JP7621729B2 - High frequency circuit, magnetic resonance imaging apparatus, and RF pulse power monitoring method - Google Patents

Description

Embodiments of the present invention relate to radio-frequency circuits and magnetic resonance imaging devices.

In magnetic resonance imaging (MRI) devices, in order to suppress the effects of high-frequency RF (radio frequency) pulses on the subject, such as an increase in temperature, IEC (International Electrotechnical Commission) requires that the specific absorption rate (SAR, the amount of RF absorbed per unit mass) be kept below a threshold value.

In order to meet safety standards for SAR, it is important to accurately monitor the power of the RF pulse applied to the subject. The power of the RF pulse is monitored, for example, by extracting a portion of the RF pulse using a directional coupler just before the output terminal to the transmission coil.

However, in an MRI device, the subject is close to the transmitting coil, and the load impedance of the RF pulse changes depending on the subject's physique, etc. This causes impedance mismatch at the output end of the RF pulse, making it difficult to monitor the accurate power of the RF pulse.

JP 2014-079573 A JP 2017-109109 A

The problem that this invention aims to solve is to accurately monitor the power of the RF pulse applied to the subject.

The high-frequency circuit according to the embodiment includes a directional coupler, a processing circuit, and an adjuster. The directional coupler has a first port that outputs at least a portion of the traveling wave, and a second port that outputs at least a portion of the reflected wave. The processing circuit calculates the impedance of the load side as seen from the directional coupler using a voltage standing wave ratio based on the outputs from the first and second ports, and the phase of the reflected wave based on the output from the second port. The adjuster adjusts the output from at least one of the first and second ports based on the impedance calculated by the processing circuit.

FIG. 1 is a block diagram showing an example of the configuration of an MRI apparatus including a high-frequency circuit according to an embodiment. FIG. 1 is a block diagram showing an example of the configuration of a conventional high-frequency circuit. FIG. 2 is a block diagram showing a configuration example of a high-frequency circuit according to the present embodiment. FIG. 4 is a block diagram showing another example of the configuration of the high-frequency circuit according to the embodiment. FIG. 1 is an explanatory diagram showing an example of a Smith chart.

Below, embodiments of a high-frequency circuit, a magnetic resonance imaging device, and an RF pulse power monitoring method are described in detail with reference to the drawings.

Figure 1 is a block diagram showing an example of the configuration of an MRI apparatus 1 including a high-frequency circuit 600 according to one embodiment. The MRI apparatus 1 has an apparatus main body (also called a magnet stand) 100, a control cabinet 300 including the high-frequency circuit 600, a console 400, a bed device 500, and an RF (Radio Frequency) coil 20. The apparatus main body 100, the control cabinet 300, and the bed device 500 are generally installed in an examination room. The console 400 is generally installed in a control room adjacent to the examination room.

The device main body 100 has a static magnetic field magnet 10, a gradient magnetic field coil 11, and a WB (Whole Body) coil 12. These components are housed in a cylindrical housing. The bed device 500 has a bed main body 50 and a tabletop 51.

The control cabinet 300 has gradient magnetic field power supplies 31 (31x for the X-axis, 31y for the Y-axis, and 31z for the Z-axis), an RF receiver 32, an RF transmitter 33, and a sequence controller 34. The RF transmitter 33 has a high-frequency circuit 600.

The console 400 has a processing circuit 40, a memory circuit 41, a display 42, and an input interface 43. The console 400 functions as a host computer.

The static magnetic field magnet 10 of the device main body 100 has a roughly cylindrical shape and generates a static magnetic field in a bore in which a subject, such as a patient, is transported. The bore is the space inside the cylinder of the device main body 100. The static magnetic field magnet 10 has a built-in superconducting coil, which is cooled to an extremely low temperature by, for example, liquid helium. The static magnetic field magnet 10 generates a static magnetic field by applying a current supplied from a static magnetic field power supply (not shown) to the superconducting coil in the excitation mode. After that, when the mode is switched to the persistent current mode, the static magnetic field power supply is disconnected. Once the mode is switched to the persistent current mode, the static magnetic field magnet 10 continues to generate a static magnetic field for a long period of time, for example, for more than one year. Note that the static magnetic field magnet 10 is not limited to a superconducting magnet, and may be, for example, a permanent magnet.

The gradient coil 11 has a roughly cylindrical shape similar to the static magnetic field magnet 10, and is fixed inside the static magnetic field magnet 10. The gradient coil 11 forms gradient magnetic fields in the X-axis, Y-axis, and Z-axis directions by currents supplied from gradient magnetic field power supplies (31x, 31y, 31z).

The bed body 50 of the bed device 500 can move the top plate 51 in the vertical and horizontal directions. For example, the bed body 50 moves the subject placed on the top plate 51 to a predetermined height before imaging. During imaging, the bed body 50 also moves the top plate 51 horizontally to move the subject into the bore.

The WB coil 12 is fixed in a roughly cylindrical shape inside the gradient coil 11 so as to surround the subject. The WB coil 12 transmits RF pulses transmitted from the high-frequency circuit 600 of the RF transmitter 33 toward the subject. The WB coil 12 is an example of an RF coil that applies RF pulses to the subject. It also receives magnetic resonance signals, i.e., MR (Magnetic Resonance) signals, emitted from the subject by excitation of hydrogen nuclei.

The MRI apparatus 1 includes an RF coil (local coil) 20 as shown in FIG. 1 in addition to the WB coil 12. The RF coil 20 is placed close to the body surface of the subject. There are various types of RF coils 20, such as a body coil placed on the subject's chest, abdomen, or legs as shown in FIG. 1, and a spine coil placed on the subject's back. The RF coil 20 is dedicated to receiving or transmitting, or performs both transmission and receiving. The RF coil 20 is an example of an RF coil that applies an RF pulse to the subject. The RF coil 20 is configured to be detachable from the tabletop 51 via a cable, for example.

The RF receiver 32 converts the channel signals, i.e., MR signals, from the WB coil 12 and the RF coil 20 into analog to digital (AD) signals and outputs them to the sequence controller 34. The digitally converted MR signals are sometimes called raw data.

The RF transmitter 33 has a high-frequency oscillator, a modulator, and a high-frequency circuit 600, and generates an RF pulse based on instructions from the sequence controller 34. The generated RF pulse is transmitted to the WB coil 12 and applied to the subject. Application of the RF pulse generates an MR signal from the subject. This MR signal is received by the RF coil 20 or the WB coil 12.

The high-frequency circuit 600 amplifies the high-frequency signal output by the modulator to generate an RF pulse, and adjusts the monitored power of the RF pulse according to the degree of mismatch in the load impedance. Details of the high-frequency circuit 600 will be described later using Figures 3-5.

The MR signals received by the RF coil 20, more specifically, the MR signals received by each element coil in the RF coil 20, are input to the RF receiver 32 via cables provided on the tabletop 51 and the bed body 50.

Under the control of the console 400, the sequence controller 34 drives the gradient magnetic field power supply 31, the RF receiver 32, and the RF transmitter 33 to scan the subject. When raw data is received from the RF receiver 32 by the scan, the sequence controller 34 transmits the raw data to the console 400.

The sequence controller 34 includes a processing circuit (not shown). This processing circuit is composed of hardware such as a processor that executes a specific program, an FPGA (Field Programmable Gate Array), or an ASIC (Application Specific Integrated Circuit).

The console 400 includes a memory circuit 41, a display 42, an input interface 43, and a processing circuit 40. The memory circuit 41 is a storage medium including a ROM (Read Only Memory), a RAM (Random Access Memory), and an external storage device such as a HDD (Hard Disk Drive) or an optical disk device. The memory circuit 41 stores various information and data, as well as various programs executed by the processor of the processing circuit 40.

The display 42 is a display device such as a liquid crystal display panel, a plasma display panel, or an organic EL panel. The input interface 43 is, for example, a mouse, a keyboard, a trackball, a touch panel, or the like, and includes various devices that allow the operator to input various information and data.

The processing circuit 40 is a circuit equipped with, for example, a CPU or a dedicated or general-purpose processor. The processor realizes various functions by executing various programs stored in the memory circuit 41. The processing circuit 40 may be configured with hardware such as an FPGA or an ASIC. The various functions described below can also be realized by such hardware. The processing circuit 40 can also realize various functions by combining software processing by a processor and a program with hardware processing.

For example, the processing circuit 40 realizes a function of calculating the SAR based on the output of the regulator of the high-frequency circuit 600. The processing circuit 40 is an example of a second processing circuit.

Next, the configuration and operation of the high-frequency circuit 600 will be explained.

Figure 2 is a block diagram showing an example of a conventional high-frequency circuit.

Conventionally, the power of the RF pulse applied to the subject has been monitored using a high-frequency circuit as shown in FIG. 2. In a conventional high-frequency circuit, an RF pulse input from a modulator to an input end 61 is amplified by an amplifier 62. In addition, in a conventional high-frequency circuit, at least a portion of the forward power of the amplified RF pulse (hereinafter referred to as forward power for monitor FWD) and at least a portion of the reflected power (hereinafter referred to as reflected power for monitor RFL) are separated by a directional coupler 63 just before the RF pulse output end 64 to the WB coil 12, and are output from a forward power monitor output end 65 and a reflected power monitor output end 66, respectively.

In conventional technology, the power of the RF pulse output from the RF pulse output terminal 64 is monitored based on the forward wave power FWD for monitoring output from the forward wave power monitor output terminal 65 and the reflected wave power RFL for monitoring output from the reflected wave power monitor output terminal 66.

However, the impedance on the load side as seen from the directional coupler 63, i.e., the impedance on the load side connected to the RF pulse output end 64, changes depending on the physique of the subject and the type of RF coil 20. On the other hand, the degree of coupling of the directional coupler 63 is set such that the load side impedance (hereinafter referred to as load impedance) R±jX is a predetermined value, for example, 50Ω±0Ω. Therefore, if an impedance mismatch occurs at the RF pulse output end 64 due to the physique of the subject, the degree of coupling of the directional coupler 63 changes, affecting the accuracy of monitoring the power of the RF pulse.

Therefore, in conventional technology, it is necessary to take into consideration that errors in the monitor power of the RF pulse caused by impedance mismatch affect the calculation of SAR. For example, for safety reasons, the upper limit of SAR may be set to a value lower than the SAR threshold (e.g., the threshold based on IEC regulations), but in this case, the power of the RF pulse must be set lower than the amount that can actually be output, which may result in longer imaging times.

Therefore, the high-frequency circuit 600 according to this embodiment corrects at least one of the monitor forward wave power FWD and the monitor reflected wave power RFL according to the degree of mismatch in the load impedance.

Figure 3 is a block diagram showing an example of the configuration of the high-frequency circuit 600 according to this embodiment. Figure 4 is a block diagram showing another example of the configuration of the high-frequency circuit 600 according to this embodiment. Also, Figure 5 is an explanatory diagram showing an example of a Smith chart.

Figure 3 shows an example of the configuration of the high-frequency circuit 600 when the corrected monitor forward power FWD and monitor reflected power RFL are output as analog signals. On the other hand, Figure 4 shows an example of the configuration of the high-frequency circuit 600 when the corrected monitor forward power FWD and monitor reflected power RFL are output as digital signals.

As shown in FIG. 3, the high-frequency circuit 600 according to this embodiment has an input terminal 601, a coupler 602 for input monitoring, an amplifier 603, a directional coupler 604, an RF pulse output terminal 605, an output terminal 606 for monitoring forward power, an output terminal 607 for monitoring reflected power, a processing circuit 610, a memory 620, a variable attenuator (ATT) 631 for correcting the forward power FWD for monitoring, and a variable ATT 632 for correcting the reflected power RFL for monitoring. The variable ATTs 631 and 632 are examples of adjusters.

The RF pulse input from the modulator to the input end 61 has its input timing monitored by the processing circuit 610 via the coupler 602, and is amplified by the amplifier 603. The amplified RF pulse is input to the directional coupler 604 as a traveling wave, output from the RF pulse output end 605 via the directional coupler 604, and applied to the subject via the WB coil 12. A portion of the traveling wave is reflected due to a mismatch in the load impedance seen from the directional coupler 604.

The directional coupler 604 has a port for extracting and outputting at least a portion of the forward power of the RF pulse amplified by the amplifier 603 (forward power for monitor FWD) and a port for extracting and outputting at least a portion of the reflected power (reflected power for monitor RFL). In the following description, an example is shown in which the degree of coupling of the directional coupler 604 is set to a load impedance of 50Ω±0Ω. Note that the forward power for monitor FWD and the reflected power for monitor RFL may be extracted by one directional coupler 604 as shown in Figures 3 and 4, or each may be extracted by a different directional coupler.

The processing circuit 610 is a processor that reads and executes a program stored in the memory 620 to perform a process of adjusting at least one of the monitor forward wave power FWD and the monitor reflected wave power RFL (hereinafter collectively referred to as monitor power) according to the degree of mismatch in the load impedance in order to accurately monitor the power of the RF pulse applied to the subject. The processing circuit 610 is an example of a first processing circuit.

The memory 620 has a configuration including a processor-readable recording medium, such as a semiconductor memory element such as a RAM (Random Access Memory), a flash memory, a hard disk, an optical disk, etc. The memory 620 stores various information, such as a table that associates the load impedance with the adjustment degree of the monitor power. The information stored in the memory 620 may be updated via a network or a portable storage medium such as an optical disk.

The processor of the processing circuit 610 realizes a VSWR detection function 611, a phase angle detection function 612, and a correction function 613. Each of these functions is stored in the memory 620 in the form of a program.

The VSWR detection function 611 detects the voltage standing wave ratio (VSWR) based on the amplitude of the monitor forward wave power FWD and the amplitude of the monitor reflected wave power RFL (see Figure 5).

The phase angle detection function 612 detects the phase angle of the load based on the phase of the monitor reflected wave power RFL (see Figure 5).

The VSWR detection function 611 and the phase angle detection function 612 may determine the timing for detecting the amplitude of the monitor forward wave power FWD and the amplitude of the monitor reflected wave power RFL, and the phase of the monitor reflected wave power RFL, based on the amplitude of the signal input to the coupler 602.

The correction function 613 determines the load impedance based on the detected VSWR value and phase angle. The VSWR value and phase angle reflect the load state. The correction function 613 may determine the load impedance by calculation using the detected VSWR value and phase angle, or may determine the load impedance using a table that associates the VSWR value, phase angle, and load impedance.

The correction function 613 also corrects the attenuation (adjustment) of at least one of the variable ATTs 631 and 632 based on the determined load impedance.

More specifically, the correction function 613 corrects the degree of adjustment of at least one of the variable ATTs 631 and 632 according to the degree of mismatch between the load impedance specified value 50 Ω and the determined load impedance so that monitor power corresponding to the power of the RF pulse actually output from the RF pulse output terminal 605 is output from the forward wave power monitor output terminal 606 and the reflected wave power monitor output terminal 607.

At this time, the correction function 613 may determine the adjustment degree of at least one of the variable ATTs 631 and 632 by calculation using the determined load impedance, or may determine the adjustment degree using a table that associates the load impedance with the adjustment degree, or may determine the adjustment degree based on the output of a comparator that outputs the difference between the determined load impedance and a specified value.

The monitor power may also be output digitally. In this case, as shown in FIG. 4, the high-frequency circuit 600 may include a detector 641 and a voltage regulator 643 instead of the variable ATT 631, and a detector 642 and a voltage regulator 644 instead of the variable ATT 632. The voltage regulators 643 and 644 are examples of regulators.

In this case, the forward monitor power FWD and the reflected monitor power RFL output from the directional coupler 604 are detected by detectors 641 and 642, respectively, and converted to DC signals. Then, the correction function 613 corrects the adjustment degree of at least one of the voltage regulators 643 and 644 based on the determined load impedance. By digitally outputting the corrected monitor power, the AD conversion process at the subsequent stage can be omitted.

The high-frequency circuit 600 according to this embodiment can actually measure the load impedance, which can vary depending on the physique of the subject. In addition, the monitor forward wave power FWD and monitor reflected wave power RFL output by the directional coupler 604 can be corrected using the variable attenuators 631 and 632 according to the actual measured value of the load impedance. Therefore, even if the load impedance deviates from the specified value of 50 Ω, the RF pulse power can be accurately monitored.

Therefore, the power of the RF pulse applied to the subject can be accurately determined, and RF power can be efficiently output from the RF amplifier.

In addition, the power of the RF pulse applied to the subject can be monitored more accurately than when load impedance mismatch is not taken into account. Therefore, a high-power RF pulse can be applied to the subject in a short time, significantly shortening the imaging time.

According to at least one of the embodiments described above, the power of the RF pulse applied to the subject can be accurately monitored.

In the above embodiment, the term "processor" refers to a circuit such as a dedicated or general-purpose CPU (Central Processing Unit), GPU (Graphics Processing Unit), or an Application Specific Integrated Circuit (ASIC), programmable logic device (e.g. Simple Programmable Logic Device (SPLD), Complex Programmable Logic Device (CPLD), and FPGA). The processor realizes various functions by reading and executing programs stored in a storage medium.

Furthermore, the functions of the processing circuit 40 and the processing circuit 610 may be realized by a single processor, such as the processor of the processing circuit 40 of the console 400.

In the above embodiment, an example was shown in which a single processor in the processing circuit 610 realizes each of the functions 611-613, but a processing circuit may be configured by combining multiple independent processors, with each processor realizing each function. In addition, when multiple processors are provided, a storage medium for storing the programs may be provided separately for each processor, or a single storage medium may store all of the programs corresponding to the functions of all of the processors.

Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the gist of the invention. These embodiments and their modifications are within the scope of the invention and its equivalents as set forth in the claims, as well as the scope and gist of the invention.

12 WB coil 20 RF coil 33 RF transmitter 600 High frequency circuit 603 Amplifier 604 Directional coupler 605 RF pulse output terminal 606 Output terminal for forward wave power monitor 607 Output terminal for reflected wave power monitor 610 Processing circuit 611 VSWR detection function 612 Phase angle detection function 613 Correction function 631, 632 Variable attenuator (adjuster)
643, 644 Voltage regulator (regulator)
FWD Forward wave power for monitor RFL Reflected wave power for monitor

Claims (7)

a directional coupler having a first port for outputting at least a portion of the traveling wave and a second port for outputting at least a portion of the reflected wave;
a processing circuit that calculates a phase angle of a load from the phase of the reflected wave based on an output from the second port, and calculates an impedance of the load side as viewed from the directional coupler using the phase angle and a voltage standing wave ratio based on the outputs from the first port and the second port;
an adjuster that adjusts an output from at least one of the first port and the second port based on the impedance determined by the processing circuit and provides the adjusted output to an output port for monitoring;
two power monitor output ports for outputting to the outside the power of the forward wave and the power of the reflected wave output from the first port and the second port of the directional coupler, at least one of which is adjusted by the adjuster;
Equipped with
(1) the adjuster has an attenuator, and adjusts an attenuation of an output from at least one of the first port and the second port based on the impedance calculated by the processing circuit, and outputs the adjusted output as an analog signal;
The two power monitor output ports each output, in analog form, the forward wave power and the reflected wave power, at least one of which has been adjusted by the adjuster, to the outside.
or
(2) the regulator has a voltage regulator, and adjusts a voltage of an output from at least one of the first port and the second port based on the impedance calculated by the processing circuit, and outputs the adjusted voltage as a digital signal;
The two power monitor output ports digitally output the power of the forward wave and the power of the reflected wave, at least one of which has been adjusted by the adjuster, to the outside.
High frequency circuits. The processing circuitry includes:
adjusting the adjustment degree of the regulator based on the impedance obtained, based on a table correlating the obtained impedance with the adjustment degree by the regulator;
2. The high frequency circuit according to claim 1. The processing circuitry includes:
a degree of adjustment of the regulator is adjusted based on an output of a comparator that outputs a difference between the impedance calculated by the processing circuit and a specified value of the impedance on the load side;
2. The high frequency circuit according to claim 1. a high-frequency amplifier that amplifies and outputs a high-frequency signal and inputs the amplified high-frequency signal as the traveling wave to the directional coupler;
Further equipped with
4. The high-frequency circuit according to claim 1. an RF coil for applying an RF pulse to a subject based on the high frequency signal amplified by the amplifier;
a directional coupler provided between the amplifier and the RF coil, the directional coupler outputting at least a part of a traveling wave input from the amplifier side from a first port and outputting at least a part of a reflected wave from a second port;
a first processing circuit that calculates a phase angle of a load from a phase of the reflected wave based on an output from the second port, and calculates an impedance of the load side as viewed from the directional coupler using the phase angle and a voltage standing wave ratio based on the outputs from the first port and the second port;
an adjuster that adjusts an output from at least one of the first port and the second port based on the impedance determined by the first processing circuit and provides the adjusted output to an output port for monitoring;
A second processing circuit that calculates a specific absorption rate (SAR) based on the output of the regulator;
two power monitor output ports for outputting to the outside the power of the forward wave and the power of the reflected wave output from the first port and the second port of the directional coupler, at least one of which is adjusted by the adjuster;
Equipped with
(1) the adjuster has an attenuator, and adjusts an attenuation of an output from at least one of the first port and the second port based on the impedance calculated by the first processing circuit, and outputs the adjusted output as an analog signal;
The two power monitor output ports each output, in analog form, the forward wave power and the reflected wave power, at least one of which has been adjusted by the adjuster, to the outside.
or
(2) the regulator has a voltage regulator, and adjusts a voltage of an output from at least one of the first port and the second port based on the impedance calculated by the first processing circuit, and outputs the adjusted voltage as a digital signal;
The two power monitor output ports digitally output the power of the forward wave and the power of the reflected wave, at least one of which has been adjusted by the adjuster, to the outside.
Magnetic resonance imaging device . a step of determining a phase angle of a load from a phase of the reflected wave based on an output from a first port of a directional coupler having a first port for outputting at least a part of a traveling wave and a second port for outputting at least a part of a reflected wave, and determining an impedance on the load side as viewed from the directional coupler using the phase angle and a voltage standing wave ratio based on the outputs from the first port and the second port;
adjusting an output from at least one of the first port and the second port based on the determined impedance;
outputting the power of the forward wave and the power of the reflected wave output from the first port and the second port of the directional coupler, at least one of which has been adjusted, to an outside from two power monitor output ports;
having
(1) The step of adjusting the output from at least one of the first port and the second port includes a step of adjusting an attenuation of the output from at least one of the first port and the second port using an attenuator based on the determined impedance, and outputting the output as an analog signal;
The step of outputting the power monitor signals from the two power monitor output ports to the outside includes:
outputting the power of the forward wave and the power of the reflected wave, at least one of which has been adjusted, to an external device in analog form,
or
(2) the step of adjusting the output from at least one of the first port and the second port includes a step of adjusting a voltage of the output from at least one of the first port and the second port using a voltage regulator based on the determined impedance, and digitally outputting the adjusted voltage;
The step of outputting the power monitor signals from the two power monitor output ports to the outside includes:
and outputting, to an outside, the power of the forward wave and the power of the reflected wave, the at least one of which has been adjusted, in digital form.
RF pulse power monitoring method . determining a specific absorption rate (SAR) based on outputs of the first port and the second port, at least one of which has been adjusted;
7. The method of claim 6, further comprising:

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