WO2010073147A1

WO2010073147A1 - Gated image reconstruction

Info

Publication number: WO2010073147A1
Application number: PCT/IB2009/055292
Authority: WO
Inventors: Holger Schmitt; Michael Grass
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2008-12-22
Filing date: 2009-11-23
Publication date: 2010-07-01

Abstract

A method includes acquiring data, during an acquisition window that corresponds to a phase of motion of a motion cycle, for a plurality of motion cycles, wherein less than full angular sampling is acquired in the acquisition window for at least two of the plurality of motion cycles and generating a full angular sampling data set for the phase of motion based on the less than full angular sampling data for at least the two of the plurality of motion cycles.

Description

GATED IMAGE RECONSTRUCTION DESCRIPTION

The following generally relates to selectively imaging moving structures during a motion phase(s) of interest and finds particular application to computed tomography (CT). However, it is also amenable to other medical and non-medical applications.

A computed tomography (CT) scanner generally includes an x-ray tube mounted on a rotatable gantry opposite a detector array including one or more rows of detector pixels. The x-ray tube rotates around an examination region located between the x-ray tube and the detector array and emits radiation that traverses the examination region and an object or subject disposed therein. The detector array detects radiation that traverses the examination region and generates projection data indicative of the examination region and the object or subject disposed therein. A reconstructor processes the projection data and generates volumetric image data indicative of the examination region and the object or subject disposed therein. The volumetric image data can be processed to generate an image that includes the scanned portion of the object or subject.

In some instances, the scanned portion of the object or subject includes a moving structure such as the heart, lung, etc., or other structure moved by the moving structure. Such movement may introduce motion artifact in the resulting images. With cardiac CT, it is often desirable to capture and reconstruct data corresponding to a particular motion phase of a heart cycle or beat such as a "quiet" phase of the heart cycle where the heart is relatively motionless as the resulting images generally are less susceptible to motion artifact relative to images generated with data from phases with more motion. Various techniques including electrocardiogram (ECG) signal gating (prospective and retrospective) are used to locate such a phase. With prospective gating, an ECG signal is concurrently measured and monitored during the imaging procedure, and x-rays are turned on only during the phase of interest, which is identified from the ECG signal.

For example, when a landmark within the ECG signal such as a peak of an R wave is identified, the x-ray source is gated to emit radiation at a predetermined time from the R wave peak for a predetermined sampling window. The sampling window generally corresponds to a set of angular views needed to obtain a complete set of projection data (e.g., 180 degrees plus a source fan of data for a "half scan, or 360 degrees of data for a "full" scan) for reconstruction. The heart is often scanned as such over multiple successive heart beats or cycles, with the data for the desired cardiac phase for each cycle being combined and reconstructed, as combining the data from different heart cycles improves temporal resolution. Although dose efficiency with prospectively gated imaging is improved relative to retrospectively gated imaging where radiation is emitted during the entirety of each motion cycle, the patient is still, unfortunately, irradiated during the entire sampling window within each motion cycle.

Aspects of the present application address the above-referenced matters and others.

In accordance with one aspect, a method includes acquiring data, during an acquisition window that corresponds to a phase of motion of a motion cycle, for a plurality of motion cycles, wherein less than full angular sampling is acquired in the acquisition window for at least two of the plurality of motion cycles and generating a full angular sampling data set for the phase of motion based on the less than full angular sampling data for at least the two of the plurality of motion cycles.

According to another aspect, an imaging system includes a radiation source that emits radiation that traverse an examination region during an imaging procedure and a detector array that detects radiation that traverses the examination region and generates a signal indicative thereof. The radiation source selectively emits the radiation during an acquisition window based on a gating signal indicative of an identification of a predetermined feature in a motion signal that is acquired during the imaging procedure. The acquisition window corresponds to a same phase of motion in each of a plurality of motion cycles. The less than full angular sampling is acquired for the phase of motion in each acquisition window. The full angular sampling is generated for the phase of motion by combining the less than full angular sampling data.

According to another aspect, a method includes selecting of motion phase of interest in a motion cycle and acquiring a set of less than full angular sampling during an acquisition window that corresponds to the motion phase of interest, wherein data acquisition is gated based on identification of a predetermined feature in a motion signal indicative of the motion cycle. The method further includes determining a time for a next acquisition based on the motion signal and the predetermined feature and estimating an angular position for the next acquisition based at least on the determined time. The method further includes determining if the estimated angular position is outside of a pre-defined angular position shift for the next acquisition and increasing or decreasing a rotational speed of a radiation source that emits radiation for data acquisition only when the estimated angular position is outside of the pre-defined angular position shift so that the next acquisition begins within the pre-defined angular position shift. The method further includes acquiring a subsequent set of less than full angular sampling during the acquisition window for a second motion cycle.

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.

FIGURE 1 illustrates an example imaging system.

FIGURE 2 illustrates an example ECG signal.

FIGURE 3 illustrates an example angular sampling pattern.

FIGURE 4 illustrates an example method.

The following generally relates to a prospective motion-gated multi-scan imaging procedure of a moving structure in which less than full angular sampling is acquired for each scan and full angular sampling (e.g., 360 degrees for a full scan) is achieved by combining data from the individual less than full angular sampling scans. In one instance, this may reduce patient dose and improve dose efficiency relative to configurations in which full angular sampling is acquired for each individual scan. In addition, the angular sampling positions between scans may be shifted, which can improve the angular sampling resolution.

FIGURE 1 illustrates an imaging system 100 such as a computed tomography (CT) scanner. The illustrate system 100 includes a generally stationary gantry 102 and a rotating gantry 104. The rotating gantry 104 is rotatably supported by the generally stationary gantry 102 via a bearing or the like and rotates around an examination region 106 about a longitudinal or z-axis 108 and emits radiation. A radiation source 110, such as an x-ray tube, is supported by the rotating gantry portion 104 and rotates therewith. The radiation source 110, as it rotates around the examination region 106, emits radiation that traverse the examination region 106. The radiation source 110 can also emit radiation while the rotating gantry 104 is at a static position, for example, for a pilot, scout, or other scan.

A radiation source controller 112 selectively turns the radiation source 110 on and off. For example, the radiation source controller 112 can gate the radiation source 110, based on a gating signal, to selectively turn x-rays on and off during an acquisition window so that, for each scan, radiation is only emitted for a subset of the available acquisition angular positions in the acquisition window. In one instance, this allows for less than full angular sampling for any particular scan of a multi-scan imaging procedure. As a result, patient dose can be reduced relative to turning x-rays on for the entire acquisition window.

A rotating gantry drive system 114 rotates the rotating gantry 104 and hence the radiation source 110 around the examination region 106. A drive controller 116 controls the drive system 114. The drive controller 116 can drive the drive system 114 so that a speed of the rotating gantry 104 increases and decreases during the multi-scan imaging procedure. A particular speed increase and decrease can be used to angularly position the rotating gantry 104 and hence the radiation source 110 based on angular start position for a scan. Without suitable positioning, incomplete angular sampling may occur since gating timing may conflict with the angular position for acquiring data that combines to form full angular sampling.

A detector array 118 subtends an angular arc opposite the examination region 106 relative to the radiation source 110. The illustrated detector array 118 includes a multi-dimensional array of photosensitive pixels. The detector array 118 generates a projection data, or a signal indicative of the detected radiation.

A reconstructor 120 reconstructs the projection data from the detector and generates volumetric data indicative thereof. The reconstructor 120 can generate individual volumetric data for one or more of the less than full angular sampling scans. Alternatively or additionally, the reconstructor 120 can generate aggregated volumetric data based on the less than full angular sampling data for two or more scans, such as less than full angular sampling data that combines to form full angular sampling. An image generator 122 generates one or more images based on the volumetric image data. In one instance, a less than full angular sampling image for at least one of the individual volumetric image data sets is generated. Alternatively or additionally, a full angular sampling image is generated based on the aggregate volumetric image data generated. One or more of the less than full angular sampling images can be normalized with the full angular sampling image. A full angular sampling image can also be generated based on the aggregate projection data acquired.

A computing system such as an operator console 124 facilitates interaction with the scanner 100. Software applications executed by the operator console 124 allow a user to configure and/or control operation of the scanner 100. For instance, the user can interact with the operator console 124 to select a motion based imaging protocol such as a cardiac protocol like a prospective, retrospective and kymogram based protocol. Executing software may also send control signals for controlling (gating) the radiation source controller 112, the gantry drive controller 116, and/or other components.

A motion sensor 126 senses motion corresponding to a portion of an object or subject disposed in the examination region 106 and generates a motion signal indicative thereof. Such structure can be related to the heart, lung, or other moving anatomy and/or other anatomical or non-anatomical moving structure. The motion signal provides information that reflects a state of the moving structure such as a motion phase from a plurality of different motion phases of the moving structure. Examples of suitable motion sensors for anatomical moving structures include, but are not limited to, an electrocardiogram (ECG), a respiratory monitor, and/or other motion sensor.

A feature identifier 128 identifies a pre-determined feature in the motion signal. The particular feature of interest may be provided to the feature identifier 128 from the operating console 124, as shown, and/or otherwise. A non-limiting example of a suitable feature for a cardiac scan includes a peak of a particular section (P or T wave, Q- R-S complex) of an ECG signal. The feature identifier 128 generates a signal indicative of the identification of the feature in the motion signal.

A scan start time determiner 130 determines a start time for a next scan based on the signal indicative of the identification of the feature in the motion signal and/or other information such as a pre-determined delay from the identification of the feature. The signal indicative of the identification of the feature in the motion signal may include a time stamp and/or other information that indicates a time at which the particular feature occurs.

An angular position sensor 132 senses an angular position of the rotating gantry 104 and, hence, the radiation source 110. As such, a current angular position of the radiation source 110 can be obtained.

A radiation source position calculator 134 calculates an estimated radiation source angular position for the next scan based on the current angular position of the radiation source 110 (e.g., as determined by the radiation source position calculator 134), the next scan start time (e.g., as determined by the scan start time determiner 130), and a speed of rotation of the rotating gantry 104, which can be provided from the console 124 based on the selected acquisition protocol, measured, estimated, and/or otherwise determined.

A speed correction factor determiner 136 determines a rotating gantry 104 rotational speed correction factor based on a difference between the estimated radiation source angular position for the start of the next scan and a pre-determined angular shift angle from the angular positions of a previous scan of the multi-scan imaging procedure. The pre-determined shift and the angular acquisition positions of the previous scan can be obtained from the console 124 based on the selected imaging protocol and/or otherwise. Generally, the correction factor can be used to increase or decrease the rotational speed of the rotating gantry 104 so that the start angular acquisition position is at an angle that is approximately equal to angular position of a previous scan shifted by the pre-determined shift angle.

A couch or patient support 138 supports a subject, such as a human or animal, or an object within the examination region 106. The support 138 is movable, which enables an operator or the system to suitably position the subject within the examination region 106 before, during and/or after scanning.

Operation for an example prospective ECG-gated multi-scan cardiac imaging procedure is described in connection with FIGURES 1 and 2. For this example, the motion sensor 126 includes an electrocardiogram (ECG) and/or other device that provides information which is indicative of electrical activity of the heart during a heart beat or cycle and which reflects the state of the heart (or phase) throughout a heart cycle. FIGURE 2 illustrates a representative ECG signal 200 that includes three heart cycles 202, 204 and 206. Each of the heart cycles 202, 204 and 206 has a systolic period 208 in which the atria (P wave) and subsequently the ventricles (QRS complex) contract and the ventricles then re -polarize (the T wave), and a subsequent diastolic period 210 in which the heart relaxes and refills with circulating blood.

With respect to FIGURES 1 and 2, in this example, the feature identifier 128 is configured to identify the R peak in the monitored ECG signal 200. Once identified, the feature identifier 128 generates a signal indicative of such identification and provides the signal to the scan start time determiner 130, which determines when x-rays should be turned on based on a motion phase of interest. As noted above, the phase can be identified by the scan protocol or otherwise. For explanatory purposes, assume the desired phase of motion is the "quiet" phase in which the motion of the heart is relative low compared to the motion during other phases of heart.

In one instance, the "quiet" phase can be approximated from the R wave peak in the ECG signal 200. For example, assuming a one (1) second heart beat, this phase can be approximated to begin at about four tenths (0.40) as shown at 212 or seven tenths (0.70) as shown at 214 of a second from the R peak. Of course, another starting point can be used. For the above two starting positions 212, 214, x-rays are respectively turned on during sub-portions of sampling or acquisition windows 216, 218, each window including a set of angular sampling angles for full angular sampling.

With respect to FIGURE 1, for the first scan the console 124 gates the radiation source 110 via the radiation source controller 112 based on the signal from the scan start time determiner 130 and a predetermined delay therefrom as discussed above, and data is acquired during the corresponding acquisition window at a first set of acquisition angles. In this example, the angle between acquisition angles within the first scan is assumed to be the same, but can be different. As noted above, the console 124 gates the radiation source 110 so that data is acquired over a sub-set of the possible acquisition angles, resulting in less than full angular sampling.

For each subsequent scan, a start data acquisition angular position is shifted by a pre-determined angular value from the acquisition angles of a previous scan(s). Hence, in one non-limiting instance each scan has a unique start acquisition angle position. A suitable shift is in a range up to twenty-five (25) degrees, such as ten (10) or twenty (20) degrees. The range may be determined via the selected imaging protocol and/or otherwise.

When the system 100 is configured so that the angle between each acquisition angle is the same for each scan, each scan includes a set of different angular acquisition positions relative to the other scans. As noted above, this can improve the angular sampling resolution. Of course, the angle between each acquisition angle may differ between scans. Generally, the angle is selected so that the resulting data can be combined to generate a full angular sampling data set.

The speed correction factor generator 136 generates a speed correction factor, if needed, for each subsequent scan based on the corresponding shifted start acquisition angular position. By way of example, assume that after an initial scan that the source position calculator 134 estimates that the radiation source 110 will be at about at a same angular position with respect to the angular positions from a previous scan.

The speed correction factor generator 136 generates a rotating gantry 104 adjustment speed value, which will result in the rotating gantry 104 speeding up or slowing down so that the start acquisition angular position for a scan will be at about the predetermined angular position shift from the angular positions of a previous scan(s). The speed of the rotating gantry 104 can be returned to the pre-adjustment or other speed once data acquisition begins.

The resulting projection data is reconstructed by the reconstructor 120 and one or more images can be generated therefrom by the image generator 122 as discussed herein. As such, full angular sampling can be generated by combining data from individual less than full or partial angular sampling. This may reduce patient dose and improve dose efficiency relative to configurations in which full angular sampling is acquired for each individual scan. In addition, sampling between scans is offset, which can increase the angular sampling resolution.

FIGURE 3 shows example angular sampling for a three (3) scan prospective ECG-gated imaging procedure. A y-axis 302 represents the acquisition or projection angle in degrees and an x-axis 304 represents time as a function of angular unit. Each scan 306, 308, 310 includes acquiring data for twelve (12) of thirty-six (36) possible angular positions, for a total of thirty-six (36) angular positions in aggregate. The first scan 306 is gated to begin at fifty (50) degrees, the second scan 308 is gated to begin at two hundred and seventy (270) degrees, and the third scan 310 is gated to begin at four hundred and eighty (480) degrees. The samples between scans are shifted along the y-axis by about ten (10) degrees.

In other embodiments the system 100 may be able to acquire data via more or less angular positions for each scan, for example, N angular positions for each scan. In addition, the procedure may entail more than three (3) such as M scans. In this case, data may be acquired at N/M different angular positions for each scan. Of course, each scan does not have to have the same number of acquisitions angles.

Moreover, the number of scans can be determined based on a predetermined amount of dose reduction. For example, the above example reduces dose by a factor of three (3). As such, M can be set to a desired reduction of dose factor. Generally, N is determined by the system configuration. In one instance, M is determined by the clinical application, i.e. the time over which image data should be acquired. For example, a perfusion scenario may have to be scanned over 20 sec, for example, at a rate of 1 image per 2 second. For this example, M=IO.

With this angular sampling pattern, partial or less than full angular sampling is achieved with each scan, and full angular sampling can be obtained through combining the individual less than full angular sampling scans. Angular sampling is improved by shifting the acquisition angular positions (y-axis) between scans. As noted above, such shifting is achieved through adjusting transiently the speed of the rotating gantry 104.

FIGURE 4 illustrates a method for performing a prospective motion-gated multi-scan imaging procedure of a moving structure in which less than full angular sampling is acquired for each scan and full angular sampling is generated by combining data from individual less than full angular sampling scans.

At 402, a motion phase of interest is selected. For cardiac CT, the phase may include one or more motion phases in which motion is relatively less than motion in other cardiac phases. Of course, motion phases such as motion phases with relatively higher motion may also be selected. In addition, the motion may correspond to respiratory or other motion.

At 404, less than full angular sampling is acquired during a sampling window, rendering a first subset of a plurality of available angular positions for a first scan during the motion phase of interest. For instance, assuming that there are N angular sampling positions for each 360 degree scan, data is acquired at less than N angular sampling positions, for example, at N/M angular sampling positions, wherein M is the number of scans or a dose reduction factor.

At 406, a time for a next scan of the motion phase of interest is determined. This can be based on a signal indicative of the motion cycle. For instance, if scanning is gated off a particular wave peak of an ECG signal, an estimated time to the next wave peak of interest is determined. This determination can be based off of information for previous heart beats, a default heart beat interval, and/or other information.

At 408, an estimated angular position of the radiation source 110 for the next scan is determined based on the time for the next scan. As discussed herein, this can be based on a current angular position of the radiation source 110, a current rotational speed of the radiation source 110, a determined next scan start time, and/or other information.

At 410, if needed, a speed of the rotating gantry 104 can be either increased or decreased so that the start acquisition angle for the next scan is at about an angular position that is shifted by a pre-defined angular value from the angular acquisition angles of the previous scan(s). The rotational speed of the rotating gantry 104 can return to the pre-adjustment speed once the next scan begins.

At 412, acts 404 to 410 are repeated for each scan of the multi-scan imaging procedure.

At 414, the less than full angular sampling data can be combined to generate full angular sampling data.

The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be constructed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims

CLAIMSWhat is claimed is:

1. A method of imaging, comprising: acquiring data, during an acquisition window that corresponds to a phase of motion of a motion cycle, for a plurality of motion cycles, wherein less than full angular sampling is acquired in the acquisition window for at least two of the plurality of motion cycles; and generating a full angular sampling data set for the phase of motion based on the less than full angular sampling data for at least the two of the plurality of motion cycles.

2. The method of claim 1, further comprising gating data acquisition based on identification of a predetermined feature in the motion cycle.

3. The method of any of claims 1 to 2, further comprising, for the at least two motion cycles, shifting a start angular acquisition position for a second acquisition during a second motion cycle by a predetermined angular shift relative to angular positions for a first acquisition of a first motion cycle.

4. The method of claim 3, wherein the predetermined angular shift is up to twenty-five degrees.

5. The method of any of claims 3 to 4, further comprising: generating an estimated start angular acquisition position for the second acquisition; and adjusting a rotational speed of a rotating gantry (104) supporting a radiation source (110) that generates radiation for data acquisition when the estimated start angular acquisition position is outside of a predetermined range about the shifted start angular acquisition position for the second acquisition so that the second acquisition begins at about the shifted start angular acquisition position.

6. The method of claim 5, further comprising generating the estimated start angular position for the second acquisition based on a current angular position of the rotating gantry (104), a scanning rotational speed of the rotating gantry (104), and the motion signal.

7. The method of claim 6, further including returning the adjusted rotational speed of the rotating gantry (104) to the scanning rotational speed of the rotating gantry (104).

8. The method of any of claims 1 to 7, further comprising generating volumetric image data for at least one set of the less than full angular sampling data.

9. The method of any of claims 1 to 8, further comprising generating aggregate volumetric image data based on the less than full angular sampling data for at least two motion cycles.

10. The method of any of claims 1 to 9, further comprising: generating a first less than full angular sampling image based on the less than full angular sampling data for a motion cycle; generating a second full angular sampling image based on the less than full angular sampling data for two or more of the motion cycles; and normalizing the first image based on the second image.

11. The method of any of claims 1 to 10, wherein the motion cycle is an ECG signal.

12. The method of claim 11, wherein the phase of motion corresponds to a phase of motion with lower motion relative to other phases of the motion cycle.

13. An imaging system (100), comprising: a radiation source (110) that emits radiation that traverse an examination region (106) during an imaging procedure, and a detector array (118) that detects radiation that traverses the examination region (106) and generates a signal indicative thereof; wherein the radiation source (110) selectively emits the radiation during an acquisition window based on a gating signal indicative of an identification of a predetermined feature in a motion signal that is acquired during the imaging procedure; wherein the acquisition window corresponds to a same phase of motion in each of a plurality of motion cycles; wherein less than full angular sampling is acquired for the phase of motion in each acquisition window; and wherein full angular sampling is generated for the phase of motion by combining the less than full angular sampling data.

14. The system of claim 13, further comprising a rotating gantry (104) that supports the radiation source (110) and rotates around the examination region (106), wherein a speed of the rotation gantry (104) is adjusted during the imaging procedure.

15. The system of claim 14, further comprising a speed correction factor determiner (136) that determines a rotating gantry adjustment speed based on an estimated angular position of the radiation source (110) for a next scan and a calculated start angular acquisition position for the next scan.

16. The system of any of claims 13 to 15, wherein a start acquisition angular position for a motion cycle is shifted by a predetermined non-zero angular value relative to an acquisition angular position in a previous motion cycle.

17. The system of any of claims 13 to 16, wherein the motion signal is an ECG signal and the predetermined feature is a peak of a wave of the ECG signal.

18. The system of any of claims 13 to 17, further comprising a reconstructor (120) that reconstructs the full angular sampling data and generates volumetric image data indicative thereof.

19. The system of claim 18, further comprising an image generator (122) that generates a full angular sampling image based on the generated volumetric image data.

20. A method, selecting of motion phase of interest in a motion cycle; acquiring a set of less than full angular sampling during an acquisition window that corresponds to the motion phase of interest, wherein data acquisition is gated based on identification of a predetermined feature in a motion signal indicative of the motion cycle; determining a time for a next acquisition based on the motion signal and the predetermined feature; estimating an angular position for the next acquisition based at least on the determined time; determining if the estimated angular position is outside of a pre-defined angular position shift for the next acquisition and increasing or decreasing a rotational speed of a radiation source (110) that emits radiation for data acquisition only when the estimated angular position is outside of the pre-defined angular position shift so that the next acquisition begins within the pre-defined angular position shift; and acquiring a subsequent set of less than full angular sampling during the acquisition window for a second motion cycle.

21. The method of claim 20, wherein a start acquisition angular position for the acquisition of the subsequent set of data is angularly different from acquisition angular positions from the acquisition of the set of data.

22. The method of any of claims 20 to 21, further comprising combining the less then full angular sampling data to form a set of full angular sampling data.

23. The method of claim 22, further comprising generating a full angular sampling image based on the combined the less then full angular sampling data.