Abstract:
An X-ray CT scanner for imaging an object is provided. The scanner includes an X-ray emitter configured to emit X-ray beams, a detector array including a plurality of detector elements, and a precollimator positioned between the X-ray emitter and the object, the precollimator configured to prevent the emitted X-ray beams from being directly incident on a first subset of the plurality of detector elements, and allow the emitted X-ray beams to be directly incident on a second subset of said plurality of detector elements. A processing device communicatively coupled to the detector array is configured to determine a signature of the object based on a first set of data acquired using the first subset of the plurality of detector elements, and tomographically reconstruct an image of the object based on a second set of data acquired using the second subset of said plurality of detector elements.

Description:
BACKGROUND 
       [0001]    The embodiments described herein relate generally to imaging objects, and more particularly, to imaging systems for reconstructing an image of an object and determining a signature of the object. 
         [0002]    In some computed tomography (CT) imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an “imaging plane”. The x-ray beam passes through an object being imaged. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated radiation beam received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam intensity at each detector location. The intensity measurements from all the detectors are acquired separately to produce a transmission profile. 
         [0003]    In third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged such that the angle at which the x-ray fan beam intersects the object constantly changes. A group of x-ray attenuation measurements (e.g., projection data) from the detector array at one gantry angle may be referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one or more revolutions of the x-ray source and detector about the object or patient being imaged. 
         [0004]    Many modern CT systems are helical scanners (also known as spiral scanners), in which the scanned object is continually moved while the projection data is being acquired. The path of the x-ray source describes a helix with respect to the scanned object. Most helical scanners have multiple rows of detectors, and the x-ray fan is collimated into a cone to illuminate the entire array of detectors. The angle between the x-ray source and the first and last detector rows is referred to as the “cone angle”. 
         [0005]    At least some known CT systems are able to reconstruct an image of the scanned object, but are unable to determine a composition of the object. Accordingly, although the shape and dimensions of the object may be ascertainable using at least some known CT systems, the composition may be indeterminable. Determining the composition of a given object, in addition to the shape and dimensions of the object, would assist in determining whether or not an object is contraband. 
       BRIEF SUMMARY 
       [0006]    In one aspect, an X-ray CT scanner for imaging an object is provided. The X-ray CT scanner includes an X-ray emitter configured to emit X-ray beams towards the object, a detector array positioned opposite the X-ray emitter and including a plurality of detector elements, and a precollimator positioned between the X-ray emitter and the object, the precollimator configured to prevent, using at least one blocking portion, the emitted X-ray beams from being directly incident on a first subset of the plurality of detector elements, and allow the emitted X-ray beams to be directly incident on a second subset of said plurality of detector elements. The X-ray CT scanner further includes a processing device communicatively coupled to the detector array, the processing device configured to determine a signature of the object based on a first set of data acquired using the first subset of the plurality of detector elements, and tomographically reconstruct an image of the object based on a second set of data acquired using the second subset of said plurality of detector elements. 
         [0007]    In another aspect, a method for imaging an object is provided. The method includes positioning the object between an X-ray emitter and a detector array that includes a plurality of detector elements, emitting X-ray beams towards the object from the X-ray emitter, preventing, using a precollimator including at least one blocking portion, the emitted X-ray beams from being directly incident on a first subset of the plurality of detector elements, allowing the emitted X-ray beams to be directly incident on a second subset of the plurality of detector elements, determining, using a processing device communicatively coupled to the detector array, a signature of the object based on a first set of data acquired using the first subset of the plurality of detector elements, and tomographically reconstructing, using the processing device, an image of the object based on a second set of data acquired using the second subset of the plurality of detector elements. 
         [0008]    In yet another aspect, a method of assembling an X-ray CT scanner for imaging an object is provided. The method includes positioning an X-ray emitter opposite a detector array that includes a plurality of detector elements, the X-ray emitter configured to emit X-ray beams towards the object, and positioning a precollimator between the X-ray emitter and the object, the precollimator including at least one blocking portion configured to prevent the emitted X-ray beams from being directly incident on a first subset of the plurality of detector elements, and allow the emitted X-ray beams to be directly incident on a second subset of the plurality of detector elements. The method further includes communicatively coupling a processing device to the detector array, the processing device configured to determine a signature of the object based on a first set of data acquired using the first subset of the plurality of detector elements, and tomographically reconstruct an image of the object based on a second set of data acquired using the second subset of the plurality of detector elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a perspective view of an exemplary CT imaging system. 
           [0010]      FIG. 2  is a schematic diagram of the CT imaging system shown in  FIG. 1 . 
           [0011]      FIG. 3  is a schematic diagram of a detector array that may be used with the CT imaging system shown in  FIG. 1 . 
           [0012]      FIG. 4  is a schematic diagram of the geometry of the CT imaging system shown in  FIG. 1 . 
           [0013]      FIG. 5  is a schematic diagram of the geometry of the CT imaging system shown in  FIG. 1 . 
           [0014]      FIG. 6  is a perspective view of a portion of an exemplary gantry that may be used with the CT imaging system shown in  FIG. 1 . 
           [0015]      FIG. 7  is a perspective view of an exemplary precollimator that may be used with the CT imaging system shown in  FIG. 1 . 
           [0016]      FIG. 8  is a perspective view of an exemplary precollimator that may be used with the CT imaging system shown in  FIG. 1 . 
           [0017]      FIG. 9  is a diagram demonstrating how often each detector element in a detector array is used for reconstructing a the CT volume in a helical scan. 
           [0018]      FIG. 10  is a schematic plan view of an exemplary precollimator that may be used with the CT imaging system shown in  FIG. 1 . 
           [0019]      FIG. 11  is a diagram demonstrating how often each detector element in a detector array is used for reconstructing the CT volume in a helical scan using a rebinning algorithm. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The embodiments described herein provide a CT imaging system that is capable of reconstructing an image of an object and determining a signature, or composition, of the object. By determining a signature in addition to imaging the object, contraband detection may be improved and false alarms may be reduced. As used herein, the “signature” may be a molecular signature (e.g., if the object is composed of a single type of molecule) or may be a more general signature that represents a mix of the molecular signatures corresponding to different types of molecules that constitute the object. 
         [0021]    As used herein, raw data refers to the actual data value read from a detector. The raw data depends on the x-ray intensity at the detector, the gain of the detector, and any bias (offset) that is added to the detector value. Furthermore and as also used herein, offset data, gain data, sample data, x-ray intensity, normalized data, converted data, x-ray source position, reconstruction circle, and reconstruction volume are as defined as follows: 
         [0022]    Offset data: Raw data measurements collected with the x-ray source off. 
         [0023]    Gain data: Raw data measurements collected with the x-ray source on, but with no sample objects in the field of view other than permanently installed objects such as the conveyor belt. 
         [0024]    Sample data: Raw data measurements collected with the x-ray source on and a sample object in the field of view. 
         [0025]    X-ray intensity: The intensity of the x-ray at each detector. X-ray intensity can be computed as K1*(Sample-Offset)/(Gain−Offset), where K1 is a calibration constant. 
         [0026]    Normalized data: A measure of the attenuation of an x-ray beam as it travels through an object. Normalized data can be computed as K2*log((Gain−Offset)/(Sample−Offset), where K2 is a calibration constant and log( ) is the natural logarithm. 
         [0027]    Converted data: Any useful representation of the scan data that may be used for a projection image. In the exemplary embodiment, converted data represents normalized data, but other representations (e.g., sample data, x-ray intensity) may be used. 
         [0028]    X-ray source position: The gantry may make several complete rotations during acquisition, creating a spiral trajectory of the x-ray source when viewed with respect to the moving scanned object. X-ray source position in this discussion refers to a single point in the spiral trajectory. 
         [0029]    Reconstruction Circle: A circle defined by the x-ray fan as the x-ray tube rotates around an object. For accurate CT reconstruction, an object must be entirely within the reconstruction circle. 
         [0030]    Reconstruction Volume: A cylinder in the scanned object&#39;s coordinates defined by the reconstruction circle and the length of the bag for which there is sufficient data to reconstruct. 
         [0031]    Referring now to  FIGS. 1 and 2 , a computed tomography (CT) imaging system  10  is shown. CT imaging system  10  is shown having a gantry  12 , which is representative of a CT scanner, a control system  14 , and a motorized conveyor belt  16  for positioning an object  18 , such as a piece of luggage, in a gantry opening  20  defined through gantry  12 . CT imaging system  10  may be, for example, a dual energy CT system. Gantry  12  includes an x-ray source  22  that projects a fan beam of x-rays  24  toward a detector array  26  on the opposite side of gantry  12 . Detector array  26  is formed by detector elements  28 , which are shown in more detail in  FIG. 3  and discussed below. Detector elements  28  are radiation detectors that each produce a signal having a magnitude that represents and is dependent on the intensity of the attenuated x-ray beam after it has passed through object  18  being imaged. During a helical scan that acquires x-ray projection data, gantry  12  along with the x-ray source  22  and detector array  26  rotate within a plane and around object  18  about a center of rotation, while object  18  is moved through gantry  12  in a z-direction  32  perpendicular to the plane of rotation. In the exemplary embodiment, detector array  26  includes a plurality of detector rings each having a plurality of detector elements  28 , the detector rings having an angular configuration corresponding to x-ray source  22 . 
         [0032]    Gantry  12  and x-ray source  22  are controlled by control system  14 , which includes a gantry controller  36 , an x-ray controller  38 , a data acquisition system (DAS)  40 , an image reconstructor  42 , a conveyor controller  44 , a computer  46 , a mass storage system  48 , an operator console  50 , and a display device  52 . Gantry controller  36  controls the rotational speed and position of gantry  12 , while x-ray controller  38  provides power and timing signals to x-ray source  22 , and data acquisition system  40  acquires analog data from detector elements  28  and converts the data to digital form for subsequent processing. Image reconstructor  42  receives the digitized x-ray data from data acquisition system  40  and performs an image reconstruction process that involves filtering the projection data using a helical reconstruction algorithm. 
         [0033]    Computer  46  is in communication with the gantry controller  36 , x-ray controller  38 , and conveyor controller  44  whereby control signals are sent from computer  46  to controllers  36 ,  38 ,  44  and information is received from controllers  36 ,  38 ,  44  by computer  46 . Computer  46  also provides commands and operational parameters to data acquisition system  40  and receives reconstructed image data from image reconstructor  42 . The reconstructed image data is stored by computer  46  in mass storage system  48  for subsequent retrieval. An operator interfaces with computer  46  through operator console  50 , which may include, for example, a keyboard and a graphical pointing device, and receives output, such as, for example, a reconstructed image, control settings and other information, on display device  52 . 
         [0034]    Communication between the various system elements of  FIG. 2  is depicted by arrowhead lines, which illustrate a means for either signal communication or mechanical operation, depending on the system element involved. Communication amongst and between the various system elements may be obtained through a hardwired or a wireless arrangement. Computer  46  may be a standalone computer or a network computer and may include instructions in a variety of computer languages for use on a variety of computer platforms and under a variety of operating systems. Other examples of computer  46  include a system having a microprocessor, microcontroller or other equivalent processing device capable of executing commands of computer readable data or program for executing a control algorithm. In order to perform the prescribed functions and desired processing, as well as the computations therefore (e.g., the execution of Fourier analysis algorithm(s), the control processes prescribed herein, and the like), computer  46  may include, but not be limited to, a processor(s), memory, storage, register(s), timing, interrupt(s), communication interfaces, and input/output signal interfaces, as well as combinations including at least one of the foregoing. For example, computer  46  may include input signal filtering to enable accurate sampling and conversion or acquisitions of such signals from communications interfaces. As described above, exemplary embodiments can be implemented through computer-implemented processes and apparatuses for practicing those processes. 
         [0035]    Referring now to  FIG. 3 , an illustration of an x-ray beam having a beam axis (iso-ray)  60  that originates at x-ray source  22  and passes through center of rotation (iso-center)  62 , relative to two-dimensional detector array  26 , having detector elements  28  arranged in rows N and columns M, is provided. While  FIG. 3  depicts only four rows (N=4 for four rings) and six columns (M=6 for six detectors per ring), it will be appreciated that any number of rows and columns may be employed as a matter of design choice. As depicted in  FIG. 3 , a detector angle γ  64  is shown as an angle formed between an x-ray intersecting a given detector element  28  and iso-ray  60  which connects x-ray source  22  and the iso-center  62 , and a projection angle β  68  is shown as an angle formed by iso-ray  60  with the y-axis. 
         [0036]    Referring now to  FIGS. 4 and 5 , and in accordance with the exemplary embodiment, the data acquired at a single x-ray source position (also referred to herein as a view or tube position) is a set of fan beams  24  corresponding to a fan angle  25 , with each x-ray beam at a slight angle to its neighbor. Also illustrated in  FIGS. 4 and 5  is the bag or object volume  70 , conveyor belt  16 , a reconstruction circle  72 , and a cone angle  74 . 
         [0037]    The systems and methods described herein facilitate determining a signature of object  18 , in addition to reconstructing an image of object  18 . This is achieved by blocking at least a portion of x-ray beam  24  such that a subset of detector elements  28  do not receive any direct incident radiation. In the exemplary embodiment, this is accomplished using a precollimator, as described herein. The detectors in the subset detect x-rays that have been scattered by object  18 , and accordingly, are referred to herein as scatter detectors. 
         [0038]      FIG. 6  is a perspective view of a portion of gantry  12  (shown in  FIG. 1 ). As show in  FIG. 6 , gantry  12  includes a precollimator  102  positioned between x-ray source  22  and detector array  26 . Precollimator  102  blocks at least a portion of x-rays  24  emitted from x-ray source  22 , as described herein. 
         [0039]      FIG. 7  is a perspective view of an exemplary precollimator  702 . Precollimator  702  includes a plurality of apertures  704  that facilitate mounting precollimator  702  to gantry  12  using fastening devices (e.g., bolts). As shown in  FIG. 7 , precollimator  702  includes an aperture  704  defined therethrough. Aperture  704  enables substantially all x-rays  24  emitted from x-ray source  22  to pass therethrough. Accordingly, because precollimator  702  does not block any x-rays  24 , detector array  26  does not include any scatter detectors when using precollimator  702 . Aperture  704  is narrower at a midpoint  706  than at ends  708 . Accordingly, aperture may have a bowed rectangular shape. 
         [0040]    In contrast,  FIG. 8  is a perspective view of an exemplary precollimator  802  that may be used with gantry  12 . As shown in  FIG. 8 , an aperture  804  of precollimator  802  is substantially different than aperture  704  of precollimator  702 . Specifically, aperture  804  includes a plurality of blocking portions  806  arranged in an alternating pattern to block sections of aperture  804 . Accordingly, detector elements  28  in detector array  26  that are aligned with blocking portions  806  will not receive direct incident radiation and, accordingly, are scatter detectors. In the exemplary embodiment, blocking portions  806  are fabricated from tungsten. Alternatively, blocking portions  806  may be fabricated from any material that enables precollimator  802  to function as described herein. 
         [0041]    The pattern of blocking portions  806  is merely an example. That is, blocking portions  806  may have any orientation and/or configuration that enables gantry  12  to function as described herein. In some embodiments, blocking portions  806  are arranged such that detector elements  28  in detector array  26  that would otherwise receive relatively little incident radiation are used as the scatter detectors. 
         [0042]    For example,  FIG. 9  is a diagram  900  demonstrating how often each detector element  28  in detector array  26  is used for reconstructing a given CT volume in a helical scan without any blocking portions  806  (i.e., with all detector elements  28  receiving direct radiation). As shown in  FIG. 9 , detector elements  28  in a lower left corner of diagram  900  and detector elements  28  in an upper right corner of diagram  900  are used the least. 
         [0043]      FIG. 10  is a schematic plan view of an exemplary precollimator  1002  designed based on diagram  900 . Precollimator  1002  includes two blocking portions  1004  that block portions of aperture  1006 . One blocking portion  1004  extends from a first corner  1008  of aperture  1006 , and the other blocking portion  1004  extends from a second, opposite, corner  1010  of aperture  1006 . In the exemplary embodiment shown in  FIG. 10 , each blocking portion  1004  has a length, L, extending from an associated corner  1008  and  1010  to a midpoint of aperture  1006 . Further, each blocking portion has a width, W. In the exemplary embodiment, width W is a distance of approximately three to four detector rows. Alternatively, blocking portions  1004  may have any dimensions that enable precollimator  1002  to function as described herein. 
         [0044]    In the systems and methods described herein, data acquired at detector elements  28  that receive direct incident radiation (i.e., unblocked detector elements) is used to tomographically reconstruct an image of object  18 . Further, data acquired at detector elements  28  that do not receive direct incident radiation (i.e., scatter detectors) is used to calculate a signature of object  18 . To avoid using data from the scatter detectors for image reconstruction, a rebinning algorithm is implemented.  FIG. 11  is a diagram  1100  demonstrating how often detector elements  28  are used for reconstructing a given CT volume when using precollimator  1002  and an exemplary rebinning algorithm. Although an image of object  18  is reconstructed using less than all of detector elements  28 , the image quality remains substantially unchanged. 
         [0045]    To facilitate limiting the amount of data acquired by the scatter detectors, in the exemplary embodiment, detector array  26  includes collimator plates that extend parallel to at least one of rows and columns of detector elements  28 . In some embodiments, because the fan angle is typically much larger than the cone angle, the collimator plates extend parallel to the columns of detector elements  28 . This configuration is advantageous, as it rejects a significant amount of unwanted scatter photons. The data acquired by the scatter detectors is processed (e.g., using computer  46  (shown in  FIG. 2 )) to tomographically reconstruct a scatter function for object  18 . The scatter function is a scatter cross section as a function of momentum transfer. In the exemplary embodiment, the scatter function is reconstructed only when object  18  has been identified as a potential threat (e.g., by applying an automated detection algorithm on the reconstructed image of object  18 ). Alternatively, the scatter function may be reconstructed for object  18  regardless of whether object  18  is previously identified as a potential threat. In the exemplary embodiment, for the scatter function reconstruction, it is assumed that object  18  consists of the same material (or material mix) throughout. 
         [0046]    In the exemplary embodiment, to reconstruct the scatter function, a forward matrix is formed by identifying possible scatter events that link a voxel in object  18  to one of the scatter detectors. For each such event, a scatter angle is computed, an incident energy spectrum at the voxel and at the scatter detector is estimated, and the contribution to the measurement at the scatter detector is estimated for each basis component of an unknown scatter function. Collimator configurations, such as those described above, are taken into consideration to limit possible detected scatter paths. 
         [0047]    Once an entire forward matrix is established (focusing on object  18  and any additional interfering objects), each scatter detector measurement is written as an equation as a function of parameters in the unknown scatter function. There will generally be many more equations than unknowns in the scatter function representation, so a minimum least-square solution for the scatter function may be possible. Iterative solutions with various types of regularization may also be implemented. 
         [0048]    In the exemplary embodiment, the scatter function may be represented by a product of a first scatter function that ignores interactions between atoms and a second, molecular scatter function that takes into account interactions between atoms. The first scatter function is a function of atomic number and momentum transfer, and may be chosen based on an estimated effective atomic number calculated, for example, based on the reconstruction image. Accordingly, only the second, molecular scatter function, which extends in a lower range of a momentum transfer scale (e.g., up to 0.25 Å −1 ), remains as an unknown. 
         [0049]    As there will be a plurality of views that transect object  18  as gantry  12  rotates around object  18 , data acquired at views that include relatively little amounts of interference (e.g., from additional objects) may be utilized to form the forward matrix. Further, if certain views contain relatively high amounts of interference, data acquired from those views may be omitted entirely. 
         [0050]    Once the scatter function is reconstructed, a signature of object  18  may be determined based on the scatter function. For example, computer  46  may compare the scatter function with a library of predetermined scatter functions stored in mass storage system  48  to determine the signature (i.e., identifying the material) of object  18 . The determined signature may be displayed to an operator on display device  52 . Further, if the determined signature indicates a contraband material (e.g., an explosive or narcotic material), computer  46  may generate an alarm to alert the operator. As used herein, the “signature” may be a molecular signature (e.g., if object  18  is composed of a single type of molecule) or may be a more general signature that represents a mix of the molecular signatures corresponding to different types of molecules that constitute object  18 . 
         [0051]    Accordingly, the systems and methods described herein provide a CT imaging system that is capable of reconstructing an image of an object and determining a signature of the object. By determining a signature in addition to imaging the object, contraband detection may be improved and false alarms may be reduced. 
         [0052]    The systems and methods described herein may be used to detect contraband. As used herein, the term “contraband” refers to illegal substances, explosives, narcotics, weapons, special nuclear materials, dirty bombs, nuclear threat materials, a threat object, and/or any other material that a person is not allowed to possess in a restricted area, such as an airport. Contraband may be hidden within a subject (e.g., in a body cavity of a subject) and/or on a subject (e.g., under the clothing of a subject). Contraband may also include objects that can be carried in exempt or licensed quantities intended to be used outside of safe operational practices, such as the construction of dispersive radiation devices. 
         [0053]    A computer, such as those described herein, includes at least one processor or processing unit and a system memory. The computer typically has at least some form of computer readable media. By way of example and not limitation, computer readable media include computer storage media and communication media. Computer storage media include volatile and nonvolatile, removable and nonremovable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules, or other data. Communication media typically embody computer readable instructions, data structures, program modules, or other data in a modulated data signal such as a carrier wave or other transport mechanism and include any information delivery media. Those skilled in the art are familiar with the modulated data signal, which has one or more of its characteristics set or changed in such a manner as to encode information in the signal. Combinations of any of the above are also included within the scope of computer readable media. 
         [0054]    Exemplary embodiments of methods and systems for imaging an object are described above in detail. The methods and systems are not limited to the specific embodiments described herein, but rather, components of systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. Accordingly, the exemplary embodiment can be implemented and utilized in connection with many other applications not specifically described herein. 
         [0055]    Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
         [0056]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.