阅读说明：本技术 包括患者运动传感器的mri系统 (MRI system comprising a patient motion sensor ) 是由 兰德尔·H·布赫瓦尔德 路易斯·杰伊·瓦纳塔 于 2020-07-22 设计创作，主要内容包括：本公开提供了一种用于MRI系统的工作台,该工作台包括：顶表面,该顶表面用于支撑要被成像的患者；以及运动传感器,该运动传感器用于感测患者的运动。运动传感器位于顶表面下方并包括自共振螺旋(SRS)线圈和耦合环。耦合环生成驱动RF信号以激励SRS线圈来辐射具有预限定的共振频率的磁场。耦合环还从SRS线圈接收反射RF信号。运动传感器被定位成使得要被成像的患者的躯干的至少一部分在磁场内。控制器被配置为基于反射RF信号来检测患者运动。(The present disclosure provides a table for an MRI system, the table comprising: a top surface for supporting a patient to be imaged; and a motion sensor for sensing motion of the patient. The motion sensor is located below the top surface and includes a self-resonant spiral (SRS) coil and a coupling loop. The coupling loop generates a drive RF signal to excite the SRS coil to radiate a magnetic field having a predefined resonant frequency. The coupling loop also receives reflected RF signals from the SRS coil. The motion sensor is positioned such that at least a portion of a torso of a patient to be imaged is within the magnetic field. The controller is configured to detect patient motion based on the reflected RF signals.)

1. A table for a magnetic resonance imaging system, the table comprising:

a top surface for supporting a patient to be imaged;

a motion sensor for sensing motion of the patient, the motion sensor being located below the top surface and comprising:

a self-resonant spiral (SRS) coil;

a coupling loop inductively coupled to the SRS coil and configured to:

generating a drive RF signal to excite the SRS coil to radiate a magnetic field having a predefined resonant frequency; and

receiving a reflected RF signal from the SRS coil;

wherein the motion sensor is positioned such that at least a portion of the torso of the patient to be imaged is within the magnetic field; and

a controller configured to detect patient motion based on the reflected RF signals.

2. The table of claim 1, wherein the motion sensor is configured to sense motion of the patient due to respiration, and wherein the controller is configured to generate a respiration signal based on changes in the reflected RF signal due to respiration of the patient.

3. The stage of claim 2, wherein the respiration signal is determined based on a change in a reflection coefficient of the SRS coil over time.

4. The stage of claim 1, wherein the stage further comprises at least one Phased Array (PA) coil, and wherein the PA coil is between the top surface of the stage and the SRS coil.

5. The table of claim 1, wherein the predefined resonant frequency is less than a larmor frequency.

6. The table of claim 1, wherein the predefined resonant frequency is between 26.957MHz and 27.283 MHz.

7. The table of claim 1, wherein the predefined resonant frequency has an industrial, scientific and medical (ISM) band.

8. The stage of claim 1, wherein the SRS coil is an elliptical spiral having between 10 and 15 turns.

9. The table of claim 1, further comprising a pair of tuning elements capacitively coupled to the SRS coil, wherein dimensions of the tuning elements are selected to calibrate the SRS coil to resonate at the predefined resonant frequency.

10. The stage of claim 1, further comprising at least one passively coupled element adjacent to and inductively coupled to the SRS coil to extend the magnetic field along a length of the stage.

11. The table of claim 1, wherein the table further comprises a second motion sensor, the second motion sensor comprising:

a second SRS coil;

a second coupling loop coupled to the second SRS coil, wherein the second coupling loop is configured to generate a driving RF signal to excite the SRS coil to radiate a magnetic field having the predefined resonant frequency, and to receive a reflected RF signal from the second SRS coil;

wherein the SRS coil and the coupling loop are positioned towards a front end of the stage and the second SRS coil and the second coupling loop are positioned towards a back end of the stage; and

a switch configured to alternately connect one of the coupling loop and the second coupling loop to the controller.

12. A magnetic resonance imaging system, the magnetic resonance imaging system comprising:

an aperture;

a table configured to support a patient to be imaged and movable to move the patient into and out of the bore;

a motion sensor for sensing motion of the patient, the motion sensor comprising:

a self-resonant spiral (SRS) coil excited by a drive signal to radiate a magnetic field having a predefined resonant frequency;

a driver-receiver coupled to the SRS coil and configured to generate the drive signal to excite the SRS coil and to receive RF signals from the SRS coil;

wherein the motion sensor is positioned such that at least a portion of the torso of the patient is within the magnetic field while the patient is being imaged in the bore; and

a controller configured to detect patient motion based on changes in the RF signal.

13. The system of claim 12, wherein the motion sensor is located below a top surface of the table.

14. The system of claim 12, wherein the driver-receiver is a coupling loop inductively coupled to the SRS coil, wherein the coupling loop is configured to generate the drive signal to excite the SRS coil to radiate a magnetic field having the predefined resonant frequency, and wherein the RF signal received from the SRS coil is a reflected RF signal; and is

Wherein the motion sensor is configured to sense motion of the patient due to respiration, and wherein the controller is configured to generate a respiration signal based on changes in the reflected RF signal due to respiration of the patient.

15. The system of claim 14, wherein the respiration signal is formatted for triggering MR image data acquisition.

16. The system of claim 14, wherein the respiration signal is determined based on a change in a reflection coefficient of the SRS coil over time.

17. A magnetic resonance imaging system, the magnetic resonance imaging system comprising:

an aperture;

a table having a top surface that supports a patient to be imaged and is movable to move the patient into and out of the bore;

a motion sensor system located below the top surface of the table and configured to sense motion of the patient due to respiration, the motion sensor system comprising:

a first resonance coil positioned toward a front end of the table;

a first coupling loop coupled to the first resonant coil, wherein the first coupling loop is configured to generate a drive RF signal to excite the first resonant coil to radiate a magnetic field having a predefined resonant frequency and to receive a reflected RF signal from the first resonant coil;

a second resonance coil positioned toward a rear end of the table;

a second coupling loop coupled to the second resonance coil, wherein the second coupling loop is configured to generate a drive RF signal to excite the second resonance coil to radiate a magnetic field having the predetermined resonance frequency and to receive a reflected RF signal from the second resonance coil;

a switch configured to alternately connect one of the first coupling loop and the second coupling loop to a controller;

a controller configured to generate a respiration signal based on a change in the reflected RF signal due to respiration.

18. The system of claim 17, wherein the respiration signal is determined based on a change in a reflection coefficient of a connected one of the first or second resonance coils over time, and wherein the respiration signal is formatted for triggering MR image data acquisition.

19. The system of claim 17, further comprising at least one Phased Array (PA) coil located between the top surface of the table and the first and second resonance coils.

20. The system of claim 17, wherein each of the first and second resonant coils is an elliptical self-resonant spiral (SRS) coil.

21. The system of claim 17, wherein the predetermined resonant frequency is between 26.957MHz and 27.283MHz, between 40.66MHz and 40.7MHz, or between 13.553MHz and 13.567 MHz.

22. The system of claim 17, further comprising at least one passively coupled element adjacent to and not actively coupled to one of the first and second resonant coils, wherein the at least one passively coupled element expands the magnetic field radiated by the respective coil along the length of the stage.

Background

The present disclosure relates generally to Magnetic Resonance Imaging (MRI), and more particularly, to an MRI system having a non-contact motion sensor for detecting patient motion, including motion of the patient due to respiration.

Magnetic Resonance (MR) imaging is commonly used to obtain internal physiological information of a patient, including for cardiac imaging and imaging of other parts or tissues within the torso of the patient (or anywhere on the patient). In certain body regions, such as portions in the torso, it is often desirable to obtain images at specific points in a variable cycle (e.g., respiratory cycle and/or cardiac cycle), such as the peak of the variable cycle, to analyze the behavior during that peak. Gating is an option to characterize different properties of organs for imaging. The most common gating techniques include cardiac, respiratory, and peripheral pulse gating, and these types of gating have utility across diagnostic modalities such as CT, MR, X-ray, ultrasound, and Positron Emission Tomography (PET) in many medical applications. For example, respiratory gating is an essential component of cardiac imaging, while imaging modalities such as CT and MR are used to minimize motion-related artifacts caused by motion due to the patient's breathing.

In MR imaging, when a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B)₀) The individual magnetic moments of the spins in the tissue attempt to align with the polarizing field, but precess around the polarizing field in a random order at their characteristic larmor frequency. If the substance or tissue is subjected to a magnetic field in the x-y plane (excitation field B) close to the Larmor frequency₁) The net alignment moment or "longitudinal magnetization" Mz can be rotated or "tilted" into the x-y plane to produce a net transverse magnetic moment Mt. In the excitation signal B₁After termination, a signal is emitted by the excited spins, and the signal can be received and processed to form an image.

When using these signals to generate an image, magnetic field gradients (G) are used_x、G_yAnd G_z). Typically, the region to be imaged is scanned by a series of measurement cycles, with these gradients varying according to the particular localization method used. The resulting set of received NMR signals is digitized and processed to reconstruct an image using a reconstruction technique.

For example, MR images of the heart region or abdominal region are commonly used by health care professionals to diagnose medical conditions. Conventional MR assessment of cardiac or abdominal regions typically relies on repeated cardiac-gated and/or respiratory-gated acquisition of MR data in order to reduce image degradation caused by continuous movement of the imaged tissue due to respiratory and/or circulatory physiological functions.

Therefore, respiratory gating and/or cardiac gating are commonly used for MR data acquisition, which rely on the detection of specific points in the motion cycle as a trigger to repeatedly acquire data at approximately the same phase of the motion cycle. The sensor system is used to sense respiratory activity and cardiac potentials. Respiratory monitors that utilize a bellows sensor are typically used to detect a respiratory waveform that utilizes a band including a pressure sensor and a bellows to detect chest inflation. An Electrocardiogram (ECG) is commonly used to monitor the cardiac cycle.

Disclosure of Invention

This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

In one embodiment, a table for an MRI system includes: a top surface for supporting a patient to be imaged; and a motion sensor for sensing motion of the patient. The motion sensor is located below the top surface and includes a self-resonant spiral (SRS) coil that is excited by a drive signal to radiate a magnetic field having a predefined resonant frequency. The driver-receiver is coupled to the SRS coil and configured to generate a drive signal to excite the SRS coil and receive an RF signal from the SRS coil. The motion sensor is positioned such that at least a portion of a torso of a patient to be imaged is within the magnetic field. The controller is configured to detect patient motion based on the reflected RF signals.

An MRI system includes: an aperture; a table configured to support a patient to be imaged and movable to move the patient into and out of the bore; and a motion sensor for sensing motion of the patient during imaging. The motion sensor includes: a self-resonant spiral (SRS) coil; and a coupling loop inductively coupled to the SRS coil and configured to generate a driving RF signal to excite the SRS coil and to receive a reflected RF signal from the SRS coil. The motion sensor is positioned such that at least a portion of the torso of the patient is within the magnetic field while the patient is being imaged in the bore. The controller is configured to detect patient motion based on changes in the reflected RF signal.

Another embodiment of an MRI system comprises: an aperture; a table having a top surface that supports a patient to be imaged and is movable to move the patient into and out of the bore; and a motion sensor located below a top surface of the table and configured to sense motion of the patient due to respiration. The motion sensor system includes: a first resonance coil positioned toward a front end of the table; and a first coupling loop coupled to the first resonant coil, wherein the first coupling loop is configured to generate a driving RF signal to excite the first resonant coil to radiate a magnetic field having a predefined frequency. The first coupling loop also receives a reflected RF signal from the first resonant coil. The motion sensor system further comprises: a second resonance coil positioned toward a rear end of the table; and a second coupling loop coupled to the second resonance coil. The second coupling loop is configured to generate a drive RF signal to excite the second resonance coil to radiate a magnetic field having a predefined resonance frequency. The second coupling loop is also configured to receive a reflected RF signal from the second resonance coil. The switch is configured to alternately connect one of the first coupling loop and the second coupling loop to the controller. The controller is configured to generate a respiration signal based on changes in the reflected RF signal due to respiration.

Various other features, objects, and advantages of the invention will become apparent from the following description taken in conjunction with the accompanying drawings.

Drawings

The present disclosure is described with reference to the following drawings.

Fig. 1 is a schematic diagram of an exemplary MRI system according to an embodiment of the present disclosure.

Fig. 2 depicts an exemplary table of an MRI system according to another embodiment of the present disclosure.

Fig. 3 depicts an embodiment of a motion sensor according to an embodiment of the present disclosure.

Fig. 4 is a circuit diagram of an exemplary coupling circuit of a motion sensor according to one embodiment of the present disclosure.

Fig. 5A depicts an exemplary motion sensor according to one embodiment of the present disclosure.

Fig. 5B depicts an exemplary self-resonant spiral coil, according to one embodiment of the present disclosure.

Fig. 6A depicts an exemplary respiration signal generated by a motion sensor according to the present disclosure.

Fig. 6B is a graph of the reflection coefficient (S11) of an exemplary resonant coil according to one embodiment of the present disclosure.

Fig. 7 is an H-field diagram of a magnetic field radiated by an exemplary resonant coil and two passively coupled elements, according to one embodiment of the present disclosure.

Detailed Description

Accurate respiratory parameter measurements are important for respiratory gating in MR imaging. The present inventors have recognized a need for an improved respiration monitoring system for use in MR imaging, such as for generating respiration signals. The present inventors have recognized that conventional bellows breath sensors are undesirable because they can be uncomfortable to the patient and require additional clinician time to place the band and bellows sensors on the patient prior to imaging.

The present inventors have recognized a need for a reliable non-contact respiration sensor that is integrated into an MRI system and that operates without the need to attach any sensors to the patient and without requiring any additional setup or engagement by the clinician performing the MR imaging. Accordingly, the inventors have developed the disclosed motion sensor system that is configured to detect respiratory motion of a patient, and may also be configured to detect other types of patient motion. The motion sensor system includes a resonance coil and a driver-receiver, such as a coupling loop inductively coupled to the resonance coil. In one embodiment, the coupling loop is configured to generate a drive RF signal to excite the resonance coil to radiate a magnetic field having a predefined resonance frequency, and the coupling loop also receives reflected RF from the resonance coil. Based on the reflected RF signal, a respiration signal can be derived. For example, the respiration signal may be determined based on a change in the reflection coefficient (S11) of the resonance coil over time. In other embodiments, different driving methods may be utilized, such as via a direct connection. In a direct drive embodiment, the resonant coil is driven directly via a connection to a voltage source, and sensing of S11 is accomplished through the same direct drive connection. Here, the driver-receiver is physically connected to one end of the coil, such as at the center.

The present inventors have recognized that prior art non-contact motion detection systems utilizing HF resonators are unreliable for detecting patient respiration due to the small sensitivity region and small magnetic field utilized. Furthermore, prior art breathing systems are not integrated into MR imaging systems, such as into a table, and may therefore require positioning and/or other intervention by a clinician in order to provide a motion sensing system for a patient to be imaged, in particular for a heavy patient or a patient with abnormal breathing patterns (chest breathing versus abdominal breathing), and/or require the use of a special receiver coil that is incorporated and designed to surround a non-contact motion sensor system.

In view of the foregoing challenges in the related art, the present inventors have developed the disclosed system that generates RF magnetic fields with greater penetration depths and greater regions of sensitivity than prior art systems. In one embodiment, the motion sensor system includes one or more sensors integrated into the table of the MR system and generating a magnetic field large enough to reliably measure patient respiration during MR imaging. For example, the non-contact sensors may be configured such that they can reliably measure patient respiration from a measurement distance of about 4 to 6 inches between the patient and the sensor coil, embodiments of which are fully described herein. The sensors may be located at various locations on the table and can be selected based on the patient's position on the table (i.e., head or feet) and/or the type of imaging to be performed.

Referring to FIG. 1, a schematic diagram of an exemplary MRI system 100 is shown, according to one embodiment. The operation of the MRI system 100 is controlled by an operator workstation 110, which includes an input device 114, a control panel 116, and a display 118. The input device 114 may be a joystick, keyboard, mouse, trackball, touch-activated screen, voice control, or any similar or equivalent input device. Control panel 116 may include a keyboard, touch-activated screen, voice controls, buttons, sliders, or any similar or equivalent control device. The operator workstation 110 is coupled to and in communication with a computer system 120 that enables an operator to control the generation and viewing of images on the display 118. Computer system 120 includes multiple components that communicate with each other via electrical and/or data connections 122. Computer system connection 122 may be a direct wired connection, a fiber optic connection, a wireless communication link, or the like. The components of computer system 120 include a Central Processing Unit (CPU)124, a memory 126, which may include a frame buffer for storing image data, and an image processor 128. In an alternative embodiment, the image processor 128 may be replaced by image processing functionality implemented in the CPU 124. The computer system 120 may be connected to an archival media device, permanent or back-up storage, or a network. The computer system 120 is coupled to and in communication with a separate MRI system controller 130.

The MRI system controller 130 includes a set of components that communicate with each other via electrical and/or data connections 132. The MRI system controller connection 132 may be a direct wired connection, a fiber optic connection, a wireless communication link, or the like. The components of the MRI system controller 130 include a CPU 131, a pulse generator 133, a transceiver 135, a memory 137 and an array processor 139, the pulse generator being coupled to and in communication with the operator workstation 110. In an alternative embodiment, the pulse generator 133 may be integrated into the resonance assembly 140 of the MRI system 100. The MRI system controller 130 is coupled to and receives commands from the operator workstation 110 to indicate an MRI scan sequence to be performed during an MRI scan. The MRI system controller 130 is also coupled to and in communication with a gradient driver system 150 that is coupled to the gradient coil assembly 142 to generate magnetic field gradients during an MRI scan.

The pulse generator 133 can also receive data from a physiological acquisition controller 155 that receives signals, including respiratory signals and/or cardiac signals (e.g., ECG signals), from a plurality of different sensors connected to a subject or patient 170 undergoing an MRI scan. And finally, the pulse generator 133 is coupled to and in communication with a scan chamber interface system 145 that receives signals from various sensors associated with the condition of the resonant assembly 140. The scan room interface system 145 is also coupled to and in communication with a patient positioning system 147, which sends and receives signals to control movement of the table 171. The table 171 is controllable to move a patient into and out of the bore 146 and to move the patient to a desired position within the bore 146 for an MRI scan.

The MRI system controller 130 provides gradient waveforms to a gradient driver system 150, which includes G_X、G_YAnd G_ZAmplifiers, and the like. Each G_X、G_YAnd G_ZThe gradient amplifiers energize corresponding gradient coils in the gradient coil assembly 142 to produce magnetic field gradients that are used to spatially encode the MR signals during the MRI scan. Included within the resonant assembly 140 is a gradient coil assembly 142 that also includes a superconducting magnet having superconducting coils 144 that, in operation, provide a uniform longitudinal magnetic field B through an aperture 146₀Or an open cylindrical imaging volume surrounded by the resonance assembly 140. The resonant assembly 140 further includes an RF body coil 148 which, in operation, provides a transverse magnetic field B through the bore 146₁The transverse magnetic field is approximately perpendicular to B₀. The resonance assembly 140 may also include an RF surface coil 149 for imaging different anatomical structures of a patient undergoing an MRI scan. The RF body coil 148 and the RF surface coil 149 may be configured to operate in a transmit and receive mode, a transmit mode, or a receive mode.

A subject or patient 170 undergoing an MRI scan may be positioned within the bore 146 of the resonance assembly 140. The transceiver 135 in the MRI system controller 130 generates RF excitation pulses that are amplified by an RF amplifier 162 and provided to the RF body coil 148 and the RF surface coil 149 through a transmit/receive switch (T/R switch) 164.

As described above, the RF body coil 148 and RF surface coil 149 and/or one or more Phased Array (PA) coils 150 may be used to transmit RF excitation pulses and/or receive resulting MR signals from a patient undergoing an MRI scan. For example, the PA coil 150 may be located in a table below the patient 170, such as in a region below the torso 170a of the patient. The resulting MR signals emitted by excited nuclei within a patient undergoing an MRI scan may be sensed and received by the RF body coil 148, the RF surface coil 149, or the PA coil 150. Each of the coils 148, 149 and 150 typically includes a respective T/R switch, and each typically includes a T/R function and a preamplifier within the surface coil/PA coil itself. Thus, a plurality of T/R switches, collectively referred to as T/R switches 164, are included in the system. Similarly, a plurality of preamplifiers, collectively referred to as preamplifiers 166, may be included. The amplified MR signals are demodulated, filtered, and digitized in a receiver portion of the transceiver 135. The appropriate T/R switch 164 is controlled by a signal from the pulse generator 133 to electrically connect the amplifier 162 to the appropriate coil 148, 149, 150 during the transmit mode and to connect the corresponding preamplifier 166 to the coil 148, 149, 150 during the receive mode. The resulting MR signals sensed and received by the RF body coil 148 or the PA coil 150 are digitized by the transceiver 135 and transferred to the memory 137 in the MRI system controller 130.

The MR scan is complete when the array of raw k-space data corresponding to the received MR signals has been acquired and temporarily stored in the memory 137 until the data is subsequently transformed to create an image. For each image to be reconstructed, the raw k-space data is rearranged into separate k-space data arrays, and each of these separate k-space data arrays is input to an array processor 139, which array processor 139 operates to fourier transform the data into an array of image data.

The array processor 139 uses known transformation methods, most commonly fourier transforms, to create images from the received MR signals. These images are transferred to computer system 120 where they are stored in memory 126. In response to commands received from the operator workstation 110, the image data may be archived in long term memory or may be further processed by the image processor 128 and communicated to the operator workstation 110 for presentation on the display 118. In various embodiments, the components of the computer system 120 and the MRI system controller 130 may be implemented on the same computer system or multiple computer systems.

A motion sensor 11 is integrated into the resonance assembly 140 to sense the motion of the patient. The detected motion information may be used to control and optimize imaging, such as to assist MR image capture based on detected periodic motion and/or to otherwise improve image quality by avoiding image degradation due to patient motion. The motion sensor 11 generates a magnetic field by which the motion of the patient can be detected, as described below. The motion information is provided to a Physiological Acquisition Controller (PAC)155, which provides information about the patient's period and/or other motion to the pulse generator 133. For example, PAC controller 155 may generate respiratory signals formatted for triggering MR image data acquisitions performed by MRI system controller 130.

Referring to fig. 2, a table 171 is shown schematically illustrating the exemplary motion sensor system 10 incorporated therein. The table 171 is outside of the bore 146 and is movable into the bore 146 for patient imaging. The table 171 has a top surface 171a for supporting a patient 170 to be imaged. In the depicted example, the stage includes a PA coil 150 located below the top surface 171 a. In the depicted example, two motion sensors 11a and 11b are located below the PA coil and are configured to sense motion of the patient. In other embodiments, the motion sensors 11a, 11b may be located elsewhere in the system 140, such as directly below the top surface 171a or elsewhere relative to the patient, such as to the side of or above the patient in the bore 146.

Each motion sensor 11a, 11b comprises a resonance coil 16a, 16b and a corresponding coupling loop 18a, 18 b. Each coupling loop 18a, 18b is configured to generate a driving RF signal to excite the corresponding resonance coil 16a, 16b to radiate a magnetic field having a predefined resonance frequency. The coupling loops 18a, 18b are further configured to receive reflected RF signals from the corresponding resonance coils 16a, 16 b. In other embodiments, different driving methods may be utilized, such as via a directly connected driver/receiver. In the case of the direct drive configuration, sensing of S11 would also be accomplished through the direct drive connection. In one such embodiment, SRS coil 36 may be comprised of two helical elements interleaved rotated 180 degrees from each other. Each interlaced helical member has a central end and an outer end. The end closest to the center of coil 36 may be directly driven using a voltage source in order to excite the SRS coil to generate a magnetic field. Receiving the RF signal and sensing S11 therefrom would also be accomplished through a direct connection. Thus, in a direct drive implementation, the coupling loop may be eliminated.

Each motion sensor 11a, 11b is positioned such that the relevant part of the patient 170 is within a region of strong magnetic field relative to the sensors 11a, 11 b. In the case where respiratory motion is detected by the motion sensors 11a, 11b, the motion sensors 11a, 11b are positioned such that at least a portion of the torso 170a of the patient 170 is within the area of a sufficiently strong magnetic field such that motion of the torso 170a due to respiration may be detected. The time-varying load of the magnetic field, i.e. the H-field, caused by changes in the absorption of the patient's tissue within the field can be measured and corresponds to the respiratory cycle.

In one embodiment, this change is detected by measuring the reflection coefficient of the RF source power emitted by the coupling loops 18a, 18b into the resonance coils 16a, 16b (S11). The reflection coefficient S11 represents how much power is reflected from the resonance coil 16a, 16b, which will be affected by the change in absorption by the patient due to breathing. Thus, the respiration signal may be determined based on the change in the reflection coefficient over the respiration cycle.

In the embodiment at fig. 2, two motion sensors 11a, 11b are included. In other embodiments, only one motion sensor 11 may be included, or more than two motion sensors 11 may be included. One or more of the motion sensors 11a, 11b are selectable via a switch 20. In the depicted example, only one of the motion sensors 11a or 11b can be selected by connecting the respective coupling loop 18a, 18b to a controller (in this example, PAC controller 155). The appropriate motion sensor 11a or 11b is selected based on the orientation of the patient to be imaged (i.e., whether the patient is positioned head-first or foot-first to move into the bore 146). In this example, the motion sensor 11a is positioned closer to the front end 172 of the table 171 (the end that enters the aperture), and the motion sensor 11b is positioned closer to the rear end 173 of the table 171 (the end that enters the aperture). If the patient is head-first positioned, the motion sensor 11a will be utilized with the patient's head at the front end 172 of the table 171. Specifically, motion sensor 11a is positioned such that it is aligned with torso 170a of the patient when the patient's head is first positioned toward aperture 146. Alternatively, if the patient's feet are first positioned toward the aperture 146, the motion sensor 11b may be selected via the switch 20, which is positioned to align with the patient's torso 170a when the patient is in the feet first position.

In one example, selection of the appropriate motion sensor 11a or 11b by the switch 20 may be controlled based on whether the patient 170 to be imaged is positioned head first or foot first. The patient position is known, for example, by the MRI system controller 130 and is a parameter used for a number of control purposes within the MRI system 100. In one embodiment, actuating the switch 20 to control the selection of the motion sensor 11a or 11b may be performed by providing a predefined DC bias on the drive signal coaxial cable, wherein a different predefined DC bias is associated with each motion sensor 11a and 11 b.

The motion sensors 11a and 11b are connected to the controller 155, such as via a coaxial cable 22. In one example, the controller 155 is a PAC controller 155 that includes a breath detection sub-controller 24 that includes circuitry for filtering and digitizing analog reflectometer measurements provided by the motion sensors 11a, 11b and software for processing the digitized signals to generate a breath signal that may be used to control MR image acquisition.

Fig. 3-5A and 5B and fig. 7 depict an exemplary embodiment of the motion sensor 11. Fig. 3 depicts an embodiment in which the resonant coil 16 is a self-resonant spiral (SRS) coil 36. In other embodiments, the resonance coil 16 may be replaced by a circular coil or an air coil positioned closer to the back of the patient 170. In one embodiment, the self-resonant spiral provides for greater H-field generation due to its multi-turn nature, with tuning capacitance dominated by the distributed capacitance between the turns of the spiral. For example, this allows the SRS coil 36 to be positioned further away from the patient than other types of resonance coils, while still providing useful detection of patient loading of the magnetic field that is sensitive enough to provide good detection of patient breathing. The SRS coil 36 with multiple turns is driven at a frequency lower than the frequency of proton scanning or larmor frequency to produce a strong H-field with a large penetration depth into the scanner object that will not form a disturbance to MR imaging.

Referring to fig. 7, in particular, SRS coil 36 has a low source impedance when excited by the driving RF signal generated by the coupling loop, with the H-field dominating the near-field environment. The depth (in meters) of the near field 66 can be estimated by the following equation:

the field decays at a rate of 1/r3, where r is the distance from the source normalized to λ/2 π. As explained in more detail below, the RF magnetic field will have a greater penetration depth when the source frequency is lower.

Fig. 3 depicts one embodiment of SRS coil 36. The exemplary SRS coil 36 includes 13 turns between a first end 37 and a second end 38 of a conductor or wire comprising the coil. In this example, the circular coils are always spaced apart, with spacing S being another parameter that affects the resonant frequency and the magnitude of the H-field. In other examples, and depending on the application, different numbers of turns and/or different spacings may be utilized, and the spacing may vary depending on the shape of SRS coil 36. For example, for an elliptical coil, the pitch will vary according to the angle of rotation about the center of the coil and according to the eccentricity of the ellipse. For example, the inventors have recognized that various numbers of turns, such as 10 to 15 turns, may be suitable depending on the desired resonant frequency and the required magnitude of the H-field (e.g., which may depend on placement within the table). In the example at fig. 3, SRS coil 36 is elliptical. In other embodiments, different shapes may be used. Fig. 5B depicts another example, which shows exemplary dimensions of the elliptical SRS coil 36. For a given excitation current, an elliptical self-resonant helical coil will generate a stronger magnetic field. Furthermore, the elliptical coil may have the additional advantage that it may be installed in a narrow space, which may be beneficial for installing the sensor 11 in a crowded space of the table 171.

Coupling loop 18 is inductively coupled to SRS coil 36 or other resonant coil 16. The coupling loop 18 is configured to generate a driving RF signal to excite the SRS coil to radiate a magnetic field at a predefined frequency. In one embodiment, it is desirable to use a 27MHz SRS coil 36 because it provides for large H-field generation due to its multi-turn nature, with the tuning capacitance dominated by the distributed capacitance between the turns of the helix. The 27MHz is advantageously within the industrial, scientific and medical (ISM) band. In other embodiments, different predefined resonant frequencies may be utilized, which may be different ISM band frequencies. To provide one example, the predefined resonant frequency may be in the ISM band between 26.975MHz and 27.283MHz, or may be between 40.66MHz and 40.7MHz, or in still other embodiments, may be between 13.553MHz and 13.567 MHz. In other embodiments, the predetermined resonant frequencies may be different and/or outside of those ISM bands. In some examples, it may be beneficial to utilize a predetermined resonance frequency that is lower than the proton scan frequency.

The coupling loop 18 also receives reflected RF signals from the SRS coil 36 so that breathing or other patient motion can be detected by measuring changes in the reflected RF signals due to changes in the load the patient is subjected to for that RF H field. For example, as the patient breathes, the amount of power reflected by SRS coil 36 will change. In one embodiment, a motion signal, such as a respiration signal, is determined based on the reflection coefficient S11 of the SRS coil 36. In the depicted implementation at fig. 4, a dual log power detection integrated circuit is used in conjunction with a directional coupler to measure the reflected power delivered at 27MHz (i.e., the reflected RF signal) divided by the forward power (i.e., the drive RF signal). The reflection coefficient S11 may then be calculated according to the following equation:

S11＝log 10(P_refl)-log 10(P_drv)

fig. 4 depicts one embodiment of a coupling plate 40 for detecting patient motion based on changes in reflected RF signals, the coupling plate including a coupling loop and a coupling circuit 41. The coupling ring 18 is, for example, of a given diameter d_clA circular ring of (a). To provide but one example, the diameter d of coupling ring 18_clMay be 50mm and the gap g between the ends of the coupling ring 18_clMay be, for example, 5 mm. The coupling circuit 41 comprises a blocking network 43 which blocks other resonance frequencies than the resonance frequency at 27MHz or the predefined resonance frequency. Also included is a lattice balun circuit 45 that converts the differential output of the ring to a single-ended coaxial feed line. The lattice balance-imbalance effectively renders the sensor assembly insensitive to frequency shifts. A filtering circuit 47, such as a diplexer, is also included to filter the output reflectometer measurement signal before transmission to the controller 155. The resulting signal is provided to a jack 49 such as a coaxial connector.

Fig. 5A depicts an exemplary sensor 11 assembly that includes a coupling plate 40 that can be connected to a coil plate 30 holding an SRS coil 36 or other resonant coil 16 at a predefined coupling distance. The coupling plate 40 and the coil plate 30 are separated by a spacer 51, such as made of foam or other material, that does not interfere with the inductive coupling between the coupling loop 18 and the coils 16, 36. For example, the coupling plate 40 may be spaced apart from the coil plate 30, such as 3/16 inches. The coupling distance or spacing between the coupling loop 18 and the coils 16, 36 controls the K-coupling factor. The coupling plate 40 and the coil plate 30 are connected together, such as via connection points 53, separated by spacers 51, thereby defining and maintaining a coupling distance.

Fig. 5B depicts an exemplary elliptical SRS coil 36, which is a 13-turn elliptical self-resonant spiral formed from 1mm copper wire. The helix is about 110mm wide and about 160mm long and has a central space of about 7.5 mm. The intra-coil spacing S varies from about 4mm on the narrow side (width) to about 6mm on the longer side (length). In certain embodiments, a pair of tuning elements 32 may be included to tune the capacitance of SRS coil 36. The tuning element is a patch of conductive material that, when properly placed and sized, provides for slightly adjusting the total distributed capacitance between the turns of the SRS coil. For example, the tuning element 32 may be formed of copper or other conductive metal and sized such that the resonant frequency of the coil 36 is perfectly correct at its mounting location in the table or for each particular table configuration. For example, tuning element 32 may be located on a side of coil plate 30 opposite SRS coil 36.

Fig. 6A depicts one embodiment of a respiration signal 60 generated by the sensor 11 according to the present disclosure. As can be seen, the breathing signal 60 is substantially periodic in nature and represents the breathing motion of the patient, i.e., the expansion and contraction of the patient's chest as the patient breathes. The respiration signal 60 can be used to control MR image capture, such as via respiratory gating. The respiration signal 60 may be derived based on the change in the reflection coefficient of the coils 16, 36. Fig. 6B is a graph of the reflection coefficient S11 of the exemplary sensor 11.

The reflection coefficient S11 varies as a result of patient breathing, as evidenced in the graph of fig. 6B. Line 62 shows S11 at the end of inspiration, where the lungs are filled with gas, while line 63 shows S11 at the end of expiration. Therefore, by tracking S11, the reflection coefficient and the breathing cycle can be tracked.

Fig. 7 is a schematic diagram of the resonant coils 16, 36 and depicts the H-field pattern in the YZ cutting plane. The H-field dominates the environment around the resonance coils 16, 36. The depth of the near field 66, where the magnetic field is strongest, is estimated in meters according to the above formula. The depth of the near field at 27MHz will be greater than the near field depth at higher frequencies such as 240 MHz. The RF magnetic field will have a greater penetration depth when the source frequency is lower. For example, the near field depth at 240MHz is about 4cm, while the depth at 27MHz increases to about 88 cm. Thus, at 27MHz, the sensor 11 can be positioned further away from the patient than with a resonance coil at 240 MHz. The electric field from the coil will decay at a rate of 1/r 2. The electric field of the coil will interact with the electrical conductivity of the tissue and may interact with the distributed capacitance used to achieve resonance tuning of the coil. Ampere-turns (At) are MKS (meters, kilograms, seconds) units of magnetomotive force (MMF), which is represented by one ampere of direct current flowing in a single-turn loop in vacuum. "turn" refers to the number of windings of the electrical conductor that make up the inductor. For example, a 2A current flowing through a coil of 10 turns produces an MMF of 20A-t.

By maintaining the same current and increasing the number of loops or turns of the coil, the strength of the magnetic field increases because each loop or turn of the coil establishes its own magnetic field. The magnetic field is combined with the fields of the other loops to generate a field around the entire coil, making the total magnetic field stronger. The larger H-field generated by this structure enables the coil to be placed at a greater distance below the patient within the table while still producing an RF field that will interact with the patient tissue. The electric field will remain close to the SRS coil and will be weakly coupled to the patient. The coil tuning is only weakly affected. Furthermore, the driven center SRS coil is flanked on both sides by passive elements, such as passive SRS coils inductively coupled to the driven SRS coil 36. The excited body region within the patient is larger than the excitation with a single driven element. However, the three-ring configuration shown in fig. 7 is not required, and it should be noted that very good results can be obtained using the single-ring configuration described and illustrated herein. Notably, as the passive components expand the field in the Z-direction, the depth in the X-Y plane will decrease. Furthermore, the single loop system is simpler because tuning interdependence is eliminated.

Referring to fig. 2 and 7, one or more passively coupled elements 26 may be included and positioned adjacent to the active coils 16, 36 to increase the magnitude of the magnetic field along the Z-axis (which extends along the length of the table 171 and from head to foot relative to the patient). In fig. 7, two coupling elements 26a and 26b are positioned on either side of the resonant coils 16, 36 along the Z-axis, which are inductively coupled into the system. Thus, a larger body area within the patient can be excited using only a single driven element 16, 36. For example, the coupling element may be a passive SRS coil and thus have no corresponding coupling loop but are inductively coupled by a magnetic field radiated centrally by the driven SRS coil 36. In other embodiments illustrated at fig. 2, only a single coupling element 26 may be passively coupled to each resonant coil 16, such that the magnetic field from the coil 16 extends in only one direction along the Z-axis.

Thus, the disclosed motion sensor system 10 is highly sensitive to respiratory motion and is more sensitive than smaller coil elements. The disclosed sensor system 10 is applicable to a wide patient population and is reliable and easy to operate without any additional work required by the clinician performing the MR imaging. The sensor system 10 may be implemented with relatively low cost signal detection circuitry and may be implemented using existing control systems with minimal additional circuitry that may be implemented as a "piggyback" board on an existing PAC. The disclosed motion sensing system 10 can be used in existing MR resonance assemblies 140 and is useful for all surface and body coils. Thus, respiration sensing can be integrated without the need for specially designed surface coils.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. Certain terms have been used for brevity, clearness, and understanding. No unnecessary limitations are to be inferred therefrom other than as required by the prior art, because such terms are used for descriptive purposes only and are intended to be broadly construed. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have features or structural elements that do not differ from the literal language of the claims, or if they include equivalent features or structural elements with insubstantial differences from the literal languages of the claims.