De-tuning MR coils involving monitoring field effect transistor switches

Abstract

The invention relates to a radio frequency receiver system (1) comprising: a radio frequency coil circuit (2); a (de) tuned circuit (3) comprising a field effect transistor (4), the (de) tuned circuit being configured to switch the radio frequency coil circuit (2) between an off (de) tuned) state and an on (tuned) state; a control source (5) configured to control conduction of the field effect transistor (4) via a control signal wherein a first pole of the control source (5) is connected to a source of the field effect transistor (4) and a second pole of the control source (5) is connected to a gate of the field effect transistor (4), the second pole of the control source (5) is connected to the drain of the field effect transistor (4) via a bypass (6), and wherein the current and / or voltage in the bypass (6) is configured to be measured via a monitoring circuit (12). In this manner, a radio frequency receiver system (1) with improved characteristics is provided.

Description

Technical Field

[0001] This invention relates to the field of magnetic resonance imaging (MRI). In particular, it relates to the field of radio frequency (RF) antenna systems for MRI equipment. The invention also relates to RF antenna or receiver assemblies comprising an RF coil loop for picking up magnetic resonance signals and a detuning circuit for switching the RF coil assembly between an off (detuning) state and an on (tuning) state. The detuning circuit includes a FET (field-effect transistor) based switch to switch the RF antenna assembly between an on state and an off state. Background Technology

[0002] Every magnetic resonance imaging (MRI) device includes a radio frequency (RF) antenna system for receiving and converting MRI signals. The RF antenna system comprises an RF coil, a matching network, a preamplifier, and an optional analog-to-digital converter. In MRI, the system-integrated coil generates an excitation magnetic field for the spin system, causing prior relaxation. The precession of the net magnetization induces a current in the RF coil via electromagnetic induction. The RF coil typically consists of wires with inductance and coil resistance.

[0003] Similar to any radio frequency operating system, radio frequency antenna systems are susceptible to noise. Therefore, the most important requirement is to achieve the lowest possible noise figure to reduce image artifacts.

[0004] Magnetic resonance (MR) coils used for signal reception typically have two operating states: an ON (conducting) state, representing a low-noise receiving state for capturing very weak (down to noise floor) nuclear signals; and an OFF (off) state, representing a passive state where the coil needs to withstand strong RF pulses designed to excite the nuclear. For the switching, PIN diode-based RF switches are typically used. The detuning circuit requires a bias current through the diode, typically on the order of 80 mA. A second feed power cable is then used to power the detuning circuit. This results in less flexibility in the coil's cable tree and can potentially destabilize the preamplifier for the RF coil. Furthermore, the control logic is expensive because the bias current can total several amperes across the multi-channel coil during one of the operating phases. Since the strong current may not interfere with the static magnetic field B0 or ​​gradient field, strong losses must be anticipated, and the feed line design is challenging.

[0005] More specifically, among other means, the on and off states are achieved by placing one or more detuning switches along the coil conductor. Impedance transformations and transmission lines may be employed so that the actual switches can be placed at the same point while remaining effective at remote locations. Currently, the switches consist of correspondingly biased diodes. The available diodes are far from ideal switches that present non-zero forward bias impedance and non-infinite impedance when reverse biased. For MR, this problem is typically addressed by adding a parallel resonant circuit with switching diodes as a switch connected in series with the coil. For patient and equipment safety, the functionality of this circuit is detected by continuously measuring the bias current through the diodes. When both desired values ​​for the bias current to the diodes ("ON" state (zero) and "OFF" state (e.g., 80mA) are not reached, a scan stop is triggered.

[0006] With the advent of (GaN)FET-based switches, RF current (caused by RF transmit pulses) is effectively blocked in the coil circuit without requiring high (de)tuning current. These newer (de)tuning circuits are fully voltage-controlled (typically 0 / 3V) and operate with almost no power in both operating states of the coil.

[0007] For FETs, there is no direct connection from the voltage control unit connected to the gate to the drain-source trace, which needs to be monitored for safety reasons. A straightforward solution would be to add a second voltage / current driver connected to the drain and source of the FET via additional wires. Monitoring the source current / voltage would enable functional checks on the diode. However, this doubles the number of circuits required for each detuning element and adds several additional cable connections that are themselves prone to failure.

[0008] US Patent Application US2014 / 0070808, which mentions an RF coil assembly, includes an RF coil and an electronic switch between the MR coil matching network and its preamplifier. This electronic switch toggles between an open and closed state to disconnect / connect one or more segments of the MR coil from the preamplifier and receiver circuitry in the event of an RF transmit pulse. Therefore, the electronic switch is used to protect sensitive electronics in the preamplifier and receiver circuitry from the effects of RF transmit pulses. Additionally, each MR coil has at least one field-effect transistor (FET) connected in series with a coil segment, as well as a bias network to interrupt current in the MR coil in the event of an RF transmit pulse. Summary of the Invention

[0009] The purpose of this invention is to provide a radio frequency antenna system with improved characteristics.

[0010] According to the invention, this objective is achieved by the subject matter of the independent claims. Preferred embodiments of the invention are described in the dependent claims.

[0011] The present invention includes a radio frequency (RF) receiver system comprising: an RF coil circuit; a (de)tuning circuit including a field-effect transistor (FET), the (de)tuning circuit being configured to be connected in parallel with the RF coil circuit to switch the RF coil circuit between an off (de)tuning state and an on (tuning) state; and a control source configured to control the on-state of the FET via a control signal, wherein a first terminal of the control source is connected to the source of the FET, and a second terminal of the control source is connected to the gate of the FET, and wherein the second terminal of the control source is connected to the drain of the FET via a bypass, wherein the current and / or voltage in the bypass is configured to be measured via a monitoring circuit.

[0012] The term "(de)tuning" describes the function of a (de)tuning circuit, including (i) tuning the RF coil circuit to its resonant state in the MR band at the Larmor frequency, making the RF coil circuit highly sensitive to weak magnetic resonance signals, where magnetic flux passes through the region of the MR coil and induces a voltage; on the other hand, (ii) detuning the RF coil so that the RF coil no longer resonates in the MR band. Tuning and detuning are typically achieved by adjusting the effective impedance of the RF coil circuit to shift the resonant frequency out of or into the MR band. Therefore, the term "(de)tuning" is an abbreviation for the following two functions: (i) tuning the RF coil from its non-resonant state to its resonant state in the MR band; and vice versa (ii) tuning the RF coil from its resonant state in the MR frequency to its non-resonant state.

[0013] This invention enables desired functional monitoring without increasing the number of current and voltage sources and individual wire connections. The principle is based on a bypass trace between the gate and drain terminals, allowing the FET's control signals to also pass through the drain-source trace without affecting the FET's switching function. This method provides efficient (de)tuning monitoring of MRI coil circuits. The bypass circuitry varies depending on the type of FET. This invention proposes solutions considering the bypass current of both depletion-type and enhancement-type FETs in the detuning circuit.

[0014] In the case of enhancement-mode FETs, a resistor can be added as a bypass between the gate and drain. A positive control voltage on the gate-source trace enables the FET to conduct, thereby effectively turning off the coil via a parallel resonant circuit. Simultaneously, the bypass allows a specific DC current to flow through the drain-source trace. This current can then be used to verify proper switching operation. The resistor is preferably used symbolically to adapt the control voltage to the gate and monitor current requirements. Setting a negative control voltage on the gate-source trace enables the FET to turn off. Therefore, no current should flow through the bypass resistor and the drain-source trace. Detecting a substantial current can indicate a fault; for example, a damaged FET exhibits a persistent short circuit between the drain and source terminals.

[0015] In the case of a depletion-mode FET, a diode can be inserted as a bypass. The control source can be implemented by generating a bipolar voltage. First, a negative voltage on the gate-source trace turns the FET off; second, a positive voltage (typically >0.7V) on the gate-source trace turns the FET on. The diode is placed to prevent current from flowing in the off state (where the diode is reverse-biased) and to allow current to flow in the on state. A diode is preferably used symbolically. Other components, such as resistors, can be added to adapt the control voltage to the gate and monitoring current requirements. This allows the monitoring current to be measured to verify proper FET operation. In the off state, the diode prevents the negative voltage at the drain from degrading the FET's switching performance. It is worth noting that in this circuit, it is impossible to directly sense the high impedance of the FET when it is off.

[0016] Compared to diode-based circuits, the bypass monitoring current can be several orders of magnitude lower than the monitoring current (80mA / 100) for switching diodes. This ensures that the aforementioned drawbacks of detuning circuits that use diodes as switching elements are not encountered.

[0017] Preferably, in the RF receiver system, the ratio of the following two is configured to regulate the gate voltage of the field-effect transistor and / or the current in the bypass: a first ohmic resistance in the bypass, and a second ohmic resistance in the connection between the second terminal of the control source and the gate of the field-effect transistor. The ratio of these ohmic resistances can be calculated according to Ohm's law and allows for cost-effective tuning of the circuit.

[0018] In another embodiment of the invention, for the radio frequency receiver system, when the field-effect transistor is a depletion-type field-effect transistor, the bypass includes a diode forward-biased towards the drain of the depletion-type field-effect transistor. In this way, the principles of the invention can be implemented in an advantageous and cost-effective manner.

[0019] Preferably, the aforementioned diode in the RF receiver system is configured to prevent current from flowing from the drain of the depletion-type field-effect transistor (FET) to the source of the FET when a predefined diode threshold voltage is exceeded. For voltages below the FET threshold voltage, no current should flow for a normally operating FET. When the threshold voltage required to turn on the FET is exceeded, the current should be measurable. For example, a negative voltage is applied to turn the FET off, and a positive voltage (typically 0.7V) turns the FET on. Therefore, knowing the specific threshold voltage of the FET prevents current direction or signal that could cause noise or unwanted background for the measurement.

[0020] In another embodiment of the invention, the radio frequency receiver system includes a diode configured to prevent a negative voltage at the drain of the depletion-mode field-effect transistor from degrading the switching performance of the transistor. In this way, noise in the measurement system can be reduced and measurement results improved.

[0021] Preferably, in the RF receiver system, the connection between the first terminal of the control source and the source terminal of the field-effect transistor includes a low-frequency path, and / or the bypass includes a low-frequency path. In another embodiment, the low-frequency path is configured to block RF current. This allows the control current to be separated from the RF current. An inductor (low-pass) can prevent RF signals from reaching the DC path. Alternatively, to protect the RF portion from DC, this can be done by a capacitor (high-pass) in the corresponding circuit.

[0022] In another embodiment of the invention, the radio frequency receiver system is configured to monitor the current in the bypass via a single wire combined with a selection / detuning signal (particularly the selection / detuning signal of an MRI sequencer available in a typical MRI system). Preferably, the aforementioned monitoring circuitry in the radio frequency receiver system is configured to detect FET failure and / or drift of the measured current and / or drift of the measured voltage. FET failure or even drift of the measured voltage is typically sensitive to FET integrity, temperature, or aging. The active monitor is capable of comparing different measured voltages across all detuning circuits and calculating a baseline. Deviations from the baseline or drift can then be measured and stored; and by activating a certain threshold, the monitoring circuitry (potentially including additional electronics and / or logic devices) allows reporting of problems.

[0023] The proposed solution also allows monitoring of the FET detuning circuit without additional active devices and only slightly affects the efficiency of the RF detuning switch (typically less than 5%). Single-wire detuning control and monitoring further enable the implementation of thin RF coils without fuses and reduce the number of components required for EMC in coils with a body coil.

[0024] Preferably, the RF receiver system is configured to compare measured voltages and / or measured currents of multiple (de)tuned circuits and / or to calculate an average value. The monitoring circuit can be implemented entirely analogously, or it can be implemented as an intelligent digital controller that uses a local artificial intelligence (AI) machine learning network for monitoring and sensing. The monitoring circuit can be controlled and powered via an optical or wireless link. The monitoring circuit can also be located in a coil, cable connector, or patient's examination table. AI-based algorithms can, for example, monitor trigger levels that may vary due to aging effects of system components and / or perform further intelligent analysis, particularly for accurate prediction of the health and lifespan of specific system components such as FETs and / or diodes and / or inductors and / or capacitors.

[0025] In another embodiment of the radio frequency receiver system, the (de)tuning circuitry is configured to be controlled by an external (de)tuning command. This allows for greater flexibility and possible centralized command even for more than one MRI system.

[0026] The present invention also includes a magnetic resonance imaging (MRI) system, the MRI system comprising a processor, a memory for storing machine-executable instructions and pulse sequence commands executed by the processor, and the aforementioned radio frequency receiver system.

[0027] The present invention also includes a method for controlling a radio frequency receiver system, the method comprising the steps of: detecting a radio frequency signal using a radio frequency coil circuit; switching the radio frequency coil circuit between an off (detuning) state and an on (tuning) state using a detuning circuit; controlling the conduction of a field-effect transistor via a control signal from a control source connected to the source and gate of the field-effect transistor; measuring current and / or voltage in bypasses of the source and drain of the field-effect transistor via a monitoring circuit; and comparing the measured voltages and / or measured currents of a plurality of (de)tuning circuits, and / or calculating an average value. The method may further include the step of receiving a magnetic resonance signal from the radio frequency coil circuit having a preamplifier, the preamplifier sending the amplified signal to a reconstructor system.

[0028] The method for controlling the radio frequency receiver system may further include the following steps: detecting faults in the field-effect transistor and / or drift of the measured current and / or drift of the measured voltage via the monitoring circuit. Attached Figure Description

[0029] These and other aspects of the invention will become apparent from the embodiments described below, and these and other aspects are illustrated with reference to the embodiments described below. However, such embodiments do not necessarily represent the full scope of the invention, and therefore the scope of the invention is explained by referring to the claims and this document.

[0030] In the attached diagram: Figure 1 A radio frequency antenna system with a diode in the (de)tuning circuit is schematically depicted according to the prior art.

[0031] Figure 2 A radio frequency antenna system with a FET in the (de)tuning circuit 5 is schematically depicted as an illustration of the background of the present invention.

[0032] Figure 3 schematically depicted Figure 2 The radio frequency antenna system with monitoring circuit 122 is provided as a further explanation of the background of the present invention.

[0033] Figure 4 A radio frequency antenna system with bypass 6 according to a preferred embodiment of the present invention is schematically depicted.

[0034] Figure 5 A radio frequency antenna system having a bypass 6 for a depletion-type FET is schematically depicted according to a preferred embodiment of the invention.

[0035] List of reference numerals in the attached diagram: 1 Radio Frequency Receiver System 2. Radio Frequency Coil Circuit 3. (Remove) Tuning circuit 4 Field-Effect Transistors / FETs 5. Control Source 6. Bypass 7 First Ohm Resistance 8 Second Ohm Resistance 9. Depletion-type field-effect transistor 10 Diodes 11 Low-frequency path 12 Monitoring Circuit Detailed Implementation

[0036] Figure 1A radio frequency antenna system 1 with a diode 10 in a (de)tuning circuit 3 according to the prior art is schematically depicted. The coil loop 2 is typically a resonator formed by a wire loop (inductance) and at least one capacitor. When the MR coil is in the on state, the diode 10 is reverse biased, thereby disabling the inductance of the parallel resonant circuit. The RF current (induced by the signal of the core) is able to circulate freely in the coil loop. Finally, one or more capacitors forming the resonant circuit of the MR coil are used for dual purposes for this detuning circuit.

[0037] The forward-biased diode 10 causes the inductor and resonant capacitor to form a parallel resonant circuit. This provides a much higher blocking impedance than the reverse-biased diode 10 itself. Radio frequency (RF) currents, typically induced by RF transmit pulses, are effectively blocked in the coil circuit, preventing device damage or situations that could endanger patient safety. When the switch is in the off state, the MR coil is "on," and vice versa.

[0038] The operation of the circuit is detected by continuously measuring the bias current through diode 10. A scan stop is triggered when the two desired values ​​for the "ON" state (zero) and the "OFF" state (e.g., 80mA) are not reached. A problem with diode 10 is that it typically requires a high turning current, which increases unwanted coil heating and the cost of generating the current. In coils with many channels, the control current can total up to several amperes.

[0039] One solution is to use a FET instead of diode 10 as the switching element in the (de)tuning circuit 5, such as Figure 2 As shown, this serves as an explanation of the background of the invention. Devices with one or more FETs that meet detuning requirements have recently become available. By utilizing FETs, the gate voltage is used to control the state of the MR coil (in which the current is essentially zero in both states), thereby reducing dissipation and power requirements. Figure 2 The figure also depicts an MR coil with capacitors (representing resonant capacitors) on the right and left sides, where the left-side capacitor and the inductor above the FET also contribute to the parallel resonant circuit 3. Reference numeral 4 again indicates the FET that controls the state together with the source 5.

[0040] Unlike diodes, control current cannot be used for both control and monitoring simultaneously. Control voltage affects the gate-source traces, while monitoring the normal operation of the drain-source traces is also required.

[0041] Figure 3 schematically depicted Figure 2As a further explanation of the background of the present invention, the radio frequency antenna system 1 has a dedicated monitoring circuit 12 for monitoring the drain-source traces of the FET. However, this requires additional effort to manufacture the monitoring circuit 12 and the corresponding wiring.

[0042] Figure 4 A radio frequency antenna system 1 with a bypass 6 according to a preferred embodiment of the present invention is schematically depicted. Figure 4 The circuitry used to enhance the FET is also shown. Ohmic resistor 8 creates a current path from control source 5 to drain. For voltages below the FET threshold voltage, no current should flow for FET 4 during normal operation. For voltages exceeding the threshold voltage required to turn on the FET, the current should be measurable. The ratio of ohmic resistors 7 and 8 can be set to adjust the gate voltage and sense current requirements. The two components with reference numeral 11 represent high-frequency (HF) blockers or low-pass filters to reduce RF current in these circuits. The monitoring circuitry is connected to or integrated into the (de)tuning circuitry.

[0043] Generally, two types of FETs need to be considered: one type is the depletion-mode FET, where the drain-source trace is essentially short-circuited when the gate voltage is zero or higher, and the FET is turned off ("off") when the gate voltage is set below the (negative) threshold voltage. Referring to switching terminology used elsewhere, the switch is normally closed (NC); the second type is the enhancement-mode FET, where the drain-source trace is essentially "off" or turned off when the gate voltage is zero, and the FET is turned on ("short-circuited") when the gate voltage is set above the (positive) threshold voltage.

[0044] Figure 5 A radio frequency antenna system 1 with a bypass 6 for a depletion-type FET 9 is schematically depicted according to a preferred embodiment of the invention. For the depletion-type FET, the circuitry has a different layout due to the change in the polarity of the threshold voltage. From a sensing perspective, the circuitry for the enhancement-type FET will also operate, but it will degrade switching performance (by also applying a negative voltage to the drain). Figure 5 In this configuration, diode 10 replaces ohmic resistor 8. Diode 10 prevents current from flowing through the drain-source trace at voltages above 0.7V (diode voltage), thus indicating that FET 4 is operating normally in this state. Diode 10 also prevents negative voltage at the drain from degrading the switching performance of the FET.

[0045] Although the invention has been detailed and described in the accompanying drawings and the foregoing description, such description is to be considered illustrative or exemplary rather than restrictive; the invention is not limited to the disclosed embodiments. Other variations of the disclosed embodiments will be understood and implemented by those skilled in the art in practicing the claimed invention upon studying the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the words "a" or "an" do not exclude multiple. The mere fact that certain measures are recited in dissimilar dependent claims does not indicate that combinations of these measures cannot be advantageously used. No reference numerals in the claims should be construed as limiting the scope. Furthermore, for clarity, not all elements in the drawings are provided with reference numerals.

Claims

1. A radio frequency receiver system (1), comprising: Radio frequency coil circuit (2). (De)tuning circuit (3), which includes field-effect transistor (4), is configured to be connected in parallel with radio frequency coil circuit (2) to switch radio frequency coil circuit (2) between off (de)tuning state and on (tuning) state. A control source (5) is configured to control the conduction of the field-effect transistor (4) via a control signal. The first terminal of the control source (5) is connected to the source terminal of the field-effect transistor (4), and the second terminal of the control source (5) is connected to the gate terminal of the field-effect transistor (4). The second electrode of the control source (5) is connected to the drain of the field-effect transistor (4) via a bypass (6), and The radio frequency receiver system (1) includes a monitoring circuit (12) configured to measure the current and / or voltage in the bypass (6).

2. The radio frequency receiver system according to claim 1, wherein, The ratio of the following two is configured to regulate the gate voltage of the field-effect transistor (4) and / or the current in the bypass (6): a first ohmic resistor (7) in the bypass (6) and a second ohmic resistor (8) in the connection between the second terminal of the control source (5) and the gate of the field-effect transistor (4).

3. The radio frequency receiver system according to claim 2, wherein, In the case that the field-effect transistor (4) is a depletion-type field-effect transistor (9), the bypass (6) includes a diode (10) that is forward-pointing to the drain of the depletion-type field-effect transistor (9).

4. The radio frequency receiver system according to claim 3, wherein, The diode (10) is configured to allow current to flow through the drain of the depletion field-effect transistor (9) to the source of the depletion field-effect transistor when the voltage exceeds a predefined diode threshold voltage.

5. The radio frequency receiver system according to claim 3, wherein, The diode (10) is configured to prevent the negative voltage at the drain from degrading the switching performance of the depletion-type field-effect transistor (9).

6. The radio frequency receiver system according to any one of the preceding claims, wherein, The connection between the first electrode of the control source (5) and the source electrode of the field-effect transistor (4) includes a low-frequency path (11), and / or the bypass includes a low-frequency path (11).

7. The radio frequency receiver system according to the preceding claim, wherein, The low-frequency path (11) is configured to block radio frequency current.

8. The radio frequency receiver system according to the preceding claim, wherein, The radio frequency receiver system (1) is configured to monitor the current in the bypass (6) via a single-wire combination of a selection / detuning signal.

9. The radio frequency receiver system according to the preceding claim, wherein, The radio frequency receiver system (1) is configured to detect faults in the FET and / or drifts in the measured current and / or drifts in the measured voltage via the monitoring circuit (12).

10. The radio frequency receiver system according to the preceding claim, wherein, The radio frequency receiver system (1) is configured to compare the measured voltages and / or measured currents of a plurality of (de)tuning circuits (3) and / or to calculate an average value.

11. The radio frequency receiver system according to any one of the preceding claims, wherein, The (de)tuning circuit (3) is configured to be controlled by an external (de)tuning command.

12. A magnetic resonance imaging system, comprising: processor, The memory is used to store machine-executable instructions and pulse sequence commands executed by the processor. The radio frequency receiver system (1) according to claim 1.

13. A method for controlling a radio frequency receiver system (1), comprising the following steps: Radio frequency signals are detected using a radio frequency coil circuit (2). The radio frequency coil circuit (2) is switched between an off (de-tuning) state and an on (tuning) state by a detuning circuit (5) configured to be connected in parallel with the radio frequency coil circuit (2) and including a field-effect transistor (4). The conduction of the field-effect transistor (4) is controlled by a control signal from a control source (5), which is connected to the source and gate of the field-effect transistor (4). The current and / or voltage in the bypass (6) from the source and drain of the field-effect transistor (4) are measured via monitoring circuit (12), and Compare the measured voltages and / or measured currents of multiple (de)tuning circuits (3), and / or calculate the average value.

14. The method for controlling a radio frequency receiver system according to claim 13, further comprising the following steps: The monitoring circuit (12) is used to detect faults in the field-effect transistor (4) and / or drifts in the measured current and / or voltage.

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