Filtering of a source of pulsed radiation

A source of pulsed radiation is coupled to a positionable filter. The positionable filter includes an element that produces an indication of a position of the filter. The source is configured to receive the indication of the position of the filter, and to regulate emission of a pulse of radiation based on the indication. A device includes an area including a material that alters a parameter of a beam of radiation that interacts with the material. The device is configured to move relative to a source of pulsed radiation. An element provides a signal to the source of pulsed radiation that indicates a position of the area relative to the source. The signal causes the source to trigger emission of a pulse at a time such that the emitted pulse is incident upon a portion of the area.

TECHNICAL FIELD

This disclosure relates to filtering a source of pulsed radiation.

BACKGROUND

X-ray radiation emitted from an x-ray source may be filtered to modify the spectral output of the x-ray source. A first filter may be used to filter x-rays having a peak energy within a first range and a second filter may be used to filter x-rays having a peak energy within a second range. These filters are selected in a predetermined manner such that the first and second filters are always used to filter x-rays having the first and second peak energies, respectively.

SUMMARY

A source of radiation may be synchronized to a moving, rotatable, and/or positionable filter wheel to allow for selection of a particular filter from among many filers included in the wheel. In some implementations, the filter wheel rotates about an axis of rotation and a measurement of the angular position of the filter wheel is used to trigger the source. The angular position of the filter wheel provides an indication of a position of the filter wheel and/or the various filters included in the filter wheel such that the source emits a pulse of radiation at a time at which a section of the filter wheel that includes the desired filtering material is in the path of a pulse emitted from the source.

Accordingly, knowledge of the position of the filter wheel together with triggering emission of the pulse from the source based on that position allows selection of a particular filtering material from among several materials included in the filter wheel. As a result, a range of filters may be introduced into the beam emitted from the source, and the filters may change between pulses.

Some prior systems apply a particular filter to a beam of radiation or a pulse of radiation depending on the energy of the radiation in a predetermined and fixed manner. In contrast, the techniques discussed below allow a pulse to be filtered by a particular filter that is selected by triggering the source to emit the pulse when the particular filter is present, or will be present, in the path of the pulse and without regard to energy of the pulse, a state of the source, or other predetermined criteria. Accordingly, the present system allows an amount of filtering to be selectively varied in real-time, or near real-time, to accommodate, for example, changes in density of an object imaged by the system. As a result, a source that emits radiation having a single peak energy and energy spectrum may be used to image an object with varying density or to image multiple objects that have a range of densities.

In one general aspect, a system includes a source of pulsed radiation, and a positionable filter coupled to the source of pulsed radiation. The positionable filter includes an element that produces an indication of a position of the filter. The source is configured to receive the indication of the position of the filter, and the source is configured to regulate emission of a pulse of radiation based on the indication.

Implementations may include one or more of the following features. The filter may include a portion that includes a material that causes alteration of one or more of a flux, energy spectrum, position, or collimation of a beam of radiation that interacts with the material. The portion may include a plurality of sections, at least one of which includes the material. At least one of the plurality of sections may be a blank section without the material such that a beam of radiation is unaltered as a result of interacting with the blank section. At least one section of the plurality of portions may include a second material different from the material.

The source of pulsed radiation may be a linear accelerator. The source of pulsed radiation may regulate emission of the pulse of radiation by determining a particular time to emit the pulse of radiation. The positionable filter may rotate about an axis of rotation such that a pulse emitted from the source strikes one of the plurality of sections at a particular time. The source of pulsed radiation may regulate emission of the pulse of radiation by delaying emission of the pulse of radiation such that the emitted pulse strikes a selected one of the plurality of sections.

In another general aspect, a device, configured to move relative to a source of pulsed radiation, includes an area including a material that alters a parameter of a beam of radiation that interacts with the material. An element provides a signal to the source of pulsed radiation indicating a position of the area relative to the source. The signal causes the source to trigger emission of a pulse at a time such that the emitted pulse is incident upon a portion of the area.

Implementations may include one or more of the following features. The device may be configured to rotate about an axis of rotation, and the indication of a position of the device may include an indication of an angular position of the device relative to the axis of rotation. The device may include a cylindrically shaped element that defines a longitudinal axis that is parallel to the axis of rotation. The cylindrically shaped element may include a first end and a second end, and the portion including the material may be oriented between the first and second ends and along the longitudinal axis. The portion including a material may include a plurality of sections, at least one of which is a blank section that does not include a material such that an emitted pulse is unaltered by interaction with the blank section.

In some implementations, the signal provided by the element may cause the source to delay the emission of the pulse such that the pulse is emitted when a selected one of the plurality of sections is in a path of the emitted pulse. The signal may be sufficient to cause the source to alter a timing of the emission of the pulse from the source.

In another general aspect, a method of filtering a pulse includes accessing a position of a movable filter that includes a plurality of sections. Each section is associated with a filtering characteristic. The method includes selecting, from among the plurality of sections, a particular section for filtering by triggering a radiation source, based on the position of the movable filter, to generate a pulse that strikes the particular section of the movable filter.

Implementations may include one or more of the following features. Selecting a section from among the plurality of sections may include selecting a section associated with a filtering characteristic that does not alter a parameter of the pulse. Accessing a position of the movable filter may include receiving an indication of the position of the movable filter generated by the movable filter. The movable filter may rotate about an axis of rotation, and accessing a position of the movable filter may include receiving an angular position of the filter. The particular section may be selected independently of an energy output of from source.

In another general aspect, a machine readable medium coupled to an electronic processor, includes instructions that, when executed, cause the processor to perform operations including accessing a position of a moving filter that includes a plurality of sections, each section associated with a filtering characteristic, determining, based on the position, a time at which a particular one of the plurality of sections is in the path of a pulsed radiation source, and generating a signal sufficient to cause the source to emit a pulse such that the pulse strikes the particular one of the plurality of sections.

Implementations may include one or more of the following features. The signal may be provided to the source. The filter may rotate about a longitudinal axis defined by the filter, and a position of the filter may be accessed by receiving an indication of an angular position of the filter relative to the longitudinal axis.

In another general aspect, a timing of a sequence of pulses generated by a pulsed x-ray source is altered such that a selected one material of multiple filter materials disposed on a rotating filter wheel that is coupled to the pulsed x-ray source is placed into a path of an x-ray beam produced by the pulsed x-ray source.

In another general aspect, a system includes a pulsed x-ray source, a rotatable wheel having multiple filtering materials mounted in slots, and a processor. The processor is configured to receive an indication of an angular position of the wheel, and to adjust a timing of an occurrence of a pulse from the x-ray source based on the indication of the angular position.

In another general aspect, a rotatable wheel includes multiple filtering materials mounted in slots formed or included in or on the wheel. The rotatable wheel is configured to be coupled to a pulsed x-ray source and to provide an indication of an angular position of the wheel to the x-ray source such that a timing of a pulse from the pulsed x-ray source is determined based on the angular position.

Implementations of the techniques discussed above may include a method or process, a system or apparatus, a device, a filter wheel, a filter drum, or computer software on a computer-accessible and/or machine readable medium.

DETAILED DESCRIPTION

Referring toFIG. 1, a system100includes a pulsed x-ray source110(such as a linac) and a rotating filter wheel120. The filter wheel120also may be referred to as a filter drum. The rotating filter wheel120includes multiple filtering materials121-125, one of which is in the path of a pulse130emitted from the x-ray source110at a given time. Passing the pulse130through any of the filtering materials121-125changes an intensity and/or an energy spectrum of the pulse130. The pulse130has a time duration “d,” and the pulse130occurs at a particular time with respect to other pulses in a train of pulses produced by the source110. The train of pulses has a frequency that nominally determines when a particular pulse is emitted from the source110. For example, the duration of the pulse130may be 3 microseconds (μs) and the time between pulses in the pulse train may be 3 milliseconds (ms). By delaying or advancing the time at which the pulse130occurs relative to an angular position (A) of the filter wheel120, a particular one of the materials121-125is selected to filter the pulse130. In other words, the timing of the occurrence of the emission of the pulse130from the x-ray source110or the phasing of the pulse130is determined by the angular position (A) of the rotating filter wheel120.

In particular, the x-ray source110emits the pulse130at a time when a particular one of the materials121-125is in the path of the pulse130as determined by the measured angular position (A) of the filter wheel120. Accordingly, the pulse130may be filtered by any one of the filtering materials121-125by selecting, controlling, and/or regulating the time at which the pulse130is emitted from the x-ray source110.

Thus, the techniques discussed below allow a particular filter to be selected (or no filter at all) by synchronizing the timing of the emission of pulse130from the pulsed x-ray source110with the angular position of the rotating filter wheel120. Synchronizing the timing of the pulse130with the angular position of the wheel120allows the pulse130to be emitted from the source110at a time in which a particular one of the materials121-125is in the path of the pulse130. Accordingly, the synchronization allows one of the materials121-125to be selected as the material to filter the pulse130.

The rotating filter wheel120may be used with the pulsed x-ray source110for material discrimination, calibration of the x-ray source110, and/or testing of the x-ray source110.

In some implementations, the filter wheel120may be part of a system that performs material discrimination. Material discrimination may be performed by determining the effective atomic number (Z) of an object140that is exposed to relatively high-energy x-ray radiation and relatively low-energy x-ray radiation. However, the ability to perform material discrimination is dependent on the amount of material and/or the density of the material of object140. When the material is relatively thick and/or dense, relatively few low-energy x-ray photons pass through the material to reach a detector150. Because little low-energy x-ray radiation reaches the detector150, there may not be enough signal from the lower energy x-ray radiation to perform material discrimination. In these cases, allowing the pulse130to reach the object140without being filtered may produce better results because more x-ray energy reaches the object140(thus maximizing penetration of the object140). Thus, in these cases, the timing of the pulse130is adjusted, regulated, or otherwise controlled such that the pulse130is emitted from the source110at a time when the angular position (A) of the filter wheel120is such that a blank region is in the path of the x-ray beam. Additionally, an optimal filtering material may depend on the type of material present in the object140. The rotating filter wheel120allows selection from among the various materials121-125by timing the pulse130to be emitted when a particular material is in the path of the beam.

The system100also may be used for calibration of the x-ray source110and/or the detector150. For example, calibration of the x-ray source110and/or the detector150may be performed by confirming that the energy of the x-ray beam from the X-ray source110is as expected. Such a determination may be achieved by measuring the amount of attenuation of the pulse130resulting from the pulse130passing through various calibration objects. The calibration objects may be mounted on the filter wheel120in the same manner as the filtering materials121-125are mounted on the filter wheel120. In some implementations, the filter wheel120includes blank sections that do not include any material at all. Selecting to pass the pulse130through a blank section may allow testing of the source110. For example, passing the pulse130through a blank section and measuring the flux of the pulse at the detector150provides an indication of whether the source110is working properly and producing an expected amount of energy.

Thus, by controlling the timing of the pulses from the x-ray source110with the angular position (A) of the filter wheel120, one of a range of filtering materials or calibration objects may be introduced into the x-ray beam without operator intervention. Additionally, by adjusting the timing of the pulse130, the filtering material or calibration objects may be changed between pulses from the x-ray source110. In some implementations, the materials121-125include materials that do not filter the pulse130.

As shown inFIG. 1, the rotating filter wheel120(or other rotating, movable, and/or positionable device that holds the filtering materials121-125) is positioned in the vicinity of the x-ray source110. An axis of rotation126of the filter wheel120is such that one of a multiple different materials or objects may be introduced into the beam from the source110depending on the angular position (A) of the filter wheel120. In some implementations, synchronizing or otherwise correlating the rotation of the filter wheel120with the pulse rate of the source110is used to introduce the same object into the beam during each pulse. For example, by rotating the filter wheel120at a rate that is half of the pulse frequency of the beam from the source110, a different object may be aligned with the beam every other pulse. The rotation rate of the filter wheel120may be adjusted so that the same type of material is in the beam two or more times per revolution.

The x-ray source110may be a linac. The pulses from a linac are short enough to allow several filter materials or calibration objects121-125to be positioned on the rotating filter wheel120such that only one of the materials or objects is in the path of the x-ray beam at the time at which the pulse130is emitted from the source110. Different objects may be selected by changing the timing of the pulse130based on the angular position (A) of the rotating filter wheel120. The timing change to select a particular filter material may be relatively slight. For example, the pulse130may be a linac pulse that is about 3-μs long whereas the time between pulses is about 3-ms. Because the filter wheel120is rotating, the time of occurrence of the 3-μs duration pulse within the 3-ms period determines through which material the pulse130passes. Because the pulse130is short, the filter objects can be therefore be thin with respect to the circumference of the rotating wheel120. Thus, only a small change in angular position (A) of the filter wheel120is needed to select a different material.

The x-ray source110may be triggered by an external signal that determines when the pulse130occurs and/or causes emission of the pulse130. For example, basing the external trigger signal on the angular position (A) of the rotating wheel120allows the linac to be triggered using the angular position (A). Synchronization between the linac and the rotating wheel may be achieved by, for example, the use of a shaft encoder or similar mechanism. The output of the shaft encoder may be used to generate a trigger pulse that causes emission of the pulse130at a particular angular position (A) of the rotating wheel120.

As discussed above, a relatively small change in the timing of the emission of the pulse130results in selection of a different filter. Thus, the filtering of the x-ray beam from the source110may be modified pulse-by-pulse in response to a signal obtained from the detector150during the previous pulse or earlier pulses. This may allow the imaging of the object140to be optimized even if the nature of the object140being examined varied. For example, the object140may be a shipping container containing cargo that is part high-dense material and part low-density material. An unfiltered beam may be applied to the high-density cargo in order to obtain maximum penetration. The unfiltered beam is produced by timing the pulse130from the source110to occur when the angular position (A) of the filter wheel is such that a blank section is in the path of the beam from the source110. In contrast, when the low-density material is imaged, the timing of the pulses from the source110is set such that the pulses alternate between passing through two different filtering materials to produce low-energy x-rays and high-energy x-rays that may be used to perform material discrimination on the low-density portion of the cargo.

In some implementations, a precise amount of filtration may be beneficial. In these implementations, each filter material or calibration object is wide enough in a direction along a circumference of the filter wheel120to maintain the same thickness in the beam for the duration of the x-ray pulse130. In some implementations, a greater variety of filter options may be provided by using filter materials or calibration objects that vary in thickness with rotation angle. In these implementations, the average thickness of the filtering material or calibration object during the pulse130depends on the timing of the pulse130with respect to the angular position. Such an approach may allow a greater choice of filter thicknesses; however, the exact amount of filtration depends on when the x-ray pulse130is triggered with respect to the angular position (A) of the rotating filter wheel120.

FIG. 2Ashows an example of a system that includes a source of pulsed radiation and a rotating filter wheel, andFIG. 2Bshows a top view of the rotating filter wheel.

The system200includes a rotating filter drum210, a source of radiation220that emits a pulse that propagates along a path222, an object to be imaged227, and detectors230. In the example shown, the filter drum210is cylindrical and defines a longitudinal axis212about which the filter drum210rotates. The filter drum210includes a portion215that includes four sections216a-216d, each of which includes a material. In this example, each of the sections216a-216dincludes a material that runs along the longitudinal axis212, and the material may be referred to as a vane.

The materials of the sections216a-216deach have physical properties that may cause alteration of a pulse that interacts with the material. The effect, or lack of effect, that a material has on the parameters of a pulse with which it interacts may be referred to as a filtering characteristic of that material. For example, interaction with the material may cause an energy spectrum of the pulse to be filtered such that certain energies present in the original pulse are no longer present or are diminished in the filtered pulse. Additionally or alternatively, interaction with the material may cause a decrease in the magnitude of energy present in the pulse. In some implementations, interaction with the material may cause a change in a position or path of the pulse or in an amount of collimation of the pulse. The filtering characteristic of a material may be such that the material does not alter one or more parameters of radiation that interacts with the material. Thus, the materials of the vanes may cause no alterations to incident pulses. In some implementations, the vanes may be blank vanes that do not include a material at all

In the filter drum210, the sections216a-216dare uniformly spaced about the circumference of the drum210with sections216aand216copposing each other and sections216band216dopposing each other. Thus, when section216ais in the path222of the pulse, section216cis also in the path222, and the pulse passes through and/or interacts with the materials of both section216aand section216c. In some implementations, each of the sections216aand216cmay include the same material. Each of the sections216a-216dmay include a different material such that the sections216a-216dare each associated with a different filtering characteristic. The amount of alteration caused by interactions between the pulse and the material for a particular material may vary with the thickness of the material. In some implementations, the thickness of the material along the direction of propagation of a pulse of radiation varies. In some implementations, one or more of the sections216a-216dmay include no material at all. Sections without material may be referred to as blank sections. In some implementations and in the example shown inFIG. 2A, the pulse expands rapidly after emission from the source220, and the vanes are longer in the direction of the longitudinal axis212than in the horizontal axis such that the entire pulse interacts with the vane.

The materials used in the vanes may include plastics, which filter radiation to remove the low end of the energy spectrum, and/or metals, such as aluminum, which have a relatively constant attenuation across the energy spectrum.

An angular position of the filter drum210is measured by a sensor214. The sensor214produces an indication of the angular position and provides the indication to a trigger pulse generator240. The indication of the angular position may be, for example, an electronic signal having an encoded value or a signal level that represents the angular position of the drum210at a particular time. The sensor214may monitor the angular position of the drum210continuously, at a preset interval, or at particular times selected by an operator or preset in the sensor214.

The trigger pulse generator240receives the indication of the angular position from the sensor214and generates a trigger pulse sufficient to cause the source220to emit a pulse of radiation. In some implementations, the trigger pulse generator240only generates the trigger pulse when the indication shows that the angular position of the drum210is equal to a particular value or falls within a range of values. In this manner, the source220is only triggered when the drum210is in a position which results in a desired one of the sections216a-216dbeing in the path222of the pulse. In some implementations, the trigger pulse is a pulse that causes the source220to delay the emission of a pulse slightly such that the pulse, once emitted, strikes one of the sections216a-216dthat is selected based on the indication of angular position of the drum210and placement of the selected one of the sections in the path of the pulse due to the motion of the drum210.

Referring toFIG. 2B, a top view of the filter drum210is shown. As seen from the top of the drum210, the sections216aand216care arranged along a line and the sections216band216dare arranged along a line. In the example shown, a gap218is formed in the middle of the drum210. The gap218is a region without material through which the pulse propagates without striking any of the sections216a-216d. Thus, the gap218allows the drum210to be positioned such that the pulse passes through the drum210without passing through any of the sections216a-216d.

In the example shown, a frame260supports the vanes and holds them in place. In other examples, the vanes may be supported by a housing (not shown) that forms an outer surface of the drum210and is centered on the longitudinal axis212. The housing may be made from a material that is penetrated by the radiation emitted from the source.

Although in the example ofFIG. 2A, the trigger pulse generator240is shown as being in communication with but physically separate from the drum210, the source220, and the sensor214, this is not necessarily the case. In some implementations, the trigger pulse generator240may be part of the source220while still being electronically coupled to the sensor214. In some implementations, the trigger pulse generator240may be part of the sensor214. The sensor214may be permanently affixed to the drum210or the sensor214may be a separate component that is removable from the drum210. The source214may be referred to as an element that produces an indication of the position of the drum210.

Referring toFIG. 3, an example process for filtering a pulse of radiation is shown. The process300may be performed on one or more processors included in the trigger pulse generator240, the sensor214, and/or the source220. The one or more processors may be processors suitable for the execution of a computer program such as a general or special purpose microprocessor, and any one or more processors of any kind of digital computer. Generally, a processor receives instructions and data from a read-only memory or a random access memory or both. The processor may be electronically coupled to an electronic storage, such as a computer-readable or machine-readable medium, that stores or otherwise includes instructions, that when executed, cause the processor to perform the process300.

A position of a movable filter that includes a plurality of sections, each of which are associated with a filtering characteristic, is accessed (310). The movable filter may be the filter drum210discussed above, and the sections may be the sections216a-216d. In some implementations, the moveable filter is a moving filter and the motion of the filter and the sections may be constant, or nearly constant. The motion of the filter may be angular motion about an axis of rotation. In some implementations, the filter may have linear motion, for example, the filter and the sections may move laterally along a direction perpendicular to the direction of propagation of the pulse and/or the filter may move along a direction parallel to the direction of propagation of the pulse.

In some implementations, the movable filter may be stationary for a finite amount of time. For example, the drum210may be rotated to place the sections216aand216cin the path of the pulse, the drum210may remain stationary while one or more pulses interact with the sections216aand216c, and then the drum210may be rotated to place sections216band216din the path of the pulse. In other implementations, the drum210may be rotated such that different a different section is moved into the path of the pulse between successive pulses emitted from the source220.

The position of the filter may be accessed by accessing an indication of the position measured by the sensor214and stored in an electronic storage in communication with the sensor214, and/or the position of the filter may be accessed by receiving the indication of the position measured by the sensor214. The indication of position may be, for example, a numeric value representing the angular position of the filter drum210.

A particular section for filtering the pulse is selected from among the plurality of sections (320). The particular section is selected by triggering a radiation source, based on the position of the filter, to emit a pulse of radiation. The section may be selected based on the position of the filter, by, for example, generating a trigger pulse when the position of the filter indicates that a desired section is in the path of the pulse, or will, accounting for motion of the filter, be in the path of the pulse. Thus, the selection of the section is based on the presence of the section in the path of the pulse. Accordingly, the selection of the section depends on adjusting the timing of an emission of a pulse from the source and is independent of an energy or other parameter of the pulse or the source.

Other implementations are within the scope of the following claims. For example, the source of pulsed radiation may be a source of neutrons. The sensor214may be a position sensor. In some implementations, the sensor214is an integral element of the drum210. The output signal of the sensor may be provided directly to the source of pulsed radiation. The drum210may include more or fewer sections than the four sections216a-216dshown inFIGS. 2A and 2B. The sections in the drum210may be arranged irregularly about the circumference of the drum rather than being uniformly placed about the circumference such that placement of one section in the path222does not result in placement of a section in the path222. The axis of rotation of the filter wheel120may be parallel with the direction of propagation of the pulse130.