Abstract:
An imaging system is provided for various non-invasive medical and non-medical imaging. The imaging system includes a tunable X-ray source for emitting X-rays having a substantially monoenergetic spectrum and an energy discriminating detector for generating a detector output signal in response to the X-rays incident on the energy discriminating detector. The imaging system also includes a system controller comprising an X-ray controller for operating the tunable X-ray source and data acquisition circuitry for acquiring the detector output signal from the energy discriminating detector.

Description:
BACKGROUND  
       [0001]     The invention relates generally to X-ray imaging and in particular to X-ray imaging using monoenergetic X-ray sources and energy discriminating detectors.  
         [0002]     X-rays have found widespread application in various non-invasive medical and non-medical imaging techniques. In general, X-ray based imaging systems direct an X-ray beam toward an object to be imaged. The X-ray beam may be generated by an X-ray tube or by other techniques. In conventional X-ray imaging systems, the generated X-rays typically have a broad spectrum that may be representative of the technique and/or materials used to generate the X-rays. The generated X-rays typically pass through an imaging volume containing an object or patient. As the X-rays pass through the object or patient, the different materials of which the object or patient are composed attenuate the X-rays to varying degrees. For example, bone, metal, water, air, and soft tissue attenuate the X-rays differently. As attenuated X-rays leave the imaging volume they typically strike a detector where they generate electrical signals that are processed to generate an image of the internal structures of the object or patient.  
         [0003]     The X-rays produced in common X-ray tubes are generally of relatively low power, and comprise long pulses or a continuous wave that pose limitations in their use. Moreover, such radiation typically comprises unpolarized, incoherent radiation having a broad energy spectrum. In general, the X-rays generated by conventional techniques may be useful for imaging techniques where the attenuation is measured to produce images, but they are less useful in techniques where energy-dependent information of the materials under inspection are also of interest.  
         [0004]     For example, X-ray attenuation through a given object is not constant and is strongly dependent on the X-ray photon energy. This phenomenon manifests itself in an image as a beam-hardening artifact, such as non-uniformity, shading and streaks. Some beam-hardening artifacts can be easily corrected by techniques such as water calibration and iterative bone correction. However, beam hardening from materials other than water and bone, such as metals and contrast agents, are difficult to correct. In addition, the same materials at different locations often show different levels of attenuation. Another limitation of conventional imaging system is lack of material characterization. For example, a highly attenuating material with a low density may result in the same degree of attenuation in the image as a less attenuating material with a high density. Thus, there is little or no information about the material composition of a scanned object based solely on the degree of attenuation. In addition, visibility of certain contrast agents in the human body may be enhanced by imaging the body with properly selected portions of the X-ray spectrum.  
         [0005]     Traditional techniques for producing monoenergetic X-ray beams such as fluorescent sources and Bragg angle scattered X-rays for energy selection are employed for various medical applications to overcome the above mentioned limitation. Filtration of a broadband bremsstrahlung radiation can also produce spectra of desired monochromaticity. For example, in mammography, rhodium-coated targets coupled with thin rhodium filtration produces relatively narrow portions of X-ray spectrum centered around the energy of interest. However, in certain cases a significant portion of the X-rays have energies too low to penetrate far into the human body, thereby failing to contribute to an image of the region of interest. In short, a wide X-ray photon energy spectrum from the X-ray source and a lack of energy resolution from the X-ray detectors limit the use of imaging systems for applications such as material characterization, tissue differentiation, scatter rejection and others.  
         [0006]     It is therefore desirable to provide an efficient imaging system having monoenergetic X-ray source and energy discriminating detectors to achieve better image contrast and high resolution while minimizing the image noise and radiation doses to the patient.  
       BRIEF DESCRIPTION  
       [0007]     Briefly in accordance with one aspect of the technique, an imaging system is provided. The imaging system includes a tunable X-ray source configured to emit X-rays having a substantially monoenergetic spectrum and an energy discriminating detector configured to generate a detector output signal in response to the X-rays incident on the energy discriminating detector. The imaging system also includes a system controller comprising an X-ray controller configured to operate the tunable X-ray source and data acquisition circuitry configured to acquire the detector output signal from the energy discriminating detector.  
         [0008]     In accordance with another aspect of the technique, an imaging system is provided. The imaging system includes an X-ray source configured to emit X-rays having a substantially monoenergetic spectrum and an energy discriminating detector configured to generate a detector output signal in response to the X-rays incident on the energy discriminating detector. The imaging system also includes a system controller comprising an X-ray controller configured to operate the X-ray source and data acquisition circuitry configured to acquire the detector output signal from the energy discriminating detector. In addition, the imaging system includes image reconstruction circuitry configured to generate at least one composition image based on the detector output signal.  
         [0009]     In accordance with a further aspect of the present technique, a method is provided for generating a composition image. The method provides for selecting a desired monoenergetic X-ray spectrum for imaging an object of interest, emitting X-rays generally at the desired monoenergetic X-ray spectrum through the object of interest, detecting the X-rays attenuated by the object of interest via an energy discriminating detector, generating a detector output signal in response to the X-rays detected by the energy discriminating detector and generating at least one composition image based on the detector output signal. Systems and computer programs that afford functionality of the type defined by this method may be provided by the present technique. 
     
    
     DRAWINGS  
       [0010]     These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:  
         [0011]      FIG. 1  depicts an exemplary imaging system using a monoenergetic X-ray source and energy sensitive detectors in accordance with one aspect of the present technique;  
         [0012]      FIG. 2  depicts an exemplary CT imaging system for volumetric imaging using a monoenergetic X-ray source and energy sensitive detectors in accordance with one aspect of the present technique; and  
         [0013]      FIG. 3  is a flowchart illustrating method of generating composition image in accordance with one aspect of the present technique. 
     
    
     DETAILED DESCRIPTION  
       [0014]     The present techniques are generally directed to X-ray imaging using monoenergetic X-rays and energy discriminating detectors. Such imaging techniques may be useful in a variety of imaging contexts, such as CT imaging, industrial inspection systems, CT metrology, X-ray radiography, nondestructive testing, heavy metals analysis, security and baggage screening, and others. Though the present discussion provides examples in a medical imaging context, one of ordinary skill in the art will readily apprehend that the application of these techniques in other contexts, such as for industrial imaging, security screening, and/or baggage or package inspection, is well within the scope of the present techniques.  
         [0015]     Referring now to  FIG. 1 , an imaging system  10  for use in accordance with the present technique is illustrated. In the illustrated embodiment, the imaging system  10  includes a radiation source  12 , such as an X-ray source. A collimator may be positioned adjacent to the radiation source  12  for regulating the size and shape of a stream of radiation  14  that emerges from the radiation source  12 . The imaging system  10 , as well as other imaging systems based on X-ray attenuation, may employ X-ray sources that generate X-rays by a variety of techniques. For example, the present technique employs a tunable X-ray source that may be configured to emit monoenergetic or nearly monoenergetic X-rays at one or more energy levels.  
         [0016]     A variety of techniques may be employed to generate the monoenergetic or nearly monoenergetic X-rays at a desired spectrum. Such techniques include but are not limited to inverse Compton scattering processes, plasma based X-ray emission, and filtration of a broadband bremsstrahlung radiation. In one embodiment, the monoenergetic X-rays are generated at a desired spectrum by an X-ray tube having a liquid metal target. A liquid metal or liquid metal suspension carrier flows through a conduit to form the target for an 80 keV to 200 keV electron beam. The electron beam creates X-rays by impact with a thin cross-section of the flowing target. Different target materials result in different spectra of X-rays. The selection of a desired characteristic X-ray spectrum and suppression of broadband bremsstrahlung radiation by filtering and proper choice of exit angle makes the source nearly monoenergetic. In one embodiment, one or more solid particles of various metals, crystals, and/or other solid materials may be suspended in the liquid carrier. Suspension of target particles in the liquid carrier allows choice of targets and spectra as well as efficient heat dissipation for relatively high average power operation. Further, the target particles enable selection of wavelength for a monochromatic or quasi-monochromatic source.  
         [0017]     In typical operation, the radiation source  12  projects a stream of radiation  14 , such as a monoenergetic X-ray beam, towards a detector array  16  placed on the opposite side of the radiation source  12 . The stream of radiation  14  passes into an imaging volume in which an object  18  to be imaged may be positioned. It should be noted that a particular region of the object  18  may be chosen by an operator for imaging so that the most useful scan of the region may be acquired.  
         [0018]     A portion of the radiation  20  passes through or around the object and impacts the detector array  16 . The detector array  16  may be a single slice detector or a multi-slice detector and is generally formed as an array of detection elements. Each detector element produces an electrical signal that represents the intensity of the incident radiation  20  at the detector element when the radiation  20  strikes the detector array  16 . These signals are acquired and processed to reconstruct an image of the features internal as well external to the object  18 .  
         [0019]     In one implementation, the detector array may be an energy discriminating detector designed to distinguish between different portions of X-ray spectra or different X-ray energy levels. There are different methods to obtain multi-energy measurements using energy sensitive detectors. For example, in one implementation energy sensitive detectors may be employed such that each X-ray incident on the detector is recorded with its energy.  
         [0020]     It should be noted that a wide variety of energy discriminating detectors may be used to detect and resolve the attenuated X-rays of different energy levels. Such energy discriminating detectors include, but are not limited to, charge integrating detectors, photon counting detectors and other energy sensitive detectors. Further, these detectors may directly convert the X-rays to electrical signals for processing. Alternatively, these detectors may use a scintillating material to convert X-rays to optical radiation that may be detected and converted to electrical signals for processing. Also, a wide variety of energy sensitive detectors such as semiconductor detectors and arrays, high density noble gas detectors, phosphors, scintillators, thin film transistor arrays, charge coupled devices, microchannel plates and calorimetric detectors may be employed for energy discrimination.  
         [0021]     Referring back to  FIG. 1 , the object  18  and the radiation source  12  are typically displaced relative to each other, allowing projection data to be acquired at various views relative to the object  18  if desired. For example, the object  18  may be positioned on a table, such as a turntable, so that the object  18  may be rotated during the examination process to expose all sides of the object  18  to the stream of radiation  14 . Alternatively, the radiation source  12  and/or the detector array  16  may be disposed on a gantry, which may be rotated around the object  18  during the examination process. As the object  18  and the radiation source  12  rotate relative to each other, the detector array  16  collects data of radiation attenuation at the various view angles relative to the object  18 . Data collected from the detector array  16  then undergoes pre-processing and calibration to condition the data to represent the line integrals of the attenuation coefficients of the scanned objects  18 . The processed data, commonly called projections, are then reconstructed to formulate one or more composition images of the scanned area, as discussed in greater detail below. Thus, an image or slice is acquired which may incorporate, in certain modes, less or more than 360 degrees of projection data, to formulate an image.  
         [0022]     Operation of the source  12  is controlled by a system controller  22 , which furnishes both power, and control signals for examination sequences. Moreover, the detector array  16  is coupled to the system controller  22 , which commands acquisition of the signals generated in the detector array  16 . The system controller  22  may also execute various signal processing and filtration functions, such as for initial adjustment of dynamic ranges, interleaving of digital image data, and so forth. In general, system controller  22  commands operation of the imaging system  10  to execute examination protocols and to process acquired data. In the present context, system controller  22  may also include signal processing circuitry and other circuitry, typically based upon a general purpose or application-specific digital computer, associated memory circuitry for storing programs and routines executed by the computer, as well as configuration parameters and image data, interface circuits, and so forth. Indeed, the system controller  22  may be implemented as hardware and software components of the depicted computer  36 .  
         [0023]     In the embodiment illustrated in  FIG. 1 , the system controller  22  is coupled to a linear positioning subsystem  24  and a rotational subsystem  26 . In particular, the system controller  22  may include a motor controller  28  that controls the operation of the linear positioning subsystem  24  and the rotational subsystem  26 . The rotational subsystem  26  enables the X-ray source assembly and/or the detector assembly to be rotated one or multiple turns around the object  18 . It should be noted that the rotational subsystem  26  might include a gantry. Thus, the system controller  22  may be utilized to control the rotational speed and position of the gantry. Alternatively, the rotational subsystem  26  may include a motorized turntable and the system controller  22  may be configured to rotate the motorized turntable, thereby rotating the object  18  one or multiple turns during an examination. The linear positioning subsystem  24  enables the object  18  to be displaced linearly, such as by moving a table or support on which the object  18  rests. Thus, in one embodiment, the table may be linearly moved within a gantry to generate images of particular areas of the object  18 .  
         [0024]     Additionally, as will be appreciated by those skilled in the art, the radiation source  12  may be controlled by a radiation controller  30  disposed within the system controller  22 . Particularly, the radiation controller  30  may be configured to provide power and timing signals to the radiation source  12 . In one embodiment, the monoenergetic spectrum of X-ray emission is user selectable and the X-ray source may be tuned via the radiation controller  30  to emit X-rays at or near the selected spectrum, thereby making the X-ray source tunable.  
         [0025]     Further, the system controller  22  may include data acquisition circuitry  32 . In this exemplary embodiment, the detector array  16  is coupled to the system controller  22 , and more particularly to the data acquisition circuitry  32 . The data acquisition circuitry  32  typically receives sampled analog signals, representative of the location and energy of the incident monoenergetic X-rays, from the detector array  16  and converts the data to digital signals for subsequent processing. An image reconstructor  34 , that is coupled to or is a part of a computer  36 , may receive sampled and digitized data from the data acquisition circuitry  32  and may perform high-speed image reconstruction to generate one or more composition image of the scanned object  18 . Alternatively, reconstruction of the image may be done by general or special purpose circuitry of the computer  36 . Once reconstructed, the image produced by the imaging system  10  reveals internal as well as external features of the object  18 .  
         [0026]     The computer  36  may include or be in communication with a memory  38 . It should be understood that any type of memory to store a large amount of data may be utilized by such an exemplary imaging system  10 . In addition, the computer  36  may be configured to receive commands and scanning parameters from an operator via an operator workstation  40 . For example, the operator workstation  40  may be equipped with a keyboard and/or other input devices by which an operator may control the imaging system  10 . Thus, the operator may observe the reconstructed image and other data relevant to the system from computer  36 , initiate imaging, select a spectrum for imaging and so forth. It should be noted that the spectrum is selected based upon the type of imaging requirement such as soft tissue imaging, bone imaging, contrast imaging, radiography of a particular metal and/or other imaging requirements.  
         [0027]     A display  42  may be coupled to one of the operator workstation  40  and the computer  36  and may be utilized to observe the one or more composition image and/or to control imaging. Additionally, the scanned image may also be printed by a printer  44  which may be coupled to the computer  36  and/or the operator workstation  40 , either directly or over a network. It should be further noted that the computer  36  and/or operator workstation  40  may be coupled to other output devices that may include standard or special purpose computer monitors and associated processing circuitry. Furthermore, additional operator workstations may be further linked in the imaging system  10  for outputting system parameters, requesting inspection, viewing images, selecting an X-ray spectrum for imaging and so forth, so that more than one operator may perform operations related to the imaging system  10 . For example, one operator may utilize one operator workstation to image acquisition while a second operator utilizes a second operator workstation to reconstruct and/or review the results of the imaging routines. In general, displays, printers, workstations, and similar devices supplied within the imaging system  10  may be local to the data acquisition components, or may be remote from these components linked to the imaging system  10  via one or more configurable networks, such as the Internet, virtual private networks, and so forth.  
         [0028]     Referring generally to  FIG. 2 , an exemplary medical imaging system utilized in a present embodiment may be a computed tomography (CT) system designed both to acquire original image data for and to process the image data for display and analysis in accordance with the present technique. The CT imaging system  46  is an energy discriminating computed tomography system as the detector subsystem is designed to record the individual photon energies of different monoenergetic X-ray spectra. The CT imaging system  46  is illustrated with a frame  48  and a gantry  50  that has an aperture (imaging volume or CT bore volume)  52 . A patient table  54  is positioned in the aperture  52  of the frame  48  and the gantry  50 . The patient table  54  is adapted so that a patient  56  may recline comfortably during the examination process. Additionally, the table  54  is configured to be displaced linearly by the linear positioning subsystem  24  (see  FIG. 1 ) as discussed above. For example, in the illustrated embodiment, a table motor controller  58  that may be a part of the system controller  22  may be adapted to operate the table  54 .  
         [0029]     The gantry  50  includes an X-ray source  12  positioned adjacent to a collimator  60 . In typical operation, the X-ray source  12  projects monoenergetic X-rays at one or more specified energy levels towards the energy discriminating detector  16  mounted on the opposite side of the gantry  50 . Collimator  60  permits a stream of radiation  14  to pass into a particular region in which a subject, such as a human patient  56  is positioned. It should be noted that the particular region of the patient  56 , for instance the liver, pancreas and so on, is typically chosen by an operator so that the most useful scan of a region may be acquired.  
         [0030]     Furthermore, the gantry  50  may be rotated around the subject  56  so that a plurality of radiographic views may be collected along an imaging trajectory described by the motion of the X-ray source  12  relative to the patient  56 . In particular, as the X-ray source  12  and the detector array  16  rotate along with the CT gantry  50 , the detector array  16  collects data of X-ray beam attenuation at the various view angles relative to the patient  56 . As described above, these data may then be processed to generate one or more composition image of the scanned area of the patient  56 .  
         [0031]     Rotation of the gantry  50  and operation of the source  12  is controlled by a system controller  22  as discussed above. As described above, the rotational subsystem  26  (see  FIG. 1 ) is configured to operate the gantry  50 . For example, in the illustrated embodiment, the system controller  22  may include a gantry motor controller  62  that controls the rotational speed and position of the gantry  50 . The computer  36  is typically used to control the entire CT system  46  and may be adapted to control features enabled by the system controller  22 . The computer  36  in turn may be configured to receive commands and scanning parameters from an operator via an operator workstation  40 .  
         [0032]     In the illustrated embodiment, the operator workstation  40  may also be coupled to a picture archiving and communications system (PACS)  64 . It should be noted that PACS  64  may be coupled to a remote system  66 , such as radiology department information system (RIS), hospital information system (HIS) or to an internal or external network, so that others at different locations may gain access to the image and to the image data.  
         [0033]     While in the present discussion reference is made to a CT scanning system in which a source and detector rotate on a gantry arrangement, it should be borne in mind that the present technique is not limited to data collected on any particular type of scanner. For example, the technique may be applied to data collected via a scanner in which an X-ray source and a detector are effectively stationary and an object is rotated, or in which the detector is stationary but an X-ray source rotates or otherwise moves relative to the detector or imaged object. Further, the data could originate in a scanner in which both the X-ray source and detector are stationary, as where the X-ray source is distributed and can generate X-rays at different locations. Similarly, while generally circular scan geometries are discussed, other geometries may be envisioned as well.  
         [0034]     The imaging system  10  and the CT imaging system  46  may generate images of the object under examination by a variety of techniques. For example, referring now to  FIG. 3 , exemplary control logic for generating one or more composition image using a monoenergetic X-ray source and an energy discriminating detector is depicted. As illustrated in the flowchart  68 , an operator may select a desired monoenergetic X-ray spectrum for imaging an object of interest at step  70 . The desired monoenergetic X-ray spectrum may be selected based on the type of imaging being performed such as contrast imaging, bone imaging, soft tissue imaging, material characterization and others. The X-rays are then emitted at the desired monoenergetic spectrum via a tunable monoenergetic X-ray source through the object of interest at step  72 . Alternatively, the X-rays may be emitted at a broader spectrum than desired and be filtered so that they are essentially monoenergetic when they reach the imaging volume.  
         [0035]     Further, the monoenergetic X-rays are attenuated by the object of interest and detected by the energy discriminating detector at step  74  that generates a detector output signal in response to the detected X-rays at step  76 . Each detector output signal contains spectral information about the composition of the scanned image based on the degree of attenuation of the monoenergetic X-rays in the scanned image. The detector output signal is therefore processed by an image processing circuitry to generate one or more composition image of the scanned object at step  78 .  
         [0036]     The imaging system  10  as described in the various embodiments discussed above, provides better diagnostic ability via better tissue differentiation, higher contrast per unit dose to the patient, better scatter rejection and better image quality. Since the X-rays are monoenergetic, selective elements of the object under scrutiny can be emphasized. In one embodiment, the present technique enables the rejection of scattered X-rays (referred to as scatter in the art) due to the ability to discriminate the energy of detected photons. The limited spectrum enables mitigation of energy-dependent differential attenuation effects which otherwise lead to beam hardening. Reduced beam hardening and scatter reduces computed tomography artifacts, thereby improving tissue differentiation and diagnostic power. In addition, the ability to tune narrow band X-ray spectra enhances material differentiation.  
         [0037]     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.