Patent Application
20140192958
The present invention relates to methods and systems of inspection using x-ray radiation, and, more particularly, to methods and systems whereby x-ray dose is reduced during the course of inspection.
Particular contents of the target object 10 (otherwise referred to herein as an “inspected object,” or simply as an “object”) may be discriminated and characterized on the basis of the transmission of penetrating radiation through the object and its detection by detector array 16 and its individual detector modules 18. (As used herein, the term “detector module” refers to one or more detector element in conjunction with its associated preprocessing electronics.) Signals from each of the detector modules, suitably pre-processed, provide inputs to processor 19, where material characteristics are computed.
In such x-ray inspection systems, image quality often depends upon the flux of radiation, in total or as a function of x-ray energy, passing through the object being inspected and reaching the detectors. Increased flux is typically concomitant with increased radiation dose to the inspected cargo as well as to the ambient environment. In order to keep the amount of radiation to the environment low, shielding is used to attenuate both direct and scattered radiation. The scattered radiation is particularly difficult to shield because shielding scattered radiation requires that attenuating material be added close to the object that is being inspected, thereby contributing, by a large fraction, to the scattered radiation. For open systems such as high energy gantries, the foregoing considerations are challenging.
Inspected objects, such as cargo containers, are not always filled with highly attenuating quantities of material, and a significant fraction of the incident x-ray beam may traverse, or be scattered out of, the container. Using the full x-ray beam power for the lightly attenuating portions has the effect of increasing the dose both to cargo and environment without providing significant image quality improvement. Methods to reduce the dose to cargo and to environment without impacting the image quality would thus be very desirable.
Methods for modulating the intensity of an x-ray beam include methods for interposing a translating or rotating filter between an x-ray source and a source collimator, as shown, for example, in U.S. Pat. No. 5,107,529 (to Boone), which describes the combination of a set of attenuating patterns. US Published Application 2006/0062353 A1 (to Yatsenko et al.) summarizes methods of modulating an X-ray beam, at pars. [0008]-[0019]. Both of the foregoing documents are incorporated herein by reference.
Some methods for real-time dose mitigation are known in the art, such as those described in U.S. Pat. No. 6,067,344 (to Grodzins et al.), entitled “X-Ray Ambient Level Safety System,” incorporated herein by reference.
In accordance with various embodiments of the present invention, methods are provided for dynamically regulating x-ray dose. The methods have steps of:
In accordance with other embodiments of the present invention, the filter may be a whole-beam filter, and, additionally or alternatively, may preferentially absorb lower-energy x-rays. Absorption by the filter may be a function of filter position. In further embodiments, the filter may be a wedge filter. Absorption by the filter may vary in a stepped manner with respect to filter position. The filter may also be a partial beam filter.
In accordance with yet other embodiments of the invention, other methods are provided for dynamically regulating x-ray dose. These methods have steps of:
The step of varying may include changing an aperture size of the source collimator, or changing a relative position of the focal spot and the source collimator.
In further embodiments, methods are provided for dynamically regulating x-ray dose, having steps of:
In any of the foregoing methods, varying a dimension characterizing the focal spot may include defocusing the focal spot.
In accordance with another aspect of the present invention, methods are provided for dynamically regulating x-ray dose by:
The characteristic of the generated x-ray beam that is varied may include spectral content of the x-ray beam, or flux of the x-ray beam, for example. It may also include a temporal characteristic of the x-ray beam such as pulse duration or frequency. It may also include variation in the frequency per unit time of interspersed pulses of electrons characterized by distinct energies, and in the ratio of the frequencies of such interspersed pulses.
In accordance with yet another aspect of the invention, an x-ray system is provided for generating an x-ray beam of dynamically regulated dose. The x-ray system has an electron accelerating structure for accelerating a beam of electrons for formation of a focal spot on an x-ray producing target and for generating an x-ray beam. The x-ray system also has a source collimator for collimating the x-ray beam, and a filter dynamically interposable by translation between the focal spot and the source collimator. The dynamically interposable filter may be a whole beam filter or a partial beam filter. The x-ray system has a detector for receiving a portion of the x-ray beam that traverses an inspected object and for generating a detector signal, and a processor adapted to dynamically interpose the filter between the focal spot and the source collimator on a basis of the detector signal.
The foregoing features of the invention will be more readily understood by reference to the following detailed description, taken with reference to the accompanying figures, in which:
Definitions. As used herein and in any appended claims, the term “beam” refers to a flux of particles (including photons or other massless particles) having a predominant direction referred to as the direction of the beam. Any plane containing the direction of the beam may be referred to as a plane of the beam.
The term “image” shall refer to any multidimensional representation, whether in tangible or otherwise perceptible form, or otherwise, whereby a value of some characteristic (such as fractional transmitted intensity through a column of an inspected object traversed by an incident beam, in the case of x-ray transmission imaging) is associated with each of a plurality of locations (or, vectors in a Euclidean space, typically 2) corresponding to dimensional coordinates of an object in physical space, though not necessarily mapped one-to-one thereonto. An image may comprise an array of numbers in a computer memory or holographic medium. Similarly, “imaging” refers to the rendering of a stated physical characteristic in terms of one or more images.
As used herein, when the terms “high” and “low” are used in conjunction with one another, the terms are to be understood in relation to one another. Thus, “low energy”, or “lower energy,” refers to radiation which is characterized by a lower endpoint energy than radiation which is characterized as “high energy” or “higher energy.” When used alone, the term “high energy” or “hard,” describing radiation, refers to radiation characterized by an endpoint energy of at least 1 MeV per particle.
As used herein, the term x-ray “dose” shall refer to the total energy fluence incident upon a specified area during a specified interval of time, such as that defined by a pulse. The term “dose rate,” while indicative of power flux, shall be used interchangeably with “dose” for all purposes, within the context of the present description.
As used herein, the term x-ray “scan” shall refer to a variation of a spatial orientation of an x-ray beam or to the relative motion of the beam relative to a medium being inspected for the purpose of characterizing a medium, as by imaging.
The term “detector” may be used without limitation herein to refer to an element of a multi-element detector array, or to an entire detector array, or to a detector module, including preprocessing electronics, as the context warrants.
The adverb “dynamically,” as applied to variation of a parameter or a position, shall refer to varying such parameter or position as a function of time, typically in response to some measurement.
The adverb “adaptively,” as applied to variation of a parameter or a position, shall refer to varying such parameter or position in response to some measurement.
As used herein and in any appended claims, an electron beam may be said to be characterized by two (or more) “distinct energies,” by which is meant that the electron beam is comprised of a chain of pulses, some of which are characterized by a first energy, and others of which are characterized by another energy. The first energy may be referred to as a lower energy (LE), for example, while another energy may be referred to as a higher energy (HE), again, for example. There may, of course, be any number of intervening energies as well.
Pulses of distinct electron energies impinging on an x-ray production target produce, through bremsstrahlung, distinct x-ray spectra, with end-point energies governed by the distinct energies of the respective incident energy beams.
In accordance with embodiments of the present invention, described now with reference to
Electron beam 203 impinges upon x-ray production target 207, (usually tungsten) and produces x-rays 209 via a bremsstrahlung process. The position where electron beam 203 impinges upon x-ray production target 207 may be referred to herein as x-ray focal spot (or “focal spot”) 211. In certain embodiments of the present invention, a beam focusing and steering system may be interposed between the electron accelerating structure 201 and the X-ray production target 207. Accelerating structure 201 may be understood as encompassing any accelerator, including a linac, for example, without limitation. The accelerating structure and x-ray production target, taken together, may be referred to herein as an “x-ray source.”
A focal spot collimator 211 for shielding unwanted x-rays is followed by one or more source collimators 213 and further shielding components. Source collimator 213 may also be referred to herein as an “inner collimator,” and may be followed by one or more subsequent outer collimators 215.
X-rays 209 emitted by x-ray emission system 200 may be characterized by an x-ray dose per pulse, in cases where electron source 205 is pulsed. Pulses emitted by x-ray emission system 200 may be referred to, for convenience herein, as “linac pulses”.
Embodiments of the present invention provide for dynamically varying and adjusting the dose per pulse during the course of an x-ray scan by changing parameters of one or more of components of x-ray emission system 200 as described above. Dynamic dose control may be performed by commands of a processor 19 (shown in
Methods for pulse-to-pulse dose reduction in accordance with the present invention may be characterized as follows, for heuristic purposes and without limitation, and understanding that some methods may employ more than one of the enumerated bases:
As stated above, the forgoing methods need not be mutually exclusive, and, for certain applications, compatible combinations of any two or more methods can be used.
Several exemplary embodiments of the present invention are now described in greater detail.
Beam filters attenuate the beam by absorbing a certain amount of x-rays. (The term “amount”, as used herein with reference to electromagnetic radiation, may refer, without limitation, to energy, power, spectral distribution, or any combination thereof) Advantage may be taken of preferential absorption of large numbers of lower-energy x-ray photons in beam filters. In fact, absorption typically decreases with energy, starkly (with an exponential coefficient of absorption decreasing as ˜ε−3) at energies below those where attenuation comes to be dominated by Compton scattering. As a consequence, the reduction in dose upon insertion of a whole beam filter is much larger than the penalty paid in image quality reduction. A translating x-ray filter 300 is depicted in
With reference to
Use of step wedges that interpose a discrete set of filtration thicknesses in the beam is sometimes desirable. A nonlinear profile or a wedge composed of multiple materials may also be employed within the scope of the present invention.
Referring to
It is often the case that only a portion of cargo undergoing x-ray inspection contains highly-attenuating materials. These dense regions of cargo often intersect only a fraction of the beam in the vertical direction. By partially blocking the beam, these dense regions can be isolated for full flux, while less-dense regions above or below it on the same scan line can be heavily filtered so that they receive reduced flux. This will modulate the intensity far more than a system that only regulates the whole line.
Two systems that may be used to accomplish the foregoing modulation within the scope of the present invention are now described with reference to
Another way to accomplish the aforesaid modulation is with a series of step filters (or wedge filters) as shown in top view in
Referring now to
In a further embodiment, depicted in
Each pixel in an x-ray image viewed by the operator usually contains information obtained from averaging or processing multiple linac pulses. In this approach the number of linac pulses per second is dynamically changed during the scan as the amount of X-ray attenuation in the object inspected varies such that the contrast-to-noise ratio per pixel in the image viewed by the operator does not decrease significantly.
The flux of the x-ray pulse may be changed on a pulse-to-pulse basis by shortening the duration of the linac pulse, as depicted in
X-ray flux produced via bremsstrahlung by electrons impinging on an x-ray production target is directly proportional to the electron current incident on the target. By varying the current on a pulse-to-pulse basis, the x-ray flux can be adjusted linearly.
For x-rays produced by bremsstrahlung targets in the MeV range, the dose rate roughly varies with the third power of the energy of the electron beam. By changing the energetic composition of the beam by even a small amount, the dose rate from pulse to pulse can be adjusted significantly. Adjustment of the linac energy or electron current, thus varying spectral or flux characteristics of the resultant x-ray beam, may be accomplished in response to radiation detected after transmission of the x-ray beam through, or scattered by, an inspected object.
More particularly, in cases where the pulse stream is based upon varying the electron beam among a multiplicity of energies from pulse to pulse, the number of pulses per unit time of each respective energy pulse may be varied on the basis of the x-ray beam detected after transmission through the inspected object. Thus, for example, if the stream of pulses is characterized by a sequence, say HE, LE, HE, LE, etc., that sequence may be modified to double the ratio of LE pulses to HE pulses, thereby lowering the average dose rate incident upon the target.
The ratio of pulses per unit time of one energy with respect to another may be referred to, herein, as “the ratio of pulses of different energies of the generated x-ray beam.”
X-ray focal spot 211 (shown in
Accordingly, when a lower dose is sought on the next pulse, just before the accelerator (an example of electron accelerating structure 201) fires, the electron beam focusing and steering system is adjusted such that the x-ray focal spot will be misaligned by a predefined distance relative to the collimator, as shown in
On a pulse-by-pulse basis, the electron beam focusing and steering system (part of electron accelerating structure 201, shown in
Joint variation of focal spot profile and position is an example of the use of multiple dynamic techniques for optimizing dose per pulse.
It is to be understood that the foregoing methods may be used in conjunction with multiple energy sources, whether multiple energies are emitted in distinct pulses or during the course of single pulses, or in conjunction with any other scheme of source configuration or operation that is known in the art.
In the case of an x-ray source configured to produce interlaced pulses with at least two different energies, referred to herein as low energy (LE) and high energy (HE), an algorithm may be employed as depicted in the flowchart of
The aforesaid x-ray pulses may be referred to as having corresponding energies WL and WH and doses per pulse DL, DH. (As used herein, the “energy” characterizing an x-ray pulse, if the pulse is characterized by a single energy, refers to the highest energy x-rays in the beam.
The following assumptions are made, for purposes of presenting an embodiment of the present invention:
In accordance with embodiments of the present invention, a controller is provided that is adapted for monitoring maximum attenuation A caused by cargo under scanning, separately as A(L) for the LE pulse and as A(H) for the HE pulse. The controller is adapted, further, to compare maximum attenuation with preset thresholds and send a signal to the x-ray source to set an appropriate LE/HE ratio, PRF and scan speed.
A scanning algorithm that may be employed in accordance with the system described above is now described with reference to
A scan starts with LE/HE=N0 at “low” PRF (e.g.,N0=9:1; PRF=100 pps). The controller then compares maximum attenuation for low energy pulse with first threshold T0(L) (defined based on low energy penetration capability). Until A(L)>T0(L), the scan runs in default mode.
If above condition (A(L)<T0(L)) is not true, the x-ray source generates the next pulse as a HE pulse.
The controller analyses attenuation for both LE and HE pulses and defines further scanning conditions as shown on
Where examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objective of x-ray dose reduction. Additionally, single device features may fulfill the requirements of separately recited elements of a claim. The embodiments of the invention described herein are intended to be merely exemplary; variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in any appended claims.
The present application claims the priority of U.S. Provisional Patent Application Ser. No. 61/748,789, filed Jan. 4, 2013, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61748789 | Jan 2013 | US |
