Patent Application
20210059633
The following generally relates to imaging and more particular to a dynamic bowtie filter utilized to reduce radiation delivered to off-centered objects in a CT-scanner.
A computed tomography (CT) scanner includes an X-ray tube that emits radiation and that is mounted on a rotatable gantry that rotates around an examination region about a z-axis. A source collimator is disposed between the x-ray tube and the examination region and collimates the radiation to produce an X-ray radiation beam. The beam traverses the examination region and an object or subject therein (which attenuates the beam as a function of the radiodensity of the object or subject) and illuminates a detector array disposed across the examination region from the X-ray tube. The detector produces projection data indicative of the detected radiation, and the projection data is reconstructed to generate volumetric image data indicative of the subject or object. A filter, referred to as a “bowtie” filter, due to its shape, is located between the x-ray tube and the source collimator and spatially attenuates the beam. Due to its shape (e.g., thinner at a central region and thicker at the periphery regions), the bowtie filter heavily attenuates regions of the beam at the periphery of the beam, lightly attenuates the region about the center of the beam, and varies the degree of attenuation there between. As a result of the beam attenuation, the X-ray radiation dose to the object or subject is reduced.
A currently available commercial CT system has a bowtie filter mounted on the rotatable gantry. The bowtie filter is fixed (static), relative to the X-ray tube, and is designed to reduce the x-ray fluence modulation at the detector array, relative to a configuration with no bowtie filter, when scanning a subject. Optimal dose reduction occurs when the geometric center of the object or subject coincides with the iso-center of the CT scanner. For a centered object or subject, the range of fluence to the detector array and the absorbed dose in the patient are minimized. For a mis-centered subject or object, the range of the fluence to the detector array and the patient dose are increased. In general, mis-centering means the geometric center of the object or subject does not coincide with (or is off-center with respect to) the iso-center of the CT scanner. As a result of mis-centering, the static bowtie filter may not produce an optimized fluence modulation at the detector array, and thus dose reduction is not optimized. The literature indicates that because of mis-centering, the increase in dose (versus a centered subject) may be thirty-three percent (33%).
An example of mis-centering of an object with respect to an iso-center of a CT scanner with a static bowtie filter is shown in
Aspects described herein address the above-referenced problems and/or others.
In one aspect, an imaging system includes a radiation source that emits radiation that traverses an examination region, a radiation detector array with a plurality of detectors that detect the radiation that traverses the examination region, a dynamic bowtie filter between the radiation source and the examination region, a first and second motor, and a controller. The dynamic bowtie filter includes a first and second half wedge with a material free space there between. 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 to increase or decrease the distance there between during an acquisition interval.
In another aspect, a computer readable storage medium is encoded with computer executable instructions, which when executed by a processor, causes the processor to obtain one of more projection image(s) of a subject or object generated with one of more survey scan(s) of the subject or object, creates a mathematical ellipse for the subject or object from the projection image(s) and calculates, for an acquisition angle, a first fluence at a detector of a detector array with the mathematical ellipse with a first half wedge or a second half wedge of a bowtie filter at a first position. The instructions, when executed by the processor, further causes the processor to compare the first calculated fluence with a predetermined fluence acceptance criterion and add the first position to a position profile only in response to the first calculated fluence satisfying the predetermined fluence acceptance criterion.
In another aspect, a method includes attenuating rays of an emitted radiation beam during a scan of a subject or object with a dynamic bowtie filter. The dynamic bowtie filter includes a first half wedge and a second half wedge. The method further includes independently moving, with a controller, the first half wedge and the second half wedge to increase or decrease a distance between the first and second half wedges during a scan based on a predetermined wedge position profile.
Those skilled in the art will recognize still other aspects of the present application upon reading and understanding the attached description.
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.
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.
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 7181 and 7182 that spatially attenuate the emitted radiation to shape the X-ray fluence profile. Each half wedge 7181 and 7182 is coupled to a half wedge holder (not visible), which are coupled to movers 7201 and 7202, which are coupled to motors 7221 and 7222. In one embodiment, the movers 7201 and 7202 are linear stages and the motors 7221 and 7222 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 7201 and 7202 may include at least a lead screw, a ball screw, a chain, a gear, or the like, driven with a suitable motor. The movers 7201 and 7202 can be coupled to the half wedge holders and/or the motors 7221 and 7222 via a fastener such as an adhesive (e.g., glue), a screw, a rivet, a clamp, and the like.
A controller 724 controls one or both of the motors 7221 and 7222 to move the movers 7201 and 7202 and hence the half wedges 7181 and 7182. In one instance, such control includes complete independent control of the two half-wedges in which movement of one of the half wedges 7181 and 7182 is independent of movement of the other of the half wedges 7181 and 7182. With this control, the half wedges 7181 and 7182 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 7181 and 7182. Optional control includes moving the half wedges 7181 and 7182 such that movement of one of the half wedges 7181 and 7182 is dependent on movement of the other of the half wedges 7181 and 7182. 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.
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 7181 and 7182 to adjust the X-ray fluence profile, e.g., through a physical movement (e.g., translation) of one or more of the wedges 7181 and 7182 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
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.
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.
In a variation, the imaging system 700 includes multiple radiation sources 708 and multiple dynamic bowtie filters 718.
In response to a command from the controller 724, the motors 7221 and 7222 may move (e.g., translate) the movers 7201 and 7202, and thereby the half wedges 7181 and 7182 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 7181 and 7182 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 7181 and 7182, 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 7221 and 7222 may move only one of the half wedges 7181 or 7182, or move both, serially or in parallel, a same or different distance, towards or away from one another at the same time.
As briefly stated above, the dynamic bowtie filter 718 is configured to dynamically move one or both (individually and/or concurrently) of the wedges 7181 and 7182 to adjust the X-ray fluence profile. In one instance, the wedges 7181 and/or 7182 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.
In
Fluence (at detector 1104N)=(Output of the radiation source 708 at angle α) (attenuation of a half wedge 7181 or 7182 at the angle α) (attenuation of the object or subject 1102 at angle α). EQUATION 1:
In
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 7181 and one for 7182.
At 1300, a survey scan of a subject is performed, thereby producing a projection image.
At 1302, a water-equivalent homogeneous ellipse of the subject or object is created from the projection image, as described herein and/or otherwise.
At 1304, the console 730 mathematically estimates a fluence of a detector 1104N with the first half wedge 7181 and the second half wedge 7182 at a current position.
At 1306, the console 730 determines if the estimated fluence satisfies a predetermined fluence acceptance criterion.
If so, then at 1308, the console 730 adds the position of the first half wedge 7181 and/or the second half wedge 7182 to a position profile.
If not, then steps 1302-1306 are repeated for a different position of the first half wedge 7181 and/or the second half wedge 7182.
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.
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.
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.
At 1400, a survey scan of a subject or object is performed, thereby producing a projection image.
At 1402 a water-equivalent homogeneous ellipse of the subject or object is created for the projection image, as described herein and/or otherwise.
At 1404, the half wedges 7181 and 7182 are first positioned at a predetermined initial location P0, e.g., the position with a maximum width between the two half wedges 7181 and 7182 or other position, as described herein and/or otherwise.
At 1406, a fluence through the water-equivalent mathematical ellipse at each detector in the detector array is calculated with the half wedges 7181 and 7182 at the location P0, e.g., with EQUATION 1 and/or otherwise.
At 1408, the fluence for all the detectors 1204i, is compared to the fluence at detector 12041.
If the fluence for all the detectors 1204i is greater than the fluence at 12041, then at 1410 the distance between half wedges 7181 and 7182 is reduced, as described herein and/or otherwise, thereby placing the half wedges 7181 and 7182 into position PN, and acts 1406 and 1408 are repeated.
However, if the fluence for any of the detectors 1204i is not greater (i.e. less) than the fluence at 12041, then at 1412, the position PN-1 (the previous position) for each half wedge 7181 and 7182 is added to a position profile.
At 1414 the imaging system 700 scans the subject or object 1102 wherein the drive system 900 moves the half wedges 7181 and 7182 based on the calculated half wedge positions during an acquisition interval.
At 1416, the controller 724 measures the half wedge 7181 and 7182 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.
In other words (and with reference to
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.
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.
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.
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.
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.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/073908 | 9/5/2018 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62555815 | Sep 2017 | US |
