Patent Grant
9980697
Modern x-ray radiography systems measure x-ray attenuation through an object. Based on the properties of an x-ray source and detector in the system, the measured x-ray attenuation can provide information about the object's density, effective atomic number, or other properties. X-ray radiography systems can be used to detect suspicious items or contraband within cargo or baggage, for example, at an airport or seaport.
Systems, devices, and methods are taught herein that enable the characterization of a high-energy x-ray source for dose and energy. Once the high-energy x-ray source is characterized for dose and energy, corrections to compensate for fluctuations in the dose and energy of an x-ray beam emitted by the high-energy x-ray source can be accomplished. In some embodiments, the disclosed systems, methods, and devices improve the quality of radiographic data indicative of x-ray radiation interacting with an object including density and atomic number data.
As taught herein, in some embodiments an x-ray imaging system is disclosed that includes a high-energy x-ray source, a detector array, a reference detector, and a processing unit. The high-energy x-ray source irradiates at least a portion of an object with a beam of x-ray radiation. The detector array detects object measurement data indicative of an interaction of x-rays with at least a portion of the object. The reference detector includes a plurality of detector elements stacked one behind the other in a stacking direction along an x-ray beam path to detect x-ray beam fluctuations in the high-energy x-ray source. The reference detector receives x-rays directly from the high-energy x-ray source. The processing unit has a central processing unit. The central processing unit is programmable to receive object measurement data from the detector array and measurements of an x-ray beam dose and the x-ray beam attenuation from the reference detector. The central processing unit is also programmable to determine a dose correction factor or energy correction factor using the measured x-ray beam dose and the measured x-ray beam attenuation from the reference detector. The central processing unit is also programmable to correct for x-ray beam fluctuations by applying the dose correction factor or energy correction factor to the object measurement data from the detector array.
As taught herein, in some embodiments a method of correcting for fluctuations in a beam of x-ray radiation from a high-energy x-ray beam source is disclosed. Performance of the method simultaneously measures an x-ray beam dose and the x-ray beam attenuation using a reference detector. The reference detector has a plurality of detector elements stacked one behind the other in a stacking direction along an x-ray beam path. The reference detector receives x-rays directly from the high-energy x-ray source. Performance of the method determines a dose correction factor or energy correction factor using the measured x-ray beam dose or the measured x-ray beam attenuation. Performance of the method corrects for the x-ray beam fluctuations by applying the dose correction factor, the energy correction factor, or both to measurement data representing density. Performance of the method corrects for the x-ray beam fluctuations by applying the dose correction factor and energy correction factor to measurement data representing effective atomic number. In some embodiments, performance of the method adjusts the high-energy x-ray beam source by applying the dose correction factor, energy correction factor, or both to the high-energy x-ray beam source.
As taught herein, in some embodiments a reference detector is disclosed that includes a plurality of detector elements. The plurality of detector elements are stacked one behind the other in a stacking direction along an x-ray beam path to detect x-ray beam fluctuations wherein a first detector element in the plurality of detector elements receives x-rays directly from a high-energy x-ray source. The stacking direction of the plurality of detector elements allows the reference detector to simultaneously detect the beam dose and the beam attenuation.
The skilled artisan will understand that the drawings are primarily for illustrative purposes and are not intended to limit the scope of the subject matter described herein. The drawings are not necessarily to scale; in some instances, various aspects of the subject matter disclosed herein may be shown exaggerated or enlarged in the drawings to facilitate an understanding of different features. In the drawings, like reference characters generally refer to like features (e.g., functionally similar or structurally similar elements).
The foregoing and other features and advantages provided by the present disclosure will be more fully understood from the following description of exemplary embodiments when read together with the accompanying drawings, in which:
Below are more detailed descriptions of various concepts related to, and examples of, methodologies, computer readable media, apparatuses, and systems for characterization of fluctuations in the dose and energy of a high-energy x-ray source. Once the high-energy x-ray source's dose and energy fluctuations are characterized, corrections can be applied to compensate for fluctuations in the x-ray beam dose and energy. Advantageously, the characterization of a high-energy x-ray source for beam dose and energy fluctuations and the subsequent corrections for these fluctuations results in improvement in the quality of the radiographic image and measurement of the atomic number of an object under observation. It should be appreciated that various concepts introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the disclosed concepts are not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes.
Systems, devices, and methods of the present disclosure enable correction of object measurement data obtained by transmission radiography systems to improve reconstructed image quality or to improve the ability to identify contraband, explosives, or other chemical or material components within objects. In particular, an x-ray imaging system is taught that that can obtain measurement data for at least a portion of an object. The system also measures the x-ray beam dose and x-ray beam attenuation using a reference detector. The system can correct the object measurement data for fluctuations of dose, energy, or both in a high-energy x-ray source using correction factors determined from the measured x-ray beam dose and attenuation. Methodologies are taught herein to enable correction for fluctuations of a high-energy x-ray source, and a device is disclosed including a stacked plurality of detector elements that simultaneously measures an x-ray beam dose and attenuation.
As used herein, “object measurement data” is data indicative of an interaction of x-ray radiation with at least a portion of an object and can include, but is not limited to, density and effective atomic number of the portion of the object.
As used herein, a “high-energy x-ray source” is an x-ray source that emits x-rays with an energy level of between 1 MeV and 20 MeV as characterized by the energy per unit mass deposited to a scanned object (‘dose’) and the energy spectrum (‘energy’).
As illustrated in
Y(t)=∫0E
where Eb is the end-point of the x-ray energy distribution (Φ) that corresponds to the electron beam energy, ε is the detector response, and μT is the x-ray attenuation coefficient of the scanned material of the object 16. The x-ray attenuation coefficient (μT) can be expressed as a sum of mass-normalized cross-sections from different sub-processes:
μT(E,Z)=a1(Z)μ1(E)+a2(Z)μ2(E)+ . . . +an(Z)μn(E) (2)
where the coefficients ai (i=1, n) depend on the atomic number, and cross sections μi (i=1, . . . , n) depend on the energy of the x-ray. A radiographic image can be formed by using the measurements of the detector array 14 to provide a measure of the x-ray attenuation inside the material of the object 16. For scanning, using more than a single energy band of x-rays in the energy spectrum allows extraction of the effective atomic number of materials that make up the object 16. The stability of the high-energy x-ray source 12 can affect the accuracy of these measurements. The total error on the yield can be expressed as:
σY2=σe2+g·Y+r·Y2 (3)
where σe is the error due to the electronic noise of a detector in the detector array 14, and the g·Y term provides a measure of the variance due to x-ray counting statistics that depend linearly on the yield. Using an approximation based on proportionality between the detected signal and the number of x-rays (N) and the number of scintillation photons and the x-ray energy, the relative error on the detector yield can be expressed as (σN/Y)2=1/N+(σE/<E>)2/N so the variance on the detected yield scales as σN2 ∝N. The r·Y2 term in Equation (3) represents the variance due to fluctuation in the beam dose or energy, and it scales as a square of the yield. The yield can be expressed based on the endpoint energy as follows: Y=γEBδ, such that σY/Y∝σB/EB. In addition, the yield can be expressed as linearly dependent on fluctuations in the electron beam current in systems that generate x-rays using a high energy beam of electrons.
Energy and dose fluctuations often occur simultaneously. The dose fluctuations can impact the visual quality of radiographic images obtained using the prior art x-ray radiography system 10. The energy fluctuations of the high-energy x-ray source 12 can affect measurement of the x-ray attenuation in scanned objects 16. Variation on the order of a few percent in the attenuation measurement can inhibit the capability of the prior art x-ray radiography system 10 to discriminate between constituent materials of the object 16. As a non-limiting example, differences between aluminum and steel are on the order of about 1%.
The example reference detectors and systems, methodologies, computer readable media, and apparatuses including the reference detectors as taught herein, can be used to correct for both dose and energy variations in the prior art x-ray radiography system 10. Non-limiting example reference detectors are described hereinbelow in connection with
The example reference detector 100 of
The first detector element 101a can include a material that is sensitive to x-ray radiation. In some embodiments, the first detector element 101a can include a scintillator crystal such as, but not limited to, cadmium tungstate (CdWO4; referred to herein as CWO), lead tungstate (PbWO4), or cesium iodide (CsI). In alternative embodiments, the first detector element 101a can include an ionization chamber.
The first detector element 101a can have a first surface 111 that faces the beam of x-ray radiation 150 and a second surface 112 opposite to the first surface 111. In some embodiments, the first surface 111 can be positioned perpendicular to a central ray of the x-ray beam 150. In some embodiments, the first detector element 101a produces a signal on exposure to the beam of x-ray radiation 150.
The second detector element 101b can include a material that is sensitive to x-ray radiation. In some embodiments, the second detector element 101b can include a scintillator crystal such as, but not limited to, cadmium tungstate (CWO), lead tungstate (PbWO4), or cesium iodide (CsI). In alternative embodiments, the second detector element 101b can include an ionization chamber.
The second detector element 101b can have a first surface 113 that faces the beam of x-ray radiation 150 and a second surface 114 opposite to the first surface 113. In some embodiments, the first surface 113 can be positioned perpendicular to the central ray of the x-ray beam 150. In some embodiments, the second detector element 101b can be formed from a different material or ionization chamber than the first detector element 101a. In some embodiments, the second detector element 101b can have a different conformation or lateral size than the first detector element 101a. In some embodiments, the second detector element 101b produces a signal on exposure to portions of the beam of x-ray radiation 150 that pass through the first detector element 101a.
In various example embodiments, the size of the gap 105 between detector elements can be set to any length that meets an application-specific requirement. In some embodiments, the gap 105 can include the same type of spacing element 106, for example, all reference detectors or a combination of spacing elements 106, for example, a combination of reference detectors and x-ray filters. In some embodiments the spacing elements abut adjacent elements. In some embodiments, one or more air gaps are included between the spacing elements 106. In an embodiment where characterized x-ray filters are used, the x-ray beam 150 can be attenuated by a known amount as it passes through the spacing elements 106. In some embodiments, the x-ray filter can be selected to have a high attenuation value to create a more compact reference detector 100′. X-ray filters can be useful in embodiments where the detector elements 101a, 101b are gaseous ionization chambers. Gaseous ionization chambers can be relatively inexpensive and long-lasting although they can have a small x-ray absorption cross-section. The addition of high attenuation value x-ray filters as spacing elements 106 can cause the reference detector 100 to be less dependent upon attenuation in the detector elements themselves.
In example reference detectors 100 and 100′, the second detector element 101b is disposed behind the first detector element 101a along the stacking direction 103 such that the first surface 113 of the second detector element 101b faces the second surface 112 of the first detector element 101a. In an exemplary embodiment, the stacking direction 103 is parallel to a central ray of the x-ray beam 150. In some embodiments, the stacking direction 103 can be perpendicular to the first surface 111 or the second surface 112 of the first detector element 101a or the first surface 113 or the second surface 114 of the second detector element 101b. Alignment of the first detector element 101a and the second detector element 101b one behind the other along the stacking direction 103 allows the simultaneous detection of the dose and the attenuation of the x-ray beam 150 by the reference detector 100, 100′. In some embodiments, a signal from the second detector element 101b can be read out separately from a signal from the first detector element 101a. The detected beam dose can be correlated to a first signal from the first detector element 101a, a second signal from the second detector element 101b, or a sum of the first signal and the second signal. The detected beam attenuation can be correlated to an energy level of the beam of x-ray radiation 150.
In some embodiments of example reference detectors 100, 100′, the second surface 112 of the first detector element 101a abuts the first surface 113 of the second detector 101b, such that there is no gap 105.
The example reference detectors according to various embodiments of the present disclosure, including any of reference detectors 100, 100′ or 100″, can be used to measure both the beam dose and the beam energy of the x-ray radiation. For ease of the discussion below, reference detector 100 will be referred to hereinafter although the below description is equally applicable to the reference detector 100′ and the reference detector 100″.
The reference detector 100 can be positioned so that it is exposed to a portion of the beam of x-ray radiation 150 that does not interact with an object being scanned, and the reference detector 100 can have direct access to the beam output. That is, the reference detector 100 is placed in the system so that an unobstructed, direct line of sight between the output of the high-energy x-ray source and the reference detector 100 is created. The example reference detector 100 can be disposed relative to the x-ray beam such that at least a portion of the first detector element 101a measures the direct x-ray beam dose and at least a portion of another detector element (such as detector elements 101b-101n) measures the dose after attenuation through a known set of materials (including detector element 101a). In some embodiments, the energy spectrum of the x-ray beam 150 changes as the beam passes through detector elements 101a-101n or spacing elements 106, i.e., as the beam is attenuated. In such embodiments, each detector element 101a-101n of the reference detector 100 can be exposed to a different energy spectrum.
While example systems, methodologies, apparatuses, and computer readable media in connection with
In many x-ray radiography systems, the high-energy x-ray source 250 can exhibit fluctuations in dose or energy from pulse-to-pulse. This instability often arises due to the method of producing high-energy x-rays. In high-energy x-ray sources that use a linear electron accelerator, for example, discrete pulses of high-energy electrons are directed onto a target that emits x-rays when struck by high-energy electrons. Subtle variations in the number of electrons or the accelerating gradient can produce fluctuations in the resulting pulse of x-rays. Because image reconstruction and material classification rely on measurement of x-ray beams that have been attenuated by passage through at least a portion of an object, fluctuations in the energy or dose of x-rays in the x-ray beam between pulses can degrade the quality of the object measurement data and, hence, the reconstructed image or material classification of the object. Systems, devices, and methods taught herein can correct for fluctuations in dose and energy of the x-ray beam by providing measurements of the dose and energy of the x-ray beam to determine correction factors that can be applied to object measurement data. This can be accomplished on a pulse-to-pulse basis.
During operation of any of the embodiments of the present disclosure, the reference detector 100 can be positioned relative to the high-energy x-ray source 250 such that the x-ray beam 150 is directed from one side and passes through the stacked detector elements 101a . . . 101n of the reference detector 100. This allows measurement of the beam dose and evaluation of the beam energy by measuring the yield across elements. Attenuation through the reference detector material is recorded as the decreasing yield through the stacked detector elements 101a . . . 101n of the reference detector 100.
The high-energy x-ray source 250 can be configured to emit a beam of x-ray radiation to irradiate at least a portion of the object 230. The detector array 260 can be configured to detect measurement data indicative of an interaction of the x-ray radiation with the portion of the object 230. As a non-limiting example, the detector array 260 can detect attenuated radiation that has passed through a portion of the object 230. The high-energy x-ray source 250 of some embodiments can include a high-energy electron beam and an extended target or array of targets. For example, the high-energy x-ray source 250 can include a linear electron accelerator (linac). In some embodiments, example imaging systems as taught herein can include more than one source 250 or more than one detector array 260.
Each pulse of x-rays from the high-energy x-ray source 250 can be characterized by its energy and dose. Values of x-ray energy and x-ray dose can fluctuate from pulse to pulse. Fluctuations in x-ray energy and dose for x-rays emitted by the high-energy x-ray source 250 can depend on the output x-ray energy. For example, high-energy x-ray sources 250 can have greater fluctuations in the x-ray energy and dose than low-energy x-ray sources. In some embodiments, the high-energy x-ray source 250 can emit x-rays with an energy level of at least 1 mega-electronvolt (MeV).
The reference detector 100 of the imaging system 200 can include a plurality of detector elements stacked one behind the other in a stacking direction along an x-ray beam path to simultaneously detect x-ray beam dose and energy fluctuations in the high-energy x-ray source. In some embodiments, the reference detector 100 is as described above with reference to any of
As shown in the example of
As illustrated in the example of
As illustrated in the example of
As shown in the example of
As shown in
The computing device 240 can use the communication links 205, 265 and feedback loop 255 to control the operation of or transmit or receive information from the reference detector 100, the high-energy x-ray source 250, and the detector array 265. In various examples, the computing device 240 including the processing unit 245 can be configured or programmed to receive object measurement data from the detector array 260 through the communication link 265. In some embodiments, the computing device 240 including the processing unit 245 can be configured or programmed to receive measurement data from the reference detector 100 through the communication link 205. In some embodiments, the computing device 240 including the processing unit 245 can control the operation of the high-energy x-ray source 250 through the feedback loop 255 including adjusting x-ray beam dose and energy.
In various examples, the computing device 240 including the processing unit 245 can be programmed to perform several operations. The processing unit 245 can receive object measurement data from the detector array 260 and measurements of an x-ray beam dose and the x-ray beam attenuation from the reference detector 100. In some embodiments, the measurements of an x-ray beam dose and the x-ray beam attenuation can include signals received from the plurality of detector elements 160 upon exposure to the x-ray beam 150. The processing unit 245 can compare object measurement data from the detector array 260 and measurements of x-ray beam dose or x-ray beam attenuation from the reference detector 100 to determine a dose correction factor, energy correction factor, or both. The processing unit 245 can correct the object measurement data from the detector array 260 by applying the dose correction factor, energy correction factor, or both. In an exemplary embodiment, the processing unit 245 can correct measurement data representing density of at least a portion of the object received from the detector array 260 by applying the dose correction factor derived from a comparison of the object measurement data to x-ray beam dose measurements from the reference detector 100 as described in greater detail below with reference to
In an exemplary embodiment, the processing unit 245 can apply a dose correction factor or energy correction factor to the high-energy x-ray source 250 through the feedback loop 255 to adjust or stabilize the output of x-ray dose or energy.
Example computation of the beam dose correction factor and beam energy correction factor and correction of the object measurement data is now described. To correct fluctuations in the beam dose, a proportionality can be assumed between the relative yield change observed at the detector array 260 and at the reference detector 100:
ΔY/Y≈−ΔRair/Rair (4)
where Rair is the yield in the reference detector 100, ΔRair is the change in the reference detector yield from the calibration value, Y is the measured yield in the detector array 260, and ΔY is the correction to the detector array yield.
In some embodiments, corrections for fluctuations of the beam energy can be made after measuring an attenuation value in the reference detector 100. The change of attenuation in the reference detector 100 is used to correct the attenuation measurement in the detector array 260. The attenuation in the detector array 260 can be computed as follows:
y=−a log(Y/Yair) (5)
where Yair is the detector array yield without any object in the beam path. The attenuation in the reference detector 100 can be computed from the yield before (Rair) the attenuation and after (R) the attenuation, as follows:
r=−a log(R/Rair) (6)
where values of R are computed from the one or more detector elements of the reference detector. In exemplary embodiments, values of R are computed from at least the first detector element 101a and the final detector element 101n. In some embodiments, there can be a linear relationship between the attenuation in the detector array 260 and the reference detector 100 due to the change in the beam energy spectrum, as follows:
Δy≈−α(y)Δr (7)
where α(y) is the slope of the correction. The term α(y) can depend on the configuration of the reference detector and can be obtained during an initial calibration.
In exemplary embodiments, the x-ray beam 150 passes through the first detector element 101a and the second detector element 101b of the reference detector 100. Variation in the signals generated by the first and second reference detectors in response to x-ray exposure can be manipulated to provide a measurement of the x-ray beam dose and an evaluation of the beam energy.
A numerical example of attenuation of an x-ray beam 150 as it passes through the reference detector 100 is illustrated in
The relative error for the energy measurement can be expressed as:
σE/E(dE/dr)(r/E)σr/r (10)
where the scaling term (dE/dr)(r/E) can be calculated by simulation.
Instead of using Equation (6) to compute the attenuation in the reference detector 100 as illustrated in
The reference detector 100 can be used to actively monitor the dose and energy of the high-energy x-ray source 250 for data quality and safety reasons. In addition to passive monitoring, the reference detector can be used in a feedback loop to improve the stability of the source in some embodiments. For example, the high-energy x-ray source 250 can rely on electron linear accelerators (linacs) that require adjustment of the RF frequency in order to keep the energy and the current of the electron beam constant. In an exemplary embodiment, the difference between a dose or energy of the x-ray beam 150 measured by the reference detector 100 and a desired dose or energy can be used to adjust the RF frequency of the electron linac via a feedback loop in order to keep the source stable as illustrated in
The utility and benefit of the example reference detector 100 was demonstrated in an example system as follows. The example system includes a gantry imaging system with a dual energy linac and the reference detector 100 including eight active detector elements 101a-101h made of CWO. A standard plastic plate including three metal wires formed of copper was used as the imaged object. The gantry speed was 0.4 m/s. The linac operated in interlaced mode at 200 pulses per second and 220 μGy/pulse and 100 μGy/pulse for 6 MeV and 4 MeV beam energies, respectively. The dose in the reference detector 100 was measured as the sum of signals from the eight detector elements 101a-101h, and the attenuation was measured as a ratio of the yields in the first two detectors 101a, 101b and the last two detectors 101g, 101h.
The impact of dose corrections on the quality of the resulting image is shown in
To evaluate the benefit of corrections to x-ray beam energy measurements, the example system was used to perform multiple scans of a one-inch aluminum target as the object. The average attenuation was measured for each scan.
The method 1100 uses the computing device 240 including the processing unit 245 to compare object measurement data received from the detector array 260 with a measured dose or attenuation value received from the reference detector 100, 100′, 100″, such as described above with reference to any of
Step 1106 can correct for the x-ray beam fluctuations by applying the dose correction factor, the energy correction factor, or both to measurement data representing density. This can be performed by using the computing device 240 including the processing unit 245 to correct x-ray beam fluctuations from the high-energy x-ray source 250 by applying the dose correction factor to reconstructed image data representing the density of a portion of an object 230 as described above with reference to
In some embodiments that include feedback loop 255, step 1110 can adjust the high-energy x-ray beam source by applying the dose correction factor, energy correction factor, or both to the high-energy x-ray beam source. This can be performed by using the computing device 240 including the processing unit 245 to adjust the high-energy x-ray source 250 by applying the dose correction factor, energy correction factor, or both to the high-energy x-ray source 250 via the feedback loop 255 as described above with reference to
In describing exemplary embodiments, specific terminology is used for the sake of clarity. For purposes of description, each specific term is intended to at least include all technical and functional equivalents that operate in a similar manner to accomplish a similar purpose. Additionally, in some instances where a particular exemplary embodiment includes a plurality of system elements, device components or method steps, those elements, components or steps may be replaced with a single element, component or step. Likewise, a single element, component or step may be replaced with a plurality of elements, components or steps that serve the same purpose. Moreover, while exemplary embodiments have been shown and described with references to particular embodiments thereof, those of ordinary skill in the art will understand that various substitutions and alterations in form and detail may be made therein without departing from the scope of the invention. Further still, other embodiments, functions and advantages are also within the scope of the invention.
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