X-ray flux reducer for a photon counting detector

An imaging system includes a radiation source (108) configured to rotate about an examination region (106) and emit radiation that traverses the examination region. The imaging system further includes an array of radiation sensitive pixels (112) configured to detect radiation traversing the examination region and output a signal indicative of the detected radiation. The array of radiation sensitive pixels is disposed opposite the radiation source, across the examination region. The imaging system further includes a rigid flux filter device (130) disposed in the examination region between the radiation source and the radiation sensitive detector array of photon counting pixels. The rigid flux filter device is configured to filter the radiation traversing the examination region and incident thereon. The radiation leaving the rigid flux filter device has a predetermined flux.

FIELD OF THE INVENTION

The following generally relates to controlling an x-ray flux incident on a photon counting detector of an imaging system, and is described with particular application to computed tomography (CT). However, the following is also amenable to flat panel, x-ray, radiotherapy and/or other imaging applications.

BACKGROUND OF THE INVENTION

A computed tomography scanner includes an x-ray tube that emits an x-ray beam. A portion of the x-ray beam traverses a subject or object located in a field of view of an examination region and is attenuated as a function of the radio density of the subject or object. Another sub-portion of the x-ray beam traverses the field of view of the examination region without traversing the subject or object. A detector array detects the radiation traversing the field of view and produces a signal indicative thereof. A reconstructor reconstructs the signal, producing volumetric image data.

A beam shaper has been positioned in the path of the x-ray beam between the x-ray tube and the examination region. The beam shaper has been referred to as a bowtie filter as its general physical shape resembles a bowtie. The beam shaper is shaped so as to attenuate the beam to a greater degree at a periphery of the beam. This makes the beam shaper well-suited for reducing the flux at the periphery in connection with direct conversion photon counting detectors, which suffer from insufficient count rate capabilities at the higher flux rates.

Unfortunately, for objects or portions of a subject (e.g., the extremities like the legs and arms) with no (or low) attenuating structure between attenuating structures (e.g., the space between the legs), the beam shaper is not well-suited for direct conversion photon counting detectors. This is because, for example, the beam shaper does not reduce the flux at this more central region enough. As a result, some centrally located detector elements of the detector array may receive and detect excessive flux, which can degrade image quality in the reconstructed volumetric image data.

This can be seen inFIG. 10in connection with objects1002and1004separated by a gap1006. InFIG. 10, an emitted beam1008is filtered by a beam shaper1010, which is thicker at peripheral regions1012and thinner at a central region1014, producing a filtered beam1016, which is filtered to a greater degree at peripheral regions1018and to a lesser degree at a central region1020. A region1022of a detector array1024, which is proximate to the gap1006, represents a region of a detector array1024which receives excessive flux.

SUMMARY OF THE INVENTION

In one aspect, an imaging system includes a radiation source configured to rotate about an examination region and emit radiation that traverses the examination region. The imaging system further includes an array of radiation sensitive pixels configured to detect radiation traversing the examination region and output a signal indicative of the detected radiation. The array of radiation sensitive pixels is disposed opposite the radiation source, across the examination region. The imaging system further includes a rigid flux filter device disposed in the examination region between the radiation source and the radiation sensitive detector array. The rigid flux filter device is configured to filter the radiation traversing the examination region and incident thereon. The radiation leaving the rigid flux filter device has a predetermined flux.

In another aspect, a method includes rotating a radiation source about an examination region. The radiation source emits radiation that traverses the examination region. The method further includes filtering the radiation that traverses the examination region with a rigid flux filter device disposed in the examination region. The method further includes detecting, with detector pixels located opposite the radiation source, across from the examination region, radiation traversing the rigid flux filter device and generating a signal indicative thereof.

In yet another aspect, a rigid flux filter device is configured to be disposed in an examination region between a radiation source and a radiation sensitive detector array of photon counting pixels, wherein the rigid flux filter device is configured to filter the radiation traversing the examination region and incident thereon, and wherein the radiation leaving the rigid flux filter device has a predetermined flux. The rigid flux filter device includes at least one of a polytetrafluoroethylene material or aluminum and having a thickness corresponding to a given radiation source voltage and a give radiation source current.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1schematically illustrates an example imaging system100, such as a computed tomography (CT) scanner. The imaging system100includes a rotating gantry102and a stationary gantry104. The rotating gantry102is rotatably supported by the stationary gantry104. The rotating gantry102is configured to rotate around an examination region106about a longitudinal or z-axis.

The imaging system100further includes a radiation source108, such as an x-ray tube, that is rotatably supported by the rotating gantry102. The radiation source108rotates with the rotating gantry102around the examination region106and is configured to emit radiation that traverses the examination region106. The imaging system100further includes a radiation source controller110. The radiation source controller110is configured to modulate radiation emission. For this, the radiation controller110can change the heating current of the cathode, the voltage supplied to the radiation source108, control a grid switch, which allows or inhibits electrons to flow, move a physical filter into and out of the radiation beam, etc.

The imaging system100further includes a beam shaper109. The beam shaper109is disposed in a path of the x-ray beam between the radiation source108and the examination region106. The beam shaper109is shaped so as to attenuate the beam to a greater degree at a periphery of the beam. This makes the beam shaper well-suited for reducing the flux at the periphery in connection with direct conversion photon counting detectors, which suffer from insufficient count rate capabilities at the higher flux rates. An example of the beam shaper109is a bowtie filter, which has a shape that resembles a bowtie.

The imaging system100further includes an array of radiation sensitive pixels112arranged along the z-axis direction. The pixels112are located opposite the radiation source108, across the examination region106, detect radiation traversing the examination region106, and generate signals indicative of the detected radiation. In the illustrated example, the pixels112include direct conversion photon counting detector pixels. With such pixels, a generated signal will include an electrical current or voltage having a peak amplitude or a peak height that is indicative of the energy of a detected photon. The direct conversion photon counting detector pixels may include any suitable direct conversion material such as CdTe, CdZnTe, Si, Ge, GaAs or other direct conversion material.

The imaging system100further includes a pulse shaper114that processes the electrical signal output by the detector pixels112and generates a pulse such as voltage or other pulse indicative of the energy of the detected photon. The imaging system100further may include an energy discriminator116that energy discriminates the pulse. In the illustrated example, the energy discriminator116includes at least one comparator118, which compares the amplitude of the pulse with at least one energy threshold that corresponds to an energy of interest. The comparator118produces an output signal indicative of whether the energy of a detected photon is above or below the threshold.

The imaging system100further includes a counter120that increments (or decrements) a count value for each threshold. For instance, when the output of the comparator118for a particular threshold indicates that the amplitude of the pulse exceeds the corresponding threshold, the count value for that threshold is incremented. The imaging system100may further include a binner122that assigns the counted pulses to energy bins, which correspond to different energy ranges. For example, a bin may be defined for the energy range between two thresholds. With this example, the binner122would assign a photon resulting in a count for the lower threshold but not for higher threshold to the bin defined for the energy range between two thresholds.

The imaging system100further includes a reconstructor124that reconstructs the binned data using a spectral and/or conventional reconstruction algorithm and generates spectral and/or conventional volumetric image data. The imaging system100further includes a computing system that serves as an operator console126, and includes an output device such as a display and an input device such as a keyboard, mouse, and/or the like. Software resident on the console126controls an operation of the system100, controlling modulation of the tube current in response to the selected scan protocol.

The imaging system100further includes a subject support128with a base132and a tabletop134. The tabletop134is moveably affixed to the base132and is configured to translate horizontally in and out of the examination region106before, during and after scanning for patient loading, patient scanning and patient unloading. The base132is affixed to or rests on a floor136in an examination room. The base132is configured to move vertically up and down and hence move the tabletop134up and down, for example, for loading and unloading a patient and for positioning the patient at a suitable height for scanning, for example, based on the region to be scanned, the iso-center of the scan field of view, and/or otherwise.

A flux filter device130is provided for scanning a portion of an object or subject that includes no structure or a low attenuating structure between attenuating structures. For such scanning, the flux filter device130is configured to at least attenuate radiation traversing towards an inner region of detector pixels of the array of radiation sensitive pixels (which corresponds to the no structure or low attenuating structure) so that the radiation exiting the flux filter device130and incident on inner region of detector pixels has a flux within a predetermined flux range. Generally, the flux filter device130is configured to uniformly attenuate the radiation across the radiation beam.

FIG. 2shows an embodiment in which the flux filter device130is placed and rests on structures202and204(e.g., the legs) with an air gap206in between the structures202and204. In this example, the filter device130attenuates radiation uniformly across the structures202and204and the air gap206.FIG. 3also shows an embodiment in which the flux filter device130is placed on and rests on the structures202and204with the air gap206in between the structures202and204. However, in this example, the filter device130includes an inner region300, which attenuates the radiation traversing the air gap206to prevent excessive flux from reaching the detector array112, and outer regions302, which only lightly attenuates radiation.

FIG. 4is similar toFIG. 2, except that the flux filter device130includes a filter portion400and brackets402and404, which are configured to rest on the subject support128and which hold the filter portion400over the structures202and204and the air gap206there between. The brackets402and404are elongate and rigid, and include a material which only lightly attenuates the x-ray radiation.FIG. 5is similar toFIG. 3, except that the flux filter device130includes brackets502and504, which hold the inner and outer portions300and302over the structures202and204and the air gap206in there between. Likewise, the brackets502and504include a material which only lightly attenuates the x-ray radiation.

FIG. 6is similar toFIG. 5, except that the flux filter device130does not include the outer portions302. With the configurations shown inFIGS. 4-6, it is to be appreciated that the flux filter device130may include only a single one of the bracket402,404,502or504, or more than two of the brackets402,404,502or504. Furthermore, one or more of the brackets402,404,502or504may be configured to be extendable, which would allow adjustment of a height of the flux filter device130based on a size of the object being scanned. An example extendable bracket may include a telescoping member, a base member and one or more extension members the affix to the base member, a set of interchangeable and different sized brackets, etc.

FIG. 7shows an example in which the flux filter device130ofFIG. 2is placed under the structures202and204and the air gap206between the structures202and204.

With continuing reference toFIGS. 1-7, in one instance, the flux filter device130is a rigid structure in that it does not flex and conform to a shape of the object it is placed on. Rather, the flux filter device130maintains its shape, regardless of the shape of the object. The illustrated flux filter device130includes a material that attenuates radiation so that the photon counting detector pixels112receiving non-attenuated radiation or low attenuated radiation are not saturated. The illustrated flux filter device130includes a material that attenuates radiation by way of photo-electric absorption and by Compton scattering. A suitable material includes a high atomic weight, Z, (e.g., Z≥13) material that has relatively higher photo-electric absorption than water or typical soft-tissue.

Generally, the flux filter device130provides a predetermined compromise between photo-electric absorption and beam hardening. An example of such a material includes Polytetrafluoroethylene (PTFE) which is a synthetic fluoropolymer of tetrafluoroethylene, aluminum (Al) or the like. An example of a suitable PTFE material is Teflon®, which is a product of DuPont Co., USA.

A thickness of the flux filter device130depends on scan protocol parameters such as tube voltage (V), tube current (I), beam conditioner (e.g., a pre patient filter) settings (B). A maximal flux of a central detector pixel can be estimated based on a function F(V,B,I) by way of a theoretical physical model of the scanner or a calibration procedure. For the latter, the flux Fcalib(V,B,Icalib) on the central detector is measured for all possible V and B settings and one current (Icalib). For a maximal flux of FMaxon the detector for a scan with scan protocol parameters FScan,BScan,IScan, the flux filter device130will have a linear absorption μAand thickness tAthat satisfies:

A set of flux filter devices130can be created for one or more different combinations of the scan protocol parameters FScan,BScan, IScan. The particular flux filter device130for a scan can then be selected by the clinician from the set of flux filter devices130that includes a flux filter device130for one or more different combinations of the scan protocol parameters FScan,BScan, IScan. In one instance, a user selects a protocol and the console126presents information that identifies a suitable flux filter device130for the protocol. The flux filter device130, depending on the configuration, can be placed on the object or subject (FIG. 2) to cover the structures and any no or low attenuation region between the structures, or on the subject support128(FIG. 3) to cover the structures and any no or low attenuation region between the structures.

FIG. 8illustrates a method in accordance with an embodiment described herein.

It is to be appreciated that the ordering of the below acts is for explanatory purposes and not limiting. As such, other orderings are also contemplated herein. In addition, one or more of the acts may be omitted and/or one or more other acts may be included.

At802, an object or subject is loaded onto the subject support.

At804, a scan protocol is selected at a console.

At806, a flux filter device is selected based on the scan protocol parameters and the object or subject.

At808, the selected flux filter device is placed on or over the object or subject.

At810, the scan is performed.

At812, the projection data is reconstructed to generate volumetric image data.

FIG. 9illustrates another method in accordance with an embodiment described herein. In this example, the subject is a human or animal patient, and the flux filter device130is placed so that the patient is between flux filter device130and the subject support128, e.g., as shown inFIGS. 2-6.

It is to be appreciated that the ordering of the below acts is for explanatory purposes and not limiting. As such, other orderings are also contemplated herein. In addition, one or more of the acts may be omitted and/or one or more other acts may be included.

At902, a patient is positioned onto the subject support128.

At904, a scan protocol is selected at the console126. In this example, the selected scan protocol causes the radiation source controller110to modulate radiation emission with a modulation pattern that modules the radiation emission between at least a first flux and a second different flux. As described next, such modulation may be dependent on the radiation source108angle.

For example, one modulation pattern will cause the controller110to modulate the radiation emission so that the flux is lower when the radiation source is rotating from the 3 o'clock position, through the 6 o'clock position at which the radiation source108is under a portion of the subject support disposed in the examination region106, to the 9 o'clock position (or from the 9 o'clock position, through the 6 o'clock position, to the 3 o'clock position if the system rotates counter-clockwise).

Furthermore, this modulation pattern will cause the controller110to modulate the radiation emission so that the flux is higher when the radiation source is rotating from the 9 o'clock position, through the 12 o'clock position at which the radiation source108is opposite the portion of the subject support disposed in the examination region106, to the 3 o'clock position (or from the 3 o'clock position, through the 12 o'clock position, to the 9 o'clock position).

The flux can be modulated through controlling a heating current in a cathode of the radiation source108. In another instance, the lower of the two fluxes is no flux, e.g., using a grid switch, a physical filter, etc. to inhibit radiation from traversing the examination region106.

At906, a flux filter device130is selected from a set of flux filter devices130based on the scan protocol parameters.

At908, the selected flux filter device130is placed on or over the patient, as described herein and/or otherwise.

At910, the patient is scanned using the modulation pattern.

For example, during the scan, the radiation source controller110modulates the radiation emission so that the flux is a lower flux as the radiation source rotates from the 3 o'clock position, through the 6 o'clock position, to the 9 o'clock position (or from the 9 o'clock position, through the 6 o'clock position, to the 3 o'clock position for a counter clockwise rotation), and a higher flux as the radiation source rotates from the 9 o'clock position, through the 12 o'clock, to the 3 o'clock position (or from the 3 o'clock position, through the 12 o'clock position, to the 9 o'clock position for a counter clockwise rotation).

At912, the projection data is reconstructed to generate volumetric image data.

InFIG. 9, the filter device130is placed opposite the subject support128with the objects202and204between the filter device130and the subject support128as shown inFIGS. 2-6. In an embodiment in which the filter device130is placed between the objects202and204and the subject support128, as shown inFIG. 7, the flux is modulated so the flux is at the lower level as the radiation source rotates from the 9 o'clock position, through the 12 o'clock, to the 3 o'clock position and at the higher flux as the radiation source rotates from the 3 o'clock position, through the 6 o'clock position, to the 9 o'clock position. In general, the particular modulation pattern utilized is selected so that it attenuates the flux before it passes the patient rather than afterwards because any attenuation before the patient implies a reduction of x-ray dose, whereas an attenuation afterwards imply a waste of dose.

More particularly, by modulating the radiation emission as such, radiation dose to the patient is reduced while the radiation source108is at a location in which radiation traversing the object or subject is subsequently filtered, relative to the location in which the radiation is filtered prior to traversing the patient. Where the radiation source108is at a location at which radiation is filtered prior to traversing the patient, the flux seen by the detector is reduced and the patient dose is lowered. Where the radiation source108is at a location at which radiation traversing the object or subject is subsequently filtered, the flux seen by the detector is reduced but without the reduction in patient dose, which results in wasted dose in that the x-ray traversing a patient are filtered and do not contribute to generation of the volumetric image data. The above modulation pattern reduces this dose inefficiency, which includes dose to the patient that is not utilized to generate the volumetric image data.

FIGS. 11-14illustrates an embodiment in which the flux filter device130removably installs within or inside of the tabletop134.FIG. 11shows a perspective view of the flux filter device130.FIG. 12shows a top down view of the flux filter device130.FIG. 13shows a first cross-sectional view of the flux filter device130along line A-A ofFIG. 12.FIG. 14shows a second cross-sectional view of the flux filter device130along line B-B ofFIG. 12.

The flux filter device130includes one or more flux reducing elements. For sake of clarity and brevity, two flux reducing elements1102and1104are shown in this example. Each flux reducing element1102(or1104) has a shape of a right triangle, with a first side1106(or1108) that extends along the z-axis, a second side1110(or1112) that extends along an x-axis perpendicular from the first side1106(or1108), and a third side1114(or1116) which is opposite the right angle formed at the intersection of the first and second sides1106(or1108) and1110(or1112). Other shapes are also contemplated herein.

The flux reducing elements1102and1104are aligned in a cavity of the tabletop134with respect to each other in an x/z plane with the first sides1106and1108facing each other. A position of the one or more flux reducing elements1102and1104is adjustable manually and/or by external control in the x and/or z directions. Based on a scout and/or other scan, the one or more flux reducing elements1102and1104are positioned so that they add x-ray absorbing material to regions with low absorption and/or no absorption.

FIG. 15schematically illustrates an example in which the one or more flux reducing elements1102and1104attenuate x-rays1502traversing lungs1504(i.e., low absorption) of a patient1506.FIG. 16schematically illustrates an example in which the one or more flux reducing elements1102and1104attenuate x-rays1602traversing an empty space1604between legs1606(i.e., no absorption) of a patient1608and/or an inner periphery1610of the legs1606(i.e., low absorption). X-rays traversing higher attenuating portions of the patients1506and1608are not shown for sake of clarity.

For a scan covering the thorax and at least a sub-portion of the lower extremities, the one or more flux reducing elements1102and1104are moved at least in the x-direction from a position in which there is a non-zero gap1508between the one or more flux reducing elements1102and1104for scanning the lungs (FIG. 15) to a different position in which the one or more flux reducing elements1102and1104abut and form a continuous additional x-ray absorbing region between the legs for scanning the lower extremities (FIG. 16). The movement can be continuous or discrete. Moving the flux-reducing device130during the scan is well suited for a scan with a large extent in z-direction, where the flux reducing elements1102and1104would not perfectly cover the regions of low absorption without such a movement. Otherwise, the flux reducing elements1102and1104remain stationary with respect to the tabletop134.

InFIGS. 11-16, the flux filter device130is installed so that the first sides1106and1108are proximate to the rotating gantry104. This configuration is well suited where the patient is lying on the tabletop134with their head proximate and their feet distal to the rotating gantry104. In a variation, the flux filter device130is installed so that the first sides1106and1108are distal to the rotating gantry104to the rotating gantry104. This configuration is well suited where the patient is lying on the tabletop134with their head distal and their feet proximate to the rotating gantry104. Furthermore, the flux filter device130is shown inFIGS. 13-16with curved sides. It is to be understood that the curvature shown is non-limiting, and the flux filter device130may have other radii of curvature, flat sides, irregular sides, and/or other shaped sides.

FIG. 17schematically illustrates an embodiment in which the one or more flux reducing elements1102and1104are installed and removed from the tabletop134from sides1702of the tabletop134.FIG. 18schematically illustrates an embodiment in which the one or more flux reducing elements1102and1104are installed and removed from the tabletop134from a back1802of the tabletop134. In another instance, the one or more flux reducing elements1102and1104can be installed in the tabletop134and removed from the tabletop134from a front1804and/or other region of the tabletop134. The one or more flux reducing elements1102and1104are installed and removed through an access.

FIG. 19shows a variation in which the flux filter device130includes a plurality of plates1902. The plurality of plates1902has invariant cross-sections over the z direction that is larger than a size of the beam in the z direction. During a scan, the plurality of plates1902can be moved within the tabletop134so that the position of the plurality of plates1902does not change with respect to a rotating gantry102. The plurality of plates1902can be installed and removed from the tabletop134as described in connection withFIGS. 17 and 18and/or otherwise.

FIG. 20shows a variation in which one or more hollow containers2002are disposed inside the tabletop134. In this variation, the one or more hollow containers2002can be filled with a highly absorbing gas2004(e.g., Xenon) through conduits2006. The absorption of the one or more hollow containers2002can be modified by adjusting a pressure of the gas inside the one or more hollow containers2002via a pressure regulator2008. The gas2004and/or hollow containers2002may reside in the base132of the subject support128, the stationary gantry104, a portion of the tabletop134that is not irradiated, and/or otherwise.

In a variation ofFIG. 20, the one or more hollow containers2002may include an expandable and/or a flexible container such as a bag, a balloon, etc. The expandable and/or a flexible container can be used inside and/or outside of the tabletop134. For example, the expandable and/or a flexible container may be filled (or pre-filled) with the highly absorbing gas2004and/or other absorbing gas, and then positioned between extremities outside of the tabletop134. In this instance, the expandable and/or a flexible container attenuates the radiation traversing the air gaps206and1604shown inFIGS. 2-6 and 16to prevent excessive flux from reaching the detector array112. The expandable and/or a flexible container may be squeezed and held in place via the extremities, a support or holding device, and/or otherwise.

With the configurations ofFIGS. 11-20, the tube current can be modulated as described herein to decrease the flux when the additional attenuating material is between the patient1506and1608and the detector array112such that x-rays first traverse the patient1506and1608and then the additional attenuating material of the flux filter device130, and to increase the flux when the additional attenuating material is not between the patient1506and1608and the detector array112such that x-rays first traverse the additional attenuating material and then the patient1506and1608. This will facilitate mitigating dose inefficiencies, as described herein.

FIG. 21illustrates another method in accordance with an embodiment described herein.

It is to be appreciated that the ordering of the below acts is for explanatory purposes and not limiting. As such, other orderings are also contemplated herein. In addition, one or more of the acts may be omitted and/or one or more other acts may be included.

At2102, a patient is positioned on the subject support128.

At2104, a scan protocol is selected at the console126. In this example, the selected scan protocol causes the radiation source controller110to modulate radiation emission with a modulation pattern that modules the radiation emission between at least a lower and a higher different flux, depending on the radiation source108angle.

At2106, a flux filter device130is selected from a set of flux filter devices130based on the scan protocol parameters.

At2108, the selected flux filter device130is moved into position in the tabletop134. As described herein, this may include moving the physical mechanical devices1102and1104into position and/or filing the one or more hollow containers2002with the highly absorbing gas2004.

At2110, the patient is scanned using the modulation pattern and moving the flux filter device130, if needed.

At2112, the projection data is reconstructed to generate volumetric image data.

Generally, the different embodiments of the flux filter device130described herein can be used with X-ray and CT systems with photon counting detectors to solve the count rate problem. The different embodiments of the flux filter device130can be used for medical applications scanning the thorax, extremity, etc. as well as dental and/or non-medical applications such as non-destructive testing, etc.