second set of detectors are arranged in alternating rows in the detector array. Optionally, 1% to 4% of the detector array comprises the first set of detectors and wherein 99% to 96%) of the detector array comprises the second set of detectors. Output signals from the first set of detectors are used to generate a first image and wherein output signals from said second set of detectors are used to generate a second image. The first image has a lower resolution than said second image. The first image has a higher energy than said second image. The first image and second image are used in combination to generate a dual energy analysis of said object.

These, and other embodiments, will described in further detail in the remainder of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will be appreciated, as they become better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 illustrates an X-ray system, in accordance with one embodiment of the present invention;

FIG. 2 illustrates the broad energy spectrum of an X-ray source, in accordance with one embodiment of the present invention;

FIG. 3 is a flowchart illustrating a method for identifying predefined threat materials present in an object being inspected;

FIG. 4 illustrates the reconstructed image pixels fitted into a curve, in accordance with one embodiment of the present invention; FIG. 5 is a graphical representation of the method for identifying predefined threat materials present in an object being inspected, in accordance with one embodiment of the present invention;

FIG. 6A illustrates a two-dimensional detector array configuration;

FIG. 6B illustrates a two-dimensional sensor array configuration;

FIG. 6C illustrates an alternative filter configuration in a tessellated pattern; FIG. 6D illustrates a detector array structure where the filter material is applied on alternating rows of the detector; and

FIG. 7 illustrates a detector array configuration for producing high resolution images, in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a source -detector configuration for use in an X-ray inspection system being used to inspect an object, whereby the energy transmitted through the object from the source to the detector is measured at a wide range of substantially mono- energetic energies. The measured energy is used to identify predefined threat material(s) in the object, if present.

X-ray sources that are typically used in screening of baggage, cargo and other similar items generally use a source that accelerates electrons in vacuum towards a target with a final kinetic energy at the target in the range of 50 keV to 200 keV depending on the intended application. Once these energetic electrons hit the target, they decelerate rapidly, liberating their energy though the Bremsstrahlung (scattering) effect or the photoelectric (absorption) effect which results in X-rays. The result is a Bremsstrahlung spectrum, a broad continuum of X-ray energies from the maximum accelerated electron energy down to zero, upon which a sharply peaked fluorescence spectrum is superimposed.

Since the linear attenuation coefficient varies rapidly with energy in the region below 500 keV, the wide range of energies that are contained within the X-ray beam can be used as a multi- energy probe of the linear attenuation coefficient of the material under inspection, thereby enabling a tight fit to the particular linear attenuation coefficient, and density, of the materials under inspection. In the present invention, it is recognized that the shape of the linear attenuation curve of a material is determined primarily by the atomic number of the material under inspection whereas the effective baseline of the curve is determined through the density of the material. For example, a gaseous material, such as Xenon, has a characteristic set of linear attenuation coefficients as a function of energy. When compressed from the gaseous state into a liquefied state, because of varying density of the states, the mass attenuation coefficient increases markedly, while the linear attenuation coefficient remains the same.

The present specification discloses 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.

Figure 1 illustrates an X-ray system in accordance with one embodiment of the present invention. The system 100 is used to inspect objects for identifying the presence of predefined threat material. The system 100 comprises an X-ray source 102 capable of emanating X-rays 104 having a broad energy spectrum, an object 106 requiring inspection, a detector array 108 comprising a plurality of detectors 110, a plurality of amplifiers 112, each of which is in data communication with at least one detector, a plurality of multi -channel analyzers 114, each of which is in data communication with at least one amplifier, an image reconstruction processor 116 in data communication with at least one multi-channel analyzer and a display unit 118 in data communication with the image reconstruction processor 116.

Figure 2 illustrates the energy spectrum of an X-ray source, in accordance with an embodiment of the present invention. The curve 200 illustrated in Figure 2 represents a typical Bremsstrahlung energy spectrum. The x-axis and the y-axis represent logarithmic values of energy and intensity, respectively. The spectrum comprises a slowly varying signal which trends toward zero 202 at higher energies and drops to zero 204 at low energies, and also displays one or more sharp peaks 206 at medium energies. The slowly varying signal is caused due to electron interactions in the target within an X-ray tube, and is known as Bremsstrahlung radiation.

The energy spectrum drops to zero 204 at low energies due to self-absorption of X-rays in the target and a vacuum window through which X-rays leave the X-ray tube. The peaked X- ray spectrum 206 is due to electron interactions in the target which result in fluorescence radiation. The spectrum peaks when all the energy of an electron which hits the target is converted to an X-ray. In various embodiments, the X-ray tubes as described in this disclosure tend to be operated with a tube voltage in the range 100 kVp to 200 kVp, and are capable of generating X-rays with energies up to 100 to 200 keV, respectively.

With reference to Figure 1, the X-rays 104 emanating from the X-ray source 102 pass through the object 106 and the transmitted X-rays are detected by detectors 110 in the detector array 108. Each detector 110 in the detector array 108 is configured to, and capable of, outputting a signal that is proportional to the amount of energy deposited in the detector 110. In one embodiment, detector materials comprising narrow bandgap semiconductors such as Ge and Si are used in the detectors 110. In another embodiment, wider bandgap semiconductors such as CdZnTe, CdTe and Hgl are used in the detector 110. In yet another embodiment, scintillation detectors with an opto-electronic readout such as, but not limited to, a Nal(Tl) scintillator with a photomultiplier tube are used in the detector array 108.

Each detector 110 is in data communication with an amplifier 112, which, in turn, is in data communication with a multi-channel analyzer 114. The signal output by each detector 110 is amplified to a pre-defined degree by the amplifier 112 and transmitted to the multi- channel analyzer 114, which accumulates the transmitted X-ray spectrum for subsequent analysis and image reconstruction via image reconstruction processor 116. The reconstructed image is displayed via the display unit 118 which may be a computer screen. In an embodiment, the circuit illustrated in Figure 1 is designed to detect the maximum output voltage from the amplifier 112 for each interacting X-ray and to convert this to a digital value. These digital values, one value per detected X-ray, are transmitted through a suitable digital interface to subsequent processing electronics such as the multi-channel analyzer 114. The electronics arrangement described above results in a detected X-ray spectrum of the form shown in Figure 2, the shape of the detected spectrum being strongly affected by the type of material which is present between the X-ray source 102 and the X-ray detector 110. As is known in the art, the energy required to liberate a signal carrier in a detector 110 varies with the type of detector material employed. In a semiconductor detector, this energy is typically below 5 eV, whereas in a scintillator detector, this energy is typically greater than 30 eV. For example, a 200 keV X-ray completely absorbed in a semiconductor detector with an efficiency of 2 eV per generated charge carrier, liberates 200/2 = 100 electrons. This corresponds

-17

to a charge of 1.6x10 ^" Coulombs.

In an embodiment, the amplifier 112 converts the liberated electrons to a voltage of IV. Hence, a gain in the range of lxlO ¹⁴ to lxlO ¹⁷ V/C is required. In an embodiment, the amplifier 112 is designed to provide a linear response, so the voltage output by the amplifier 112 increases with an increase in the amount of energy deposited therein. Also, in an embodiment, a saturation point of the amplifier 112 is set such that it corresponds to an absorbed energy in the detector 110 which is greater than the maximum energy that can be generated by the combination of X- ray source 102 and detector 110. As would be evident to a person having ordinary skill in the art, a plurality of suitable amplifier 112 configurations such as, in one embodiment, an integrator with a reset switch which results in a step voltage output and integrator with resistive feedback which results in a smoothly varying pulsed output voltage, may be employed.

In various embodiments, at least one amplifier 112 and at least one multi -channel analyzer 114 is in data communication with each detector 110 in the detector array 108, resulting in a system with hundreds to thousands of individual energy sensitive detectors. The plurality of detectors 110 enable measurement of the energy spectrum transmitted through the object 106, rather than just the total energy deposited. Since, the energy spectrum is characteristic of the specific material of the object 106, measurement of the spectrum energy may be used to identify the presence of threat materials.

In various embodiments, a plurality of image reconstruction algorithms is used to analyze the accumulated X-ray spectrum for identifying the presence of predefined threat materials in the object 106. In an embodiment, the energy spectrum data from each of the multi-channel analyzers 114 is transmitted to an image reconstruction processor 116 in parallel for reconstructing a set of images by using one or more image reconstruction algorithms, each reconstructed image being characteristic of a particular energy in the detected spectrum.

Figure 3 is a flowchart illustrating a method for identifying materials present in an object being inspected using the system arrangement described above. At step 302, the object is placed between a broad spectrum X-ray source and an array of detectors. At step 304, X-rays transmitted through the object are measured at each detector. In various embodiments, the energy transmitted through the object from the X-ray source to the detector array is measured at a wide range of substantially mono -energetic energies. The range varies from the lowest energy generated (typically 20 keV and above), to the highest energy that is capable of being generated by the X-ray source (typically 250 keV and below).

The array of detectors comprises a plurality of detectors and each detector outputs a signal that is proportional to the amount of energy deposited in the detector by the impinging X rays, transmits to an amplifier and then to a multi-channel analyzer. At step 306, an image reconstruction algorithm is used to reconstruct a set of images, one image for each energy bin that is created by the detector array. In an embodiment, reconstructive algorithms like a filtered back projection algorithm and iterative reconstruction methods are used to reconstruct the set of images at step 306. For example, with a peak electron energy of 200 keV of the X-ray source, and a detector energy resolution of 10 keV, a set of 20 different images are reconstructed, at energies of 10 keV, 20 keV, 30 keV and so on up to 200 keV. In an exemplary scenario where typical energies are in the range 100 kVp to 250 kVp and the width of an energy bin is 10 keV to 50 keV a minimum of five images at 50 keV energy intervals and a maximum of 25 images at 10 keV intervals are generated. The underlying detector resolution lies in the range 1 keV (for a semiconductor detector) to 50 keV (for a typical scintillation detector). Thus, a system with 200 keV full scale with 10 keV energy bins would result in 20 reconstructed images whereas a system with 200 keV full scale and 50 keV bins would result in 4 reconstructed images.

At step 308, each pixel in each reconstructed image represents the mass attenuation coefficient of the material within that volume element of the object being inspected, at that particular energy. At step 310, each pixel in each reconstructed image is fitted to a curve to obtain a description of the mass attenuation coefficient at the point in space.

Figure 4 illustrates the reconstructed image pixels fitted in a curve, in accordance with an embodiment of the present invention. The vertical axis 402 represents mass attenuation coefficient which may be represented as (μ/ρ) (linear attenuation coefficient/density) whereas the horizontal axis 404 represents logarithmic values of Energy. The curve 406 illustrates the variation of mass attenuation coefficient as a function of X-ray energy. The measurement points 408 on the curve 406 indicate the reconstructed pixel values that are reconstructed for each of the energy bins.

Referring back to Figure 3, at step 312, the resulting measured curve is a convolution of the energy response of the detector array (at each energy within the X-ray source output spectrum) multiplied by a function of linear attenuation coefficient and density. At step 314, the function of linear attenuation coefficient is evaluated with respect to a library of data defining linear attenuation coefficient functions of predefined materials. In an embodiment, the functions are evaluated experimentally. In another embodiment, the functions are calculated from known linear attenuation and density data for the materials. In either case, it should be appreciated that the known liner attenuation coefficient functions are stored in a data library which is stored, and accessed, either local to, or remote from, the system in a memory, such as a RAID array, hard disk, or any other form of memory.

At step 316, a fit is made between the empirical data measured during the X-ray imaging process (obtained at step 312) and the pre-computed material data for identifying threat materials. In an embodiment, the fit is made to determine the effective mass attenuation coefficient of a given material in the object under inspection. The mass attenuation coefficient is specific to each material in the pre-computed material data and is used to identify threat materials from benign materials. In an embodiment of the present invention, a least squares fitting method is used to provide a known statistical fit between sets of data to produce the most accurate analysis of material type. In an embodiment, the pixels which are determined to fall into the category of threat material are highlighted to an image inspector, for example through the use of color or image annotation. It should be appreciated that all of the image processing and data processing steps are performed by a processor executing on a plurality of programmatic instructions stored in a memory. The processor may be local to, or remote from, the X-ray source and detector array. The memory and programmatic instructions may be local to, or remote from, the X-ray source and detector array.

Figure 5 is a graphical representation of the method for identifying predefined materials present in an object being inspected, in accordance with an embodiment of the present invention. The graph 500 represents a fitted curve between the empirical data measured during the X-ray imaging process and the pre-computed material data for identifying predefined materials as described with reference to Figure 3. The curve 502 represents the experimentally measured mass coefficient data with respect to a material having a high atomic number. The curve 504 represents the experimentally measured mass coefficient data with respect to a material having a low atomic number. Data generated from the empirical data measured during the X-ray imaging process is fitted to the curves 502, 504, and 506, as represented by data points 508.

Curves 502 and 504 represent a poor fit. The curve 506 represents the experimentally measured mass coefficient data with respect to a material having an atomic number lying between the high and the low atomic numbers corresponding to curves 502 and 504 respectively. The curve 506 demonstrates a good fit. Accordingly, in operation, the pixels corresponding to data points 508 would be deemed to correlate with, and potentially represent, medium atomic number materials. In the display unit, the pixels would be graphically highlighted as being medium atomic number materials, either by color, by intensity, or by any other visual means.

A plurality of detector array configurations that may be used in conjunction with the X- ray system illustrated in Figure 1 are provided in Figures 6 A, 6B, 6C, and 6D. Figure 6 A illustrates a two-dimensional detector array comprising a front detector 602 and a rear detector 604. Some X-rays pass through the front detector 602 to the rear detector 604, such that the rear detector 604 sees a higher average energy spectrum than the front detector 602. The front detector 602 is typically thin (0.2 mm to 1.0 mm scintillator material) compared to the rear detector 604 (typically 2.0 to 4.0 mm scintillator material).

Figure 6B illustrates a two-dimensional sensor array in which a portion, such as a third or a half, of the rings 606 of a detector array are covered in a filter material while the other portion of the rings 608 are not. In most cases, the filter material is a metal, such as copper with a thickness ranging between 0.2 to 0.5 mm. The detectors 606 covered with the filter material experience a higher net energy X-ray spectrum than the uncovered detectors 608, and this difference is used to determine material type.

Figure 6C illustrates an alternative filter configuration in a tessellated pattern in which the filter material covers detectors 610 while detectors 612 remain uncovered. Hence, the filter material is spread at even intervals over the detector array.

Figure 6D shows a detector array structure where the filter material is applied on alternating rows of the detector. Rows 614 are covered with a detector material while rows 616 remain uncovered. In the detector array configurations illustrated in Figures 6A, 6B, 6C, and 6D, a large fraction of the imaging detector is dedicated to both filtered and non-filtered detectors, and this may compromise the quality of each of the reconstructed images produced by using the detector arrays in conjunction with the X-ray system illustrated in Figure 1. It is preferred that the reconstructed image is of the highest quality in order to provide the best possible representation of the object under inspection. For example, a system with a stationary gantry may be configured with approximately 10,000 individual X-ray detectors and hundreds of individual X-ray source points resulting in a reconstructed image with a spatial resolution of approximately one millimeter and a mass attenuation coefficient resolution of better than one percent.

In order to provide material type through analysis of the shape of the mass attenuation coefficient curve, it is recognized that objects of interest tend to be large compared to the intrinsic pixel dimension and that the materials analysis data may be generated over a length scale which is long compared to the intrinsic image data. As an example, an improvised explosive device may contain a sizeable volume of explosive material, a detonating device, a trigger device, an energy source to supply the trigger and detonator devices, and/or some interconnecting wires. The intrinsic image resolution will ideally be able to provide good visualization of most, if not all, components of such a device, but the material type is only necessary to be estimated for the sizeable explosive component.

In various embodiments of the present invention, an image inspector processing unit is used to analyze intrinsically high resolution images having a high signal-to-noise ratio with a much lower resolution material type analysis that can be applied over larger contiguous volumes. As is known in the art, at an individual pixel level, there is a degree of noise which leads to uncertainty in the absolute value of detected mass attenuation coefficient, the uncertainty being described as signal-to-noise ratio. The signal-to-noise ratio is directly related back to the number of X-rays that interact in each detector. In order to improve signal-to-noise ratio, and hence the ability to discriminate between materials with similar mass attenuation coefficient, the signals are required to be averaged over a larger region. The system of the present invention enables analysis of data at multiple resolution levels, each with varying signal-to-noise ratio, in order to parameterize the measurement. Analysis at a high resolution level results in a higher degree of noise but better visualization of structure, whereas analysis at a low resolution level results in poor visualization of structure but leads to a closer identification of material type. The present invention provides a detector array configuration for use in conjunction with the X-ray system illustrated in Figure 1 to produce high resolution images. Figure 7 illustrates a detector array configuration for producing high resolution images, in accordance with an embodiment of the present invention. The figure illustrates a multi -element pixel array 700. In an embodiment, the pixel array 700 is one commonly known in the art, such as those used in contemporary tomographic X-ray scanners. In an exemplary embodiment, the pixel array has 128 pixels arranged in a 16 x 8 pixel matrix. One of the pixels 702 of the pixel array 700 is covered with a filter material such as 0.2 mm to 1.0 mm of copper or aluminium. The effect of covering one pixel with the filter material has a minimal impact on the whole reconstructed image. In various embodiments, the exact numbers of filtered detectors depend on detailed system designs, but typically lie in the range of 1% to 4% of the total number of detector channels.

In an embodiment, a complete detector array comprises approximately 50 to 200 pixel arrays such as the one illustrated in Figure 7, thereby providing over one hundred filtered pixel values that are more or less equally dispersed over the entire imaging volume. The values from just these sparsely located pixels may then be reconstructed using either a filtered backprojection or iterative reconstruction method to generate a low resolution but net high energy image which can then be fitted to the high resolution but lower average energy image to create materials specific data.

By placing a filter material over a small percentage of the detector array, i.e. from 1% to 4% of all detector elements, the system can generate a primary image which is reconstructed from the detector elements which are not filtered, i.e. 96% to 99% of all detector elements. The primary image is acquired by integrating over all X-ray energies, including the lowest X-ray energies, at a high spatial resolution. A secondary image, which has a lower resolution but higher mean X-ray energy, can also be generated using the filtered detector elements, i.e. 1% to 4% of the detector array. Once generated, the two images, namely the high resolution, lower energy primary image and lower resolution, higher energy secondary image, can be used in combination to generate, engage in, or otherwise conduct a dual energy materials analysis of materials under inspection.

While the exemplary embodiments of the present invention are described and illustrated herein, it will be appreciated that they are merely illustrative. It will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from or offending the spirit and scope of the appended claims.