MRI apparatus and RF amplification circuit

According to one embodiment, an MRI apparatus includes an amplifier and a control circuit. The amplifier amplifies an RF pulse and supplies it to an RF coil. The control circuit is configured to determine whether an output RF pulse outputted from the amplifier is fed back to an input side of the amplifier to correct an input RF pulse to be inputted into the amplifier, based on a determination value being set according to a slew rate of the input RF pulse.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of Japanese Patent Application No. 2018-131384, filed Jul. 11, 2018, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to an MRI (Magnetic Resonance Imaging) apparatus and an RF (Radio Frequency) amplification circuit.

BACKGROUND

Since an MRI apparatus has no problem with exposure to X-rays and can perform internal examination in a minimally invasive manner, an MRI apparatus is an essential modality in the current medical treatment.

An MRI apparatus is an imaging apparatus that magnetically excites nuclear spin of an object placed in a static magnetic field by applying an RF pulse having the Larmor frequency and reconstructs an image on the basis of MR (Magnetic Resonance) signals emitted from the object due to the excitation. In order to acquire the MR signals, high frequency pulses, i.e., RF pulses are applied to the object. Each RF pulse is amplified by an amplifier so as to have a predetermined power and radiated to the object. From the viewpoint of the safety of the object and accuracy of each image to be acquired, each RF pulse outputted from the amplifier is required to have stability and linearity.

As a method for improving stability and correcting nonlinearity of each RF pulse outputted from the amplifier, feedback processing is included. The feedback processing is processing of monitoring at least a part of an RF pulse actually outputted from the amplifier and then correcting the RF pulse to be inputted to the amplifier on the basis of the outputted RF pulse.

In addition to sinc pulses, ultrashort square waves and trapezoidal waves have come to be used as RF pulses applied to the object. However, square waves and trapezoidal waves have a sharp rise (a high slew rate). For this reason, when the feedback processing is applied to RF pulses that are square waves or trapezoidal waves, the rise of each RF pulse outputted from the amplifier is deteriorated to degrade the image quality in some cases.

DETAILED DESCRIPTION

Hereinbelow, a description will be given of respective embodiments of an MRI apparatus and an RF amplification circuit in detail with reference to the accompanying drawings. In general, according to one embodiment, an MRI apparatus includes an amplifier and a control circuit. The amplifier amplifies an RF pulse and supplies it to an RF coil. The control circuit is configured to determine whether an output RF pulse outputted from the amplifier is fed back to an input side of the amplifier to correct an input RF pulse to be inputted into the amplifier, based on a determination value being set according to a slew rate of the input RF pulse.

FIG. 1is a block diagram illustrating a configuration of an MRI apparatus1according to one embodiment. The MRI apparatus1includes a main body (also referred to as a gantry)100, a control cabinet300, a console400, a bed500, and local RF coils20. The main body100, the control cabinet300, and the bed500are generally installed in an examination room. The console400is generally installed in a control room adjacent to the examination room.

The main body100includes a static magnetic field magnet10, a gradient coil11, and a WB (whole body) coil12, and these components are housed in a cylindrical housing.

The control cabinet300includes three gradient coil power supplies31(31xfor an X-axis,31yfor a Y-axis, and31zfor a Z-axis), an RF receiver32, an RF transmitter33, and a sequence controller34.

The console400includes processing circuitry40, a memory41, a display42, and an input interface43. The console400functions as a host computer.

The bed500includes a bed body50and a table51.

The static magnetic field magnet10of the main body100is substantially in the form of a cylinder, and generates a static magnetic field inside a bore into which an object P, e.g., a patient, is transported. The bore is a space inside the cylindrical structure of the main body100. The static magnetic field magnet10may include a superconducting coil inside, and the superconducting coil is cooled down to an extremely low temperature by liquid helium. The static magnetic field magnet10generates a static magnetic field by supplying the superconducting coil with an electric current provided from a non-illustrated static magnetic field power supply in an excitation mode. Afterward, the static magnetic field magnet10shifts to a permanent current mode, and the static magnetic field power supply is separated. Once it enters the permanent current mode, the static magnetic field magnet10continues to generate a static magnetic field for a long time, e.g., over one year. Note that the static magnetic field magnet10is not limited to a superconducting magnet including a superconducting coil but may be a permanent magnet.

The gradient coil11is also substantially in the form of a cylinder similarly to the static magnetic field magnet10, and is fixed to the inside of the static magnetic field magnet10. The gradient coil11forms gradient magnetic fields in the respective directions of the x-axis, the y-axis, and the z-axis by using electric currents supplied from the gradient coil power supplies31x,31y, and31z.

The bed body50of the bed500can move the table51in the vertical direction and in the horizontal direction. For instance, the bed body50moves the table51with an object loaded thereon to a predetermined height before imaging. Afterward, when the object is imaged, the bed body50moves the table51in the horizontal direction so as to move the object to the inside of the bore.

The WB body coil12is shaped substantially in the form of a cylinder to surround the object, and is fixed to the inside of the gradient coil11. The WB coil12applies RF pulses transmitted from the RF transmitter33to the object. The WB coil12receives magnetic resonance signals, i.e., MR signals emitted from the object due to excitation of hydrogen nuclei.

The MRI apparatus1may include the local RF coils20as shown inFIG. 1in addition to the WB coil12. Each of the local RF coils20is placed close to the body surface of the object. There are various types for the local RF coils20. For instance, as the types of the local RF coils20, as shown inFIG. 1, there are a body coil attached to the chest, abdomen, or legs of the object and a spine coil attached to the backside of the object. The local RF coils20may be of a type dedicated for receiving MR signals, another type dedicated for transmitting RF pulses, or still another type for performing both of transmitting RF pulses and receiving MR signals. The local RF coils20are configured to be attachable to and detachable from the table51via a cable, for instance.

The RF receiver32performs A/D (Analog to Digital) conversion on the channel signal from the WB coil12and/or the local RF coils20, i.e., the MR signals, and outputs the converted MR signals to the sequence controller34. The MR signals converted into digital signals are sometimes referred to as raw data.

The RF transmitter33generates an RF pulse based on an instruction from the sequence controller34. The generated RF pulse is transmitted to the WB coil12and applied to the object. MR signals are generated from the object by the application of the RF pulse. The MR signals are received by the local RF coils20and/or the WB coil12. The RF transmitter33will be described in detail later with reference toFIGS. 3-6.

The sequence controller34performs a scan of the object by driving each of the gradient coil power supplies31, the RF receiver32, and the RF transmitter33and under the control of the console400. When the sequence controller34receives raw data from the RF receiver32by performing a scan, the sequence controller34transmits the received raw data to the console400. The sequence controller34will be described in detail later with reference toFIGS. 3-6.

The sequence controller34includes non-illustrated processing circuitry. This processing circuitry is configured of hardware such as a processor for executing predetermined programs, an FPGA (Field Programmable Gate Array), and an ASIC (Application Specific Integrated Circuit).

The console400includes a memory41, a display42, an input interface43, and processing circuitry40.

The memory41is a recording medium including a ROM (Read Only memory) and a RAM (Random Access Memory) in addition to an external memory device such as a HDD (Hard Disk Drive) and an optical disc device. The memory41stores various programs executed by the processor of the processing circuitry40as well as various types of data and information.

The display42is a display device such as a liquid crystal display panel, a plasma display panel, and an organic EL panel.

The input interface43includes various devices for an operator to input various types of information and data. The input interface43is configured of, e.g., a mouse, a keyboard, a trackball, and/or a touch panel.

The processing circuitry40is, e.g., a circuit equipped with a CPU (Central Processing Unit) and/or a special-purpose or general-purpose processor. The processor implements various functions by executing the various programs stored in the memory41. The processing circuitry40may be configured of hardware such as an FPGA and an ASIC. The various functions described below can also be implemented by such hardware. Additionally, the processing circuitry40can implement the various functions by combining hardware processing and software processing based on its processor and programs.

Next, relationship between the RF pulse and the feedback processing will be described.

FIG. 2is a schematic diagram illustrating the influence of the feedback processing on the rise of the RF pulse in the case of using a trapezoidal wave as the RF pulse.

FIG. 3is a schematic diagram illustrating a conventional configuration for performing the feedback processing.

As shown inFIG. 3, a conventional RF transmitter33includes, e.g., an RF waveform generator331and an RF amplifier332. The digital waveform of the RF pulse generated by the sequence controller34is converted from digital to analog by the RF waveform generator331and inputted to the RF amplifier332.

The RF amplifier332includes a correction circuit61, an amplifier62, and a directional coupler63. The RF pulse inputted to the RF amplifier332(hereinafter, referred to as the input RF pulse) is amplified by the amplifier62and outputted. The RF pulse outputted from the amplifier62(hereinafter, referred to as the output RF pulse) is inputted to the directional coupler63, then outputted via the directional coupler63, and then applied to the object via, e.g., the WB coil12. The directional coupler63extracts a part of the output RF pulse and supplies to the correction circuit61. The correction circuit61corrects, i.e., performs the feedback processing, at least one of the phase and gain of the input RF pulse so as to improve the stability and nonlinearity of the output RF pulse of the amplifier62, thereby generating the RF pulse subjected to the correction (hereinafter referred to as the corrected RF pulse).

Recently, imaging methods with very short TE (Echo Time) such as UTE (Ultrashort TE) and ZTE (Zero TE) have come to be used. In this type of imaging method, an ultrashort square wave and/or an ultrashort trapezoidal wave (e.g., pulse width of about 20 to 40 μsec) is used as an RF pulse.

In the feedback processing, an RF pulse is delayed in order to avoid the risk of oscillation in some cases. For this reason, when the input RF pulse is a square wave or trapezoidal wave, its rise is steep and the slew rate (voltage fluctuation value per unit time [V/sec]) is high, and consequently, the input RF pulse is greatly affected by the delay due to the feedback processing.

Accordingly, the waveform of the output RF pulse of the amplifier62differs from the desired or intended waveform, and the area also changes significantly as shown in the upper right ofFIG. 2. When the pulse width of the input RF pulse is short, the output RF pulse may fall before rising completely. When the waveform of the output RF pulse of the amplifier62differs from the desired waveform, deterioration of slice characteristics and insufficient flip angle may be caused, which degrades image quality in some cases.

When a sinc pulse having a pulse width of, e.g., about 1000 μsec is used as the input RF pulse, the influence on image quality is negligible even when its rise is delayed. Thus, in the case of using this type of input RF pulse, it is preferable to perform the feedback processing in order to improve the stability and nonlinearity of the output RF pulse of the amplifier62in view of variations in the characteristics of the amplifier62.

For this reason, the MRI apparatus1according to the present embodiment determines whether the feedback processing is to be performed on the input RF pulse, on the basis of the determination value having been set according to the slew rate of the input RF pulse. The MRI apparatus1may determine the slew rate of the rising edge of the input RF pulse of the amplifier62and control whether the feedback processing is to be performed on the input RF pulse according to the determined slew rate. The control and the calculation of the slew rate are performed by at least one of the RF transmitter33and the sequence controller34.

Next, a description will be given of the configuration and operation of the RF transmitter33and the sequence controller34according to the present embodiment.

FIG. 4is a block diagram for illustrating the configuration according to the first control method of selectively switching the feedback processing to be performed or not according to the slew rate in the present embodiment. The configuration according to the first control method has a configuration in which a slew-rate measurement circuit333and a multiplexer71are added to the conventional configuration.

In the configuration according to the first control method of selectively switching the feedback processing to be performed or not according to a slew rate, the RF transmitter33includes the RF waveform generator331, the RF amplifier332, and the slew-rate measurement circuit333.

The RF waveform generator331includes a circuit equipped with a dedicated or general-purpose processor. The RF waveform generator331may be configured of hardware such as an FPGA or an ASIC. The RF waveform generator331is provided separately from the RF amplifier332and is provided in the output stage of the sequence controller34and also provided in the input stage of the slew-rate measurement circuit333. The RF waveform generator331performs digital-to-analog conversion on the digital waveform generated by the sequence controller34so as to generate the input RF pulse, and then outputs the input RF pulse to the slew-rate measurement circuit333. The RF waveform generator331is an example of the RF waveform generation circuit.

The RF amplifier332further includes a multiplexer71in addition to the configuration of the conventional RF amplifier332shown inFIG. 3. The RF amplifier332is an example of an RF amplification circuit.

The multiplexer71receives the input RF pulse and the corrected RF pulse subjected to the feedback processing by the correction circuit61. The multiplexer71then selects either the input RF pulse without correction or the corrected RF pulse according to the control signal outputted from the slew-rate measurement circuit333, and outputs the selected RF pulse to the amplifier62. The correction circuit61is an example of the correction circuit. The multiplexer71is an example of the switching circuit.

In the configuration according to the first control method shown inFIG. 4, the slew-rate measurement circuit333is provided separately from the RF amplifier332and is connected to the output stage of the RF waveform generator331and also connected to the input stage of the RF amplifier332. The slew-rate measurement circuit333includes a calculation circuit configured to determine the slew rate of the rising edge of the input RF pulse of the amplifier62, and further includes a control circuit configured to control whether the feedback processing is to be performed on the input RF pulse or not, depending on the slew rate of the input RF pulse. The slew-rate measurement circuit333is an example of the measurement circuit, and is also an example of the calculation circuit and the control circuit that are included in the measurement circuit.

Specifically, the slew-rate measurement circuit333calculates the slew rate of the rising edge of the input RF pulse on a real-time basis each time the input RF pulse is inputted from the RF waveform generator331. When the calculated slew rate is smaller than the threshold value, the slew-rate measurement circuit333supplies a control signal (hereinafter referred to as the feedback control signal) to the multiplexer71, whereby the slew-rate measurement circuit333issues a command to the multiplexer71to output the corrected RF pulse. When the calculated slew rate is equal to or larger than the threshold value, the slew-rate measurement circuit333supplies a feedback control signal to the multiplexer71, whereby the slew-rate measurement circuit333instructs the multiplexer71to output the input RF pulse to the amplifier62without correcting the input RF pulse.

According to the configuration according to the first control method shown inFIG. 4, whether to perform the feedback processing on the input RF pulse can be switched according to the slew rate of the input RF pulse. For this reason, when the input RF pulse used in an imaging sequence such as UTE or ZTE is an ultrashort square wave and/or an ultrashort trapezoidal wave, an uncorrected input RF pulse without being subjected to the feedback processing can be inputted to the amplifier62, and thus, deterioration of the rising waveform can be prevented. When the input RF pulse has a low slew rate waveform like a sinc pulse, the corrected RF pulse having been subjected to the feedback processing can be inputted to the amplifier62, and thus, stability and nonlinearity of the output RF pulse of the amplifier62can be reliably improved.

Although a description has been given of the case where the slew-rate measurement circuit333is provided separately from the RF amplifier332in the configuration according to the first control method shown inFIG. 4, the slew-rate measurement circuit333may be provided in the RF amplifier332as its modification. In this case, the configuration equivalent to the configuration shown inFIG. 4can be realized only by replacing the RF amplifier332without changing the respective configurations of the sequence controller34and the RF waveform generator331from their conventional configurations.

FIG. 5is a block diagram for illustrating a configuration according to the second control method of selectively switching the feedback processing to be performed or not according to the slew rate in the present embodiment. The configuration according to the second control method differs from the configuration according to the first control method in that the sequence controller34includes the calculation circuit and the control circuit of the slew-rate measurement circuit333, and the slew-rate measurement circuit333is omitted. Since the other configuration and functions are substantially the same as the configuration according to the first control method shown inFIG. 4, the same reference signs are assigned to the same components as the first control method and duplicate description is omitted.

In the configuration according to the second control method shown inFIG. 5, the sequence controller34determines the conditions of the RF pulse supplied to the amplifier62on the basis of the imaging sequence. Specifically, the sequence controller34generates a digital waveform that is the source of the input RF pulse. Additionally, the sequence controller34includes the calculation circuit and the control circuit described above. Specifically, the sequence controller34calculates the slew rate of the rising edge of the input RF pulse. When the calculated slew rate is smaller than the threshold value, the sequence controller34supplies the multiplexer71with the control signal (hereinafter referred to as the feedback control signal), whereby the sequence controller34instructs the multiplexer71to output the corrected RF pulse. When the calculated slew rate is equal to or larger than the threshold value, the sequence controller34supplies the multiplexer71with the feedback control signal, whereby the sequence controller34instructs the multiplexer71to output the input RF pulse to the amplifier62without correcting the input RF pulse.

The sequence controller34can obtain information on the slew rate of the input RF pulse in order to determine the conditions of the RF pulse supplied to the amplifier62. The information on the slew rate of the input RF pulse includes information on the imaging sequence. The sequence controller34can acquire information on the imaging sequence. When the imaging sequence and the slew rate of the input RF pulse are associated with each other in advance, the sequence controller34may set, for each imaging sequence, a determination value as to whether to correct the input RF pulse according to the slew rate corresponding to the imaging sequence. In this case, when the control circuit included in the sequence controller34acquires the information on the imaging sequence, the control circuit may control whether to correct the input RF pulse, on the basis of the determination value having been set for each imaging sequence. In this case, the sequence controller34does not have to calculate the slew rate directly from the waveform of the input RF pulse in real time and may not include the calculation circuit.

The configuration according to the second control method shown inFIG. 5also achieves the same effects as the configuration according to the first control method shown inFIG. 4. The configuration according to the second control method shown inFIG. 5can be said to be simpler than the configuration according to the first control method shown inFIG. 4in that the slew-rate measurement circuit333is omitted.

FIG. 6is a block diagram for illustrating a configuration according to the third control method of selectively switching the feedback processing to be performed or not according to the slew rate in the present embodiment. The difference in configuration between the first and third control methods is that an FPGA81of the RF waveform generator331achieves the functions of the calculation circuit and the control circuit of the slew-rate measurement circuit333, and the slew-rate measurement circuit333is omitted.

In the configuration according to the third control method shown inFIG. 6, the RF amplifier332has an amplifier62and a directional coupler63, and the correction circuit61and the multiplexer71are included in the RF waveform generator331.

In the configuration according to the third control method shown inFIG. 6, the RF waveform generator331is provided separately from the RF amplifier332and is connected to the output stage of the sequence controller34and also connected to the input stage of the RF amplifier332, similarly to the configuration according to the second control method shown inFIG. 5. In the configuration according to the third control method shown inFIG. 6, the RF waveform generator331includes the correction circuit61and the multiplexer71in addition to the FPGA81and a DAC (Digital-to-Analog Converter)82.

The FPGA81of the RF waveform generator331calculates the slew rate of the rising edge of the input RF pulse on the basis of the digital waveform inputted from the multiplexer71, and outputs the feedback control signal according to the calculated slew rate. The DAC82converts the digital waveform generated by the sequence controller34from digital to analog so as to generate the input RF pulse.

The configuration according to the third control method shown inFIG. 6also achieves the same effects as the configuration according to the first control method shown inFIG. 4. As compared with the respective configurations of the first and second control methods shown inFIG. 4andFIG. 5, the configuration according to the third control method shown inFIG. 6is not required to add the multiplexer71to the RF amplifier332and can use an RF amplification circuit as the general-purpose RF amplifier332.

As a modification of the first to third control methods described above, the delay time (response time) of the feedback processing by the correction circuit61may be controlled according to the type and/or slew rate of the waveform of the input RF pulse. For example, when the input RF pulse is a trapezoidal wave, to use a delay time shorter than a delay time of the sinc wave can reduce the influence of the delay due to the feedback processing while advantages of the feedback processing kept.

According to at least one embodiment described above, whether to perform the feedback processing on the input RF pulse of the amplifier62can be switched according to the slew rate of the input RF pulse.

The processing circuitry in the above-described embodiments is an example of the processing circuitry described in the claims. In addition, the term “processor” used in the explanation in the above-described embodiments, for instance, refer to circuitry such as dedicated or general purpose CPUs (Central Processing Units), dedicated or general-purpose GPUs (Graphics Processing Units), or ASICs (Application Specific Integrated Circuits), programmable logic devices including SPLDs (Simple Programmable Logic Devices), CPLDs (Complex Programmable Logic Devices), and FPGAs (Field Programmable Gate Arrays), and the like. The processor implements various types of functions by reading out and executing programs stored in the memory circuitry.

In addition, instead of storing programs in the memory circuitry, the programs may be directly incorporated into the circuitry of the processor. In this case, the processor implements each function by reading out and executing each program incorporated in its own circuitry. Moreover, although in the above-described embodiments an example is shown in which the processing circuitry configured of a single processor implements every function, the processing circuitry may be configured by combining plural processors independent of each other so that each processor implements each function of the processing circuitry by executing corresponding program. When a plurality of processors are provided for the processing circuitry, the memory medium for storing programs may be individually provided for each processor, or one memory circuitry may collectively store programs corresponding to all the functions of the processors.