JP3335710B2 - Magnetic resonance imaging equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a nuclear magnetic resonance (hereinafter, referred to as "nuclear magnetic resonance").
The present invention relates to a magnetic resonance imaging (hereinafter, referred to as MRI) apparatus for obtaining a tomographic image of a desired portion of a subject using a phenomenon (referred to as NMR), and particularly to an MRI apparatus with improved safety.

[0002]

2. Description of the Related Art An MRI apparatus irradiates a subject placed in a static magnetic field with an electromagnetic wave from a high-frequency coil to cause nuclear magnetic resonance in the nuclei of atoms constituting living tissue, and generates a magnetic resonance signal ( This is hereinafter referred to as an NMR signal) by a receiving coil, and Fourier transforms the received NMR signal to reconstruct an image, which is widely used to obtain a tomographic image at an arbitrary position of a subject.

[0003] Such a magnetic resonance imaging apparatus includes:
As shown in FIG. 3, a static magnetic field generating magnet 2 for applying a static magnetic field to the subject 1, a magnetic field gradient generating system 3 for applying a gradient magnetic field to the subject 1, and a nucleus in an atomic nucleus of a living tissue of the subject 1 A sequencer 7 for repeatedly applying a high-frequency pulse for causing magnetic resonance in a predetermined pulse sequence, and a high-frequency pulse for causing nuclear nuclei of atoms constituting the living tissue of the subject 1 to undergo nuclear magnetic resonance by the high-frequency pulse from the sequencer 7 A transmission system 4 for irradiating a magnetic field, a reception system 5 for detecting an NMR signal emitted by nuclear magnetic resonance, and a reception system 5
And a signal processing system 6 for performing an image reconstruction operation using the NMR signal detected by the NM.
The tomographic image is obtained by repeatedly measuring the R signal.

The transmitting system 4 includes a high-frequency oscillator 11, a modulator 12, a high-frequency amplifier 13, and a high-frequency coil 14, and the high-frequency pulse generated by the high-frequency oscillator 11 is
After being modulated to a predetermined frequency by the modulator 12, it is amplified by the high frequency amplifier 13 and applied to the high frequency coil 14. Here, if the irradiation power by the high-frequency coil 14, that is, the output of the electromagnetic wave is too large, the subject 1 may be adversely affected, and SAR (Specific Abs) is used as a safety standard.
orption rate) (irradiation power per unit weight, w / kg). Therefore, in the conventional MRI apparatus, the high-frequency amplifier 13 uses one whose irradiation power does not exceed the SAR even when used at its maximum capacity, or the output of the high-frequency amplifier 13 A method of controlling by software is applied so that an input signal not exceeding the input signal is supplied to the amplifier.

[0005]

However, if the maximum capability of the high-frequency amplifier is limited to a value that does not exceed the SAR, it is not possible to cope with the high-performance measurement of the MRI apparatus. Further, in the method of limiting the input to the high-frequency amplifier by software, if there is a software error or runaway, the SAR may be exceeded, and safety cannot be ensured.

SUMMARY OF THE INVENTION The present invention has been made to solve such a conventional problem. In order to cope with the enhancement of the function of an MRI apparatus, the safety of a high-frequency amplifier is maintained while maintaining a relatively high capability. It is an object of the present invention to provide an MRI apparatus having an improved MRI.

[0007]

An MRI apparatus according to the present invention for achieving the above object is provided for causing nuclear magnetic resonance to occur in nuclei of atoms constituting a living tissue of a subject placed in a static magnetic field. In a magnetic resonance imaging apparatus including a high-frequency coil for applying a high-frequency pulse in a predetermined pulse sequence and a high-frequency amplifier for amplifying the high-frequency pulse, the irradiation power of the high-frequency coil must not exceed a predetermined value. It is provided with two protection circuits for interrupting the output from the high-frequency amplifier to the high-frequency coil. First
The protection circuit detects the output power of the high-frequency amplifier and generates an error signal when the detected output power exceeds a preset value. Switch means for interrupting the output to the coil . In addition, the second security
The protection circuit detects a power supply current of the high-frequency amplifier, and generates a error signal when the detected power supply current exceeds a preset value, and a high-frequency coil of the high-frequency amplifier based on the error signal from the comparator. Switch means for interrupting the output to the switch .

[0008]

The protection circuit of the high-frequency amplifier compares the value obtained by detecting the power supply current and converting it into a voltage with a reference voltage set in advance by another circuit using a comparator, and determines that the voltage value corresponding to the power supply current is equal to the reference voltage. Is exceeded, an error signal is generated, the power supply of the high frequency amplifier and the bias ON signal are cut off via the switch means, and the operation is stopped. The protection circuit of the high-frequency amplifier compares the output power of the high-frequency amplifier into a voltage value with a predetermined reference voltage value using a comparator, and the voltage value corresponding to the output power is the reference voltage value. Also, an error signal is generated when the frequency exceeds the limit, and the operation of the high-frequency amplifier is similarly stopped via the switch means. By appropriately setting the reference values of these voltages,
It is possible to prevent the irradiation power from exceeding the safety standard SAR. Further, by providing two protection circuits, one that functions with the detected output power and one that functions with the power supply current, the protection function can be ensured.

[0009]

Embodiments of the present invention will be described below in detail with reference to the drawings. A magnetic resonance imaging (MRI) apparatus to which the present invention is applied obtains a tomographic image of a subject using an NMR phenomenon as shown in the block diagram of FIG. And a magnetic field gradient generating system 3,
It comprises a transmission system 4, a reception system 5, a signal processing system 6, a sequencer 7, and a central processing unit (CPU) 8.

The static magnetic field generating magnet 2 generates a uniform static magnetic field around the subject 1 in an arbitrary direction.
A permanent magnet type, a normal conduction type, or a superconducting type magnetic field generating means is disposed in a space having an expanse around. The magnetic field gradient generating system 3 includes a gradient magnetic field coil 9 wound in Cartesian coordinate axis directions orthogonal to each other, that is, three axes of X, Y, and Z, and a gradient magnetic field power supply 10 for driving each gradient magnetic field coil. By driving the gradient magnetic field power supplies 10 of the respective coils in accordance with a command from a sequencer 7 described later, X, Y, and Z gradient magnetic fields are applied to the subject 1. A slice plane for the subject 1 can be set by how to apply the static magnetic field.

The transmission system 4 irradiates a high-frequency magnetic field to cause nuclear magnetic resonance of the nuclei of the atoms constituting the living tissue of the subject 1 by high-frequency pulses. The high-frequency oscillator 11, the modulator 12, and the high-frequency amplifier 13 And a high-frequency coil 14 on the transmission side. The high-frequency pulse output from the high-frequency oscillator 11 is transmitted to the modulator 1 according to a command from the sequencer 7.
2, the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 13, and then supplied to the high-frequency coil 14 arranged close to the subject 1, so that the subject is irradiated with the electromagnetic wave. It has become.

The receiving system 5 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of the nucleus of the living tissue of the subject 1, and includes a high-frequency coil 15 on the receiving side, an amplifier 16, a quadrature phase detector. 17 and an A / D converter 18, and an electromagnetic wave (NMR signal) of the response of the subject 1 due to the electromagnetic wave radiated from the high-frequency coil 14 on the transmitting side is transmitted to the high-frequency coil 15 disposed close to the subject 1. A / D via the amplifier 16 and the quadrature phase detector 17
The data is input to the converter 18, converted into a digital quantity, and further converted into two series of collected data sampled by the quadrature phase detector 17 at a timing according to a command from the sequencer 7, and the signal is sent to the signal processing system 6. It has become.

The signal processing system 6 comprises a CPU 8, a recording device such as a magnetic disk 19 and a magnetic tape 20, a display 21 such as a CRT, and an input device 22 such as a keyboard. Processing such as image reconstruction is performed, and a signal intensity distribution of an arbitrary cross section or a distribution obtained by performing an appropriate operation on a plurality of signals is imaged and displayed on the display 21 as a tomographic image. In FIG. 3, the high-frequency coils 14 and 15 on the transmission side and the reception side and the gradient magnetic field coil 9 are arranged in the magnetic field space of the static magnetic field generating magnet 2 arranged in the space around the subject 1.

The sequencer 7 repeatedly applies a high-frequency pulse for causing nuclear magnetic resonance to the nuclei of the atoms constituting the living tissue of the subject 1 in a predetermined pulse sequence. Various commands necessary for data collection of the tomographic image are transmitted to the transmission system 4, the magnetic field gradient generation system 3, and the reception system 5. Here, since the high-frequency pulse applied to the subject 1 by the transmission system 4 does not exceed the safety standard SAR (Specific Absorption Rate), this MRI apparatus includes the high-frequency amplifier 13 of the transmission system 4 shown in FIGS. A protection circuit 30 is provided as shown in FIG. The protection circuit 30 detects the output power of the high-frequency amplifier 13, generates an error signal when the detected output power exceeds a preset value, and cuts off the output of the high-frequency amplifier 13 to the high-frequency coil 14. The power supply current of the first system and the high-frequency amplifier is detected, and when the detected power supply current exceeds a preset value, an error signal is generated and the output of the high-frequency amplifier 13 to the high-frequency coil 14 is cut off. The second system is composed of two systems.

The protection circuit of the first system is a high-frequency amplifier 1
Rectifier 31 which converts the output of 3 into a voltage and averages it
And the rectifier 31 compares the output of the rectifier 31 with the reference voltage.
And a first reference voltage generation circuit 33 that generates an error signal when the output of the first reference voltage exceeds a reference voltage. The protection circuit of the second system includes a detector 34 for detecting the current of the power supply 23, a current-voltage converter 35 for converting the current detected by the detector 34 to a voltage,
A second comparator for comparing the output of the voltage converter with the reference voltage and generating an error signal when the output of the current-to-voltage converter exceeds the reference voltage; and a second reference voltage generating circuit. And

As the detector 34, a known current transformer or the like is used, and it detects either the primary or secondary current value of the power transformer or the output current value when the power source 23 has a stabilizing circuit. You may make it.
Since the operation of the high-frequency amplifier is a pulse operation, the current-voltage converter 35 outputs a voltage obtained by averaging this voltage.

The outputs (error signals) of the first and second comparators 32 and 36 are both input to the high-frequency amplifier 13 via an OR circuit 38 and provided between the AC input 24 and the power supply 23, respectively. It is supplied to the relays SSR1 and SSR2, which are the switching means, to drive the relays. The output of the OR circuit 38 is input to a switching transistor TR that is a switch. The switching transistor TR switches the operational amplifier 25 for controlling the bias signal of the high frequency amplifier 13. That is, the collector of the switching transistor TR is connected to the (+) terminal of the operational amplifier 25 for controlling the bias signal of the high-frequency amplifier 13, and when the switching transistor TR is turned on, the collector side drops to the ground GND. The output of the operational amplifier 25 becomes 0, and the bias O
The N signal is shut off.

The reference voltage generation circuits 33 and 37 are set to values such that the irradiation power from the high-frequency coil does not exceed SAR. In such a configuration, first, a predetermined slice plane is set by the static magnetic field generating magnet 2 and the magnetic field gradient generating system 3, and a static magnetic field of about 0.02 to 2 Tesla is applied to the subject 1. At this time, the nuclear spin of the atoms in the subject 1 precesses at a frequency (Larmor frequency) determined by the strength of the static magnetic field.

An electromagnetic wave having a frequency equal to the Larmor frequency of the nucleus to be measured is applied by the high-frequency coil 14 in the transmission system 4 to cause the nucleus to generate nuclear magnetic resonance. When the high-frequency magnetic field is discontinued, the spin returns to the original low energy state with a time constant corresponding to each state, and the electromagnetic waves (NMR signals) emitted at this time are received by the high-frequency receiving coil 15 and amplified by the amplifier 16. After shaping the waveform,
It is digitized by the A / D converter 18 and sent to the CPU 8. C
The PU 8 reconstructs an image based on the data and displays a tomographic image of the subject 1 on the display 21.

Here, in the transmission system 4, the high-frequency pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 in accordance with a command of the sequencer 7, and the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 13. The voltage is supplied to the high-frequency coil 14. The protection circuit 30 of the high-frequency amplifier 13 detects the current of the power supply 23, and a value obtained by converting the current into a voltage is larger than the voltage value set by the second reference voltage generation circuit 37. In this case, the comparator 36 outputs an error signal. The error signal activates the relay SSR2 to cut off the power supply of the high frequency amplifier 13. When an error signal is input to the switching transistor TR via the OR circuit 38, the switching transistor TR is turned on. Thereby, the operational amplifier 2 for controlling the bias signal
The output of 5 becomes 0, and the bias ON signal is cut off.
Therefore, the high-frequency amplifier 13 is stopped, and a current of a predetermined value or more is prevented from being supplied to the high-frequency coil 14.

The protection circuit 30 detects the output of the high-frequency amplifier 13 and detects the voltage of the output from the first reference voltage generation circuit 3.
If the voltage value is higher than the voltage value set in step 3, an error signal is output from the comparator 32. The error signal activates the relay SSR1 to cut off the power supply of the high frequency amplifier 13. When an error signal is input to the switching transistor TR via the OR circuit 38, the switching transistor TR is turned on and the bias ON signal is cut off. This prevents a current equal to or greater than a predetermined value from being supplied to the high-frequency coil 14. In this case, it is possible to stop the operation of the amplifier by the error signal, and at the same time, activate an alarm such as a warning light or a warning sound (not shown).

As described above, when the power supply current exceeds a predetermined value and when the output of the high-frequency amplifier exceeds a predetermined value, the high-frequency amplifier is stopped, so that the power of the electromagnetic wave applied to the subject meets the safety standard SAR. Will not exceed
Safety can be ensured. Further, by using two protection circuits together as the protection circuit, the operation can be ensured and the safety can be improved.

In the above embodiment, the power supply is cut off and the bias O of the amplifier is controlled by the error signal output from the protection circuit.
Although the N signal is cut off together, either the power supply or the bias ON signal may be cut off.

[0024]

As is apparent from the above description, according to the MRI apparatus of the present invention, the protection circuit for preventing the irradiation power from exceeding a predetermined value in the high-frequency amplifier of the high-frequency coil for irradiating the subject with the electromagnetic wave. Is provided, there is no possibility that the subject is irradiated with electromagnetic waves exceeding the safety standard SAR, and M
The safety of the RI device can be improved. When two protection circuits are used as the protection circuit, the safety can be further ensured. Further, according to the MRI apparatus of the present invention, a high-performance power supply can be used as the power supply of the high-frequency amplifier, so that the measurement function can be enhanced.

[Brief description of the drawings]

FIG. 1 is a block diagram of a protection circuit of a magnetic resonance imaging apparatus according to the present invention.

FIG. 2 is a block diagram of a transmission system of the magnetic resonance imaging apparatus of the present invention.

FIG. 3 is a block diagram showing the overall configuration of the magnetic resonance imaging apparatus of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Subject 2 ... Magnetic field generator 3 ... Magnetic field gradient generation system 4 ... Transmission system 5 ... Reception system 6 ... Signal processing system 7 ... Sequencer 8 ... CPU 13 ... High frequency amplifier 14 ... High frequency coil 23 ... High frequency amplifier power supply 30 ... Protection circuit 32: First comparator 36: Second comparator SSR1, SSR2 ... Relay (switch means) TR: Switching transistor (switch means)

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-1-185253 (JP, A) JP-A-4-352943 (JP, A) JP-A-5-237078 (JP, A) JP-A-5-37078 317287 (JP, A) JP-A-4-104806 (JP, U) JP-A-4-105712 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) A61B 5/055 G01R 33 / 20

Claims (2)

(57) [Claims] 1. A method for causing nuclear magnetic resonance to occur in nuclei of atoms constituting a living tissue of a subject placed in a static magnetic field,
In a magnetic resonance imaging apparatus including a high-frequency coil for applying a high-frequency pulse in a predetermined pulse sequence, and a high-frequency amplifier for amplifying the high-frequency pulse, the output power of the high-frequency amplifier is detected.
High frequency amplification when force power exceeds a preset value
Protection circuit to cut off the output to the high-frequency coil of the heater
And the power supply current of the high-frequency amplifier is detected and detected.
High frequency amplification when the power supply current exceeds a preset value
A second protection circuit for interrupting output to a high-frequency coil of the device. 2. The power supply for a high-frequency amplifier according to claim 2, wherein :
Source current is detected and the detected power supply current is preset
A comparator that generates an error signal when the value is exceeded
High-frequency coil of high-frequency amplifier by error signal from
2. A magnetic resonance imaging apparatus according to claim 1, further comprising: switch means for interrupting output to said controller.