Patent Application
20180081007
Embodiments of the subject matter disclosed herein relate to magnetic resonance imaging (MRI), and more particularly, to disconnecting MRI radio frequency (RF) coils.
Magnetic resonance imaging (MRI) is a medical imaging modality that can create pictures of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a superconducting magnet to create a strong, uniform, static magnetic field. When a human body, or part of a human body, is placed in the magnetic field, the nuclear spins associated with the hydrogen nuclei in tissue water become polarized, wherein the magnetic moments associated with these spins become preferentially aligned along the direction of the magnetic field, resulting in a small net tissue magnetization along that axis. MRI systems also include gradient coils that produce smaller amplitude, spatially-varying magnetic fields with orthogonal axes to spatially encode the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are then used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei, which add energy to the nuclear spin system. As the nuclear spins relax back to their rest energy state, they release the absorbed energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
As mentioned, RF coils are used in MRI systems to transmit RF excitation signals (“transmit coil”), and to receive the RF signals emitted by an imaging subject (“receive coil”). Coil-interfacing cables may be used to transmit signals between the RF coils and other aspects of the processing system, for example to control the RF coils and/or to receive information from the RF coils. The coil-interfacing cables may be disposed within the bore of the MRI system and subjected to electro-magnetic fields produced and used by the MRI system. The cables may support transmitter-driven common-mode currents, which create field distortions and/or unpredictable heating of components. These field distortions may result in a shadow of the cables appearing within an image reconstructed from received MR signals.
Conventionally, baluns (balanced to unbalanced) networks or common-mode traps that provide high common-mode impedances may be utilized to mitigate the effect of transmitter-driven currents. However, placing the common-mode traps or blocking circuits at appropriate locations may be difficult, as the appropriate placement may vary based on the positioning of a cable or coil associated with the common-mode traps. In addition, the common-mode traps may be difficult to manufacture and assemble. Furthermore, excessive voltage and/or power dissipation may occur even if conventional common-mode traps or blocking circuits are placed at appropriate locations. Further still, baluns or common-mode traps positioned too close to each other on a cable may become coupled due to fringe magnetic fields, thereby resulting in a detuning of the baluns, which may adversely affect the functioning of the baluns.
In one embodiment, a device may include a device having a first terminal and a second terminal, the first device operably coupling a first terminal of a radio frequency (RF) coil with one or more data acquisition elements, and a second device having a third terminal and a fourth terminal, the second device operably coupling a second, different terminal of the RF coil with one or more data acquisition elements. The first device and the second device may be operated to couple and decouple a receive RF coil from one or more of a transmit RF coil and the data acquisition elements. As such, a coupling circuit may electrically couple the one or more terminals of the first device and the second device to the one or more acquisition elements. The first and the second device and the coupling circuit may be placed along a receive path of the receive RF coil. Herein, the receive path may include one or more of coupling circuits, processing system, acquisition elements, feedboards, coil-interfacing cables, and the like. The first and the second devices may include a plurality of switches that can be opened or closed. Specifically during a transmit operation, the first and the second devices may be operated such that the plurality of switches are open, thus altering the impedance to provide isolation to the receive RF coil. In this way, the device may function as common-mode choke in MRI systems.
It should be understood that the brief description above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.
The present invention will be better understood from reading the following description of non-limiting embodiments, with reference to the attached drawings, wherein below:
The following description relates to various embodiments of a radio frequency (RF) coil in MRI systems. In particular, systems and methods are provided for disconnecting a receive RF coil from a transmit RF coil during a transmit operation in an MRI system, such as the MRI system depicted in
The magnetostatic field magnet unit 12 includes, for example, typically an annular superconducting magnet, which is mounted within a toroidal vacuum vessel. The magnet defines a cylindrical space surrounding the subject 16, and generates a constant primary magnetostatic field along the Z direction of the cylinder space.
The MRI apparatus 10 also includes the gradient coil unit 13 that generates a gradient magnetic field in the imaging space 18 so as to provide the magnetic resonance signals received by the RF coil unit 14 with three-dimensional positional information. The gradient coil unit 13 includes three gradient coil systems, each of which generates a gradient magnetic field which inclines into one of three spatial axes perpendicular to each other, and generates a gradient field in each of frequency encoding direction, phase encoding direction, and slice selection direction in accordance with the imaging condition. More specifically, the gradient coil unit 13 applies a gradient field in the slice selection direction of the subject 16, to select the slice; and the RF coil unit 14 transmits an RF pulse to a selected slice of the subject 16 and excites it. The gradient coil unit 13 also applies a gradient field in the phase encoding direction of the subject 16 to phase encode the magnetic resonance signals from the slice excited by the RF pulse. The gradient coil unit 13 then applies a gradient field in the frequency encoding direction of the subject 16 to frequency encode the magnetic resonance signals from the slice excited by the RF pulse.
The RF coil unit 14 is disposed, for example, to enclose the region to be imaged of the subject 16. In some examples, the RF coil unit 14 may be referred to as the surface coil or the receive coil. In the static magnetic field space or imaging space 18 where a static magnetic field is formed by the magnetostatic field magnet unit 12, the RF coil unit 14 transmits, based on a control signal from the controller unit 25, an RF pulse that is an electromagnet wave to the subject 16 and thereby generates a high-frequency magnetic field. This excites a spin of protons in the slice to be imaged of the subject 16. The RF coil unit 14 receives, as a magnetic resonance signal, the electromagnetic wave generated when the proton spin thus excited in the slice to be imaged of the subject 16 returns into alignment with the initial magnetization vector. The RF coil unit 14 may transmit and receive an RF pulse using the same RF coil.
The RF body coil unit 15 is disposed, for example, to enclose the imaging space 18, and produces RF magnetic field pulses orthogonal to the main magnetic field produced by the magnetostatic field magnet unit 12 within the imaging space 18 to excite the nuclei. In contrast to the RF coil unit 14, which may be easily disconnected from the MRI apparatus 10 and replaced with another RF coil unit, the RF body coil unit 15 is fixedly attached and connected to the MRI apparatus 10. Furthermore, whereas local coils such as those comprising the RF coil unit 14 can transmit to or receive signals from only a localized region of the subject 16, the RF body coil unit 15 generally has a larger coverage area and can be used to transmit or receive signals to the whole body of the subject 16. Using receive-only local coils and transmit body coils provides a uniform RF excitation and good image uniformity at the expense of high RF power deposited in the subject. For a transmit-receive local coil, the local coil provides the RF excitation to the region of interest and receives the MR signal, thereby decreasing the RF power deposited in the subject. It should be appreciated that the particular use of the RF coil unit 14 and/or the RF body coil unit 15 depends on the imaging application.
The T/R switch 20 can selectively electrically connect the RF body coil unit 15 to the data acquisition unit 24 when operating in receive mode, and to the RF driver unit 22 when operating in transmit mode. Similarly, the T/R switch 20 can selectively electrically connect the RF coil unit 14 to the data acquisition unit 24 when the RF coil unit 14 operates in receive mode, and to the RF driver unit 22 when operating in transmit mode. When the RF coil unit 14 and the RF body coil unit 15 are both used in a single scan, for example if the RF coil unit 14 is configured to receive MR signals and the RF body coil unit 15 is configured to transmit RF signals, then the T/R switch 20 may direct control signals from the RF driver unit 22 to the RF body coil unit 15 while directing received MR signals from the RF coil unit 14 to the data acquisition unit 24. The coils of the RF body coil unit 15 may be configured to operate in a transmit-only mode, a receive-only mode, or a transmit-receive mode. The coils of the local RF coil unit 14 may be configured to operate in a transmit-receive mode or a receive-only mode.
The RF driver unit 22 includes a gate modulator (not shown), an RF power amplifier (not shown), and an RF oscillator (not shown) that are used to drive the RF coil unit 14 and form a high-frequency magnetic field in the imaging space 18. The RF driver unit 22 modulates, based on a control signal from the controller unit 25 and using the gate modulator, the RF signal received from the RF oscillator into a signal of predetermined timing having a predetermined envelope. The RF signal modulated by the gate modulator is amplified by the RF power amplifier and then output to the RF coil unit 14.
The gradient coil driver unit 23 drives the gradient coil unit 13 based on a control signal from the controller unit 25 and thereby generates a gradient magnetic field in the imaging space 18. The gradient coil driver unit 23 includes three systems of driver circuits (not shown) corresponding to the three gradient coil systems included in the gradient coil unit 13.
The data acquisition unit 24 includes a preamplifier (not shown), a phase detector (not shown), and an analog/digital converter (not shown) used to acquire the magnetic resonance signals received by the RF coil unit 14. In the data acquisition unit 24, the phase detector phase detects, using the output from the RF oscillator of the RF driver unit 22 as a reference signal, the magnetic resonance signals received from the RF coil unit 14 and amplified by the preamplifier, and outputs the phase-detected analog magnetic resonance signals to the analog/digital converter for conversion into digital signals. The digital signals thus obtained are output to the data processing unit 31.
The MRI apparatus 10 includes a table 26 for placing the subject 16 thereon. The subject 16 may be moved inside and outside the imaging space 18 by moving the table 26 based on control signals from the controller unit 25.
The controller unit 25 includes a computer and a recording medium on which a program to be executed by the computer is recorded. The program when executed by the computer causes various parts of the apparatus to carry out operations corresponding to pre-determined scanning. The recording medium may comprise, for example, a ROM, flexible disk, hard disk, optical disk, magneto-optical disk, CD-ROM, or non-volatile memory card. The controller unit 25 is connected to the operating console unit 32 and processes the operation signals input to the operating console unit 32 and furthermore controls the table 26, RF driver unit 22, gradient coil driver unit 23, and data acquisition unit 24 by outputting control signals to them. The controller unit 25 also controls, to obtain a desired image, the data processing unit 31 and the display unit 33 based on operation signals received from the operating console unit 32.
The operating console unit 32 includes user input devices such as a keyboard and a mouse. The operating console unit 32 is used by an operator, for example, to input such data as an imaging protocol and to set a region where an imaging sequence is to be executed. The data about the imaging protocol and the imaging sequence execution region are output to the controller unit 25.
The data processing unit 31 includes a computer and a recording medium on which a program to be executed by the computer to perform predetermined data processing is recorded. The data processing unit 31 is connected to the controller unit 25 and performs data processing based on control signals received from the controller unit 25. The data processing unit 31 is also connected to the data acquisition unit 24 and generates spectrum data by applying various image processing operations to the magnetic resonance signals output from the data acquisition unit 24.
The display unit 33 includes a display device and displays an image on the display screen of the display device based on control signals received from the controller unit 25. The display unit 33 displays, for example, an image regarding an input item about which the operator inputs operation data from the operating console unit 32. The display unit 33 also displays a slice image of the subject 16 generated by the data processing unit 31.
During a scan, coil-interfacing cables (not shown) may be used to transmit signals between the RF coils (e.g., RF coil unit 14 and RF coil unit 15) and other aspects of the processing system (e.g., data acquisition unit 24, controller unit 25, and so on), for example to control the RF coils and/or to receive information from the RF coils. As explained previously, the RF body coil 15 is a transmit coil that transmits MR signals, and the local surface RF coil 14 receives the MR signals. More generally, RF coils are used to transmit RF excitation signals (“transmit coil”), and to receive the RF signals emitted by an imaging subject (“receive coil”). In an example, the transmit and receive coils are a single mechanical and electrical structure or array of structures, with transmit/receive mode switchable by auxiliary circuitry. In other examples, the transmit body coil (e.g., RF coil unit 15) and the surface receive coil (RF coil unit 14) may be independent structures that are physically coupled to each other via a data acquisition unit or other processing unit. For enhanced image quality, however, it may be desirable to provide a receive coil that is mechanically and electrically isolated from the transmit coil. In such case it is highly desirable that the receive coil, in its receive mode, be electromagnetically coupled to and resonant with the RF pulse that was provided from the transmit coil. Further, it may be desirable that the receive coil is electromagnetically decoupled from and therefore not resonant with the transmit coil, during actual transmission of the RF pulse. Such decoupling averts a potential problem of noise produced within the auxiliary circuitry when the receive coil couples to the full power of the RF pulse. The inventors have recognized that it may be possible to electromagnetically couple the transmit coil to the receive coil through microelectromechanical systems (MEMS) switches. In this way, it may be possible to electromagnetically decouple or disconnect the receive coil from the transmit coil, particularly during the transmit operation.
The inventors have recognized that it may be possible to employ a plurality of MR compatible MEMS switches to decouple the coil during a transmit operation of the MRI apparatus, and further disconnect the coil from the interfacing feedboard and cables. An example embodiment using a MEMS switch to rapidly decouple the receive coil from the transmit coil is shown in
Turning now to
Coil-interfacing cables 218 may be used to transmit signals between the RF coils and other aspects of the processing system, for example to control the RF coils and/or to receive information from the RF coils. The coil-interfacing cables may be disposed within the bore or imaging space of the MRI apparatus (such as MRI apparatus 10 of
Typically, the RF coil 202 in its receive mode may be coupled to a body coil (such as RF body coil unit 15) in order to receive echoes of the RF pulse transmitted during the transmit mode. As mentioned earlier, if the RF coil 202 is not used for transmission, then it may be necessary to decouple the RF coil 202 from the body coil while the body coil is transmitting the RF pulse. Traditionally, the decoupling of the receive coil from the transmit coil is achieved using resonance circuits and PIN diodes. Herein, the diodes may activate detuning circuits operatively connected to the RF coil 202. However, diode-activated detuning circuits may be slow and may cause loss in signals. Instead of using diode activated detuning circuits, MEMS device 208 may be used to decouple the receive coil from the transmit coil as discussed below. Herein, the MEMS device 208 may include a pair of MEMS switches in a back-to-back configuration as shown in
Turning now to
Each of the first MEMS switch 310 and the second MEMS switch 312 of the MEMS device 300 are electrically coupled to a passive bypass circuit 332. The MEMS device 300 includes a single driver or gate 318 that controls each of the first MEMS switch 310 and the second MEMS switch 312. The passive bypass circuit 332 includes a first resistor 334 coupled across the first MEMS switch 310 and a second resistor 336 coupled across the second MEMS switch 312. An example value of resistance of the first and second resistor is 100K ohm. It should be understood that the passive elements in the passive bypass circuit in this example are shown as resistive elements 334, 336; however, other passive elements including inductors may be used. The passive bypass circuit 332 may shield the MEMS switches 310 and 312 by receiving at least a portion of electrical energy transferred across contacts of the MEMS switches 310 and 312 during transition from open to closed or from closed to open states, for example.
The first MEMS switch 310 includes a first beam element 320, a first contact 324, and a first gate 322. Herein, the first beam element 320 may contact the first contact 324 when an actuation voltage is applied to the first gate 322. When the first beam element 320 is in contact with the first contact 324, the first MEMS switch 310 may be in closed state. When no actuation voltage is applied to the first gate 322, the first MEMS switch 310 may be in open state wherein the first beam element 320 may not be in electrical contact with the first contact 324. Thus, the first gate 322 influences whether or not the first beam element 320 makes electrical connection to the first contact 324, and thereby controls whether the first MEMS switch 310 is in a closed or an open state.
Similar to the first MEMS switch 310, the second MEMS switch 312 includes a second beam element 326, a second contact 330, and a second gate 328. The second beam element 326 may contact the second contact 330 based on an actuation voltage that is applied to the second gate 328. The first gate 322 of the first MEMS switch 310 and the second gate 328 of the second MEMS switch 312 may be electrically coupled together to form a common gate or driver 318. It may be possible to employ a single (or common) actuation voltage to control the actuation of both the first and the second MEMS switches as described below.
As an example, a switch controller 302 may apply an actuation voltage to the driver 318 to switch the MEMS device 300 from an open state to a closed state. Likewise, the switch controller 302 may cease the application of the actuation voltage to the driver 318 to switch the MEMS device 300 from a closed to an open state. For example, the switch controller 302 may apply an actuation voltage to the driver 318, and may bias each of the first beam element 320 and the second beam element 326 in such a way that the first beam element 320 may contact the first contact 324 and the second beam element 326 may contact the second contact 330. Thus, current may flow from the first and second beam elements to the respective first and second contact elements, and MEMS device 300 may be in a “closed” state. Herein, the actuation voltage is equally applied to the first gate 322 and the second gate 328. In a similar way, when the switch controller 302 does not apply an actuation voltage to the driver 318, the first and the second beam elements 320 and 326 may be spaced apart from the first and the second contact elements 324 and 330. Thus, the MEMS device 300 may be in an “open” state. The actuation voltage may be a single voltage value (e.g., 80V), or a range of voltage values (e.g., 10-100V). In an example configuration, when the switch controller 302 applies a voltage greater than a threshold voltage to the driver 318, the beam elements of each of the first and the second MEMS switch may be electrically coupled to the respective first and second contacts, thereby closing the MEMS device 300. When the switch controller applies a voltage, less than the threshold voltage (e.g., threshold voltage=50V), the beam elements of each of the first and the second MEMS switches may be electrically decoupled from the respective first and second contacts, thereby opening the MEMS device 300. In this way, the MEMS device 300 including a pair of MEMS switches in back-to-back configuration may be used to coupled and decouple the receive RF coil from the transmit RF coil, for example. As such, the MEMS switches may be in open state providing coil decoupling when the receive RF coil is unpowered (or the receive coil being unplugged, for example). As mentioned earlier, the coupling and decoupling of receive and transmit coils are typically achieved using resonance circuits and diodes. However, these resonance circuits and diodes are slow. Specifically, the switch turn-off time can be slow if larger amounts of charge are stored in the resonance circuits and diodes to carry high RF currents. Typically, to discharge or remove that charge, the resonance circuits and diodes may take longer than 10 μsec. By replacing these circuits and diodes with the MEMS device, it may be possible to transition the RF coils from a coupled to a decoupled state and vice versa at a faster rate. As an example, the time that the MEMS device takes to transition from a closed to an open state is less than 1 μsec and from open to closed state is about 4 μsec. Another advantage of using the MEMS devices to decouple the RF coils is that the MEMS devices are electrostatically driven, and thus are low power devices, which can operate in the RF ranges.
In one example, the back-to-back MEMS are configured such that the actuating elements are mechanically coupled to each other. By way of example, during the transmit operation, the switch controller 302 may switch the MEMS device 300 to the open state to decouple the RF coil from an RF receiver before selective application of RF transmit signals to a patient. Further, during the receive operation, the switch controller 302 may switch the MEMS device 300 to the closed state for coupling the RF coil to the RF receiver to enable detection of the MR signals corresponding to the resulting excitation in the patient. The detected MR signals, in turn, may be communicated to a processing system (not shown in
Thus, by including MEMS switches to decouple the receive coil from the transmit coil in the MRI apparatus, currents in the loop may be minimized during transmit operation, for example. As such, the MEMS switches may be in open state providing coil decoupling when the MR receive coil is unpowered (or unplugged, for example). Returning to
The circuit 204 may be a coupling circuit and may include several baluns, preamplifiers, and so on. In one example embodiment, the circuit 204 may include an input balun 210 and an output balun 216. Typically, the input and the output baluns are resonance circuits that cause currents to be equal in magnitude, but opposite in phase, resulting in a zero imbalance current. As such, baluns provide high common-mode impedances, which may be utilized to mitigate the effect of transmitter-driven common-mode currents. Examples of baluns include solenoid baluns, transformer-like baluns, bazooka baluns, lattice baluns, and the like. Typically, the structures of the input and the output baluns may be the same, although the output baluns tend to be more robust. In application, the input baluns may be generally used to break the loop from coil element to coil element during the receive state. The output baluns stop the common mode flowing down the interfacing cables during transmit state.
Baluns present low impedance to the signal current that is in differential mode, and thus allows DC coupling. However, for common-mode current, the baluns act as a high impedance choke. The common-mode current (which is the algebraic sum or the net current in the cables) generates unnecessary coupling and heating of the RF coils and cables. Baluns positioned at an appropriate boundary between the RF coil and the interfacing cables may be necessary to reduce common-mode currents. In addition, baluns positioned too close to each other on a cable may become coupled due to fringe magnetic fields, thereby resulting in a detuning of the baluns, which may adversely affect the functioning of the baluns. Thus, the baluns may require additional tuning which may in turn add to the cost of the MRI system. Decoupling the RF coil MEMS switches as described above may not completely block the common-mode currents. The finite Q (or quality factor) of the resonant balun, plus any tuning imperfections, will lower the common mode blocking impedance, for example. Herein, the Q factor of the resonant baluns is a measure of the quality of the resonant circuits. More formally, Q is the ratio of power stored to power dissipated in the circuit reactance and resistance, respectively.
The inventors have recognized that it may be possible to operate a plurality of devices as common-mode chokes. For example, the devices may include switches and/or relays such as GaNFET, PIN diodes, MEMS devices, and the like. Herein, the plurality of devices are coupled across the terminals of the RF coil and serve to completely disconnect the RF coil from the interfacing feedboards and cables, as shown in
Turning now to
The pair of devices 416 may be operated to disconnect and isolate the RF coil 402 during transmit operation. In the case of the pair of devices 416 having MEMS switches, during transmit operation, the MEMS switches may act as floating arrays, wherein the beam element of each of the MEMS switch is disconnected from the respective contact of the MEMS switch. Specifically, during transmit operation, the MEMS switches may be in an open state providing coil decoupling when the MR surface coil (e.g., receive RF coil) is unpowered (or unplugged).
As described previously, the switches may include one or more of MEMS switches, GaNFET switches, relays, and the like. In the case of MEMS switches, an actuation voltage applied between a gate and a beam electrode of the MEMS switches may close the switch. However, when the actuation voltage is not applied between the gate and the beam electrode, the MEMS switches may be open. In the case of GaNFET switches, an actuation voltage or bias applied to a gate relative to a source of the GaNFET switch may close the switch; and when the bias is removed, the switch may be opened. In case of relays, which are electrically operated switches, a control signal or actuation voltage given to the relay may be able to control the opening and closing of the relay switch.
For example, terminal 1 of the RF coil is coupled to the first device 406 (herein also referred to as device 1), and likewise, terminal 2 of the RF coil is coupled to the second device 408 (herein also referred to as device 2). Specifically, terminal 1 of the RF coil 402 is coupled to a first terminal of the first device 406. A second, different terminal of the first device 406 is coupled to terminal 3 of the coil-interfacing cable 420. Likewise, terminal 2 of the RF coil 402 is coupled to a first terminal of the second device 408, and a second, different terminal of the device 408 is coupled to a terminal 4 of the coil-interfacing cables 420. Optionally, an inductive coupling element 404 may also be coupled across the terminals 1 and 2 of the RF coil 402, and an inductive coupling element 405 may be coupled across the wires of the coil-interfacing cable 420. As an example, the coupling elements may include an inductor, a capacitor, and the like. It may be appreciated that the magnitude of the current flowing through each of the terminals of the device may be within 10% of the current through the other terminal of the device. As such, the magnitude of current flowing through all four terminals of the first and the second devices may be within 10% of each other.
When the two switches of the first device 406 are closed, then terminal 1 of the RF coil is electrically connected to terminal 3 of coil-interfacing cables 420 and when the two switches of the second device 408 are closed, then terminal 2 of the RF coil is electrically connected to terminal 4 of the coil-interfacing cables 420. As such, the coil-interfacing cables couple the RF coil to the data acquisition elements (or processing system 414) through one or more of coupling circuits, and preamplifiers, resonance circuits, and the like. Herein, the devices coupled to the coil-interfacing cables 420 couple one or more RF terminals to one or more data acquisition elements. However, when the two switches 407 of the first device 406 are open, then the RF terminal 1 is electrically disconnected from terminal 3 of coil-interfacing cables 420 (
The first and second devices 406 and 408 may each include MEMS devices with pairs of MEMS switches as shown in
In one example, the first electric circuit 508 may electrically couple both MEMS 1 and MEMS 2 using coupling elements (e.g., resistors, capacitors, and the like). Likewise, the second electric circuit 510 may include coupling elements (e.g., resistors) that electrically couple MEMS 3 and MEMS 4. Herein, the first electric circuit 508 and the second electric circuit 510 may be in a single housing (e.g., formed as a single package) that may be coupled to RF coils of MRI systems, specifically to the terminals of the RF coil as described below to disconnect and isolate the RF coils during specific operation of the MRI system. As such, MEMS 1 and MEMS 2 are in a back-to-back configuration, and MEMS 3 and MEMS 4 are in back-to-back configuration, as described with reference to
When the two MEMS switches of the first MEMS device 512 are open, then the RF terminal 1 is electrically disconnected from terminal 3 of the coil-interfacing cables 420 (
MEMS 1 and MEMS 2 of the first MEMS device 512 are MEMS switches and may include contact elements, beam elements, and a gate as explained previously with reference to
As an example, a gate of the first MEMS device 512 may be coupled to a gate 502 of the pair of MEMS devices 501 via a resistor (100 Kohm, for example) of the coupling circuit 506. Likewise, a first beam element and a second beam element of the first MEMS device 512 may each be coupled to a beam electrode 504 of the pair of MEMS devices 501 via a resistor (100 Kohm, for example) of the coupling circuit 506. In a similar way, a gate of the second MEMS device 514 may be coupled to the gate 502 via a resistor of the coupling circuit 506, and a first beam element and a second beam element of the second MEMS device 514 may each be coupled to the beam electrode 504 via a resistor of the coupling circuit 506. In one example, the resistors of the coupling circuit 506 may be 100 Kohm resistors. The gate 502 and the beam electrode 504 may include additional coupling resistors and capacitors.
When the switch controller 518 applies a common actuation voltage between the gate 502 and the beam electrode 504, the actuation voltage may drive each of the MEMS switches. Herein, the actuation voltage may enable the first and the second beam elements of the first MEMS device 512 to contact respective first and second contact elements of the first MEMS device 512. In addition, the same actuation voltage applied between the gate 502 and the beam electrode 504 may additionally enable the first and the second beam elements of the second MEMS device 514 to contact respective first and second contact elements of the second MEMS device 514. In this way, the switch controller 518 may concurrently close all the MEMS switches with a common actuation voltage. In a similar way, when the controller stops applying the actuation voltage between the gate 502 and the beam electrode 504, the first and the second beam elements of the first MEMS device 512 may be mechanically and electrically disconnected from the respective first and second contact elements of the first MEMS device 512. Additionally, the first and the second beam elements of the second MEMS device 514 may be mechanically and electrically disconnected from the respective first and second contact elements of the second MEMS device 514 when the controller stops applying the actuation voltage between the gate 502 and the beam electrode 504. In this way, the switch controller 518 may apply a common actuation voltage to simultaneously close all the MEMS switches and cease or stop the application of the actuation voltage to simultaneously open all the MEMS switches.
As an example, the switch controller 518 may apply an actuation voltage (e.g., −80V) between the gate 502 and the beam electrode 504 to bias the beam elements of the first MEMS device 512. Biasing the beam elements of the first MEMS device 512 includes biasing both the first beam element and the second beam element of the switches of the first MEMS device 512. In addition, the beam elements of the second MEMS device 514 are also biased when the switch controller applies the actuation voltage between the gate 502 and beam electrode 504. Biasing the beam elements of the second MEMS device 514 includes biasing both the first beam element and the second beam element of the switches of the second MEMS device 514.
Thus, when the actuation voltage is applied by the switch controller 518, the beam elements of the switches of the first MEMS device 512 may contact the corresponding contact elements of the MEMS device 512, thereby closing both the switches of the MEMS device 512. Likewise, the beam elements of the switches of the second MEMS device 514 additionally contact the respective contact elements, thereby closing both the switches of the second MEMS device 514. Consequently, both the pairs of the MEMS devices are closed.
The first and the second beam elements of the first MEMS device 512 may together form terminal A of the first MEMS device 512, and the first and the second beam elements of the second MEMS device 514 may together form terminal C of the second MEMS device 514. Herein, terminal A of the first MEMS device 512 may be electrically coupled to terminal 1 of the RF coil 402 of
However, when the switch controller 518 does not apply an actuation voltage (or ceases application of actuation voltage, for example) to the gate 502 and the beam electrode 504, the first and the second beam elements of each of the MEMS switches may be spaced apart from the respective first and the second contact elements. Thus, each of the MEMS devices 512 and 514 may be in an “open” state. Thus, terminal A may be disconnected from terminal B of the first MEMS device 512, and likewise, terminal C may be disconnected from terminal D of the second MEMS device 514. This in turn may result in isolating each of the terminals 1 and 2 of the RF coil from the corresponding terminals 3 and 4 of the coil-interfacing cables. In one example, the capacitive isolation across the MEMS switch is between 0.3 and 1 pF and the passive resistors provide 200K total resistance across the switch. The advantage of including the passive elements in the MEMS switches is that excessive current may not be leaked though the actuation connections. Thus, terminal 1 of the RF coil 402 may be isolated from terminal 3 of the coil-interfacing cables 420, and terminal 2 of the RF coil 402 may be isolated from terminal 4 of the coil-interfacing cables 420. In this way, the pair of MEMS devices 50 leach including a pair of MEMS switches in a back-to-back configuration may be used to selectively connect and disconnect the receive RF coil from the transmit RF coil, for example. The advantages of using the MEMS device as RF switches is that the MEMS devices are low power devices which can provide high RF isolation when open and low insertion loss when closed The PIN diode based switch may require tens of milliwatts of power to operate in the resonant blocking state, for example. Disconnecting the terminals of the RF coil from the terminals of the coil-interfacing cables may also decouple the receive RF coil from the transmit RF coil and vice versa. Thus, during transmit operation, when larger fields are transmitted, the pairs of MEMS switches decouple the RF coil/loop by disconnecting the loop.
It may be appreciated that coupling the pair of MEMS switches across each of the RF coil 402 terminals has several advantages. As explained above, by opening the MEMS switches of each MEMS device during transmit operation, each terminal of the RF coil 402 may be disconnected with the coil-interfacing cables 420 and the feedboards (including the differential preamplifier 412 and the processing system 414, for example). In this way, the receive coil may be completely isolated or disconnected from the transmit coil.
As another advantage, additional resonance circuits may be reduced. Instead of using additional baluns as common-mode chokes to block the common-mode currents flowing along the wires of the coil-interfacing cables 420, it may be possible to use the pair of MEMS devices 501 to act as the common-mode choke. Herein, when the pair of switches MEMS 1 and MEMS 2 of the first MEMS device 512 are open, the terminal 1 of the RF coil 402 is electrically disconnected or isolated from terminal 3 of coil-interfacing cables 420. Likewise, when the pair of switches MEMS 3 and MEMS 4 of the second MEMS device 514 are open, the terminal 2 of the RF coil 402 is electrically disconnected or isolated from terminal 4 of coil-interfacing cables 420. Thus, when all four MEMS switches are open, there is high isolation between the RF coil 402 and the coil-interfacing cables 420, and high isolation between the RF coil and the feedboards, and thus, common-mode currents are blocked from flowing into the preamplifier 412, for example. In this way, the pair of MEMS devices 501 having a plurality of MEMS switches may block the common-mode currents without the need for additional resonance circuits. As such, the MEMS switches have reduced interaction with the transmitting field, and thus have reduced detuning effects on the transmitting system, for example. In one example embodiment, the size of the MEMS switches may be smaller in size when compared to conventional baluns, and may additionally be packaged closer together, to form a single common-mode blocker. In some examples, packaging several MEMS switches together may eliminate the need for additional output baluns. The differential preamplifier 412 (shown in
Using MEMS devices as common-mode choke to block the common-mode currents may reduce preamplifier saturation, for example. Traditionally, in silent imaging, the preamplifier may be easily saturated, and recovery from the saturation may be relatively lengthy (e.g., on the order of tens of μsec). However, when the MEMS devices are used as common-mode chokes, the preamplifier may not be saturated. Since the preamplifier is not saturated, the additional time loss due to recovery from saturation in traditional set-ups may be avoided when MEMS switches are used as common-mode chokes.
As a common-mode blocker, the pair of MEMS devices 501 may be positioned at the input of the differential preamplifier 412 as shown in
Traditionally baluns are positioned along coil-interfacing cables to block common-mode currents as shown in
Typically, baluns 616 are coupled to the coil-interfacing cables 614 at high current locations. A standing wave current distribution 610 shows the distribution of the current 612 as it travels along the coil-interfacing cables 614. As an example, the standing wave current distribution 610 shows the current being high at two locations (marked by vertical dashed lines 613). The baluns 616 are typically placed in the high current locations, and the baluns 616 provide high impedance for the common-mode currents. However, the RF coil 604 is generally placed arbitrarily inside the body coil 602. Thus, the high current locations may not be known, and positioning the baluns at appropriate locations may be difficult. However, by using the switches 620 as explained previously, device 608 may be positioned anywhere along the coil-interfacing cables 614. An example configuration is shown in
Turning now to
In the example where the device 608 is a MEMS device, the MEMS switches of the MEMS device may be actuated by a common actuation voltage. The operation of the MEMS devices may be controlled by a switch controller 625. The switch controller 625 is one non-limiting example of switch controller 418 of
When the switch controller 625 applies an actuation voltage to the device 608, the same actuation voltage is applied across each of the first and the second switches 620 and 622. As a result, all the switches may be closed, and current may be transmitted along the coil-interfacing cable 614. When the device 608 is coupled across the terminals of the RF coil 604, closing the switches result in connecting the RF coil to the pre-amplifier 606 in the receive path. Thus, the RF coil 604 may be able to receive MR signals from the transmit coil.
When an actuation voltage is not applied to both the first and the second switches 620 and 622, all the switches may be in open state, thereby blocking common-mode currents from being transmitted along the coil-interfacing cables 614. By opening all the switches, each of the wires of the coil-interfacing cables 614 may be disconnected from the coil and the feedboards, thus opening the circuit to block common-mode currents. In this way, circulating currents flowing in the coil-interfacing cables may be reduced, particularly during transmit operation.
Turning now to
Method 700 begins at 702, where the method includes during transmit operation, operating a first device and a second device in a first mode. In one example, the first and second device may each be a MEMS device. Herein, operating the first MEMS device and the second MEMS device in the first mode includes ceasing application of an actuation voltage between the gate and the beam of the first and second MEMS device at 704. In one example, first and the second MEMS devices may each include a pair of MEMS switches in a back-to-back configuration. As such, the MEMS switches are coupled together such that the switches are gated by a common actuation voltage. Herein, a controller (such as a switch controller 418 of
In another example, the first and the second device may include GaNFET switches. Herein, an actuation voltage or bias applied to a gate relative to a source of the GaNFET closes the switch, and when the bias is removed, the switch is open. In yet another example, the first and the second device may each include relays. Relays are electrically operated switches. Similar to the MEMS switches and GaNFET described thus far, a control signal or actuation voltage given to the relay may control the opening and closing of the relay switch. At 706, method 700 includes disconnecting the receive RF coil from the transmit RF coil. The receive RF coil is disconnected from the receive path. Herein, the receive path may include one or more of coupling circuits, processing system, feedboards, coil-interfacing cables, and the like. In one example, the devices are coupled across each terminal of the RF coil, and opening the devices includes disconnecting each of the terminals of the RF coil from the terminals of the coupling electronics at 708. In this way, the receive RF coil may be disconnected from the transmit RF coil, and any distortion of the RF fields due to coupling of the coils may be minimized.
Optionally, the devices may be coupled along different wires of the coil-interfacing cables that couple the RF coil to the feedboards. Disconnecting the RF coil may be alternatively achieved by opening the switches coupled to the coil-interfacing cables at 710. In this way, common-mode currents may be blocked during transmit operation.
At 712, method includes during a receive operation, operating the first and the second devices in a second mode. Operating the devices in the second mode includes applying the actuation voltage to the switches of each of the device to close the switches at 714.
In one example, where the switches are MEMS switches, the controller may stop applying the actuation voltage between the gate and beam to open the MEMS switches. As a result, the beam element of each of the MEMS switch is separated from the corresponding contact element, thus opening all the MEMS switches of the first MEMS device and the second MEMS device. In another example, where the switches are GaNFET switches, the actuation voltage, or bias may be removed, and the GaNFET switch may be opened.
At 716, the method includes reconnecting the receive coil to the transmit coil by closing the switches. In one example, when the system transitions from the transmit mode to the receive mode, the method includes transitioning the switches from the first mode to the second mode by moving the switches from the open position to closed position. Moving the switches from the open to closed position includes applying the common actuation voltage to the gate and the beam of the first and the second devices to close all the switches of the first and the second devices. At 718, closing the switches coupled across the terminals of the RF coils may include connecting the receive coil to the RF transmit coil, for example. Optionally, closing the switches may include reconnecting the wires of the coil-interfacing cables coupling the RF coils to the feedboards at 720. Method 700 ends.
In this way, the receive coil may be completely disconnected from the transmit coil during transmit operation. The technical effect of using pairs of MEMS switches is that the switches can be opened to isolate the coils during transmit, and further function as a common-mode choke, thus reducing the propagation of common-mode current to the feedboards and cables. Common-mode currents flowing along the cables may excite anatomy, and may generate unwarranted MR signals. In some examples, the common-mode current may suppress MR signals from an area that is being imaged. The local fields created by the currents may lead to local B1 field distortions. Thus, by reducing the common-mode current by opening the MEMS switches coupled to the coil-interfacing cables, distortions in MR signals may be reduced. As such, when the MEMS switches are open, each of the beam electrode of the MEMS switches are disconnected from the corresponding contact element, thereby rendering floating MEMS switches. Further, the floating MEMS arrays of switches may have reduced interaction with the environment, and/or the body coil, thereby reducing the need to for additional resonance circuits to be built into the MRI apparatus. Furthermore, the MEMS devices are electrostatically driven, thus can quickly transition from an open to a closed state. Thus, the MRI apparatus may be switched from transmit to receive operation at higher speed, thereby allowing the imaging to be performed at a faster rate. This in turn may reduce the exposure of the patient to radiation.
In another representation, a method includes, during transmit operation, operating a pair of MEMS devices to disconnect each cable of coil-interfacing cables operably coupling a receive coil to a transmit coil to block common-mode currents from flowing along the coil-interfacing cables. In an example, each of the MEMS devices includes two MEMS switches in a back-to-back configuration. The operating may include concurrently opening the two MEMS switches of each of the MEMS devices to isolate each cable of the coil-interfacing cables to block flow of common-mode currents. The method may further include, during receive operation, reconnecting each cable of the coil-interfacing cables to allow currents to flow along the coil-interfacing cables. The reconnecting may include applying a common actuation voltage to terminals of the two MEMS switches of each of the pair of MEMS devices to close the two MEMS switches of each pair of MEMS devices.
The systems and methods described above also provide for a device, the device comprising a device having a first terminal and a second terminal, the first device operably coupling a first terminal of a radio frequency (RF) coil with one or more data acquisition elements, and a second device having a third terminal and a fourth terminal, the second device operably coupling a second, different terminal of the RF coil with one or more data acquisition elements. In a first example of the device, the device may additionally or alternatively include wherein the first terminal of the first device is electrically coupled to the first terminal of the RF coil, and wherein the third terminal of the second device is electrically coupled to the second terminal of the RF coil. A second example of the device optionally includes the first example and further includes wherein the second terminal of the first device is electrically coupled to a first terminal of a coupling circuit and the fourth terminal of the second devices is coupled to a second terminal of a coupling circuit, the coupling circuit electrically coupling one or more terminals of the first device and the second device to the one or more acquisition elements. A third example of the device optionally includes one or more of the first and the second examples, and further includes wherein the first terminal and the second terminal of the coupling circuit are input terminals. A fourth example of the device optionally includes one or more of the first through the third examples, and further includes wherein the first terminal and the second terminal of the coupling circuit are output terminals. A fifth example of the device optionally includes one or more of the first through the fourth examples, and further wherein each of the first device and the second device comprise one or more of a GaNFET switch, a diode, a microelectromechanical systems (MEMS) device, and a relay, the MEMS device having a pair of MEMS switches coupled in a back-to-back configuration. A sixth example of the device optionally includes one or more of the first through the fifth examples, and further includes a controller operably coupled to each of the first device and the second device, and configured with instructions in non-transitory memory that when executed cause the controller to: during a receive operation, apply a common actuation voltage to each of the first device and the second device to connect the first terminal and the second terminal of the RF coil to respective input terminals of the coupling circuit. A seventh example of the device optionally includes one or more of the first through the sixth examples, and further includes wherein the controller is configured with instructions in non-transitory memory that when executed cause the controller to: during a transmit operation, cease application of the actuation voltage to each of the first device and the second MEMS device to disconnect the first terminal and the second terminal of the RF coil from respective input terminals of the coupling circuit when the RF coil is unpowered.
The systems and methods described above also provide for a method, the method, comprising during a transmit operation, operating a first) device and a second device in a first mode to disconnect a receive radio frequency (RF) coil from a transmit RF coil, the first device and the second device operably coupling the receive RF coil to the transmit RF coil of an imaging apparatus, and during a receive operation, operating each the of the first device and the second device in a second mode to reconnect the receive RF coil to the transmit RF coil. In a first example of the method, the method may additionally or alternatively include wherein the first device and the second device each include two micro electromechanical systems (MEMS) switches coupled in a back-to-back configuration, and wherein a magnitude of current through each terminal of the first device and the second device is within 10% of each other. A second example of the method optionally includes the first example, and further includes wherein operating the first device and the second device in the first mode includes ceasing application of an actuation voltage to open the two switches of each of the first MEMS device and the second device to isolate the receive RF coil from the transmit RF coil, and wherein a magnitude of current through a first terminal of the RF coil is within 10% of a magnitude of current through a second, different terminal of the RF coil. A third example of the method optionally includes one or more of the first and the second examples, and further includes wherein operating the first device and the second device in the second mode includes applying the actuation voltage to close the two switches of each of the first device and the second device to connect the receive RF coil to the transmit RF coil. A fourth example of the method optionally includes one or more of the first through the third examples, and further includes wherein the first device and the second device are positioned at an input of a coupling circuit. A fifth example of the method optionally includes one or more of the first through the fourth examples, and further includes wherein closing the two switches of each of the first device and the second device includes electrically coupling a terminal of the receive RF coil to an input terminal of the coupling circuit, and wherein opening the two switches of each of the first device and the second device includes electrically decoupling the terminal of the receive RF coil from the input terminal of the coupling circuit. A sixth example of the method optionally includes one or more of the first through the fifth examples, and further includes wherein the coupling circuit includes one or more of a balun, a matching circuit, a feedboard, and a preamplifier. A seventh example of the method optionally includes one or more of the first through the third examples, and further includes wherein first device and the second device include one or more relays, PIN diodes, and GaNFET coupled to different wires of a coil-interfacing cable. An eighth example of the method optionally includes one or more of the first through the third examples, and further includes wherein opening the two switches of each of the first device and the second device includes blocking a common-mode current flowing through the coil-interfacing cable, and wherein closing the two MEMS switches each of the first MEMS device and the second device includes allowing the common-mode current to flow through the coil-interfacing cable.
The systems and methods described above also provide for a system, the system, comprising a receive RF coil configured to receive RF signals and send the RF signals along a receive path, a plurality of MEMS switches placed along the receive path, including a first MEMS switch coupled to a first terminal of the RF coil and a second MEMS switch coupled to a second terminal of the RF coil, and a controller operably coupled to the plurality of MEMS switches and configured with instructions in non-transitory memory that when executed cause the controller to: in response to a receive to transmit transition, adjust the plurality of MEMS switches from an open position to a closed position to electrically disconnect the receive RF coil from the receive path, and in response to a transmit to receive transition, adjust the plurality of MEMS switches from the closed position to the open position to electrically connect the receive RF coil to the receive path. In a first example of the system, the system may additionally or alternatively include wherein the plurality of MEMS switches includes a first pair of MEMS switches coupled to each of a first terminal of the receive RF coil and a first input of a coupling circuit, the first pair of MEMS switches including the first MEMS switch, and a second pair of MEMS switches coupled to each of a second terminal of the receive RF coil and a second input of the coupling circuit, the second pair of MEMS switches including the second MEMS switch, an output of the coupling circuit operably coupled to the transmit RF coil. A second example of the system optionally includes one or more of the first and the second example, and further includes wherein opening the plurality of MEMS switches includes opening each of the first pair of MEMS switches and the second pair of MEMS switches to electrically disconnect both the first terminal of the receive RF coil from the first input of the coupling circuit and second terminal of the receive RF coil from the second input of the coupling circuit.
As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising,” “including,” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property. The terms “including” and “in which” are used as the plain-language equivalents of the respective terms “comprising” and “wherein.” Moreover, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements or a particular positional order on their objects.
This written description uses examples to disclose the invention, including the best mode, and also to enable a person of ordinary skill in the relevant art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those of ordinary skill in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
