Radio frequency receive array coil for a magnetic resonance imaging system, method of using said coil, and MRI system including said coil

Abstract

The device receives magnetic resonance signals 250 from a region of interest of the patient 20 during an RF receive cycle, which are generated in response to an RF transmit signal 350 of the MRI system 100 during the RF receive cycle. The device includes an RF receive coil 301, a detector 330, and a first coupling device that couples a signal 350 proportional to the current through the RF receive coil 301 during the RF transmit cycle to an input of the detector 330. The detector 330 outputs a signal 350 indicative of the magnitude and / or phase of the current through the RF receive coil 301 during the RF transmit cycle. The digital signal 350 is used to stop the MR scan and / or notify an operator of the system 100 before harm to the patient 20 occurs due to excessive current flowing through the RF receive coil 301 during the RF transmit cycle.

Description

[Technical field]

[0001] The present system relates generally to a magnetic resonance imaging (MRI) system having a radio frequency (RF) receive array coil that includes one or more RF coil elements or loops, and a method of operation thereof. [Background technology]

[0002]

[0001] Many MRI systems use one or more RF receiving antenna devices to detect magnetic resonance (MR) signals emanating from one or more regions of interest in a subject or patient under MRI examination to reconstruct an image of the regions of interest. These RF receiving antenna devices typically include one or more RF receiving coils or loops.

[0003] These RF receiving coils are generally 1 The MR imaging system is used in an environment of high amplitude RF magnetic fields, called magnetic fields. This field naturally induces currents in the RF receiving coils used to receive the MR signals. If these currents are not controlled, the RF receiving coils will 1 The homogeneity of the magnetic field can be distorted, and more importantly, the high local magnetic fields due to these induced currents can cause harm to the patient. Summary of the Invention [Problem to be solved by the invention]

[0004]

[0003] It would therefore be desirable to provide an RF receive antenna device for a magnetic resonance imaging (MRI) system that includes means for preventing high intensity local magnetic fields due to induced currents in one or more RF receive array coils from harming a patient. In particular, it would be desirable to provide means and methods for directly measuring the loop currents in the RF receive array coils of the RF receive antenna device and providing a signal that can notify a controller and / or operator of the MRI system when the induced currents exceed safe limits. [Means for solving the problem]

[0005]

[0004] As disclosed herein, an MRI system includes a magnet generating a magnetic field, the magnetic field generating a net nuclear magnetization in a patient placed within the magnetic field, a gradient coil at least partially surrounding at least a portion of the patient to be imaged, a radio frequency (RF) transmit coil unit applying a transmit RF magnetic field to at least a portion of the patient during an RF transmit cycle, perturbing the orientation of magnetization relative to the magnetic field in the portion of the patient, and at least one RF receive antenna device positioned adjacent to a region of interest of the patient and receiving magnetic resonance signals from the region of interest of the patient generated in response to the transmit RF magnetic field during an RF receive cycle. The at least one RF receive antenna device includes at least one RF receive coil array, a detector, and a first coupling device coupling to an input of the detector a signal proportional to the current flowing through the at least one RF receive coil during at least the RF transmit cycle. The MRI system includes a main controller, the detector having a signal as an output coupled to the main controller for providing to the main controller. The signal is indicative of the magnitude and / or phase of the current flowing through the at least one RF receive coil during at least the RF transmit cycle.

[0006]

[0005] Also as disclosed herein, the method includes generating an RF magnetic field; applying a transmit RF magnetic field to at least a portion of a patient being imaged during a radio frequency (RF) transmit cycle to perturb the orientation of magnetization relative to the magnetic field; detecting magnetic resonance signals emanating from a region of interest of the patient during the RF receive cycle in response to the transmit RF magnetic field using an RF receive antenna device having at least one RF receive coil; coupling to an input of the detector a first signal proportional to a current flowing through the at least one RF receive coil during at least the RF transmit cycle; outputting from the detector to a processor a digital signal indicative of a magnitude and / or phase of the current flowing through the at least one RF receive coil during at least the RF transmit cycle; and interrupting the transmit RF magnetic field when the digital signal indicates that the magnitude and / or phase of the current flowing through the at least one RF receive coil during the transmit RF receive cycle is greater than a threshold value.

[0007] As further disclosed herein, a device receives magnetic resonance signals from a region of interest of a patient during a radio frequency (RF) receive cycle. The magnetic resonance signals are generated in response to a transmit RF magnetic field during an RF transmit cycle of a magnetic resonance imaging (MRI) system. The device includes at least one RF receive coil element and a first coupling device for coupling a signal proportional to a current flowing through the at least one RF receive coil element during at least the RF transmit cycle to an input of a detector. The detector has an output for providing a signal (350, 650b) indicative of a magnitude and / or a phase of the current flowing through the at least one RF receive coil element during at least the RF transmit cycle. [Brief description of the drawings]

[0008]

[0007] The present invention will be more readily understood from the following detailed description of illustrative embodiments when considered in conjunction with the accompanying drawings.

[0009] [Figure 1]FIG. 1 illustrates an exemplary embodiment of a magnetic resonance imaging (MRI) system. [Diagram 2] FIG. 2 is a block diagram of an exemplary embodiment of an MRI system that uses at least one RF receiving antenna device, as described in more detail below. [Diagram 3]

[0010] FIG. 3 shows a first embodiment of an RF receiving antenna device. [Figure 4]

[0011] FIG. 4 shows a second embodiment of the RF receiving antenna device. [Diagram 5]

[0012] FIG. 5 shows a third embodiment of the RF receiving antenna device. [Figure 6]

[0013] FIG. 6 shows a fourth embodiment of the RF receiving antenna device. [Figure 7]

[0014] FIG. 7 shows a fifth embodiment of the RF receiving antenna device. [Figure 8]

[0015] FIG. 8 illustrates an exemplary embodiment of a method of operation of an MRI system that includes one or more RF receive antenna devices that directly sample, measure, and digitize the current flowing through an RF receive array coil during an RF transmit cycle. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010]

[0016] In the following detailed description, for purposes of explanation and not limitation, representative embodiments disclosing specific details are described to provide a thorough understanding of the embodiments according to the present teachings. Descriptions of known systems, devices, materials, methods of operation, and methods of manufacture may be omitted so as not to obscure the description of the representative embodiments. However, systems, devices, materials, and methods within the purview of one of ordinary skill in the art are within the scope of the present teachings and may be used in accordance with the representative embodiments. It is to be understood that the terms used herein are for the purpose of describing particular embodiments only and are not intended to be limiting. Defined terms are to be given the technical and scientific meaning of the defined terms as commonly understood and accepted in the art of the present teachings.

[0011]

[0017] In this specification, terms such as "first," "second," and "third" are used to describe various elements or components, but it should be understood that these elements or components are not limited by these terms. These terms are used only to distinguish one element or component from another. Thus, a first element or component discussed below may be referred to as a second element or component without departing from the teachings of the inventive concept.

[0012]

[0018] The terms used herein are for the purpose of describing particular embodiments only and are not intended to be limiting. As used in this specification and the appended claims, singular elements are intended to include both the singular and the plural, unless the context clearly indicates otherwise. Additionally, as used herein, the term "comprises" and / or similar terms specify the presence of the stated features, elements, and / or components, but do not exclude the presence or addition of one or more other features, elements, components, and / or groups thereof. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0013]

[0019] Unless otherwise noted, when an element or component is said to be "connected" or "coupled" to another element or component, it will be understood that the element or component may be directly connected or coupled to the other element or component, or there may be intervening elements or components. That is, these and similar terms encompass the cases where one or more intermediate elements or components may be used to connect the two elements or components. However, when an element or component is said to be "directly connected" to another element or component, this only encompasses the cases where the two elements or components are connected to each other with no intermediate or intervening elements or components.

[0014]

[0020] Unless otherwise noted, as used herein, the term "coil" includes one or more RF coil elements or loops, and may be referred to as an RF array coil.

[0015]

[0021] 1 shows an exemplary embodiment of a magnetic resonance imaging (MRI) system 100. The MRI system 100 includes a magnet system 101, a patient table 104 for holding a subject or patient 20, magnetic field gradient coils 103 that at least partially surround at least a portion of the patient 20 for which the MRI system 100 generates an image, and a radio frequency coil 106 that applies a transmit RF magnetic field to at least a portion of the subject or patient 20 being imaged and perturbs the orientation of magnetization relative to the magnetic field, as well as one or more sensors 100 that detect changes in magnetization induced by the transmit RF magnetic field and the patient 20.

[0016]

[0022] FIG. 2 is a block diagram of an exemplary embodiment of an MRI system 200 that uses at least one RF receiving antenna device, as described in more detail below.

[0017]

[0023] The MRI system 200 includes a static magnetic field magnet 201, a gradient coil 202, a gradient power supply 203, a patient table or bed 204, a patient table (or bed) controller 205, an RF transmit coil unit 206a, one or more RF receive antenna devices 206b (each including one or more RF receive coils 206c), a transmitter 207, a clock generator 208, an RF / gradient controller 209, a driver 210, a reconstruction front end 215, a reconstruction system 220, a storage device 221, a display 222, an input unit 223, a main controller 224, and a data generator 225.

[0018]

[0024] The main unit is divided into a gantry and a processing system. In this case, for example, a static magnetic field magnet 201, a gradient magnetic field coil 202, a gradient magnetic field power supply 203, a patient table 204, a patient table controller 205, an RF transmit coil unit 206a, a transmitter 207, and an RF / gradient magnetic field controller 209 can be provided in the gantry, and a clock generator 208, a driver 210, a reconstruction front end 215, a reconstruction system 220, a storage device 221, a display 222, an input unit 223, and a main controller 224 can be provided in the processing system.

[0019]

[0025] The static magnetic field magnet 201 has a hollow cylindrical shape and generates a uniform static magnetic field in its internal space. For example, a permanent magnet or a superconducting magnet can be used as the static magnetic field magnet 201.

[0020]

[0026] The gradient coil 202 also has a hollow cylindrical shape and is disposed in the static magnetic field magnet 201. The gradient coil 202 includes a combination of three types of coils corresponding to the X-axis, Y-axis, and Z-axis that are orthogonal to each other. The gradient coil 202 generates gradient magnetic fields having intensities that are gradient along the X-axis, Y-axis, and Z-axis by individually supplying currents from a gradient magnetic field power supply 203 to the three types of coils. Furthermore, the Z-axis is, for example, in the same direction as the direction of the static magnetic field. The gradient magnetic fields of the X-axis, Y-axis, and Z-axis correspond, for example, to a slice selection gradient magnetic field Gs, a phase encoding gradient magnetic field Ge, and a readout gradient magnetic field Gr, respectively. The slice selection gradient magnetic field Gs is used to determine a specific imaging section. The phase encoding gradient magnetic field Ge is used to change the phase of a magnetic resonance signal according to a spatial position. The readout gradient magnetic field Gr is used to change the frequency of a magnetic resonance signal according to a spatial position.

[0021]

[0027] The RF transmit coil unit 206a includes one or more coils encased in a cylindrical case. The RF transmit coil unit 206a can be disposed inside the gradient coil 202. The RF transmit coil unit 206a is supplied with radio frequency pulses (RF pulses) corresponding to a Larmor frequency by a transmitter 207 to generate a radio frequency magnetic field. The RF transmit coil unit 206a applies a transmit RF magnetic field to at least a portion of the patient 20 including one or more regions of interest during an RF transmit cycle, and in conjunction with the RF receive antenna device 206b, perturbs the orientation of the magnetic field in the portion of the patient 20 to generate MR image data of the regions of interest.

[0022]

[0028] In particular, "perturbing" as used herein includes the use of transmit pulses to excite / generate a signal for collection by a receive coil, to refocus the magnetization so that the signal can be utilized at a later time, or to saturate or suppress the signal to nothing. Often such perturbations are performed over a particular frequency band.

[0023]

[0029] The patient 20 is placed on a top plate 204a of a patient table 204 and is placed in the internal space (imaging space) of the gradient magnetic field coil 202. The patient table 204 is controlled by a patient table controller 205 to move the top plate 204a in its longitudinal direction (left and right direction in FIG. 2) and vertical direction. Usually, the patient table 204 is installed so that the longitudinal direction is parallel to the central axis of the static magnetic field magnet 201.

[0024]

[0030] The RF receiving antenna device 206b can be mounted on or integrated into the tabletop 204a, placed on or attached to the patient 20. During imaging, the RF receiving antenna device 206b is placed in the imaging space with the patient 20 to receive or detect magnetic resonance signals as electromagnetic waves emanating from the patient 20 in response to the transmitting RF magnetic field from the RF transmitting coil unit 206a, and in response, generate digital data representative of the detected magnetic resonance signals. The RF receiving antenna device 206b may include or be attached to one, two, or more receiving RF receiving coils 206c, and may include any type of other element that functions as a sensor to detect the magnetic resonance signals emanating from the patient 20. Although only one RF receiving antenna device 206b is shown in FIG. 2, four, eight, sixteen, thirty-two, or any other desired number of RF receiving antenna devices 206b may be used in different embodiments of the MRI system 200. Each RF receive antenna device 206b is positioned adjacent to a region of interest of the patient 20 and receives magnetic resonance signals from the region of interest of the patient 20 that are generated in response to the transmitted RF magnetic field from the RF transmit coil unit 206a during an RF receive cycle.

[0025]

[0031] The clock generator 208 (also referred to herein as the main clock or the first clock) generates a first clock signal having a predetermined frequency. The clock generator 208 can be used as a system clock that serves as a timing reference for the entire operation of the MRI system 200.

[0026]

[0032] The RF / gradient magnetic field controller 209 changes the gradient magnetic field in accordance with a required pulse sequence under the control of the main controller 224, and also controls the gradient magnetic field power supply 203 and the transmitter 207 so as to transmit RF pulses. Furthermore, the RF / gradient magnetic field controller 209 is provided with a first clock signal whose level has been appropriately adjusted by a driver 210. The RF / gradient magnetic field controller 209 executes the pulse sequence in synchronization with this first clock signal.

[0027]

[0033] The reconstruction front end 215 receives the magnetic resonance signal 250 provided from the RF receiving antenna device 206b.

[0028]

[0034] The reconstruction system 220 reconstructs an image of the patient 20 based on at least one of the magnetic resonance signals processed by the reconstruction front end 215 .

[0029]

[0035] The storage device 221 stores various types of data, such as image data representing images reconstructed by the reconstruction system 220 .

[0030]

[0036] The display 222 displays various information such as images reconstructed by the reconstruction system 220 and various operation screens for a user to operate the MRI system 200 under the control of the main controller 224. Any convenient display device such as a liquid crystal display can be used as the display 22.

[0031]

[0037] The input unit 223 accepts various commands and information input from an operator of the MRI system 200. The input unit 223 includes an input device such as a pointing device, such as a mouse or a trackball, a selection device, such as a mode change switch, and / or a keyboard.

[0032]

[0038] The main controller 224 (also called an MRI system controller) has a CPU, a memory, etc. (not shown), and controls the entire MRI system 200 .

[0033]

[0039] The RF / gradient controller 209 and main controller 224 are tangible and non-transient. As used herein, the term "non-transient" is to be interpreted as a property of a state that persists for a period of time, rather than an eternal property of a state. The term "non-transient" specifically negates momentary properties, such as carrier waves or signals or other types of properties that exist only temporarily at any time and in any place. The RF / gradient controller 209 and main controller 224 of the present teachings are products and / or machine parts.

[0034]

[0040] The RF / gradient controller 209 and the main controller 224 execute software instructions stored in the storage device 221 to perform the functions described in the various embodiments herein. Each of the RF / gradient controller 209 and the main controller 224 may be a general-purpose processor or part of an application specific integrated circuit (ASIC). The RF / gradient controller 209 and the main controller 224 may also be (or include) a processor, microprocessor, microcomputer, processor chip, controller, microcontroller, digital signal processor (DSP), state machine, or programmable logic device. The RF / gradient controller 209 and the main controller 224 may also be (or include) a logic circuit including a programmable gate array (PGA) such as an FPGA, or another type of circuit including discrete gate and / or transistor logic. The main controller 224 may be (or include) a central processing unit (CPU), a graphics processing unit (GPU), or both. Additionally, each of the RF / gradient controller 209 and the main controller 224 may include multiple processors, parallel processors, or both. Multiple processors may be included or coupled in a single device or multiple devices.

[0035]

[0041] The storage device 221 may include a main memory, a static memory, or both, and the memories may communicate with each other via a bus (not shown). The storage device 221 described herein is a tangible storage medium capable of storing data and executable instructions, and is non-transient while the instructions are stored therein. As used herein, the term "non-transient" is to be interpreted as a property of a state that persists for a period of time, rather than a permanent property of a state. The term "non-transient" specifically negates the properties of a momentary nature, such as a carrier wave or signal or other type of property that exists only temporarily at any time and place.

[0036]

[0042] The storage device 221 of the present teachings is a product and / or a machine part. The storage device 221 includes one or more computer-readable media from which a computer can read data and executable instructions (e.g., to execute the process described in connection with FIG. 4). The memory described herein may be a random access memory (RAM), a read-only memory (ROM), a flash memory, an electrically programmable read-only memory (EPROM), an electrically erasable programmable read-only memory (EEPROM), a register, a hard disk, a removable disk, a tape, a compact disk read-only memory (CD-ROM), a digital versatile disk (DVD), a floppy disk, a Blu-ray disk, or any other form of storage medium known to those skilled in the art. The memory of the present teachings may be a volatile or non-volatile memory, a secure and / or encrypted memory, a non-secure and / or unencrypted memory. The main controller 224, storage device 221, and display 222 may be housed in or linked to a workstation (not shown), such as a computer, or another assembly of one or more computing devices, a display / monitor, and one or more input devices (e.g., keyboard, joystick, mouse), in the form of a standalone computing system, desktop or tablet, etc.

[0037]

[0043] The storage device 221 stores instructions that are executed by the RF / gradient controller, the main controller 224, or both to perform aspects of the methods described herein. For example, the main controller 224, the RF / gradient controller 209, or both can provide instructions to perform an exemplary method 800 of operating an MRI system, which is described in more detail below.

[0038]

[0044] The general operation of an MRI system is well known and therefore a further description thereof will not be repeated here.

[0039]

[0045] As previously mentioned, the RF receiving coil 206c is generally 1 The receiver operates in an environment of high amplitude transmit RF magnetic fields, referred to as B magnetic fields. These fields naturally induce currents in the RF receive coil 206c. If these currents are not controlled, they can 1 These induced currents can distort the uniformity of the magnetic field. Furthermore, the high strength local magnetic fields generated from these induced currents can be harmful to the patient 20.

[0040]

[0046] One approach to prevent these currents and maintain a safe state is to use a switchable high impedance resonant circuit, or "decoupler" or "trap," placed in series with the loop path. For example, an LC tank circuit consists of two parallel circuit legs, with a capacitor (or inductor) forming one parallel path and a series combination of a switching device, such as a PIN diode, and an inductor (or capacitor) forming the other parallel path. A PIN diode is turned "on" by a direct current (DC) current and turned "off" by the removal of the DC. Additionally, a PIN diode can be driven into a conducting state without DC current if the applied RF signal is large enough.

[0041]

[0047] The decouplers are simple and robust circuits, but given the criticality of their function, the failure of even one of them could render the MR coil unsafe.

[0042]

[0048] Several options are available to provide a "fail-safe" in the event of a failure of the decoupler. One option for such a "fail-safe" mechanism is a monitoring circuit that measures the DC switching current flowing through the decoupler. If this current is interrupted, the MRI system controller is prompted to abort the scan and / or notify the operator before harm occurs to the patient. Unfortunately, this monitoring method is indirect and the monitoring circuitry may detect faults that do not result in an unsafe condition. Alternatively, and more importantly, faults may occur in the decoupler that cannot be directly detected by the monitoring circuitry. For this reason, a redundant fail-safe may be created using an auxiliary safety mechanism.

[0043]

[0049] However, the inventors have realized that a better method would be to measure the loop current directly and notify the MRI system controller and / or operator when the induced current exceeds a safe limit. Heretofore, this has been difficult due to the architecture of MR receive antenna devices, which are only designed to measure the current (or voltage) during the receive portion of the RF cycle, when the signal strength is orders of magnitude lower than the transmit portion of the RF cycle. During the RF transmit cycle, the preamplifiers and measurement circuitry are often placed into a protection mode or turned off completely to prevent damage.

[0044]

[0050] However, as disclosed in more detail below, a system capable of sampling signals emanating from the entire MR cycle can directly measure the safe state of the RF antenna receiving device, thereby allowing for immediate determination of an unsafe condition.

[0045]

[0051] FIG. 3 shows a first embodiment of an RF receive antenna device 300 that may use one or more of these principles.

[0046]

[0052] The RF receiving antenna device 300 includes an RF receiving coil or loop 310, a tuning capacitor 312, one or more decouplers 314, a coupling circuit 316a, a sampling device 316b, a first amplifier (e.g., preamplifier) ​​320a, an optional second amplifier (e.g., preamplifier) ​​320b, a switching device 325, and a detector 330. In particular, as shown in FIG. 3, the sampling device 316b can be either a sampling resistor, a sampling capacitance, or a sampling inductance. Furthermore, the detector 330 can include an analog-to-digital converter (ADC) or alternatively be an analog detector known to those skilled in the art. By way of example, the analog detector is an analog RF power detector.

[0047]

[0053] 3 shows an example embodiment having a single RF receive coil 310 configured in a single loop, in other examples, two or more RF receive coils or loops may be provided, for example, two or more RF receive coils or loops connected in parallel with each other.

[0048]

[0054] Here, the coupling circuit 316a includes an LC matching circuit that attempts to couple the coil 310 to the input impedance of the first preamplifier 320a. The switching device 325 may include a transistor, a diode, or other convenient switching device and operates under the control of a switching control signal 322 provided from a processor, such as the main controller 224. The switching control signal 322 may be synchronized with the RF transmit and receive cycles of the MRI system.

[0049]

[0055] During the RF transmit cycle, the current through the RF receive coil 310 induces a voltage across the sampling device 316b, which is provided to the second preamplifier 320b as a sampling signal. Meanwhile, the switching device 325 is controlled by the switching control signal 322 to connect the output of the second preamplifier 320b to the input of the detector 330 during the RF transmit cycle. By switching the input signal to the detector 330 to the second preamplifier 320b during the RF transmit cycle, the current through the RF receive coil 310 during the RF transmit cycle can be directly measured by the detector 330 and optionally converted into a digital word or value. Thus, the detector 330 becomes an analog-to-digital converter (ADC). In the RF receive antenna device 300, the sampling device 316b can be considered as a first coupling device that couples a signal proportional to the current through the RF receive coil 310 during the RF transmit cycle to the input of the detector 330.

[0050]

[0056] The detector 330 has an output coupled to a processor (e.g., an MRI system controller) of the MRI system to provide the processor with a signal 350 indicative of or representative of the magnitude of the current through the RF receive coil 310 during an RF transmit cycle. As previously mentioned, the detector 330 may include an ADC, in which case the signal 350 is a digital signal indicative of or representative of the magnitude of the current through the RF receive coil 310 during an RF transmit cycle. Alternatively, the detector 330 may include an analog RF power detector. In this case, the signal 350 is still digital, but is considered to have a reduced bit depth. Thus, the main controller 224 compares the value of the signal 350 (e.g., the value of the digital word of the signal 350, or the magnitude of the signal 350) to a threshold established by the designer or operator of the MRI system as representing the maximum allowable magnitude of the current through the RF receive coil 310 during an RF transmit cycle for safety or other reasons. In this case, if the value of signal 350 exceeds a threshold value, a processor (eg, an MRI system controller) may abort the MR scan and / or notify a system operator before harm occurs to the subject or patient.

[0051]

[0057] Furthermore, during the RF receive cycle, the coupling circuit 316a couples a signal proportional to the current flowing through the RF receive coil 310 during the RF receive cycle to an input of the first preamplifier 320a. Meanwhile, the switching device 325 connects the output of the first preamplifier 320a to the input of the detector 330 during the RF receive cycle. By switching the input signal to the detector 330 to the first preamplifier 320a during the RF receive cycle, the current flowing through the RF receive coil 310 during the RF receive cycle can be directly measured by the detector 330, converted to a digital word or value, and provided to a reconstruction system, for example, to generate an MR image of a region of interest of a subject or patient.

[0052]

[0058] In some variations of the RF receive antenna device 300, the second preamplifier 320b is omitted.

[0053]

[0059] FIG. 4 shows a second embodiment of an RF receive antenna device 400 .

[0054]

[0060] Since the RF receive antenna device 400 is partially similar to the RF receive antenna device 300, for the sake of brevity, only the differences between the two will be discussed and described herein.

[0055]

[0061] Instead of the first preamplifier 320a, the second preamplifier 320b, and the switching device 325, the RF receive antenna device 400 includes a single amplifier (e.g., preamplifier) ​​420 having a switchable gain under the control of a switching control signal 422. In particular, the preamplifier 420 is switchable between at least a first gain and a second gain greater than the first gain under the control of the switching control signal 422, which may be provided from a processor, such as, for example, an MRI system controller. More specifically, the switching control signal 422 controls the preamplifier 420 to have a first gain during an RF transmit cycle and a second gain during an RF receive cycle, the second gain being greater than the first gain.

[0056]

[0062] The RF receive antenna device 400 retains the existing basic architecture of RF receive antenna devices, where an RF receive coil 310 is connected to a preamplifier 420 via a coupling circuit 316a (an LC matching circuit). The impedance of such a circuit can be easily characterized such that by measuring the voltage across the circuit, the current in the RF receive coil 310 can be calculated directly from the signal 350 output by the detector 330.

[0057]

[0063] Preamplifier 420, as a practical matter, has at least two gain states so that the signal seen at the input of detector 330 is appropriately scaled to prevent overflow. However, it is believed that, at least in theory, and as the cost and capabilities of suitable preamplifiers and detectors continue to improve, the switchable gain preamplifier 420 could be replaced by an amplifier (e.g., a preamplifier) ​​whose gain does not need to be switched between RF transmit and RF receive cycles.

[0058]

[0064] 5 shows a third embodiment of an RF receive antenna device 500 including such a preamplifier 520. The RF receive antenna device 500 is otherwise identical to the RF receive antenna device 400, and therefore for the sake of brevity, a description thereof will not be repeated.

[0059]

[0065] FIG. 6 shows a fourth embodiment of an RF receive antenna device 600 .

[0060]

[0066] Since the RF receive antenna device 600 is partially similar to the RF receive antenna device 300, for the sake of brevity, only the differences between the two will be discussed and described in detail.

[0061]

[0067] The RF receive antenna device 600 uses a single amplifier (eg, a preamplifier) ​​320, a first detector 630a, and a second detector 630b.

[0062]

[0068] The second detector 630b directly samples the voltage across the sampling device 316b at least during the RF transmit cycle and responsively outputs a first digital signal 650b indicative or representative of the magnitude of the current passing through the RF receive coil 310 during the RF receive cycle. The second detector 630b provides the first digital signal 650b to a processor (e.g., an MRI system controller) indicative or representative of the magnitude of the current flowing through the RF receive coil 310 during the RF transmit cycle. The processor therefore compares the value of the first digital signal 650b (e.g., the value of the digital word of the first digital signal 650b) to a threshold established by the MRI system designer or operator as representing the maximum allowable magnitude of the current flowing through the RF receive coil 310 during the RF transmit cycle for safety or other reasons. In this case, if the value of the first digital signal 650b exceeds the threshold, the processor (e.g., an MRI system controller) will abort the MR scan and / or notify the system operator before harm occurs to the subject or patient.

[0063]

[0069] However, as before, the coupling circuit 316a couples a signal proportional to the current through the RF receive coil 310 during an RF receive cycle to the input of the preamplifier 320. Thus, the current through the RF receive coil 310 during an RF receive cycle can be measured directly by the first detector 630a, converted to a digital word or value, and provided via the second digital signal 650a to a reconstruction system for generating, for example, an MR image of a region of interest of a subject or patient.

[0064]

[0070] The second detector 630b may be very expensive if it is matched to the specifications of the first detector 630a used in the RF receive cycle to generate MR imaging data from the current in the RF receive coil 310. However, if the current in the RF receive coil 310 is measured for safety purposes, a detector with the same bit depth, or even in some cases the same sampling rate (which is required to sample the MR imaging signal from the patient acquired via the RF receive coil 310 in the RF receive cycle) is not needed. In this case, a degraded and less expensive device with fewer bits and / or a lower sampling rate can be used for the second detector 630b, thereby reducing the cost of the RF receive antenna device 600.

[0065]

[0071] The embodiments described above with respect to Figures 3-6 are not intended to be exclusive or limiting, but are intended to be exemplary examples of an RF receive antenna device that directly samples and measures the current through the RF receive coil 310 and generates a digital representation thereof at least during the RF transmit cycle. This digital representation is provided to a processor (e.g., an MRI system controller) to abort the MR scan and / or notify a system operator before harm occurs to the subject or patient due to excessive current in the RF receive coil 310 during the RF transmit cycle (e.g., due to a fault or failure of the decoupler 314). Also, measuring the current directly reduces or eliminates the possibility of false positives of faults in the decoupler 314 that do not actually result in an unsafe condition. This improves the reliability of the RF receive antenna device.

[0066]

[0072] When the current through the RF receive coil 310 is directly measured, sampled, and digitized, and the digital signal is processed, the amplitude and phase of the current through the RF receive coil 310 is determined to be the magnitude and phase of the current through the RF receive coil 310 in a known and controlled manner. 1Alternatively, the current through the RF receive coil 310 can be measured by a second detector 630b and the output used to control the amplitude and phase.

[0067]

[0073] Manipulating the transmit magnetic field in this manner results in B 1 Called "shimming," this technique can be effective in localizing RF excitation of tissue to reduce the Specific Absorption Rate (SAR), countering local image intensity shading effects caused by the body's natural conductivity, and / or creating RF phase gradients from the RF receive coil 310 for the purposes of spatial encoding, or generally altered k-space trajectories.

[0068]

[0074] Once the current induced in the RF receive coil 310 is measured directly, it can be modulated by adding a controllable variable impedance element in the series path of the RF receive coil 310. The impedance of the controllable variable impedance element can be controlled using a local processing device or circuitry on the RF receive antenna device itself. Alternatively, in some embodiments, it can be controlled by the MRI system (e.g., an MRI system controller) rather than locally on the RF receive antenna device, if a suitable connection infrastructure is available.

[0069]

[0075] To this end, FIG. 7 shows a fifth embodiment of an RF receiving antenna device 700 .

[0070]

[0076] Since the RF receive antenna device 700 is partially similar to the RF receive antenna device 600, for the sake of brevity, only the differences between the two will be discussed and described herein.

[0071]

[0077] The RF receive antenna device 700 includes a data processor or data processing circuit 710 connected to the output of the second ADC 730b to receive a first digital signal 650b indicative of the magnitude of the current through the RF receive coil 301 during the RF transmit cycle. The RF receive antenna device 700 also includes a controllable variable impedance element 720 connected in the series path of the RF receive coil 310. In various embodiments, the controllable variable impedance element 720 includes a field effect transistor (FET) placed in the gap of the RF receive coil 310 and gated by a voltage generated by a digital-to-analog converter (DAC) of the data processing circuit 710 in response to the first digital signal 650b. The controllable variable impedance element 720 is controlled by a control signal 715 from the data processing circuit 710 to have a very low impedance during the RF receive cycle. Furthermore, the controllable variable impedance element 720 is controlled by the data processing circuit 710 to have any desired impedance during the RF transmit cycle, thereby controlling the B as described above. 1 Distorting the magnetic field in a known and controlled way (B 1 The amplitude and phase of the current through the RF receive coil 310 is controlled for purposes of "shimming" (see FIG. 1). Also, as previously mentioned, this can be effective to localize the RF excitation in tissue to reduce the Specific Absorption Rate (SAR), to counter local image intensity shading effects caused by the body's natural conductivity, and / or to create RF phase gradients from the RF receive coil 310 for purposes of spatial encoding, or modified k-space trajectories in general.

[0072]

[0078] The additional processing circuitry and controllable variable impedance elements that provide these capabilities are shown in FIG. 7 as having been added to the RF receive antenna device 600 of FIG. 6, but in principle similar elements could be added to the RF receive antenna device 300 of FIG. 3, the RF receive antenna device 400 of FIG. 4, or the RF receive antenna device 500 of FIG. 5 to achieve similar advantages.

[0073]

[0079] Further, while FIG. 7 only shows the controllable variable impedance element 720 controlled by the data processing circuit 710 in response to the first digital signal 650b from the second ADC 730b, many variations are possible. In some variations, the data processing circuit 710 also receives the second digital signal 650a from the first ADC 730a and controls the controllable variable impedance element 720 in response to both the first digital signal 650b and the second digital signal 650a. In this case, the control of the controllable variable impedance element 720 relies on information collected from both the transmit RF cycle and the receive RF cycle. In other variations, the data processing circuit 710 is omitted and the control signal 715 for the controllable variable impedance element 720 is instead provided by a processor of the MRI system (e.g., an MRI system controller, etc.) in response to the first digital signal 650b (and optionally also the second digital signal 650a). Again, many variations are possible.

[0074]

[0080] 8 illustrates an exemplary embodiment of a method 800 for operating an MRI system, such as MRI system 100 or 200, that includes an RF receive antenna device, such as RF receive antenna device 300, 400, 500, 600, 700, that can directly sample, measure, and digitize current through an RF receive coil during an RF transmit cycle. Additionally, and as previously discussed, method 800 may be stored in storage device 221 as instructions that are executed by RF / gradient controller 209, main controller 224, or both, to implement aspects of the methods described herein.

[0075]

[0081] In step 810, a magnetic field is generated using the MRI system.

[0076]

[0082] In step 820, an RF transmit signal is applied to at least a portion of the patient or subject being imaged to perturb the orientation of magnetization relative to the magnetic field.

[0077]

[0083] In step 830, a magnetic resonance (MR) signal emitted from the patient in response to the transmitted RF magnetic field is sensed during an RF receive cycle using an RF receive antenna device, where the RF receive antenna unit includes at least one RF receive coil. In some embodiments, the RF receive antenna device is RF receive antenna device 300, 400, 500, 600, 700, or a variation thereof.

[0078]

[0084] In step 840, a first signal proportional to the current through at least one RF receive coil is coupled to a detector (or analog or digital) input of the RF receive antenna device at least during the RF transmit cycle. In some embodiments, the first signal is coupled to the detector input also during the RF receive cycle. In other embodiments, separate detectors are provided to measure the current during the RF receive cycle and the RF transmit cycle. Many variations are possible, at least some of which are described with respect to Figures 3-7.

[0079]

[0085] In step 850, the detector outputs to the processor an analog or digital signal (e.g., signal 350) indicative of or representative of the magnitude of the current through the at least one RF receive coil during at least the RF transmit cycle. Again, in some embodiments, the detector also outputs a signal during the RF receive cycle. In other embodiments, separate detectors are provided to measure the current during the RF receive cycle and the RF transmit cycle.

[0080]

[0086] In step 860, the transmit RF magnetic field is discontinued when the signal indicates that the magnitude of the current passing through at least one RF receive coil during an RF transmit cycle is greater than a threshold value, the threshold value being selected by the MRI system designer or MRI system operator as representing the maximum allowable magnitude of current passing through an RF receive coil during an RF transmit cycle for safety or other reasons.

[0081]

[0087] In step 870, corresponding to step 830 above, a magnetic resonance image is generated of one or more regions of interest in the patient or subject based at least in part on the digital signals output by the detector during the RF receive cycles.

[0082]

[0088] In step 880, a controllable variable impedance element connected in a series path of at least one RF receive coil is controlled based on the digital signal output by the detector. For example, during an RF receive cycle, the controllable variable impedance element is controlled to have a very low impedance. Meanwhile, during an RF transmit cycle, the impedance of the controllable variable impedance element is controlled to have a very low impedance, as described above. 1 Distorting the magnetic field in a known and controlled way (B 1 The RF receiver coil is selected to control the amplitude and phase of the current through the RF receiver coil for purposes of "shimming" (see FIG. 1). Also, as previously mentioned, this can be effective to localize the RF excitation of tissue to reduce the Specific Absorption Rate (SAR), to counter local image intensity shading effects caused by the body's natural conductivity, and / or to create RF phase gradients from the RF receiver coil for purposes of spatial encoding, or modified k-space trajectories in general.

[0083]

[0089] In some variations where a second ADC 730b is present to measure the loop current during the RF receive cycle as described in Figure 7, step 880 further includes controlling the controllable variable impedance element based on the second digital signal output by the second ADC 730b. In this case, the control of the variable impedance element depends on information collected from both the transmit RF cycle and the receive RF cycle. Many variations are possible.

[0084]

[0090] It should be understood that the order of steps shown in Figure 8 is merely an order for illustrating the steps and does not indicate that the steps are necessarily performed in that order. In practice, many of the steps may be performed simultaneously with one another and many of the steps may be performed iteratively. In fact, it should be understood that the steps may generally be performed continuously in a manner that collects magnetic resonance data and ensures safe operation of the MRI system. Further, for example, in some embodiments, step 880 may be omitted.

[0085]

[0091] The inventive concept also encompasses computer readable media that store instructions that cause a data processing system (such as processor 118) to perform the methods described herein. Computer readable media is defined as any medium that constitutes patentable subject matter under 35 U.S.C. 101, and excludes any medium that does not constitute patentable subject matter under 35 U.S.C. 101. Examples of such media include non-transitory media, such as computer memory devices that store information in a form readable by a computer or data processing system. More specific examples of non-transitory media are described above.

[0086]

[0092] Although trend-based analysis has been described with reference to certain exemplary embodiments, it is understood that the words used herein are words of description and illustration, rather than words of limitation. Changes may be made within the purview of the appended claims, as presently presented and as amended, without departing from the scope and spirit of perfusion shift measurement in its aspects. Although perfusion shift measurement has been described with reference to particular means, materials, and embodiments, the perfusion shift measurement is not intended to be limited to the details disclosed. Rather, perfusion shift measurement extends to all functionally equivalent structures, methods, and uses as fall within the scope of the appended claims.

[0087]

[0093] The illustrations of the embodiments described herein are intended to provide a general understanding of the structure of various embodiments. The illustrations do not completely describe all elements and features of the disclosure described herein. Many other embodiments will be apparent to those skilled in the art upon review of the present disclosure. Other embodiments can be utilized and derived from the present disclosure, such as structural and logical substitutions and changes can be made without departing from the scope of the present disclosure. Moreover, the illustrations are merely representative and may not be to scale. Certain proportions in the illustrations may be exaggerated and other proportions may be minimized. Thus, the present disclosure and the figures should be considered illustrative and not restrictive.

[0088]

[0094] One or more embodiments of the present disclosure may be referred to herein, individually and / or collectively, by the term "invention" for convenience only and without any intention to spontaneously limit the scope of the present application to any particular invention or inventive concept. Also, although specific embodiments are illustrated and described herein, it should be understood that subsequent arrangements designed to achieve the same or similar purpose may be substituted for the specific embodiment shown. The present disclosure is intended to cover any and all subsequent adaptations or variations of the various embodiments. Combinations of the above embodiments with other embodiments not specifically described herein will be apparent to those of skill in the art upon review of the description.

[0089]

[0095] The Abstract of the Disclosure is provided to comply with 37 CFR Rule 1.72(b) and is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the above Detailed Description, various features may be grouped together or described in a single embodiment for the purpose of streamlining the disclosure. This disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter may be directed to less than all features of any of the disclosed embodiments. Accordingly, the following claims are incorporated into the Detailed Description, with each claim standing on its own as defining separate claimed subject matter.

[0090]

[0096] The foregoing description of the disclosed embodiments is provided to enable any person skilled in the art to practice the concepts described in the present disclosure. Therefore, the above disclosed subject matter is considered to be illustrative and not restrictive. Moreover, the appended claims are intended to cover all such modifications, enhancements, and other implementations that fall within the true spirit and scope of the present disclosure. Thus, to the maximum extent permitted by law, the scope of the present disclosure shall be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be limited or restricted by the foregoing detailed description.

Claims

1. A magnet that generates a magnetic field, A gradient magnetic field coil that at least partially surrounds at least a part of a patient to be imaged, An RF transmission coil that applies a transmitted RF magnetic field to at least a part of the patient during an RF transmission cycle and perturbs the orientation of magnetization with respect to the magnetic field in the part of the patient, At least one RF reception antenna device that is positioned adjacent to a region of interest of the patient and receives a magnetic resonance signal from the region of interest of the patient generated in response to the transmitted RF magnetic field during an RF reception cycle An MRI system comprising: The at least one RF reception antenna device includes: At least one RF reception coil array, A detector, A first coupling device that measures a current flowing through the at least one RF reception coil at least during the RF transmission cycle and couples a signal proportional to the current flowing through the at least one RF reception coil at least during the RF transmission cycle to an input portion of the detector, and The MRI system includes a main controller, The detector has an output portion coupled to the main controller to provide a signal to the main controller, and the signal indicates the magnitude and / or phase of the current flowing through the at least one RF reception coil at least during the RF transmission cycle. An MRI system.

2. The first coupling device includes: A first amplifier, A sampling device that measures a current flowing through the at least one RF reception coil at least during the RF transmission cycle and couples a sampling signal proportional to the current flowing through the at least one RF reception coil at least during the RF transmission cycle to an input portion of the first amplifier, and The output of the first amplifier is coupled to the input of the detector, the MRI system according to claim 1.

3. The first coupling device is a second amplifier, measures the current flowing through the at least one RF receiving coil at least during the RF reception cycle, and couples a second sampling signal proportional to the current flowing through the at least one RF receiving coil during the RF reception cycle to the input of the second amplifier, a coupling circuit a switching device having a first input terminal connected to the output of the first amplifier, a second input terminal connected to the output of the second amplifier, and an output connected to the input of the detector, further comprising the MRI system connects the output of the first amplifier to the input of the detector during the RF transmission cycle, and connects the output of the second amplifier to the input of the detector during the RF reception cycle, controls the switching device, during the RF reception cycle, the digital signal indicates the magnitude and / or phase of the current flowing through the at least one RF receiving coil during the RF reception cycle, the MRI system according to claim 2.

4. The sampling device includes a resistive element, a capacitive element, or an inductive element connected in series with the at least one RF receiving coil, and the coupling circuit includes an impedance matching network, the MRI system according to claim 3.

5. The coupling circuit further couples a second signal proportional to the current flowing through the at least one RF receiving coil during the RF reception cycle to the input of the detector, Each of the first amplifier and the detector has a sufficient dynamic range such that the signal indicates the magnitude and / or phase of the current flowing through the at least one RF receive coil during the RF receive cycle and during the RF transmit cycle, the MRI system according to claim 3.

6. The coupling circuit also couples a second signal proportional to the current flowing through the at least one RF receive coil during the RF receive cycle to the input of the detector, The first amplifier has a gain that is switchable under the control of a control signal of the MRI system, and the control signal controls the first amplifier to have a first gain during the RF transmit cycle and a second gain during the RF receive cycle, and the second gain is greater than the first gain, The signal indicates the magnitude and / or phase of the current flowing through the at least one RF receive coil during the RF receive cycle and during the RF transmit cycle, the MRI system according to claim 3.

7. The at least one RF receive antenna device, A second detector, A second coupling device that couples a second signal proportional to the current flowing through the at least one RF receive coil during the RF receive cycle to the input of the second detector, and further includes, The second detector has an output coupled to the main controller for providing a digital signal to the main controller, and the second signal indicates the magnitude and / or phase of the current flowing through the at least one RF receive coil during the RF receive cycle, the MRI system according to claim 1.

8. The at least one RF receive antenna device, A variable impedance element connected in series with the at least one RF receive coil and controlled by a control signal, It has an input part connected to the output part of the detector, and a data processing circuit that generates the control signal in response to the output part of the detector. It further includes The data processing circuit generates the control signal to cause the variable impedance element to have a first impedance during the RF transmission cycle and a second impedance during the RF reception cycle. The first impedance is greater than the second impedance. The MRI system according to claim 1.

9. The step of generating a magnetic field. During the RF transmission cycle, applying an RF transmission signal to at least a part of a patient as an imaging target and perturbing the orientation of the magnetic field. Using an RF reception antenna device having at least one RF reception coil to detect a magnetic resonance signal emitted from a region of interest of the patient during the RF reception cycle in response to the RF transmission signal. Measuring at least the current flowing through the at least one RF reception coil during at least the RF transmission cycle, and coupling a first signal proportional to at least the current flowing through the at least one RF reception coil during at least the RF transmission cycle to the input part of the detector. Outputting a digital signal indicating at least the magnitude and / or phase of the current flowing through the at least one RF reception coil during at least the RF transmission cycle from the detector to the processor. Interrupting the RF transmission signal when the digital signal indicates that at least the magnitude and / or phase of the current flowing through the at least one RF reception coil during the RF transmission cycle is greater than a threshold value. A method including. **Claim 10**: A step of measuring a current flowing through the at least one RF receiving coil during at least the RF receiving cycle and coupling, to the input portion of the detector, a second signal proportional to the current flowing through the at least one RF receiving coil during the RF receiving cycle, wherein the digital signal indicates the magnitude and / or phase of the current flowing through the at least one RF receiving coil during the RF receiving cycle and during the RF transmitting cycle; a step of coupling. A step of generating a magnetic resonance image of the region of interest based at least in part on the digital signal during the RF receiving cycle. The method according to claim 9, further comprising. **Claim 11** The method according to claim 10, further comprising a step of controlling a switch to selectively couple the first signal to the input portion of the detector during the RF transmitting cycle and to selectively couple the second signal to the input portion of the detector during the RF receiving cycle. **Claim 12** The method according to claim 9, further comprising a step of controlling a variable impedance element connected in series with the at least one RF receiving coil based on the RF transmission signal output by the detector, such that the variable impedance element has a first impedance during the RF transmitting cycle and a second impedance during the RF receiving cycle, wherein the first impedance is greater than the second impedance. **Claim 13** A step of coupling, to the input portion of a second detector, a second signal proportional to the current flowing through the at least one RF receiving coil during the RF receiving cycle. A step of outputting, from the second detector to the processor, a second digital signal indicating the magnitude and / or phase of the current flowing through the at least one RF receiving coil during the RF receiving cycle. The method according to claim 9, further comprising. **Claim 14** A device that receives, during an RF reception cycle, a magnetic resonance signal from a region of interest of a patient, which is generated in response to an RF transmission signal during an RF transmission cycle of an MRI system, at least one RF reception coil element, a first coupling device that measures a current flowing through the at least one RF reception coil during at least the RF transmission cycle and couples, to an input part of a detector, a signal proportional to the current flowing through the at least one RF reception coil element during at least the RF transmission cycle, and comprising the detector having an output part for providing a signal indicating at least a magnitude and / or a phase of the current flowing through the at least one RF reception coil element during at least the RF transmission cycle. **Claim 15** The first coupling device comprises a first amplifier, a first sampling device that measures a current flowing through the at least one RF reception coil during at least the RF transmission cycle and couples, to an input part of the first amplifier, a sampling signal proportional to the current flowing through the at least one RF reception coil during at least the RF transmission cycle, and comprising The output part of the first amplifier is coupled to the input part of the detector. The device according to claim 14. **Claim 16** The first coupling device comprises a second amplifier, a second sampling device that measures a current flowing through the at least one RF reception coil during at least the RF reception cycle and couples, to an input part of the second amplifier, a second sampling signal proportional to the current flowing through the at least one RF reception coil during the RF reception cycle, a switching device having a first input terminal connected to the output part of the first amplifier, a second input terminal connected to the output part of the second amplifier, and an output part connected to the input part of the detector, further includes The switching device is controlled to connect the output of the first amplifier to the input of the detector during the RF transmission cycle and to connect the output of the second amplifier to the input of the detector during the RF reception cycle. The digital signal indicates the magnitude and / or phase of the current flowing through the at least one RF receiving coil during the RF reception cycle, according to the device of claim 15.

17. The first coupling device also measures the current flowing through the at least one RF receiving coil, at least during the RF reception cycle, and couples a second signal proportional to the current flowing through the at least one RF receiving coil during the RF reception cycle to the input of the detector. Each of the first amplifier and the detector has a sufficient dynamic range such that the signal indicates the magnitude and / or phase of the current flowing through the at least one RF receiving coil during the RF reception cycle and during the RF transmission cycle, according to the device of claim 15.

18. The first coupling device also measures the current flowing through the at least one RF receiving coil, at least during the RF reception cycle, and couples a second signal proportional to the current flowing through the at least one RF receiving coil during the RF reception cycle to the input of the detector. The first amplifier has a gain that is switchable under the control of a control signal, and the control signal controls the first amplifier to have a first gain during the RF transmission cycle and a second gain during the RF reception cycle, and the second gain is greater than the first gain. The signal indicates the magnitude and / or phase of the current flowing through the at least one RF receiving coil during the RF reception cycle and during the RF transmission cycle, according to the device of claim 15.

19. A second detector, Measure the current flowing through the at least one RF receiving coil during at least the RF receiving cycle, and a second coupling device that couples a second signal proportional to the current flowing through the at least one RF receiving coil during the RF receiving cycle to an input of the second detector. further comprising The second detector has an output that provides a second signal indicating the magnitude and / or phase of the current flowing through the at least one RF receiving coil during the RF receiving cycle. The device according to claim 14.

20. A variable impedance element connected in series with the at least one RF receiving coil and controlled by a control signal. It has an input connected to the output of the detector, and a data processing circuit that generates the control signal in response to the output of the detector. further comprising The data processing circuit generates the control signal to cause the variable impedance element to have a first impedance during the RF transmission cycle and a second impedance during the RF receiving cycle. The first impedance is greater than the second impedance. The device according to claim 14.

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