CN115697202A

CN115697202A - System and method for controlling image contrast in an X-ray system

Info

Publication number: CN115697202A
Application number: CN202080101575.6A
Authority: CN
Inventors: 杰弗里·R·舒伯特
Original assignee: American Science and Engineering Inc
Current assignee: American Science and Engineering Inc
Priority date: 2020-06-01
Filing date: 2020-06-01
Publication date: 2023-02-03
Also published as: GB2609588A; WO2021246998A1; EP4157093A4; GB202216885D0; EP4157093A1

Abstract

An X-ray inspection system for scanning an object and providing a corresponding contrast controlled scan image is provided. The system comprises: an X-ray source configured to generate an X-ray beam for irradiating the object, wherein the X-ray source is coupled with at least a first beam filter having a first thickness and a second beam filter having a second thickness, the second thickness being greater than the first thickness; a detector array; a processing unit; a user interface configured to receive user input indicating a desired level of contrast in an image; and a controller configured to adjust a position of at least one of the first or second beam filters based on a user input indicative of a desired level of contrast in the at least one image.

Description

System and method for controlling image contrast in an X-ray system

Technical Field

The present invention relates to an apparatus and method for controlling image contrast in a transmission or backscatter X-ray inspection system.

Background

X-ray inspection systems typically include a beam filter for filtering the inspection X-ray beam before it strikes the object under inspection. The beam filter limits the total dose of the X-ray beam by limiting the flux and lowest energy portions of the X-ray beam spectrum. This may be desirable in certain applications, for example medical applications where energy below 30keV is generally considered to be absorbed by soft tissue of the patient. Similarly, in a backscatter X-ray inspection system for inspecting cargo, the lowest radiant energy may be absorbed in the cargo being illuminated. Some of the scattered radiation that produces low energy radiation may be absorbed by the housing of the inspection system. These low energy photons are believed to contribute to the dose but do not provide any benefit in exchange for the fact that even without specific dose limitations, the principle of ALARA (dose should be as low as possible) would stimulate the removal of low energy radiation.

The application of the beam filter and the reduction of the low energy beam spectral components may also suppress the display of only certain portions of the image data outside of the imaged object, thereby providing an image with a clearer view of the interior of the object.

In a backscatter X-ray inspection system, the shape of the beam spectrum affects the quality of the acquired image. Thus, controlling the beam spectral shape of a backscatter X-ray inspection system can optimize the ability of the system to highlight organic (or non-organic) threats in the image, particularly with reference to threats located behind steel shields.

U.S. Pat. No. 9,014,339 discloses "(a) scanning apparatus for scanning a light beam in one-dimensional scanning, the apparatus comprising: a. a radiation source for generating a fan-shaped beam of radiation, the beam of radiation being effectively emitted from a source axis and characterized by a width; b. the angle selector is fixed in the scanning process and used for limiting the scanning range; a multi-aperture unit rotatable about a central axis such that the flux of the beam incident on the target is the same for each revolution of the beam on the target, wherein the multi-aperture unit comprises an inner multi-aperture collar characterized by a collar axis, the inner multi-aperture collar being made of a material that is opaque to the beam, and wherein the inner multi-aperture collar comprises aperture rings spaced laterally along the collar axis, the aperture rings being positioned in the beam in such a way that axial movement of the multi-aperture collar places the aperture rings in the beam, the beam being collimated by the respective opening angles in the angle selector. "

U.S. Pat. No. 9,291,582 discloses "a tunable collimator for shaping a particle beam, the particle beam being characterized by a dynamic sweep propagation radial direction radially with respect to an aperture ring rotating about an axis of rotation and serving to interrupt the beam, the sweep propagation direction being transverse to the axis of rotation of the aperture ring, the collimator comprising: a. a shading element substantially opaque to passage of particles in the dynamic sweep propagation direction; b. the gap in the shading elements is adapted to be a shading element passing particles in a dynamic sweep propagation direction, the gap being characterized by a length taken along a long dimension and a nip pitch taken along a narrow dimension, both the long dimension and the nip pitch being transverse to the dynamic sweep propagation direction, wherein at least one of the length of the gap and the nip pitch is adjusted. "

The above-mentioned prior art patents disclose the use of beam filters to reduce dose, alter the energy distribution of the beam, and promote dual energy backscattering. However, the cited art does not disclose the use of a beam filter to control the contrast value of the scanned image.

There is a need for a system and method for controlling the contrast value of a scanned image through the use of a beam filter to improve the detectability of threat materials concealed behind a metal shield and to enable an operator to adjust the image to optimize the image display according to his or her particular preferences. Enabling the operator to control the contrast of the images may improve his or her ability to detect threats, identify contraband, and/or improve scanning throughput.

Disclosure of Invention

The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools, and methods, which are meant to be exemplary and illustrative, not limiting in scope. This application discloses a number of embodiments.

In some embodiments, the present specification is directed to an X-ray inspection system for scanning an object, the system comprising: an X-ray source configured to generate an X-ray beam for irradiating an object, wherein the X-ray beam irradiating the object defines a field of view, and wherein the X-ray source is coupled with at least a first beam filter having a first thickness and a second beam filter having a second thickness greater than the first thickness; a detector array adapted to receive radiation from the X-ray beam, the radiation transmitted through or scattered from the object, and to generate data representing at least one image; a processing unit configured to receive data representing at least one image and generate for display at least one image based on the data representing the at least one image; a user interface configured to receive user input indicative of a desired level of contrast in at least one image; a controller configured to adjust a position of at least one of the first beam filter or the second beam filter based on a user input indicative of a desired level of contrast in the at least one image.

Optionally, the desired contrast level comprises at least one of a first contrast level, a second contrast level, a third contrast level, and a fourth contrast level, and wherein the first contrast level is less than the second contrast level, the second contrast level is less than the third contrast level, and the third contrast level is less than the fourth contrast level.

Optionally, the controller is configured to cause the first beam filter and the second beam filter to be out of view of the X-ray source when the user interface receives a user input of a first contrast level.

Optionally, when the user interface receives a user input of a second contrast level, the controller is configured to cause the first beam filter to be in the field of view of the X-ray source and to cause the second beam filter not to be in the field of view of the X-ray source.

Optionally, when the user interface receives a user input of a third contrast level, the controller is configured to cause the first beam filter to be out of view of the X-ray source and the second beam filter to be located in view of the X-ray source.

Optionally, when the user interface receives a user input of a fourth contrast level, the controller is configured to cause the first beam filter to be in the field of view of the X-ray source and the second beam filter to be in the field of view of the X-ray source.

Optionally, the first and second beam filters comprise a metallic material having a high atomic number.

Optionally, the first and second filters comprise at least one of bronze, tin, tungsten, pure copper (copper), and a copper matrix with tungsten particles embedded therein.

Optionally, the first and second beam filters comprise a first layer made of tungsten or lead and a second layer made of steel or pure copper configured to absorb fluorescent light emitted by the first layer.

Optionally, the system further comprises a shield coupled to the first beam filter and the second beam filter, the shield configured to reduce radiation leakage.

Optionally, the system further comprises a pencil beam forming aperture placed in front of the X-ray source, wherein the first beam filter is located between the X-ray source and the pencil beam forming aperture, and wherein the image contrast is increased by: increasing the distance between the pencil beam forming aperture and the first beam filter and decreasing the distance between the first beam filter and the X-ray source.

Optionally, the system further comprises a third beam filter.

Optionally, the first, second and third beam filters comprise 0.5mm thick pure copper material, 1.0mm thick pure copper material and 2.0mm thick pure copper material, respectively.

Optionally, the processing unit is further configured to modify one or more non-linear transfer functions adapted to process data representing the at least one image based on a desired contrast level. Optionally, the nonlinear transfer function comprises at least one of a gamma function or an S-curve function.

Optionally, the processing unit is further configured to implement at least one of the first set of program instructions and the second set of program instructions based on a desired contrast level. Optionally, the processing unit is further configured to implement a first set of program instructions based on at least one of the first contrast level and the second contrast level, and wherein the first set of program instructions comprises one or more contrast enhancement functions. Optionally, the processing unit is further configured to implement a second set of program instructions based on at least one of the third contrast level or the fourth contrast level, and wherein the second set of program instructions comprises one or more edge enhancement functions.

In some embodiments, the present description is directed to a method of scanning an object using an X-ray inspection system, the method comprising: irradiating an object with an X-ray beam generated by an X-ray source, wherein the X-ray beam irradiating the object defines a field of view, and wherein the X-ray source is coupled with at least a first beam filter having a first thickness and a second beam filter having a second thickness greater than the first thickness; detecting radiation from the X-ray beam, the radiation transmitted through or scattered from the object, and generating data representative of at least one image; generating at least one image for display based on data representing the at least one image; receiving user input indicating a desired level of contrast in at least one image; and controlling a position of at least one of the first and second beam filters based on a user input indicative of a desired level of contrast in the at least one image.

Optionally, receiving the user input indicating the desired contrast level comprises receiving at least one of a first contrast level, a second contrast level, a third contrast level, and a fourth contrast level, and wherein the first contrast level is less than the second contrast level, the second contrast level is less than the third contrast level, and the third contrast level is less than the fourth contrast level.

Optionally, controlling the position of at least one of the first beam filter and the second beam filter comprises leaving the first beam filter and the second beam filter out of view of the X-ray source if a user input indicative of the first contrast level is received.

Optionally, controlling the position of at least one of the first beam filter and the second beam filter comprises causing the first beam filter to be in the field of view of the X-ray source and the second beam filter not to be in the field of view of the X-ray source if a user input indicating a second contrast level is received.

Optionally, controlling the position of at least one of the first beam filter or the second beam filter comprises leaving the first beam filter out of view of the X-ray source and leaving the second beam filter in view of the X-ray source.

Optionally, controlling the position of at least one of the first beam filter or the second beam filter comprises causing both the first beam filter and the second beam filter to be in the field of view of the X-ray source if a user input indicative of a fourth contrast level is received.

Optionally, the first and second filters comprise at least one of bronze, tin, tungsten, pure copper, or a copper matrix embedded with tungsten particles.

Optionally, the first and second beam filters comprise a first layer made of tungsten or lead and a second layer made of steel or copper configured to absorb fluorescence emitted by the first layer.

Optionally, generating for display at least one image based on data representative of the at least one image comprises modifying one or more non-linear transfer functions adapted to process the data representative of the at least one image based on a desired contrast level.

Optionally, the one or more non-linear transfer functions comprise at least one of a gamma function and an S-curve function.

Optionally, generating for display at least one image based on the data representative of the at least one image comprises implementing at least one of the first set of program instructions or the second set of program instructions based on a desired level of contrast.

Optionally, the method further comprises implementing a first set of programming instructions based on at least one of the first contrast level or the second contrast level, wherein the first set of programming instructions includes one or more contrast enhancement functions.

Optionally, the method further comprises implementing a second set of programming instructions based on at least one of the third contrast level or the fourth contrast level, and wherein the second set of programming instructions includes one or more edge enhancement functions.

In some embodiments, the present specification is directed to a method of controlling contrast of a scanned image obtained by using an X-ray inspection system, the method comprising: irradiating an object with an X-ray beam generated by an X-ray source, wherein the X-ray beam irradiating the object defines a field of view, and wherein the X-ray source is coupled to at least a first beam filter having a first thickness and a second beam filter having a second thickness greater than the first thickness; detecting radiation from the X-ray beam, the radiation transmitted through or scattered from the object, and generating data representing at least one image; generating at least one scanned image for display based on data representing at least one image; receiving a user input indicating a desired contrast level in the scanned image as one of a first contrast level, a second contrast level, a third contrast level, and a fourth contrast level, and wherein the first contrast level is less than the second contrast level, the second contrast level is less than the third contrast level, and the third contrast level is less than the fourth contrast level; and changing a position of at least one of the first beam filter and the second beam filter relative to a field of view of the X-ray source based on a user input indicative of a desired level of contrast in the scanned image.

Optionally, changing the position of at least one of the first beam filter and the second beam filter comprises leaving the first beam filter and the second beam filter out of view of the X-ray source if a user input indicating a first contrast level is received.

Optionally, controlling the position of at least one of the first beam filter and the second beam filter comprises causing the first beam filter to be in the field of view of the X-ray source and the second beam filter to be out of the field of view of the X-ray source if a user input is received indicating the second contrast level.

In some embodiments, the present description is directed to a method of combining scan images obtained using an X-ray inspection system to obtain an image with improved detection quality, the method comprising: irradiating an object with an X-ray beam generated by an X-ray source, wherein the X-ray beam irradiating the object defines a field of view, and wherein the X-ray source is coupled with at least a first beam filter having a first thickness and a second beam filter having a second thickness greater than the first thickness; obtaining a first scanned image of the object with a first beam filter; obtaining a second scanned image of the object with a second beam filter; determining one or more regions containing edges in the second image by using the first image as a guide; applying an edge enhancement algorithm only to the determined region containing the edge in the second image; a smoothing algorithm is applied to all regions of the second image except the determined edge-containing region to mitigate noise of the second image to obtain an image with improved detection quality.

Optionally, the first image is a high resolution image.

Optionally, the second image is a high contrast image.

Optionally, determining one or more regions in the second image that contain edges by using the first image as a guide comprises: one or more regions in the first image that contain edges are determined.

Optionally, applying only the edge enhancement algorithm to the determined region containing the edge in the second image comprises: the edge enhancement algorithm is not applied to the region of the second image that has no edges to prevent the edge enhancement algorithm from enhancing noise in the region of the second image that has no edges.

Optionally, the method further comprises determining a potential threat located in the second image; and applying a graphical indicator to the first image to guide an operator in analyzing the threat zone based on the determined potential threats located in the second image.

The above described and other embodiments of the present specification are more fully described in the drawings and detailed description provided below.

Drawings

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

FIG. 1A is a diagram illustrating a system for integrating a variable beam filter with a mechanical beam shutter of an X-ray source according to embodiments herein;

FIG. 1B is a side view illustrating a system for integrating a variable beam filter with a mechanical beam shutter of an X-ray source according to another embodiment of the present description;

FIG. 1C shows a top view of the shutter and filter mechanism shown in FIG. 1B;

FIG. 1D shows a top view of an X-ray source coupled with a shutter and a plurality of filters according to embodiments of the present description;

FIG. 2A illustrates the effect of applying a beam filter on a backscatter X-ray image of an automobile containing several organic threats and metal projectiles;

FIG. 2B illustrates the effect of applying a stronger beam filter than shown in FIG. 2A on a backscatter X-ray image of an automobile containing several organic threats and metal projectiles;

FIG. 3A is an image of a laboratory test setup for studying organic and metallic image features located behind a metal plate;

FIG. 3B shows an X-ray image obtained using the apparatus shown in FIG. 3A, in which organic and metallic image features are visible behind the metal plate shown in FIG. 3A;

FIG. 3C shows a radiation image of the apparatus of FIG. 3A obtained by using four different beam filtering conditions;

FIG. 3D shows a radiation image of the apparatus of FIG. 3A obtained by using different filter thicknesses, beam powers and scan speeds;

FIG. 3E is a graph illustrating the relationship between X-ray doses corresponding to different thickness beam filters;

FIG. 4 is a diagram of an imaging module coupled to a turntable according to an embodiment of the present description;

FIG. 5A is a diagram of an exemplary beam filtering system configuration for obtaining at least three beam spectra in accordance with an embodiment of the present description;

FIG. 5B is a diagram of an exemplary beam filtering system configuration for obtaining multiple beam spectra in accordance with an embodiment of the present description;

FIG. 5C is a diagram of an exemplary beam filtering system configuration employing multiple filter rings around an X-ray source to obtain multiple beam spectra, in accordance with embodiments of the present description;

FIG. 5D is a diagram of a top view of an exemplary beam filtering system configuration employing a plurality of filters arranged linearly in front of a source to obtain a plurality of beam spectra, in accordance with an embodiment of the present description;

FIG. 6A is a diagram of a contrast beam filter employed in an X-ray collimation system in accordance with an embodiment of the present description;

FIG. 6B is a diagram of a contrast beam filter employed in an X-ray collimation system in accordance with another embodiment of the present description;

FIG. 6C illustrates a scatter shield used in conjunction with the comparative beam filter inspection system shown in FIG. 6B according to another embodiment of the present description;

FIG. 7A is a side cross-sectional view of a pencil beam collimation system employing a beam filter in accordance with an embodiment of the present description;

FIG. 7B is a top sectional view of a pencil beam collimation system employing the filter shown in FIG. 7A;

FIG. 8A depicts an exemplary graphical user interface for enabling an operator to adjust the contrast of an image in accordance with embodiments of the present description;

FIG. 8B is a flow diagram illustrating a method for automatically determining a signal-to-noise ratio in response to a user selecting a beam filter thickness and a scan speed for an inspection system in accordance with embodiments of the present description;

FIG. 8C is a flow diagram illustrating a method for automatically determining an image contrast value in response to a user selecting a signal-to-noise ratio and a scan speed for an inspection system in accordance with embodiments of the present description;

FIG. 8D is a flow diagram illustrating a method for automatically determining a maximum scan speed in response to a user selecting a signal-to-noise ratio and an image contrast value for an inspection system in accordance with an embodiment of the present description;

FIG. 8E is a table illustrating the correlation between beam filter thickness, scan speed, and signal-to-noise ratio values according to embodiments herein;

FIG. 9A is a graph illustrating a transfer function that may be used to process a backscatter X-ray image in accordance with embodiments of the present description;

FIG. 9B is a backscatter image processed by using a linear function according to embodiments of the present description;

FIG. 9C shows the backscatter image of FIG. 9B processed by using a gamma transfer function in accordance with an embodiment of the description;

FIG. 9D shows the backscatter image of FIG. 9B processed by using an S-curve transfer function in accordance with embodiments of the disclosure;

FIG. 10 is a table showing filter chains for different beam filter configurations that may employ a variable filter system in accordance with embodiments of the present description;

FIG. 11 illustrates image display adjustment of a scanned image that has been processed using one or more beam filters according to embodiments of the present description;

FIG. 12 is a flow diagram illustrating a method of controlling contrast values of a scanned image obtained from a backscatter X-ray inspection system in accordance with an embodiment of the present description;

FIG. 13 is a flow diagram illustrating a method of combining scanned images to obtain an image with improved detection quality in accordance with an embodiment of the present description.

Detailed Description

The present specification provides a method of controlling backscatter image quality based on the degree of beam filtering provided in an X-ray inspection system. In an embodiment, the present specification provides a backscatter inspection system in which a plurality of different thickness beam filters are deployed at different locations to filter an X-ray beam configured to illuminate an object to produce scanned images having different contrast values, depending on the thickness and location of the beam filter used.

Defining:

the terms "image penetration" and "penetration contrast" refer to the contrast characteristic between an image object and its surroundings, the object being located behind a mask in the radiation image.

The term "image contrast" or "contrast" refers to the level of brightness or color of pixels in an image such that objects represented by the pixels are visually distinguishable from one another. Modifying contrast, therefore, means increasing, decreasing, or otherwise changing the brightness or color level of one or more pixels in an image to affect how the pixels are visually distinguished from one another.

The term "signal-to-noise ratio (SNR)" is defined as (average signal/pixel)/(standard deviation).

The term "flux" is defined as a measure of the number of X-ray photons in a radiation image used to form the image.

The term "spectral hardness" is defined as the bremsstrahlung spectrum filtered to preferentially attenuate lower energy components.

When an element is referred to as being "on," "connected to," or "coupled to" another element, it can be directly on, connected or coupled to the other element or one or more intervening elements may be present unless otherwise specified.

In various embodiments, a "computing device/controller" includes an input/output interface, at least one communication interface, and a system memory. In various embodiments, the computing device/controller includes conventional computer components such as a processor, necessary non-transitory memory or storage devices such as RAM (random access memory) and disk drives, a monitor or display, and one or more user input devices, such as a keyboard and mouse. In embodiments, the user input device allows the user to interact with the device by a command such as clicking a button on a mouse or keyboard, or alternatively by touching in embodiments where the display is touch screen enabled. The computing device/controller may also include software to support wireless or wired communications over a network (e.g., HTTP, TCP/IP, and RTP/RTSP protocols). These elements communicate with a Central Processing Unit (CPU) to enable operation of the computing device/controller. In various embodiments, the computing device/controller may be a conventional standalone computer, mobile phone, tablet computer, or laptop computer. In some embodiments, the functionality of the computing device/controller may be distributed across multiple computer systems and architectures.

In some embodiments, execution of a plurality of program instructions or code sequences causes or results in the CPU of the computing device/controller to perform various functions and processes. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the processes of the systems and methods described herein. Thus, the described systems and methods are not limited to any specific combination of hardware and software.

The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting. The singular forms "a", "an" and "the" are intended to include the plural forms as well.

This description is directed to various embodiments. The following disclosure is provided to enable one of ordinary skill in the art to practice the description. No language in the specification should be construed as indicating any non-claimed embodiment as essential to the invention or as limiting the scope of the claims beyond the meaning of the terms in which they are used. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the specification. Also, the phraseology and terminology used are for the purpose of describing the exemplary embodiments and should not be regarded as limiting. Thus, the present description is to be accorded the widest scope encompassing numerous alternatives, modifications and equivalents consistent with the principles and features disclosed. For the purpose of clarity, details relating to technical material that is known in the technical fields related to the description have not been described in detail so as not to unnecessarily obscure the description.

In the description and claims of this application, each of the words "comprise," "include," and "have," and forms thereof, is not necessarily limited to members of the list with which the word is associated. It should be noted herein that any feature or component described in connection with a particular embodiment may be used and practiced with any other embodiment unless specifically stated otherwise.

In an embodiment, the present description provides a system for varying a thickness of a beam filter applied to an X-ray beam. Fig. 1A is a diagram illustrating a system for integrating a variable beam filter with a mechanical beam shutter of an X-ray source according to embodiments of the present description. As shown, the X-ray source 150 is coupled to a shutter 152, and the shutter 152 may cover the source 150 if desired. The shutter 152 is coupled to a first beam filter 154 and a second beam filter 156. At least three different beam spectral shapes can be generated by using two beam filters, one with each

filter

154, 156 and one without either of the two filters. In alternative embodiments, a different design of beam filters using a sliding mechanism may also be used to obtain the desired number of beam filters and corresponding beam spectral shapes.

Fig. 1B is a side view illustrating a system for integrating a variable beam filter with a mechanical beam shutter of an X-ray source according to another embodiment of the present description. In an embodiment, an X-ray beam is generated by using an X-ray source 160 and a beam forming aperture 161. As shown in FIG. 1B, the X-ray source 160 is coupled to a shutter 162, and the shutter 162 can be slid over to cover the beam forming aperture 161, if desired. Shutter 162 is coupled to beam filter 164 and beam filter 166. In the embodiment shown in fig. 1B, the shutter 162 and

filters

164, 166 are configured to move parallel to the axis of the X-ray source 160, covering the beam forming aperture 161 and used to obtain different filter configurations. In the exemplary configuration shown in fig. 1B, aperture 161 is covered by beam filter 166. The aperture 161 and the beam filter 166 are in the same plane. To obtain different filter configurations, beam filter 166 is moved out of the plane of aperture 161 and beam filter 164 is moved into the plane of aperture 161 and is thus configured to cover aperture 161.

FIG. 1C illustrates a top view of the shutter and filter mechanism shown in FIG. 1B. In operation, as shown in FIG. 1C, filter 166 is translated and placed in front of beam forming aperture 161 to produce a filtered X-ray beam 168, while shutter 162 and filter 164 are slid to a position offset from beam forming aperture 161 and not in front of beam forming aperture 161, as shown. Dashed line 170 depicts the position of shutter 162 and

filters

164, 166 in an open position, wherein shutter 162 and

filters

164, 166 do not cover beam forming aperture 161. In an embodiment, an infinite number of filters may be used in the design shown in FIG. 1B, as the viewing angle of the light source has no limit on the number of filters that may be employed. Fig. 1D illustrates a top view of an X-ray source 180 coupled with a shutter and a plurality of filters according to an embodiment of the present description. As shown, any of the filters 182a, 182b.. 182n or the shutter 184 can be translated to cover the beam forming aperture 181 coupled to the X-ray source 180 to obtain the desired beam filtering. As shown, a filtered X-ray fan beam 188 is obtained when one of the filters 182a, 182b.. 182n is positioned in front of the beam forming aperture 181.

Referring to FIG. 1A, in various embodiments, the minimum beam is used to enhance the lowest energy component of the beam spectrum by filtering, for example, applying a first filter 154. By using minimal beam filtering, an aesthetically high signal-to-noise ratio image can be obtained. In addition, minimal beam filtering results in maximization of contrast between low atomic number materials (e.g., plastic) and high atomic number materials (e.g., steel), and maximization of spatial resolution, particularly for features outside the target. This enhances the detectability of organic threats that may be hidden behind thin shelters such as, but not limited to, cloth covered trucks. Minimal beam filtering may also result in better contrast, and thus better detection of thin organic threats on thick or opaque organic backgrounds. The contrast of thin organic targets to thick organic backgrounds also benefits from a softer beam spectrum. This form of contrast is referred to as "layer contrast" in ANSI-N42.46 cargo backscatter X-ray inspection system image quality standardization testing.

In various embodiments, beam filtering, which is added by, for example, applying the second filter 156, is applied in order to maximize the contrast between organic objects and metallic objects in the radiation image for targets behind a metallic shield, such as within a vehicle. The low energy components of the X-ray beam spectrum have a low probability of penetrating the mask but a high probability of interacting near the mask surface and are therefore only used for generating an image of the mask. The application of the beam filter reduces the low energy components, thereby enabling suppression of displaying only image data outside of the imaged object, thereby enhancing the view of the interior of the object.

Fig. 2A shows the effect of applying a beam filter on a backscatter X-ray image of an automobile containing a variety of organic threats and metal projectiles. Fig. 2B illustrates the effect of applying a stronger beam filter than that shown in fig. 2A on a backscatter X-ray image of an automobile containing multiple organic threats and metal projectiles. Referring to fig. 2A and 2B, an automobile 200 includes a trunk 202 containing two large explosive simulant tanks 204, and 155mm cannonballs 206 (seen as black objects in the figures) near the bottom of the tanks 204. The rear door 208 contains several packs of simulated drugs 210 within the door panel. Front door 212 contains a smaller simulated drug pack 214. Another simulated drug pack 216 is located in a front wheel well 218. Since the beam filter applied in fig. 2A is smaller than that applied in fig. 2B, it can be seen in fig. 2A that the headlights 222 and the plastic side fascia 224 appear brighter and darker in the image than the metal body of the automobile 200. In FIG. 2B, all organic threats in the automobile appear brighter relative to their surroundings, since more beam filtering is applied than in FIG. 2A. In fig. 2A, the cannonball 206 is only seen as a shadow in the organic material tank 204 in the backdrop box 202, with better visual contrast in fig. 2B, because the organic background 204 appears brighter than in fig. 2A.

The contrast between the metal object and the air also increases with increasing beam filtering. As can be seen from fig. 2A, the trunk 202 and hood 220 of the car 200 almost blend into the background, whereas in fig. 2B the transition from the steel object to the background is clearly defined. Further, it can be seen that the cannonball 206 appears brighter in FIG. 2B. Thus, if the cannonball 206 is presented with a black background, it would be detectable in FIG. 2B, but would be difficult to detect in FIG. 2A.

Fig. 3A is an image of a laboratory test setup used to study organic and metallic image features located behind a metal plate. FIG. 3B shows an X-ray image obtained using the apparatus shown in FIG. 3A, in which organic and metallic image features are visible behind the metal plate shown in FIG. 3A. Fig. 3A and 3B show a laboratory test setup with various image targets, including an image quality indicator for resolution, a step wedge for penetration, and several actual image targets intended to simulate the physical conditions of threats or contraband concealed in a vehicle. Referring to fig. 3A and 3B, the apparatus 300 includes a metal safe 302 that includes a pistol 304, a first metal plate 306, and a simulant 308 with drugs, explosives, and cash behind the plate, a second metal plate 310, and a simulant tubular bomb 312 hidden behind the plate 310. The candy bag 314, as a simulator of all types of organic threats, including drugs and explosives, is also placed behind the plate 310, the plate 310 being made of steel, 0.048 inches (1.2 mm) thick, simulating an automobile backbone that may represent a VBIED or smuggled scene. The thickness of the

plates

306, 310 and the metal safe 302 is 1 to 2mm, and the material is steel. The test apparatus 300 is positioned on a conveyor belt (not shown) in front of an imaging module (not shown) containing a 220keV X-ray source and imaging hardware, similar to commercial backscatter imaging products designed specifically for inspection of cars and trucks.

Fig. 3C shows a radiation image of the device of fig. 3A obtained by using four different beam filter conditions. Image 350 was obtained using a 0.8mm beryllium vacuum window over the X-ray source used to image device 300. Thus, image 350 is obtained using a negligible or zero-beam filter.

Images

360, 370, 380 were obtained using 0.5mm, 1.0mm, 2.0mm thick copper beam filters, respectively. As can be seen in FIG. 3C, the sugar pocket 314 has increased visibility from the unfiltered image 350 to the maximum filtered image 380, indicating that the visual detectability of organic threats increases with increasing thickness of the applied beam filter.

In an embodiment, the image 350 has a flux of 125%, a signal-to-noise ratio (SNR) of 37.1 or 112% at a radiation dose of 10.11R or 124%; image 360 has a flux of 78%, a signal-to-noise ratio (SNR) of 29.4 or 88% at a radiation dose of 6.lur or 74%; image 370 has a flux of 57%, a signal-to-noise ratio (SNR) of 25 or 75% at a radiation dose of 4.7uR or 58%; and image 380 has a flux of 37%, a signal-to-noise ratio (SNR) of 20.1 or 60% at a radiation dose of 3.1uR or 37%. Thus, while the low-filtered image 350 has a darker background and sharper edges for better aesthetics, the higher-filtered

images

360, 370, and 380 provide better visual detection of threat items concealed behind 1mm steel at significantly lower radiation doses. The addition of 2mm copper beam filtering (image 380) shifts the average energy of the beam from 73keV in image 350 to 120keV in image 380, reducing the X-ray dose by a factor of 3 and by a factor of 1.8 of the SNR. This SNR reduction corresponds to a 3, 4 times reduction in photon flux.

As shown in FIG. 3C, image 360 with a 0.5mm beam filter provides a significant increase in visual detectability and increases the average energy of the primary beam spectrum by-30%, from 73keV in image 350 to 95keV in image 360. Each of the

subsequent images

370 and 380 provides a higher level of detectability and increases the average energy by 12% to 13% from 95keV in image 360 to 106keV in image 370 to 120keV in image 380. Each increase in average energy allows a relatively large portion of the primary X-ray beam to penetrate the steel shield and produce a scatter signal from the target behind the shield. Thus, it is clear that a larger relative signal from the object of interest provides a larger signal and a larger contrast with respect to the surrounding environment for a given threat. The image study of fig. 3C empirically shows that the removal of the total flux from the beam to obtain a stiffer beam with lower energy photons removed outweighs the cost (in terms of SNR and aesthetic image smoothness).

As shown in fig. 3C, the thin plastic (organic) plate on the handle and the metal side of pistol 304 inside safe 302 become progressively more visible as the thickness of the beam filter increases, in addition to enhancing the detectability of organic sugar 314 behind plate 310. Furthermore, while in

images

360, 370, 380 the shape of the simulant 308 concealed behind the plate 306 by drugs, explosives and cash is progressively masked by the reduced flux of the greater filter thickness, the apparent brightness of the simulant 308 is enhanced by the increased beam filter thickness, and some smaller simulants 308 that are barely detectable or completely invisible in the unfiltered image 350 become more apparent after the beam filter is used, as can be seen in

images

360, 370, 380.

As is evident from fig. 3C, the spatial resolution of the filter-free image 350 is better than the

images

360, 370, 380 obtained after applying the beam filtering. In different scenarios, the attribute of spatial resolution or contrast may be more popular. However, to enhance the visibility of objects near detectable edges, such as the metal slide of pistol 304 or mock-object 308, spatial resolution provides little value for detection, and the increased contrast of the beam filter is critical.

For quantitative measurements of relative image signals, each of

images

350, 360, 370, and 380 is scaled to have the same minimum and maximum pixel values. The average signal from a set of pixels in a generally uniform region of interest at the location of one of the organic threats (e.g., the sugar pocket 314) behind a steel mask (e.g., the plate 310) in each scaled image is then compared.

Fig. 3D shows a radiation image of the device of fig. 3A obtained by using different filter thicknesses, beam powers and scanning speeds. Referring to fig. 3A, 3B, 3C, and 3D, an image 395 is obtained using a 0.8mm beryllium vacuum window on the X-ray source used to image the apparatus 300. Thus, the image 395 is obtained using negligible or zero beam filtering. Image 397 was obtained using a 2.0mm thick copper beam filter. Further, the beam power used to obtain image 397 is three times that used to obtain image 395. As shown, the 3D image 397 is more granular than the image 395. However, the contrast advantage of the 2mm copper beam filter is the same as the image 380 shown in FIG. 3C. In addition, the detectability of the sugar bag 314 and the simulant of drugs, explosives, and cash 308 in image 397 is greatly improved compared to image 395. Thus, even with different power adjustments, the contrast excitation beam filter of the present description provides enhanced detection of organic threat materials hidden behind the steel plate.

It has been observed that the scan speed has the same effect on dose and image quality as the effect of varying the beam power. Changing the scanning speed by 1/2x is the same as increasing the beam power by 2x, so by decreasing the scanning speed, the same image quality as the image 397 can be obtained. In different embodiments, multiple different thickness beam filters with different scan speeds may be employed to provide the same dose in each case to obtain a high contrast image, such as image 397.

In an embodiment, the variable beam filter of the present description may be combined with a mobile backscatter vehicle inspection system, which is typically an imaging module built into a panel van or minitruck. The driving speed, i.e. the horizontal scanning speed, is controlled by the accelerator pedal of the truck. To achieve high quality low speed scanning, some mobile backscatter vehicle inspection systems use mechanical scanning drives. As an example, an AS & E Mobile Search Backscatter truck (Mobile Search Backscatter truck) has two scan speeds, 3 inches/second (about 1/4 km/h) and 6 inches/second (about 0.5 km/h), both controlled by a motorized high friction drum that engages one of the tires of the truck to turn the wheels and drive the vehicle. In an embodiment, a thick filter is used in conjunction with a mechanical scan drive of the inspection system to obtain a low scan speed, thereby providing the radiation image with improved contrast and threat visibility.

In an embodiment, where cargo dose limiting is applicable to the inspection system, the mechanical control of the selection beam filter is coordinated with the control of the selection scan speed to maintain a constant dose in the inspection system. In various embodiments, the relationship between beam filter thickness and dose reduction is non-linear and non-trivial, but can be calculated by using known methods. Fig. 3E is a graph showing a relationship between X-ray doses corresponding to beam filters of different thicknesses. The Y-axis 3012 of the graph 3010 depicts dose relative to the baseline intrinsic filter and the X-axis 3014 depicts filter thickness. As shown by plot 3010, the X-ray dose decreases as the filter thickness increases. In an embodiment, in case the cargo dose limitation is not applicable, the system operator may select a beam filter for the required contrast level and individually select the scanning speed for the required flux level in exchange for the scanning time. In various embodiments, the contrast level obtained at any signal-to-noise ratio is greater than the contrast level demonstrated at 220keV in image 380 shown in figure 3C using a 2mm copper filter, due to the low scanning speed facilitated by the mechanical scan drive.

In embodiments where the system operator selects high contrast, high scan speed, and high signal-to-noise ratio, system power may be limited, resulting in a trade-off between high contrast and scan speed versus low power. Thus, in an embodiment, the system automatically performs a trade-off analysis to determine the highest contrast possible for the desired, input scan speed and signal-to-noise ratio, and scans at as high a contrast as possible, which may be lower than the desired contrast level input by the operator. In another embodiment, the system automatically performs a trade-off analysis to determine the highest possible signal-to-noise ratio for the desired input scan speed and contrast, and runs the scan at the highest possible signal-to-noise ratio, which may be lower than the desired signal-to-noise ratio input by the operator. In another embodiment, the system automatically performs a trade-off analysis to determine the fastest possible scan speed for the desired, input signal-to-noise ratio and contrast and to run the scan at a scan speed that may be lower than the highest possible scan speed input by the operator.

In various embodiments, the scanning speed of a typical cargo inspection system ranges from about 1 kilometer per hour (km/h) to 10km/h. For example, for inspection systems that may be transported on rails, for scanning systems where the target vehicle is towed past the imager by a car wash mechanism, and in some embodiments, for on-board scanning systems with a scanning drive, a minimum scan speed in the range of approximately between 0.1km/h and 0.3km/h may be achieved. In another example, for vehicle-based scanning systems where speed is controlled by the driver (using an accelerator pedal or other device), and for stationary scanning systems where the target vehicle is required to drive past the stationary scanning system, it is often difficult to control and achieve scanning speeds below about 1km/h to 2 km/h. In various embodiments, a maximum scanning speed of 10km/h may be achieved, since scanning speeds higher than 10km/h result in low image quality and reduced security. However, in some embodiments, there is no limit to the maximum scan speed that can be allowed. Thus, in one embodiment, the system performs a tradeoff analysis bounded by a minimum scan speed of 0.5km/h, more preferably 1km/hr, and bounded by a maximum scan speed of 20km/h, preferably 10km/h.

In various embodiments, the scanning system delivers a radiation dose to the cargo that ranges between 1 μ R/scan and 25 μ R/scan. In an embodiment, a minimum dose value of 1 μ R/scan may be delivered to the object being scanned, since a dose below 1 μ R/scan results in low image quality. It is well known that the relationship between dose and image quality depends on a number of parameters, such as, but not limited to, detector size and/or beam size. In some embodiments, there is no lower limit on the permissible dosage. The maximum dose of a vehicle being scanned with a person still in the vehicle is typically limited by local laws or generally accepted safety standards, which in some embodiments does not exceed 10 μ R/scan or does not exceed 25 μ R/scan. Even for an unmanned vehicle, the maximum dose threshold may be the same, as humans may be present in the vehicle as smuggling goods.

The SNR index is based on various parameters of the imaging system and the method used to measure the SNR value. In an embodiment of the present description, a typical SNR value corresponds to the baseline image 370 shown in fig. 3C. The image 380 shown in fig. 3C has an SNR value of about 0.84x relative to the baseline image 370. In an embodiment, the minimum acceptable SNR value is approximately in the range of one-half (l/2) x or one-third (l/3) x relative to the SNR value of the baseline image 370. In an embodiment, the minimum acceptable SNR value of a is in the range of about one quarter (1/4) to one tenth (1/10) of the image flux. In an embodiment, there is no maximum acceptable SNR value because the value of SNR is limited by the scanning system's capabilities with respect to parameters such as, but not limited to, cost, weight, and scanning time. Thus, in an embodiment, the trade-off analysis performed by the system is limited to a minimum acceptable SNR in the range of 1/4 to 1/10 of the image flux.

In an embodiment, the mechanical scan drive is replaced by a turntable carrying the imaging module of the inspection system and rotating about a vertical axis, thereby causing an X-ray fan beam to sweep in stages over the object. Fig. 4 is a diagram of an imaging module coupled to a turntable according to an embodiment of the present description. Inspection system 400 includes an imaging module 402 fixedly positioned on a turntable 404, the turntable 404 in turn being supported by a base 406. The imaging module 402 includes at least an X-ray source 408 coupled to a shutter 410, and the shutter 410 may cover the source 408 if desired. Shutter 410 is coupled to a first beam filter 412 and a second beam filter 414. The operation of the shutter 410 and

filters

412, 414 is the same as described above with respect to fig. 1A, 1B, 1C, and 1D. The turntable 404 may be rotated to position the imaging module 402 such that the filtered fan beam of X-rays 416 is emitted in a desired direction. As shown in fig. 4, the beam 416 is scanned in the vertical direction. In an embodiment, the variable beam filter of the present description may be coordinated with the rotational speed of the turntable to achieve the same radiation quality as described above.

The following U.S. patent numbers, which are incorporated herein by reference in their entirety, describe, among other features, cargo scanning systems commonly owned by the applicant or by the applicant's parent: 6,542,580;6,542,580;6,658,087;7,099,434;7,218,704;7,322,745;7,369,643;7,400,701;7,486,768;7,517,149;7,519,148;7,593,506;7,720,195;7,783,004;7,817,776;7,860,213;7,876,880;7,963,695;7,991,113;7,995,707;7,995,705;8,054,937;8,059,781;8,170,177;8,194,822;8,275,091;8,345,819;8,356,937;8,385,501;8,433,036;8,437,448;8,457,275;8,503,605;8,579,506;8,644,453;8,668,386;8,687,765;8,774,357;8,781,067;8,824,632;8,837,670;8,840,303;8,903,046;8,908,831;8,929,509;8,971,485;9,020,096;9,025,731;9,036,779;9,052,403;9,052,264;9,057,679;9,121,958;9,158,027;9,223,049;9,223,052;9,274,065;9,279,901;9,285,498;9,429,530;9,541,540;9,562,866;9,632,205;9,688,517;9,791,590;9,817,151;9,823,201;9,835,756;9,958,569;10,007,021;10,007,019;10,098,214;10,228,487;10,302,807;10,317,566;10,408,967;10,422,919;10,585,207; and 10,591,629.

In one embodiment, the system operator selects a beam filtering range such that the dose output can be varied by a certain factor (e.g., 8 times from minimum to maximum beam filtering), and the controller can be configured to scan at the same modulation scan speed factor (8 times). If the controller determines that the maximum scan speed and the minimum beam filter provide an SNR acceptable to the operator, the controller may automatically determine that the minimum scan speed and the maximum beam filter provide the same dose and a similar SNR. In embodiments, a greater or lesser range for the beam filter or scan speed parameter may be adjusted depending on the use of the system. For example, without dose-to-cargo limitations, the minimum allowable scan speed may be arbitrarily low, and the final dose may be arbitrarily high, even if for mechanical reasons, it is not feasible to continue adding beam filters and utilize greater contrast at constant dose at the lowest scan speed.

Mechanical design of variable beam filter

Referring back to fig. 1A, in one embodiment, an X-ray tube is positioned after each of the first filter 154 and the second filter 156 relative to the object being scanned. Preferably, the first filter 154 comprises a curved surface that is concentrically positioned relative to the X-ray tube 150 and is movable through an arc to be alternately positioned within or not positioned within the field of view 167 of the X-ray tube 150. Similarly, the second filter 156 includes a curved surface that is concentrically positioned relative to the X-ray tube 150 and is movable through an arc to be alternately positioned within or not positioned within the field of view 167 of the X-ray tube 150. The first filter 154 and the second filter 156 are also preferably concentrically positioned relative to each other such that they may be simultaneously moved through an arc to simultaneously be in or out of the field of view 167 of the X-rays.

Fig. 5A is a diagram of an exemplary beam filtering system configuration for obtaining at least three beam spectra in accordance with an embodiment of the present description. As shown in fig. 5A, filters 502, 504, and 506 of different thicknesses are positioned radially or concentrically around the beam forming aperture 513 of the X-ray source 500 such that each filter occupies a portion of an arc around the source 500, in a rotating wheel configuration, where a portion 508 of the arc remains open. Three types of filtered beam spectra with field of view 510 may be obtained by positioning each of three

filters

502, 504, and 506 (one after the other) in front of beam forming aperture 513. This configuration does not use a shutter, as shown, for example, in FIG. 1A. Without a shutter, the arc-shaped open portion 508 may be occupied by any of the

filters

502, 504, and 506 in a manner that allows the open portion 508 to be positioned in front of the beam forming aperture 513 to obtain a fourth unfiltered beam spectrum. For example, referring to fig. 5A, an unfiltered light beam can be obtained by sliding the filter 506 into the open portion 508 and sliding the filter 504 into the position previously occupied by the filter 506, thereby opening the arc-shaped portion in front of the beam forming aperture 513 (previously occupied by the filter 504). A spectral field of view of approximately 90 degrees is obtained by using the filter placement/configuration shown in fig. 5A.

It will be appreciated that the described radial translation filter system defines a field of view for each filtering level that is less than the angular extent of the filter, where the angular extent is defined by the angle formed by one end of the filter, the center of the X-ray source and the other end of the filter, as shown by angle 192 in fig. 1A relative to filter 2. In one embodiment, the field of view is on the order of 0.5 to 5 degrees less than the angular range of the filter.

Fig. 5B is a diagram of an exemplary beam filtering system configuration for obtaining multiple beam spectra in accordance with an embodiment of the present description. As shown in FIG. 5B, by reducing the field of view 510 of the beam spectrum obtained in FIG. 5A, multiple filters along with mechanical shutters may be employed in an arc around the source 500 to increase the number of available beam spectra using the system. In the embodiment shown in FIG. 5B, six

filters

502, 504, 506, 512, 514, and 516 of different thicknesses are used, enabling the system operator to obtain six different beam spectra with fields of view 518 corresponding to each filter. As explained with respect to fig. 5A, the open portion 508 of the arc provides an unfiltered beam spectrum when brought in front of the beam forming aperture 513. A mechanical shutter 520 is also employed in the configuration shown in fig. 5B. In various embodiments, the "open" position provides zero millimeter filtering of any material (i.e., allows all components of the X-ray beam to pass), while the shutter can be considered to provide infinite filtering (i.e., does not allow any components of the X-ray beam to pass). Although only six filters are shown in fig. 5B, it will be apparent to those skilled in the art that any number of filters may be employed by decreasing the field of view/increasing the distance of the filter ring from the source.

Fig. 5C is a diagram of an exemplary beam filtering system configuration employing multiple filter rings around an X-ray source to obtain multiple beam spectra in accordance with an embodiment of the present description. In the configuration shown in fig. 5C, two concentric filter rings have been positioned around the source 500. The inner ring includes two

filters

532, 534 of different thicknesses, a shutter 536, and an opening portion 538; and the outer ring includes three

filters

542, 544, 546 of different thicknesses and an open portion 548. By positioning various combinations of five filters and two opening portions before beam forming, apertures 513 of filtered beams 540 source 500 of different characteristics can be obtained. In the exemplary filter combination shown in fig. 5C, beam spectrum 540 is obtained by placing filters 532 (from the inner loop) and 544 (from the outer loop) in front of beam forming aperture 513. In various embodiments, this is possible when shutter 536 is used in the configuration shown in fig. 5C for twelve filter combinations (including combinations where no filter is placed in front of beam forming aperture 513, i.e., where both

open portions

538, 548 are located in front of beam forming aperture 513) in a field of view of approximately 90 degrees. If shutter 536 is not included, a total of 16 filter combinations would be available for the configuration of FIG. 5C. In various embodiments,

filters

532, 534 and 542, 544, 546 may be made of the same or different materials. In an embodiment, where a filter having a first thickness in the outer ring is placed before a filter having a second thickness in the inner ring, and both filters are made of the same material, the combined filtering effect will be the sum of the first and second thicknesses with the thickness of the single filter.

Fig. 5D is an illustration of a top view of an exemplary beam filtering system configuration employing a plurality of filters linearly arranged in front of a beam forming aperture of a source to obtain a plurality of beam spectra, in accordance with an embodiment of the present description. As shown in fig. 5D, the filters 582, 584 and the shutter 586 are arranged in a first vertical row in front of the source 501, and the

filters

588, 590 and 592 are arranged in a second vertical row (582, 584) in front of the source 501. As explained with reference to fig. 1C, the filters 582, 584 and the shutter 586, as well as the

filters

588, 590, and 592 shown in fig. 5D, may be vertically translated to position any of the filters (or shutters) at the beam forming aperture 515 of the source 501 to obtain the filtered beam 594. Dashed line 596 depicts the position of filters 582, 584 and shutter 586 in the open position, i.e., not covering beam forming aperture 515, and dashed line 598 depicts the position of

filters

588, 590 and 592 in the open position. In an embodiment, a large number of filters may be used in the design shown in FIG. 5D, as the angular field of view of the source has no limit on the number of filters that may be positioned in front of the beam forming aperture 515. In various embodiments, when the filters are employed in a concentric ring configuration, as explained with respect to fig. 5A-5C, the field of view of the filter placed in front of the pencil beam forming aperture of the X-ray source is approximately 2 ° to 4 ° less than the angle of the filter. This is because the width of the pencil beam is limited according to known mechanical and optical rules and the field of view of the filter is less than 360/N due to timing and data management, where N is an integer value. Thus, if the field of view is <360/N, N spectra can be obtained by using the filter configuration as shown in fig. 5A to 5C.

In an embodiment, an exemplary contrast beam filter that can produce a high contrast image of an organic threat and that can be used to enhance the visibility of objects near detectable edges is made of dense materials with high atomic numbers. More specifically, the contrast beam filter is made of a metal, such as, but not limited to, bronze, tin, tungsten, or a copper matrix embedded with tungsten particles. In an embodiment, if the filter is made of a material such as tungsten or lead with fluorescence energy high enough to be detected in the backscatter detector (e.g., 60keV for tungsten and 75keV for lead), the filter is designed to absorb substantially all of the fluorescence energy fluorescence. In an embodiment, the filter is designed from a composite material or with a multi-layer design that employs a secondary shielding layer made of steel or copper to absorb fluorescence from lead or tungsten. In an embodiment, the secondary shielding layer thickness is designed to absorb a predetermined portion of the fluorescent light. For example, a secondary shielding layer made from a 0.5mm thick copper sheet may attenuate the fluorescence of tungsten to half its initial value.

In an embodiment, the contrast beam filter of the present description is used in conjunction with a beam collimation system designed to prevent scatter from exiting the system without passing through the beam filter. This is because any compton or rayleigh scattering that escapes the beam filter but still leaves through the beam forming aperture forms a "halo" around the main X-ray pencil beam, which in turn reduces the spatial resolution of the acquired image. Fig. 6A is a diagram of a contrast beam filter employed in an X-ray collimation system in accordance with an embodiment of the present description. Fig. 6B is a diagram of a contrast beam filter employed in an X-ray collimation system in accordance with another embodiment of the present description. The system shown in fig. 6A and 6B includes a shutter 602 coupled to a filter 604 for filtering the X-ray beam 606. X-ray beam 606 is generated using X-ray tube 608 and pencil beam forming aperture 610. The aperture 610 is provided in a collar 612, the collar 612 being mounted to a rotatable disk-like structure 614 which leaves room for the X-ray tube 608. As shown in fig. 6A, the X-ray tube 608 is not positioned on the shaft 616 of the rotating disk 614. A motor 618 enables the rotation of the disc 614. To minimize unwanted scatter, the beam 606 is collimated into a fan beam by a second collimator 620 that fills the space between the X-ray tube housing 622 and the shield ring 624. The exit opening 626 of the X-ray tube housing 622 also contributes to the fan-beam collimation and is referred to as the first fan collimator 621. As shown in fig. 6A and 6B, the shutter 602 and the variable beam filter 604 are located between the exit openings 626 and the collimator 620. The shutter 602 coupled with the filter 604 may slide to at least partially cover the exit opening 626 to attenuate the light beam 606, as explained above with reference to fig. 1A-1D.

The system shown in fig. 6A provides radiation safety for the person operating the system and the person being scanned by the system when in operation. This is because all of the direct path of the light beam 606 from the focal point 628 to the aperture 610 passes through the filter 604. In the system shown in fig. 6A, a thin filter is used, which allows the maximum number of filters to be used within a given available space.

However, this system allows scattered radiation 630 to exit the system through aperture 610 without first being attenuated by filter 604. This scattered radiation 630 may not pose a safety problem because the scatter is a relatively small fraction of the total dose. Furthermore, since the safety of personnel around the system depends on the total dose, the safety parameters can be met by increasing the beam filtering in proportion to the dose. However, even if the system is safe for humans, the scanned image produced by the system may not be contrast-effective (as described above) due to the scattered radiation 630. As shown in fig. 6A, the scattered radiation 630 contributes to the formation of a halo around the main beam envelope 606. This unattenuated scatter 630 halo becomes proportionally stronger as the primary beam 606 is attenuated by the added filter, resulting in further image efficiency degradation. In addition to reducing spatial resolution, the scatter 630 halo can also cause all forms of contrast reduction, as halos can cause background fogging in the scanned image.

Fig. 6B shows a modification of the collimation system of fig. 6A to produce a high contrast scan image. As shown in fig. 6B, exit opening 626 of X-ray tube housing 622 is narrowed and beam filter 604 is widened such that beam 606 has no direct path to exit opening 626 without traversing through filter 604. Since this results in a reduction of the direct light beam 606 falling on the walls of the second sector collimator 620, the thickness of the collimator 620 may be reduced since there is less scattering to be shielded. Thus, scatter is reduced in the system of fig. 6B as compared to the system of fig. 6A (and not shown in fig. 6B), resulting in the system of fig. 6B producing a more efficient high contrast image.

In various embodiments, the scatter shields used in conjunction with the filters of the present description are designed to eliminate any signal noise/scatter. FIG. 6C illustrates a scatter shield used in conjunction with the comparative beam filter inspection system shown in FIG. 6B according to another embodiment of the present description. As shown in fig. 6C, a backscatter detector 630 may be employed in the system to detect radiation backscattered from the imaged object. In various embodiments, backscatter detector 630 also inevitably detects radiation scattered by filter 604 as part of sliding/rotating filter exchange system 631. A shield 632 made of a material such as, but not limited to, lead is provided around the X-ray tube housing 622 and aperture 610. In various embodiments, additional shielding features are incorporated into the system to eliminate leakage around the mobile filter changer assembly 631 of filter 604. A scatter path such as 634 shown in fig. 6C may be created due to scattering of the main beam 606 by the components of the filter 604 and/or filter replacement hardware. Additional shielding 636 is provided around the system to eliminate X-ray leakage along the scatter path 634. Because the hardware components of the system (e.g., disk 614, which may be made of metal such as aluminum or steel) do not provide the required shielding, in embodiments, additional shielding 638 made of lead or tungsten is provided around the components to prevent radiation leakage, as any leakage captured by the backscatter detectors will degrade the quality of the resulting scanned image.

In an embodiment, to prevent isotropic scattering from the beam filter from escaping the X-ray inspection system, the contrast beam filter is located close to the X-ray source when used in the X-ray inspection system. The further the beam forming aperture of the beam collimating system of the X-ray inspection system is from the origin of this scattering (i.e. the beam filter), the smaller the solid angle of the aperture with respect to the origin is, and less scattering escapes the system. The same is true for any multiple scattering that finds a path around the beam filter. To maximize contrast by minimizing the effects of X-ray scatter, in one embodiment, the present system implements a beam filter that is as close as possible to the X-ray tube and as far as possible from pencil beam forming aperture 610. The beam filter may be located between the tube and the fan collimator or, due to space constraints, it may be located between the fan collimator and the pencil beam forming aperture 610. As shown in fig. 6, in one embodiment, the beam filter is positioned as close as possible to the X-ray tube (608) and the first sector collimator (621), and as far as possible from the pencil beam aperture (610), to maximize contrast.

Fig. 7A is a top cross-sectional view of a pencil beam collimation system employing a beam filter in accordance with an embodiment of the present description. Fig. 7B is a side cross-sectional view of a pencil beam collimation system employing a filter as shown in fig. 7A. For illustrative purposes, a single beam filter 702 is shown in fig. 7A, 7B, however the single filter 702 may be part of the multiple beam filters employed in the rotating or translating beam filter changing systems described above. As shown, a main beam 704 emitted from an X-ray tube 706 is attenuated by a filter 702 and collimated by a pencil beam forming collimator 708. The isotropic scattering 710 from the beam filter 702 constitutes a photon source that can escape the collimator 708 through the aperture 712 of the X-ray tube. The scattered photons 710 collimated by the aperture 712 constitute a single beam spot or "halo" 714 around a beam spot 716 formed by the primary beam 704, as shown. Halo 714 reduces spatial resolution because scattered photons 710 sample a large area and effectively average the backscatter signal from that area, which in combination with a good image signal from the main beam spot, halo 714 reduces all forms of contrast in the scanned image obtained by the system.

The intensity of the halo 714 signal (proportional to the flux of scattered photons escaping through aperture 712) is proportional to the distance (L) from filter 702 to aperture 712 (L) _filter ) Is inversely proportional to the square of. The intensity of the main beam 704 and the distance (L) from the focal point 718 to the aperture 712 of the X-ray tube 706 _fs ) Is inversely proportional to the square of. In various embodiments, the distance (Δ) from the focal point 718 of the X-ray tube 706 to the filter 702 is:

△＝L _fs -L _filter equation 1

The image degradation caused by the halo 714 is proportional to the ratio of the poor flux from the scattered halo to the good flux from the main pencil beam 704 and can be defined as:

L _fs ^2/ L _filter ² ＝L _fs ² /(L _fs -△) ² equation 2

Thus, by positioning the beam filter 702 in close contact with the X-ray tube 706, a minimum halo 714 will be achieved. Furthermore, if Δ is from 0.1 × L _fs Increase to 0.4 × L _fs Then the scattered halo intensity will increase by a factor of 2.25; the delta further increases to 0.6 x L _fs Relative to 0.1 × L _fs And the halo intensity increased by a factor of 5.l. The effect of the halo 714 on spatial resolution is a function of halo intensity and halo size, the latter being the size of aperture 712, the distance of aperture 712 from focal point 718, and the size of beam filter 702, which will be larger than focal point 718. These factors vary from inspection system to inspection system, but for narrow main beam 704 and narrow collimating aperture 712, and target distance>10*L _fs The ratio of the size of the halo 714 (in one dimension) to the size of the main beam spot 716 is proportional to:

L _fs /L _filter ＝L _fs /(L _fs -. DELTA) formula 3

Equation 3 is the square root of equation 2. This relationship between equation 2 and equation 3 indicates that if Δ is from 0.1 × l _fs Increase to 0.4 × L _fs Then the scattered halo diameter will increase by a factor of 1.5; and relative to 0.1 × l _fs A further increase of Δ to 0.6 × l _fs The halo diameter is increased by a factor of 2.25.

In an embodiment, the contrast beam filter of the present description is used in combination with a shielding device designed to contain all scatter from the beam filter or related components, such that scattered radiation does not escape the beam collimation system and does not enter the detector or detectors of the X-ray inspection system. Escaping scattered radiation (leakage) can cause haze in the backscatter image obtained from the detector, thereby reducing image contrast. In a typical backscatter X-ray imaging system, a backscatter detector is located near the X-ray source (rather than distal to the image target) and the detector subtends a much larger area than a typical transmission X-ray detector. The sensitivity of the backscatter detectors to X-ray dose is much lower than the dose for safety considerations, and any leakage or scatter dose into the detectors causes signal shifts, thereby reducing contrast. To enhance image contrast, the image SNR can be affected, and therefore it becomes critical to eliminate all other noise sources or unwanted signals. Thus, the scatter shields used in conjunction with the filters of the present description are designed to eliminate any signal noise/scatter, such as described with reference to fig. 6C. Thus, any leakage of all other parts of the beam forming and collimating system of the X-ray inspection system, such as, but not limited to: direct leakage and scattering from the source housing escape; scatter escaping through a gap between the source housing and a rotating chopper forming a scanning pencil beam; and is attenuated by using the shielding design by scattering of the aperture which is only present to accommodate the mechanical structures that move the beam filter into and out of the beam.

Software design for variable beam filters

FIG. 8A depicts an exemplary graphical user interface for enabling an operator to adjust the contrast of an image according to embodiments of the present description.

Referring to fig. 8A, in one embodiment, a graphical user interface 801 has a display portion 804 and a plurality of controls 802, one of which may be a contrast level input mechanism, a scan speed input mechanism, and a signal-to-noise ratio input mechanism, which may be icons, buttons, toggles, switches, dialog boxes, or other input mechanisms. In one embodiment, the operator may interact with the input mechanism 802 to increase or decrease the contrast level of the generated image.

In one embodiment, the system may have an initial default contrast level that corresponds to a plurality of other default settings in the X-ray inspection system, including a default filter configuration. Upon modifying the contrast level, a controller 811 coupled to the X-ray inspection system is in data communication with the graphical user interface via a network 815, resulting in reconfiguration of any default contrast settings in the X-ray inspection system, including the filter positions as described above and some other software settings as described below, to accommodate the new contrast settings.

In an embodiment, for safety purposes, the controller 811 may modify any default settings for a minimum allowed user selectable scan speed in order to limit the maximum dose per scan. In an embodiment, the controller 811 may modify any default setting for the maximum duty cycle to limit the amount of dose received by the operator or bystander.

In an embodiment, the controller 811 may inhibit scanning for a predetermined period of time after the predetermined scan time is reached, along with modifying the maximum duty cycle. For example, the system may be configured to emit X-rays for 45 minutes over any 60 minute time period. In an embodiment, the time at which scanning is inhibited is different for each beam filter used in the system; where the time to inhibit scanning is proportional to the dose and calculated as a function of time and beam filter. This enables the user to operate one filter for part of an hour using a filter operating system and the remainder of the hour using a different filter, the controller 811 taking into account the combined dose for that hour to determine when to apply the scan ban.

In an embodiment, controller 811 automatically determines one of the following in response to user selection of at least two of the parameters: beam filter thickness, scan speed and signal-to-noise ratio. FIG. 8B is a flow diagram illustrating a method for automatically determining a signal-to-noise ratio in response to a user selecting a beam filter thickness and a scan speed for a detection system in accordance with embodiments of the present description. At step 850, the user selects a beam filter thickness and a maximum scan speed for the inspection system. In an embodiment, as described above, the contrast of the scanned image depends on the selected beam filter thickness. In an embodiment, a thick filter may produce a high contrast scan image.

Fig. 8E is an exemplary table illustrating a correlation between beam filter thickness, scan speed, and signal-to-noise ratio values according to an embodiment of the present description. Table 840 includes a column 842 listing a plurality of beam filters, a column 844 listing a plurality of corresponding scan speeds, and a column 846 listing a plurality of SNR values relative to a baseline system that includes a 1mm copper filter and operates at a scan speed of 1km/h with variable parameters including, but not limited to, scan distance and detector array size. For example, the user may select: 1) A 2mm thick copper beam filter to obtain maximum penetration contrast, and 2) a maximum scan speed of 10km/h to allow rapid inspection of in-line vehicles. Referring to table 840, the controller (811 in fig. 8A) automatically determines that the SNR value may drop to 0.26 times the baseline configuration value shown in column 846. In an embodiment, the predefined threshold for SNR may set the SNR drop to be no less than 0.32 times the baseline value (corresponding to one tenth of the baseline flux). In such embodiments, the user is informed that the selected beam thickness and scanning speed will provide a very low SNR value below a predetermined threshold. In an embodiment, the controller may prompt the user to limit the scan speed to 3km/h for use with a 2.0mm copper filter (or scan speed range between 3km/h and 10 km/h). In an embodiment, the controller may prompt the user to limit the beam filter thickness to 1mm copper at a scan speed of 10km/h.

Referring back to fig. 8B, at step 852, the signal-to-noise ratio or amplifier gain level is automatically determined by using the selected beam filter thickness and maximum scan speed. In an embodiment, the controller 811 is programmed to determine the signal-to-noise ratio by using the user selected beam filter thickness and the maximum scan speed. At step 854, the automatically determined signal-to-noise ratio or amplifier gain is presented to the user via graphical user interface 801. At step 856, a determination is made as to whether the automatically determined signal-to-noise ratio or amplifier gain level is greater than a predetermined threshold, where the threshold is a baseline value indicating a minimum level of acceptable signal-to-noise ratio or amplifier gain level. If the signal-to-noise ratio or amplifier gain level is less than the predetermined threshold, the user is notified of the same at step 858. At step 860, the user is presented with an option to modify the beam filter thickness and/or the maximum scan speed selected at step 850. In an embodiment, a user may modify the beam filter thickness and/or the maximum scan speed by logging onto the plurality of controls 802 using the graphical user interface 801. If the signal-to-noise ratio or amplifier gain level is greater than a predetermined threshold, the user will be notified of the same, and the inspection system will operate at the selected beam filter thickness, maximum scan speed, and determined signal-to-noise ratio or amplifier gain level, step 862.

Fig. 8C is a flow diagram illustrating a method for automatically determining an image contrast value in response to a user selected signal-to-noise ratio and scan speed for a detection system in accordance with an embodiment of the present description. At step 870, the user selects a signal-to-noise ratio and a maximum scan speed for the inspection system. In an embodiment, as described above, the contrast of the scanned image depends on the selected beam filter thickness. In an embodiment, a thick filter will result in a high contrast scanned image. For example, the user may select a higher signal-to-noise ratio value, such as 1.4 times the typical baseline value, to obtain a smooth and sharp image, and a maximum scan speed of 1km/h for medium speed inspection of small vehicles. Then, referring to the table 840 shown in fig. 8E, the controller (811 in fig. 8A) automatically provides the only compatible available beam filter thickness is the 0.8mm beryllium beam window (effective zero beam filtering), which will provide moderate penetration contrast (below typical baseline values). Based on the purpose of the scanning system, in an embodiment, a predefined threshold value of penetration contrast may be defined. For example, a scanning system operated by the military may need to detect organic IEDs in the trunk of an automobile. In such an embodiment, the controller may prompt the user to select a thicker beam filter, for example a copper filter having a thickness of 1.0 mm. As shown in FIG. 8E, a scan speed of 1km/h shown in column 842 corresponds to an SNR value of 1.45x shown in column 846, which is higher than the desired SNR with a 1mm copper filter. In an embodiment, if the scanning system is capable of providing such a speed, the controller determines a lower scanning speed (e.g., 0.5 km/h). If the scanning system is unable to provide such a low speed, the user is prompted to select a lower SNR value.

At step 872, an image contrast value or beam filter thickness is automatically determined by using the selected signal-to-noise ratio and the maximum scan speed. In an embodiment, the controller 811 is programmed to determine the image contrast value by using the user selected signal-to-noise ratio and the maximum scan speed. At step 874, the automatically determined image contrast values are presented to the user through the graphical user interface 801. At step 876, it is determined whether the automatically determined image contrast value is greater than a predetermined threshold, where the threshold is a baseline value indicating a minimum level of acceptable image contrast value. If the image contrast value is less than the predetermined threshold, the user is notified of the same value at step 878. At step 880, the user is presented with the option of modifying the signal-to-noise ratio and/or the maximum scan speed selected at step 870. In an embodiment, a user may modify the signal-to-noise ratio and/or the selected maximum scan speed by logging onto a plurality of controls 802 using graphical user interface 801. At step 882, if the image contrast value is greater than the predetermined threshold, the user is notified of this and the inspection system operates at the selected signal-to-noise ratio, the maximum scan speed, and the determined image contrast value.

Fig. 8D is a flow diagram illustrating a method for automatically determining a maximum scan speed in response to a user selecting a signal-to-noise ratio and an image contrast value for a detection system in accordance with an embodiment of the present description. At step 890, the user selects a signal-to-noise ratio and an image contrast value for the inspection system. In an embodiment, as described above, the contrast of the scanned image depends on the selected beam filter thickness. In an embodiment, a thick filter will result in a high contrast scanned image. For example, the user may select an SNR value of 0.8 times the typical baseline value to obtain an image approximately equivalent to an image having a typical image grain level and a high penetration contrast value to maximize ease of use and effectiveness in detecting organic anomalies such as illegal drugs hidden behind the body panel of the vehicle. Then, referring to the table 840 shown in FIG. 8E, the controller (811 in FIG. 8A) automatically determines that the ideal beam filter is a copper filter with a thickness of 2.0mm and the maximum compatible scanning speed is 1km/h. In an embodiment, a maximum scan speed of 3km/h has been predefined based on the user's requirements, and to ensure reasonably high scan throughput, the controller prompts the user to select a lower SNR value (e.g., 0.48 times as a typical baseline value) or a lower level of penetration contrast.

At step 892, the maximum scan speed is automatically determined by using the selected signal-to-noise ratio and image contrast values. In an embodiment, the controller 811 is programmed to determine the maximum scan speed by using the user selected signal-to-noise ratio and image contrast value. At step 894, the automatically determined maximum scan speed is presented to the user through the graphical user interface 801. At step 896, it is determined whether the automatically determined maximum scanning speed is greater than a predetermined threshold, where the threshold is a baseline value indicating a minimum level of acceptable maximum scanning speed. If the maximum scan speed is less than the predetermined threshold, the user is notified of the situation at step 898. At step 8100, the user is presented with the option of modifying the signal-to-noise ratio and/or image contrast value selected at step 890. In embodiments, a user may modify a selected signal-to-noise ratio and/or image contrast value by using graphical user interface 801 and plurality of controls 802. If the maximum scan speed is greater than the predetermined threshold, the user will receive the same notification and the inspection system will operate at the selected signal-to-noise ratio, image contrast value, and determined maximum scan speed, step 8102.

In various embodiments, the non-linear transfer functions used, such as the "gamma" and "S-curve" transfer functions, may have to be configured when selecting different contrast levels, since the ideal gamma or S-curve transfer function depends on the bright scale/dark pixels in the image. In embodiments where only one beam filter is used, only one set of parameters is required to apply the transfer function, whereas in the case where 'n' number of beam filters are used, an 'n' set of parameters (i.e. one set of parameters for each filter) is necessary. In an embodiment, the gamma function is defined by one parameter, while the S-curve is defined by a plurality of parameters. The choice of gamma or S-curve transfer function is based on the desired characteristics or "look" of the scanned image.

In an embodiment, a processing unit of the inspection system employs a contrast-excited beam filtering system, receives data representing the image and performs an S-curve transfer function from the detector array to suppress unsightly fogging and/or noise in a dark background region of the backscattered radiation image. In operation, the controller accesses a memory that stores a plurality of parameters specific to each filter. For example, if there are N beam filters, the memory preferably stores and the controller preferably has access to N sets of respective parameters, wherein each set of respective parameters defines at least one of the above-described gamma function or S-curve transfer function. A number of parameters are pre-selected according to the desired aesthetic appearance of a given filter type. For backscatter radiation images, different detection materials correspond to a change in signal level and this transfer function should be reconfigured according to the new contrast level. The addition of the beam filter of the present specification increases both the signal in the air region and the noise (due to flux reduction), increases the need to modify the S-curve transfer function, and increases the S-curve that requires a specific imaging scene for carefully optimizing shape.

Fig. 9A is a graph illustrating a transfer function that may be used to process a backscatter X-ray image in accordance with an embodiment of the present description. Fig. 9B is a backscatter image processed by using a linear function according to an embodiment of the description. FIG. 9C shows the backscatter image of FIG. 9B processed by using a gamma transfer function in accordance with an embodiment of the description. Fig. 9D illustrates the backscatter image of fig. 9B processed by using an S-curve transfer function in accordance with an embodiment of the description. Referring to fig. 9A-9 d, the shape of the s-

curves

918, 922 is a function of the parameters "a" and "C", where parameter a controls high signal amplification and parameter C controls low signal suppression. Referring to fig. 9A and 9B, image 910 was obtained by using a 1.0mm thick copper beam filter, where only the linear function depicted by plot 912 was applied, corresponding to a being 1 and C being a value of 0.0001, so plot 910 represents raw scan data without processing. Referring to fig. 9A and 9C, when the image 910 is processed by using a gamma function such as "square root filter" shown by a graph 914, an image 916 is obtained. As can be seen by comparing fig. 9B and 9C, the image 916 has a higher brightness and relative contrast in darker areas in exchange for a reduced contrast in lighter areas as compared to the original image 910. Referring to fig. 9A and 9D, when the image 910 is processed by using the S-line function depicted corresponding to the line 918 having the a value of 2.25 and the C value of 0.25, an image 920 is obtained. The S-curve of plot 918 is compatible with the use of a relatively thick beam filter and may be applied to images obtained by using a thick filter to darken any foggy background areas in the image. As can be seen in fig. 9A, the S-curve of plot 918 has a steeper slope in the mid-range of the input signal, providing more contrast in this range, and a shallower slope in the low-end range of the input signal, demonstrating more low signal rejection. 9B, 9C, and 9D, the image 920 has reduced fog 915 and enhanced high-to-high signal organic threats 917 as compared to the original image 910 and the gamma function processed image 916. Referring to FIG. 9A, plot 922 depicts an S-curve for an A value of 1.75 and a C value of 0.1. The S-curve of plot 922 is compatible with the use of relatively thin beam filters and may be applied to images obtained by brightening mid-high signal areas using thin filters, such as, for example, organic threat simulators hidden behind steel.

In various embodiments, due to contrast reduction in the image, a number of contrast enhancement algorithms have been developed for processing images obtained using minimum beam filtering as described above. In various embodiments, a plurality of edge enhancement algorithms are developed for processing images obtained using increased beam filtering as described above. In some embodiments, contrast enhancement is achieved by employing methods such as, but not limited to, histogram stretching or by adaptive contrast using adaptive histogram equalization and/or contrast limited adaptive histogram equalization. In an embodiment, image edges are enhanced by using methods such as, but not limited to, 'Unshirp Masking' or Sobel filters, in combination with noise reduction achieved by using non-local mean filters.

In various embodiments, the backscatter and transmission X-ray images are typically processed by a series of software image enhancement filters prior to display, with the filters having an associated "intensity" or degree to which each filter acts on the image. In some embodiments, the filter is defined by a plurality of parameters that adjust the degree of different factors within the filter. In various embodiments, the selection of filters and the degree to which the selected filters are combined before being applied to an image to process the image is unique for each given system. Fig. 10 is a table illustrating filter chains for different beam filter configurations that may employ a variable filter system according to embodiments of the present description.

Columns

1002, 1004, and 1006 of table 1000 provide the characteristics of image processing obtained by not using (or very thin) filters, medium filters, and thick filters, respectively. As shown in table 1000, for an exemplary 220KV bremsstrahlung spectrum, a 0.8mm thick beryllium or 0.5mm thick copper filter would correspond to no (or very thin) filter, a 1.0mm copper filter to medium filter and a 2.0mm copper filter to thick filter. The absence of a (or very thin) filter provides high SNR and high spatial resolution but provides low contrast for any object placed behind the shade in the processed image, while the use of a thick filter provides high contrast behind a blurred but low SNR for any object placed behind the shade, resulting in haze in low signal areas of the processed image. Not using the filter (column 1002) provides little or no image smoothing, low S-curve signal rejection factor, high non-linear transfer function contrast enhancement, and, if desired, high edge enhancement in the processed image. The use of a median filter (column 1004) provides median image smoothing, S-curve signal suppression factors, and non-linear transfer function contrast enhancement, and if desired, median edge enhancement in the processed image. The use of a thick filter (column 1006) provides strong image smoothing, high S-curve signal rejection factor, low non-linear transfer function contrast enhancement, and moderate edge enhancement in the processed image.

In various embodiments, an X-ray inspection system employing two or more variable contrast filters of the present description includes a corresponding number of default display parameters that are automatically selected based on the selection of the beam filter.

In various embodiments, after processing the scanned image using one or more beam filters, the user may perform image adjustments, such as, but not limited to, "histogram stretching," as is known in the art, which may adjust the brightness and contrast of a Cathode Ray Tube (CRT) television set with the user. This type of image adjustment does not change any physical properties of the image as does the beam filter. User image adjustments result in only brightness and contrast adjustments of the final display parameters of the image, representing only linear scaling of the available image data.

FIG. 11 illustrates image display adjustment of a scanned image that has been processed using one or more beam filters according to embodiments of the present description. Image 1102 has been processed using a thin (0.8 mm) Be beam filter. Image adjustments such as, but not limited to, "histogram stretching" are applied to image 1102 to obtain image 1104. The image adjustments applied to image 1102 are depicted in chart 1106. As described above, a thin filter requires a mid-range signal of greater brightness than a thick or medium filter, so the default display window depicted by line 1108, as depicted in graph 1106, is set to allow the brightest pixels to saturate, as depicted by line 1110, in exchange for making the remaining pixels appear brighter, relative to the full-size display depicted by line 1112. Graph 1108 depicts the area between the high and low signal limits of the displayed data, referred to as the display window. As can be seen when comparing image 1102 to image 1104, the features 1114 in image 1104 are saturated and the features 1116 have become brighter.

Image 1120 has been processed by using a thick (2 mm) Cu beam filter. Image adjustments such as, but not limited to, "histogram stretching" are applied to image 1120 to obtain image 1124. Image adjustments applied to image 1120 are depicted in chart 1126. As described above, even after processing with a thick filter, the resulting image may still have unsightly noise or fog in the background, so the default display window depicted by plot 1128 is set to allow low signal data to be "cropped" as depicted in graph 1126, for initial display as depicted by plot 1130. Plot 1128 depicts the area between the high signal limit and the low signal limit of the displayed data and is referred to as the display window. Since clipping low signal data may cause the entire image to appear darker, a small fraction of the brightest pixels may be allowed to saturate, as shown in graph 1126, to increase the overall image brightness such that the average brightness level is approximately similar to that of full-size display 1132 in bright areas. Comparing the image 1120 to the image 1124, it can be seen that the features 1134 in the image 1124 are darkened.

FIG. 12 is a flow diagram illustrating a method of controlling contrast values of a scanned image obtained from a backscatter X-ray inspection system in accordance with an embodiment of the disclosure. The system includes a plurality of increased thickness beam filters coupled to the X-ray source for filtering the X-ray beam generated by the X-ray source before the X-ray beam strikes the subject. At step 1202, a beam filter for filtering the X-ray beam is selected based on a default contrast setting or based on a desired contrast level input by an operator. The contrast selection is based on the desired contrast value of the scanned image, and it will be appreciated that the increase in contrast value is proportional to the increase in filter thickness.

At step 1204, the default settings of the system are reconfigured to correspond to the selected filter. Reconfiguration of the default settings enables the system to accommodate any changes in the scanned image that occur corresponding to changes in the beam filter. At step 1206, one or more non-linear transfer functions of an image processing module coupled to the inspection system are optimized to obtain a scanned image having a desired contrast value. In an embodiment, the acquired scan image is subjected to at least one of a contrast enhancement algorithm or an edge enhancement algorithm based on a desired contrast value and a desired signal-to-noise ratio value of the image.

In different embodiments, information from two images of the same target, each obtained by using a different thickness beam filter, may be combined to obtain an image with improved detection quality. FIG. 13 is a flow diagram illustrating a method of combining scanned images to obtain an image with improved detection quality in accordance with an embodiment of the present description. At step 1302, a first high resolution scan image of the subject is obtained. At step 1304, a second high contrast scan image of the same object is obtained. In an embodiment, the first high resolution scan image is obtained by using an inspection system in which the impinging X-ray beam is filtered using a thin/minimal filter; the second high contrast image scan image is obtained by an inspection system that filters the impinging X-ray beam using a thick filter. In various embodiments, the edge located in the first high resolution image serves as a guide to enhance the edge located in the second high contrast image. In step 1306, a region in the second image that contains the edge is determined by using the first image as a guide. At step 1308, an edge enhancement routine is applied to regions known to contain edges in the second image while leaving other regions untouched, thereby preventing the edge enhancement algorithm from enhancing noise in regions in the image that do not have edges. Once located, the edge is used as a guide for edge preservation in the second image, while applying a smoothing algorithm to mitigate noise of the second high contrast image, step 1310. In an embodiment, a potential threat located in the second high contrast image may be used as a guide to apply a graphical bounding box or other indicator to the first high resolution/high SNR image in order to guide the operator in analyzing the threat zone.

The above-described examples are merely illustrative of many applications of the systems and methods of the present disclosure. Although only a few embodiments of the present specification have been described herein, it should be understood that the present specification may be embodied in many other specific forms without departing from the spirit or scope of the specification. Accordingly, the present examples and embodiments are to be considered as illustrative and not restrictive, and the description may be modified within the scope of the appended claims.

Claims

1. An X-ray inspection system for scanning an object, the system comprising:

an X-ray source configured to generate an X-ray beam for irradiating an object, wherein the X-ray beam irradiating the object defines a field of view, and wherein the X-ray source is coupled with at least a first beam filter having a first thickness and a second beam filter having a second thickness greater than the first thickness;

a detector array adapted to receive radiation from the X-ray beam, the radiation transmitted through or scattered from the object, and to generate data representing at least one image;

a processing unit configured to receive the data representing at least one image and to generate the at least one image for display based on the data representing at least one image;

a user interface configured to receive user input indicating a desired level of contrast in the at least one image;

a controller configured to adjust a position of at least one of the first and second beam filters based on a user input indicative of a desired level of contrast in the at least one image.

2. The X-ray inspection system of claim 1, wherein the desired contrast level comprises at least one of a first contrast level, a second contrast level, a third contrast level, and a fourth contrast level, and wherein the first contrast level is less than the second contrast level, the second contrast level is less than the third contrast level, and the third contrast level is less than the fourth contrast level.

3. An X-ray inspection system according to claim 2, wherein, on receipt of user input of the first contrast level by the user interface, the controller is configured to cause the first and second beam filters to be out of view of the X-ray source.

4. An X-ray inspection system according to claim 2, wherein, on receipt of user input of the second contrast level by the user interface, the controller is configured to cause the first beam filter to be in the field of view of the X-ray source and the second beam filter to be out of the field of view of the X-ray source.

5. An X-ray inspection system according to claim 2, wherein, on receipt of user input of the third contrast level by the user interface, the controller is configured to cause the first beam filter to be out of view of the X-ray source and the second beam filter to be located in view of the X-ray source.

6. The X-ray inspection system of claim 2, wherein, upon the user interface receiving user input of the fourth contrast level, the controller is configured to cause the first beam filter to be in a field of view of the X-ray source and the second beam filter to be in the field of view of the X-ray source.

7. The X-ray inspection system of claim 1 wherein said first and second beam filters comprise a metallic material having a high atomic number.

8. The X-ray inspection system of claim 1, wherein the first and second beam filters comprise at least one of bronze, tin, tungsten, pure copper, and a copper matrix embedded with tungsten particles.

9. The X-ray inspection system of claim 1, wherein the first and second beam filters comprise a first layer made of tungsten or lead and a second layer made of steel or pure copper configured to absorb fluorescence emitted by the first layer.

10. The X-ray inspection system of claim 1, further comprising a shield coupled with the first and second beam filters, the shield configured to reduce radiation leakage.

11. The X-ray inspection system of claim 1, further comprising a pencil beam forming aperture placed in front of the X-ray source, wherein the first beam filter is located between the X-ray source and the pencil beam forming aperture, and wherein image contrast is increased by: increasing the distance between the pencil beam forming aperture and the first beam filter and decreasing the distance between the first beam filter and the X-ray source.

12. The X-ray inspection system of claim 1, further comprising a third beam filter.

13. The X-ray inspection system of claim 12, wherein the first, second, and third beam filters comprise 0.5mm thick pure copper material, 1.0mm thick pure copper material, and 2.0mm thick pure copper material, respectively.

14. The X-ray inspection system of claim 1, wherein the processing unit is further configured to modify one or more non-linear transfer functions adapted to process the data representing at least one image based on a desired contrast level.

15. The X-ray inspection system of claim 14, wherein the non-linear transfer function comprises at least one of a gamma function and an S-curve function.

16. The X-ray inspection system of claim 2, wherein the processing unit is further configured to implement at least one of a first set of program instructions and a second set of program instructions based on the desired contrast level.

17. The X-ray inspection system of claim 16, wherein the processing unit is further configured to implement the first set of program instructions based on at least one of the first contrast level and the second contrast level, and wherein the first set of program instructions comprises one or more contrast enhancement functions.

18. The X-ray inspection system of claim 16, wherein the processing unit is further configured to implement the second set of program instructions based on at least one of the third contrast level and the fourth contrast level, and wherein the second set of program instructions comprises one or more edge enhancement functions.