Dynamic bowtie filter and methods of using the same

Abstract

An imaging system (100) includes a radiation source (708) that emits radiation that traverses an examination region (706), a radiation detector array (716) with a plurality of detectors (1104N) that detect the radiation that traverses the examination region, a dynamic bowtie filter (718) between the radiation source and the examination region, a first motor (7221) and a second motor (7222), and a controller (724). The dynamic bowtie filter includes a first half wedge (7181) and a second half wedge (7182). The first motor is in mechanical communication with the first half wedge and moves the first half wedge and the second motor is in mechanical communication with the second half wedge and moves the second half wedge. The controller independently controls the first and second motors to move the first and second half wedges.

Claims

1. An imaging system, comprising: a radiation source configured to emit radiation that traverses an examination region; a radiation detector array having a plurality of detectors configured to detect the radiation traversing the examination region; a dynamic bowtie filter disposed between the radiation source and the examination region, wherein the dynamic bowtie filter comprises a first half wedge and a separate and distinct second half wedge with a material free space therebetween; a first motor in mechanical communication with the first half wedge, wherein the first motor is configured to move the first half wedge; a second motor in mechanical communication with the second half wedge, wherein the second motor is configured to move the second half wedge; and controller circuitry configured to independently control the first and second motors to move the first and second half wedges to increase or decrease a distance therebetween during an acquisition interval, wherein the controller circuitry is further configured to, prior to the moving of the first half wedge and the second half wedge: perform a survey scan of the subject of object to produce a projection image of the subject of object; identify a contour and a center point of the subject or object from the projection image; create a wafer-equivalent homogenous ellipse for the subject of object using the center point; and determine the predetermined wedge position profile based on the water-equivalent homogeneous ellipse.

2. The system of claim 1, wherein the first half wedge and the second half wedge have a same size, shape, and density.

3. The system of claim 1, wherein the controller circuitry is configured to move the first half wedge and second half wedge in a same direction at a point in time.

4. The system of claim 1, wherein the controller circuitry is configured to move the first half wedge and second half wedge in opposite directions at a point in time.

5. The system of claim 1, wherein the first and second motors are linear motors.

6. The system of claim 5, wherein the controller circuitry is further configured to determine a geometric center of an object or subject by creating a mathematical body from a survey scan of the object or subject.

7. The system of claim 1, wherein the controller circuitry is configured to move at least one of the first half wedge and the second half wedge from a first location to a different location, which reduces an x-ray fluence at the radiation detector array.

8. The system of claim 1, further comprising: a first mover and a second mover, wherein the first mover is coupled to a first half wedge holder and the first motor, and the second mover is coupled to the second half wedge holder and the second motor, and the first half wedge holder is coupled to the first half wedge, and the second half wedge holder is coupled to the second half wedge.

9. The imaging system according to claim 1, wherein the first half wedge and the second half wedge do not overlap in a direction from the radiation source to the radiation detector array.

10. An imaging method, comprising: emitting, from a radiation source, radiation that traverses an examination region; detecting, by a radiation detector array, the radiation traversing the examination region; providing a dynamic bowtie filter between the radiation source and the examination region; attenuating rays of the emitted radiation during a scan of a subject or object with the dynamic bowtie filter, wherein the dynamic bowtie filter comprises a first half wedge and a second half wedge with a material free space therebetween; and independently moving, with a controller, the first half wedge and the second half wedge to increase or decrease a distance therebetween during a scan based on a predetermined wedge position profile, wherein the method further comprises, prior to the independently moving of the first half wedge and the second half wedge: performing a survey scan of the subject or object to produce a projection image of the subject or object; identifying a contour and a center point of the subject or object from the projection image; creating a water-equivalent homogeneous ellipse for the subject or object using the center point; and determine the predetermined wedge position profile based on the water-equivalent homogeneous ellipse.

11. The method of claim 10, further comprising: determining a position of the first half wedge or the second half wedge at which a fluence at a detector satisfies a predetermined fluence acceptance criterion.

12. The method of claim 11, further comprising: creating the predetermined wedge position profile with positions of the first half wedge and the second half wedge at which fluences at the detectors satisfy the predetermined fluence acceptance criterion.

13. The method according to claim 10, wherein the first half wedge and the second half wedge do not overlap in a direction from the radiation source to the radiation detector array.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention.

(2) FIG. 1 illustrates a CT scanner with a prior art static bowtie filter where a center of an object is centered at an iso-center.

(3) FIG. 2 illustrates the CT scanner with the prior art static bowtie filter where the center of the object is above (off-center, or not center with) the iso-center.

(4) FIG. 3 illustrates the CT scanner with the prior art static bowtie filter where the center of the object is centered at the iso-center.

(5) FIG. 4 illustrates the CT scanner with the prior art static bowtie filter where the center of the object is below the iso-center.

(6) FIG. 5 illustrates the CT scanner with the prior art static bowtie filter where the center of the object is to the right of the iso-center.

(7) FIG. 6 illustrates the CT scanner with the prior art static bowtie filter where the center of the object is to the left of the iso-center.

(8) FIG. 7 diagrammatically illustrates an example imaging system with a dynamic bowtie filter in accordance with an embodiment described herein.

(9) FIG. 8 diagrammatically illustrates an example of the dynamic bowtie filter in accordance with an embodiment herein.

(10) FIG. 9 diagrammatically illustrates a side view of an example of a drive system for the dynamic bowtie filter in accordance with an embodiment herein.

(11) FIG. 10 diagrammatically illustrates a bottom view of the example of the drive system for the dynamic bowtie filter in accordance with an embodiment herein.

(12) FIG. 11 diagrammatically illustrates an example of a subject or object in the imaging system in accordance with an embodiment herein.

(13) FIG. 12 diagrammatically illustrates an example of a mathematical body in the imaging system in accordance with an embodiment herein.

(14) FIG. 13 diagrammatically illustrates an example of a method in accordance with an embodiment herein.

(15) FIG. 14 shows a flowchart of a method in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

(16) FIG. 7 diagrammatically illustrates an imaging system 700 such as a computed tomography (CT) scanner. The imaging system 700 includes a generally stationary gantry 702 and a rotating gantry 704, which is supported by the stationary gantry 702 via a bearing (not visible) or the like. The rotating gantry 704 rotates around an examination region 706 about a longitudinal or z-axis (“Z”). A radiation source 708, such as an X-ray tube, is supported by and rotates with the rotating gantry 704 and emits X-ray radiation via a focal spot 710 of the radiation source 708.

(17) A source collimator 712 is disposed between the radiation source 708 and the examination region 706 and collimates the emitted radiation to produce a collimated beam 714 having a pre-determined geometrical shape (e.g., fan, wedge, cone, etc.). The collimated beam 714 traverses the examination region 706 (and a portion of any object or subject therein, which attenuates the beam as a function of the radiodensity of the object or subject) and illuminates a radiation sensitive detector array 716. The radiation sensitive detector array 716 subtends an angular arc opposite the radiation source 708 across the examination region 706 and includes a plurality of detectors that detect radiation traversing the examination region 706 and outputs an electrical signal (line integrals, intensity data, or projection data) indicative thereof.

(18) A dynamic bowtie filter 718 is arranged between the x-ray tube 708 and the collimator 712 and attenuates the collimated beam. In the illustrated example, the dynamic bowtie filter 718 comprises two half wedges 718.sub.1 and 718.sub.2 that spatially attenuate the emitted radiation to shape the X-ray fluence profile. Each half wedge 718.sub.1 and 718.sub.2 is coupled to a half wedge holder (not visible), which are coupled to movers 720.sub.1 and 720.sub.2, which are coupled to motors 722.sub.1 and 722.sub.2. In one embodiment, the movers 720.sub.1 and 720.sub.2 are linear stages and the motors 722.sub.1 and 722.sub.2 are linear motors. An example of a suitable linear stage includes products of Chieftek Precision Co. (CPC), LTD, Tainan, Taiwan. In other embodiments, the movers 720.sub.1 and 720.sub.2 may include at least a lead screw, a ball screw, a chain, a gear, or the like, driven with a suitable motor. The movers 720.sub.1 and 720.sub.2 can be coupled to the half wedge holders and/or the motors 722.sub.1 and 722.sub.2 via a fastener such as an adhesive (e.g., glue), a screw, a rivet, a clamp, and the like.

(19) A controller 724 controls one or both of the motors 722.sub.1 and 722.sub.2 to move the movers 720.sub.1 and 720.sub.2 and hence the half wedges 718.sub.1 and 718.sub.2. In one instance, such control includes complete independent control of the two half-wedges in which movement of one of the half wedges 718.sub.1 and 718.sub.2 is independent of movement of the other of the half wedges 718.sub.1 and 718.sub.2. With this control, the half wedges 718.sub.1 and 718.sub.2 can both be moved in a same direction or in different directions at any point in time. This also includes moving only one of the half wedges 718.sub.1 and 718.sub.2. Optional control includes moving the half wedges 718.sub.1 and 718.sub.2 such that movement of one of the half wedges 718.sub.1 and 718.sub.2 is dependent on movement of the other of the half wedges 718.sub.1 and 718.sub.2. This may include moving them in a same direction, by an approximately same distance, with an approximately same velocity, at an approximately same time, all within pre-determined tolerances.

(20) As described in greater detail below, the dynamic bowtie filter 718 is configured to dynamically move one or both (individually and/or concurrently) of the wedges 718.sub.1 and 718.sub.2 to adjust the X-ray fluence profile, e.g., through a physical movement (e.g., translation) of one or more of the wedges 718.sub.1 and 718.sub.2 relative to source 708. In one instance, the movement compensates for an object or subject positioned off-center with respect to iso-center 732 of the imaging system 700, where a center of geometry of the object is above, below, to the left of, or to the right, or a combination thereof, of the iso-center 732. As such, the system(s) and/or method(s) described herein, in one instance, can mitigate overdose resulting from mis-centering (described herein in connection with FIGS. 1-6) an object or subject in the examination region 706 of the imaging system 700.

(21) A reconstructor 726 reconstructs the electrical signals and generates three-dimensional volumetric image data. Example processing when using a filtered back-projection reconstruction algorithm includes, for each view, normalize the intensity data output by the detector array 716, perform a mathematical logarithm operation on the normalized intensity data, remove attenuation of the dynamic bowtie filter 718 from the logged data, optionally perform a correction (e.g., a beam hardening correction), back-project the corrected data, and convolve the back-projected data with a high pass filter. The volumetric image data (tomographic images) is generated therefrom. Generally, a view is the data collected across the detector array for an acquisition angle, and an acquisition interval refers to a period of time wherein the collimated beam illuminates the detector array and the detector array 716 detects the radiation over a predetermined angular increment of the rotating gantry. This is referred to as an integration period.

(22) A subject support 728, such as a couch, supports a subject or an object in the examination region 706. A general purpose computing system serves as an operator console 730, which includes human readable output devices such as a display and/or printer and input devices such as a keyboard and/or mouse and allows the operator to control the operation of the system 700, for example, allowing the operator to select a protocol that employs the dynamic bowtie filter 718, initiate scanning, etc. The console 730 includes one or more computer processors (e.g., a central processing unit or CPU, a microprocessor, etc.) and computer readable storage medium, which excludes transitory medium, such as physical memory, a memory device, and/or other non-transitory storage medium. The computer readable storage medium includes one or more computer readable instructions. The one or more computer processors are configured to execute at least one of the one or more computer readable instructions and/or instructions carried by a carrier wave, a signal and/or other transitory medium.

(23) In a variation, the imaging system 700 includes multiple radiation sources 708 and multiple dynamic bowtie filters 718.

(24) FIG. 8 diagrammatically illustrates an example embodiment 800 of the dynamic bowtie filter 718 comprising separate and distinct (not connected) half wedges 718.sub.1 and 718.sub.2. The half wedges 718.sub.1 and 718.sub.2 have a greater thickness (in a direction 806 from the source 708 to the detector array 716) at an outer region 802 of the half wedges 718.sub.1 and 718.sub.2 relative to an inner region 804 of the half wedges 718.sub.1 and 718.sub.2. Half wedges 718.sub.1 and 718.sub.2 more heavily filter the collimated beam 714 passing through the thicker outer regions 802 of the half wedges 718.sub.1 and 718.sub.2 relative to X-rays passing through the thinner inner region 804 of the half wedges 718.sub.1 and 718.sub.2. Due to its shape, the dynamic bowtie filter 718 varies the degree of the beam attenuation there between. The half wedges 718.sub.1 and 718.sub.2 may comprise a material(s) such as Teflon®, a product of Chemours, USA, aluminum oxide (Al.sub.2O.sub.3), and/or other material suitable for shaping a fluence profile of the beam 714 for CT scanning.

(25) FIG. 9 diagrammatically illustrates a side view of an embodiment of a drive system 900 supporting the half wedges 718.sub.1 and 718.sub.2, and including half wedge holders 902.sub.1 and 902.sub.2, the movers 720.sub.1 and 720.sub.2, and the motors 722.sub.1 and 722.sub.2 from a view along a Z-axis direction. FIG. 10 diagrammatically illustrates a bottom view of the drive system 900. The half wedge holders 902.sub.1 and 902.sub.2 are coupled to outer regions 904.sub.1 and 904.sub.2 of the half wedges 718.sub.1 and 718.sub.2. The motors 722.sub.1 and 722.sub.2, in response to a command from the controller 730, adjust the position of the half wedge holders 902.sub.1 and 902.sub.2 so that the collimated beam 714 (collimator 712 not shown), emitted from the focal spot 710 of the source 708, transverses portions of inner region 906.sub.1 and 906.sub.2 of the half wedges 718.sub.1 and 718.sub.2, and not the half wedge holders 902.sub.1 and 902.sub.2.

(26) In response to a command from the controller 724, the motors 722.sub.1 and 722.sub.2 may move (e.g., translate) the movers 720.sub.1 and 720.sub.2, and thereby the half wedges 718.sub.1 and 718.sub.2 in an X/Y-plane (e.g., a direction 908), which is transverse to the Z-axis direction or Z direction. In another embodiment, the half wedges 718.sub.1 and 718.sub.2 are moved in the X, Y, and/or Z direction at any distance. The movement may increase or decrease a distance 910 between the half wedges 718.sub.1 and 718.sub.2, before, during, and/or after an acquisition interval. Again, an acquisition interval, as used herein, refers to a period of time wherein the collimated beam 714 illuminates the detector array 716 and the detector array 716 detects the radiation over a predetermined angular increment of the rotating gantry 704 (also referred to as an integration period). Furthermore, the motors 722.sub.1 and 722.sub.2 may move only one of the half wedges 718.sub.1 or 718.sub.2, or move both, serially or in parallel, a same or different distance, towards or away from one another at the same time.

(27) As briefly stated above, the dynamic bowtie filter 718 is configured to dynamically move one or both (individually and/or concurrently) of the wedges 718.sub.1 and 718.sub.2 to adjust the X-ray fluence profile. In one instance, the wedges 718.sub.1 and/or 718.sub.2 are dynamically translated (in or out on the X/Y-plane) until a fluence profile, across the detectors in the detector array 716, after traversing the dynamic bowtie filter 718 and the object or subject, satisfies a given acceptance criterion. FIGS. 11 and 12 describe a non-limiting approach for determining a fluence at the detectors in the detector array 716. FIG. 11 diagrammatically illustrates an example of a mis-centered object or subject 1102, and FIG. 12 diagrammatically illustrates using, conceptually, a water-equivalent mathematical ellipse 1200 to estimate the fluence at the detectors for the mis-centered object or subject in FIG. 11.

(28) In FIG. 11, a center point 1110 of the mis-centered object or subject 1102 is below the iso-center 732. In general, an X-ray radiation fluence at a given detector 1104.sub.N of the detector array 716 is a function of an output of the radiation source 708 at an angle α from an x-ray 1112, an attenuation of the collimated beam 714 by the half wedges 718.sub.1 and 718.sub.2 (a half wedge path length 1106), an attenuation of the collimated beam 714 by the object or subject 1102 (an object or subject path length 1108), and scattered radiation from the object or subject being scanned incident on a detector 1104.sub.N of the detector array 716. Where a magnitude of the scattered radiation is negligible compared to a magnitude of the attenuated primary radiation (non-scatter radiation traversing from the source 708 to the detector array 716), a fluence at the given detector 1104.sub.N in the detector array can be determined as shown in EQUATION 1:
Fluence (at detector 1104.sub.N)=(Output of the radiation source 708 at angle α) (attenuation of a half wedge 718.sub.1 or 718.sub.2 at the angle α) (attenuation of the object or subject 1102 at angle α).  EQUATION 1:
In FIG. 12, the console 730 conceptually replaces the object or subject 1102 with the water-equivalent mathematical ellipse 1200 centered at 1202, which is the center point 1110 of the mis-centered object or subject 1102.

(29) For this, the imaging system 700 is operated to perform a survey scan (e.g., a scout, a surview, a pilot, etc. scan) to acquire a two or three-dimensional projection image(s) of the subject or object 1102. A contour of the subject or object is identified in the projection image(s) to estimate the boundary of the subject or object 1102. From the boundary, the console 730 creates the water-equivalent homogeneous ellipse 1200, wherein lengths of the major and minor axes of the ellipse are derived from the boundary of the object 1102. The console 730 determines the center point 1202 of the water-equivalent mathematical ellipse from projection image(s). The coordinates of the center point 1202 are determined for each axial slice and used to generate two half wedge position profiles, one for half wedge 718.sub.1 and one for 718.sub.2.

(30) FIG. 13 illustrates a method which dynamically moves the wedges 7181 and/or 7182 to adjust the X-ray fluence profile based on the fluence estimate.

(31) At 1300, a survey scan of a subject is performed, thereby producing a projection image.

(32) At 1302, a water-equivalent homogeneous ellipse of the subject or object is created from the projection image, as described herein and/or otherwise.

(33) At 1304, the console 730 mathematically estimates a fluence of a detector 1104.sub.N with the first half wedge 718.sub.1 and the second half wedge 718.sub.2 at a current position.

(34) At 1306, the console 730 determines if the estimated fluence satisfies a predetermined fluence acceptance criterion.

(35) If so, then at 1308, the console 730 adds the position of the first half wedge 718.sub.1 and/or the second half wedge 718.sub.2 to a position profile.

(36) If not, then steps 1302-1306 are repeated for a different position of the first half wedge 718.sub.1 and/or the second half wedge 718.sub.2.

(37) Where there are more than one gantry rotation angle and/or more than one projection image the steps 1300-1308 are repeated for more than one gantry rotation angle and/or more than one projection image.

(38) While the above example estimates the fluence at a single detector and determines if the estimated fluence for a single detector satisfies a predetermined fluence acceptance criterion, these steps may apply to all or less than all of the detectors.

(39) A non-limiting example of the predetermined fluence acceptance criterion includes uniformity of signal across the detector. Examples of uniformity criteria include: no detector reading below a designated minimum and/or a % of the designated minimum, a least mean squared difference (from a designated value) for all/limited set of detector readings, etc. For sake of brevity, an example where the acceptance criteria include uniformity of the signal across the detectors is described below.

(40) FIG. 14 illustrates a method which dynamically moves the wedges 7181 and/or 7182 to adjust the X-ray fluence profile based on the fluence estimate and a uniformity acceptance criterion of uniformity of the signal across all detectors.

(41) At 1400, a survey scan of a subject or object is performed, thereby producing a projection image.

(42) At 1402 a water-equivalent homogeneous ellipse of the subject or object is created for the projection image, as described herein and/or otherwise.

(43) At 1404, the half wedges 718.sub.1 and 718.sub.2 are first positioned at a predetermined initial location P.sub.0, e.g., the position with a maximum width between the two half wedges 718.sub.1 and 718.sub.2 or other position, as described herein and/or otherwise.

(44) At 1406, a fluence through the water-equivalent mathematical ellipse at each detector in the detector array is calculated with the half wedges 718.sub.1 and 718.sub.2 at the location P.sub.0, e.g., with EQUATION 1 and/or otherwise.

(45) At 1408, the fluence for all the detectors 1204.sub.i, is compared to the fluence at detector 1204.sub.1.

(46) If the fluence for all the detectors 1204.sub.i is greater than the fluence at 1204.sub.1, then at 1410 the distance between half wedges 718.sub.1 and 718.sub.2 is reduced, as described herein and/or otherwise, thereby placing the half wedges 718.sub.1 and 718.sub.2 into position P.sub.N, and acts 1406 and 1408 are repeated.

(47) However, if the fluence for any of the detectors 1204.sub.i is not greater (i.e. less) than the fluence at 1204.sub.1, then at 1412, the position P.sub.N-1 (the previous position) for each half wedge 718.sub.1 and 718.sub.2 is added to a position profile.

(48) At 1414 the imaging system 700 scans the subject or object 1102 wherein the drive system 900 moves the half wedges 718.sub.1 and 718.sub.2 based on the calculated half wedge positions during an acquisition interval.

(49) At 1416, the controller 724 measures the half wedge 718.sub.1 and 718.sub.2 during an integration period, averages the measured half wedge positions for that integration period, and sends the averaged half wedge positions to the reconstructor 726.

(50) In other words (and with reference to FIG. 12), with using the acceptance criterion of uniformity of the signal across all detectors 1204.sub.N (and thereby minimizing over dose to the object or subject being scanned), the fluence for each angle is modulated (via moving the half wedge 7181 and/or 7182) so that the fluence at the detector 1204.sub.1 is a maximum fluence as compared to fluences at the other detectors 1204.sub.i and the fluences at the other detectors 1204.sub.i are modulated, via a half wedge setting, so that the fluences at the other detectors 1204.sub.i are closer to the minimum fluence.

(51) Where there are more than one gantry rotation angles and/or more than one projection images, then steps 1400-1412 are repeated for each gantry rotation angle/projection image. For the sake of brevity, the above example includes one gantry rotation angle and one projection image.

(52) The above may be implemented by way of computer readable instructions, encoded or embedded on computer readable storage medium, which, when executed by a computer processor(s), cause the processor(s) to carry out the described acts. Additionally or alternatively, at least one of the computer readable instructions is carried by a signal, carrier wave or other transitory medium, which is not computer readable storage medium.

(53) While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

(54) In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

(55) A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with, or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.