JPH11290294A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent degrading of the quality of a tomographic picture by reducing effect as caused by changes in the surrounding environment of a magnetic field of a magnetic field generation means. SOLUTION: A magnetic field sensor 21 is arranged near an electrostatic magnetic field generation magnet 2 to measure the environment F of a magnetic field. Any change in the environment F of the magnetic field is detected by a magnetic field sensor 21 and a detection signal is inputted into a sequencer 7 through an A/D converter 22. The sequencer 7 translates the change in the magnetic field into a frequency and outputs a frequency signal after the changes in the magnetic field to a transmitting system 4 and a receiving system 5. This eliminates the need for a correction magnetic field coil and a correction control circuit to cancel changes in the surrounding environment of the magnetic field thereby significantly improving installing availability and cost.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging apparatus that obtains an image of an examination part of a subject by utilizing a nuclear magnetic resonance (NMR) phenomenon, and in particular, to reduce the influence of a magnetic field environment fluctuation around a magnetic field generating means and tomography The present invention relates to an MRI apparatus capable of preventing image quality deterioration.

[0002]

2. Description of the Related Art In a conventional MRI apparatus, six surfaces around the entire apparatus are made of permalloy, amorphous metal or the like in order to reduce the influence of magnetic field environment fluctuation around the magnetic field generating means and to prevent image quality deterioration of tomographic images. Magnetically shielded with a high permeability material. However, in this method, one M
Regarding the RI apparatus, the material cost and the construction cost are high, and the prevention of the image quality deterioration is not perfect.

On the other hand, in recent years, Japanese Patent Laid-Open Publication No.
An MRI apparatus as described in Japanese Patent Laid-Open Publication No. 5 has been proposed. That is, a magnetic field environment fluctuation around the magnetic field generating means is detected by a magnetic field sensor, an output waveform detected by the magnetic field sensor is amplified by an amplifier, and a correction current is generated by a correction magnetic field coil power supply according to the output signal. The correction current is caused to flow through a loop-shaped correction magnetic field coil provided in parallel in a magnetic circuit for generating a static magnetic field so as to be vertically opposed to each other, thereby generating a correction magnetic field in a direction opposite to the above-described magnetic field environment fluctuation. This cancels out the magnetic field environment fluctuation.

[0004]

In the conventional MRI apparatus, a correction control circuit and a correction magnetic field coil are required in addition to the magnetic field sensor. In recent years, in open MRI apparatuses, the gantry structure has become complicated, and a method of installing a correction magnetic field coil outside the apparatus has been proposed. Thus, there is a problem in that it is difficult to accurately perform the correction due to the restriction on the installation location of the magnetic field sensor.

Accordingly, the present invention solves the above-mentioned problems, and prevents MRI from deteriorating the image quality due to the influence of an external magnetic field.
In order to provide an apparatus, an irradiation electromagnetic wave for causing nuclear magnetic resonance in a subject and a reception for collecting electromagnetic waves emitted by nuclear magnetic resonance of the subject based on a signal from a magnetic field sensor are provided. The image deterioration is prevented by shifting the frequency of the detector 16 of the system by an amount corresponding to the change of the resonance frequency due to the magnetic field environment fluctuation.

[0006]

In order to achieve the above object, an MRI apparatus according to the present invention comprises: a magnetic field generating means for applying a static magnetic field and a gradient magnetic field to a subject; A transmission system that irradiates an electromagnetic wave to cause nuclear magnetic resonance in a nucleus, a reception system that detects an electromagnetic wave emitted by the above-described nuclear magnetic resonance, and an image reconstruction using a signal of the electromagnetic wave detected by the reception system In a magnetic resonance imaging apparatus including a signal processing system for performing an operation and a magnetic field sensor for detecting a change in a magnetic field environment around the magnetic field generating means, based on a signal from the magnetic field sensor,
Means is provided for controlling the frequency of the irradiation electromagnetic wave output from the transmission system and the detection frequency of the reception system to be the same.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of an overall configuration showing an embodiment of an MRI apparatus according to the present invention. This MRI
The apparatus obtains a tomographic image of a subject by using a nuclear magnetic resonance phenomenon. As shown in FIG.
Magnetic field gradient generating system (9, 10), transmitting system 4, and receiving system 5
, A signal processing system 6, a sequencer 7, a central processing unit (CPU) 8, and a magnetic field sensor 21.

The static magnetic field generating magnet 2 generates a uniform static magnetic field around the subject 1 in the body axis direction (horizontal direction) or the direction perpendicular to the body axis (vertical direction). A static magnetic field is generated by a permanent magnet type, a normal conduction type, or a superconducting type magnetic field generating means in a space having a certain extent around the above 1. In FIG. 1, the direction of the static magnetic field is indicated by the direction of arrow A in the figure. The magnetic field gradient generating system includes a gradient magnetic field coil 9 wound in three directions of X, Y, and Z, and a gradient magnetic field power supply 10 for driving each coil. By driving the gradient magnetic field power supply 10, gradient magnetic fields Gx, Gy,
Gz is applied to the subject 1. The slice plane with respect to the subject 1 can be set by the method of applying the gradient magnetic field.

The transmission system 4 irradiates an electromagnetic wave to cause nuclear magnetic resonance to the nuclei of the atoms constituting the living tissue of the subject 1. The transmission system 4 transmits a high-frequency oscillator 11, a modulator 12, and a high-frequency amplifier 13. A high-frequency pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 in accordance with a command from the sequencer 7 and the high-frequency pulse is output from the high-frequency amplifier 1.
After being amplified by 3 and supplied to the high-frequency coil 14a arranged close to the subject 1, the subject 1 is irradiated with electromagnetic waves.

The receiving system 5 detects electromagnetic waves (NMR signals) emitted by nuclear magnetic resonance of atomic nuclei of the living tissue of the subject 1. The receiving system 5 includes a high-frequency coil 14b on the receiving side, an amplifier 15, and a quadrature phase detector. An electromagnetic wave (NMR) of a response of the subject 1 due to an electromagnetic wave emitted from the high-frequency coil 14a on the transmitting side.
Signal) is a high-frequency coil 1 disposed close to the subject 1
4b, input to the A / D converter 17 via the amplifier 15 and the quadrature detector 16 and converted into a digital quantity, and further sampled by the quadrature detector 16 at a timing according to a command from the sequencer 7. The collected data is two-series data, and the signal is sent to the signal processing system 6.

The signal processing system 6 includes a CPU 8, a recording device such as a magnetic disk 18 and a magnetic tape 19, and a CRT.
The CPU 8 performs processing such as Fourier transform, correction coefficient calculation, image reconstruction, and the like.
The signal intensity distribution at an arbitrary cross section or the distribution obtained by performing an appropriate operation on a plurality of signals is imaged and displayed on the display 20.
Is displayed as a tomographic image.

The sequencer 7 operates under the control of the CPU 8 and sends various commands necessary for collecting data of tomographic images of the subject 1 to the transmission system 4, the magnetic field gradient generation systems (9, 10) and the reception system 5, This is a means for generating a sequence for measuring the NMR signal. In FIG. 1, the high-frequency coil 14a on the transmitting side and the high-frequency coil 1 on the receiving side
4 b and the gradient magnetic field coils 9, 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 magnetic field sensor 21 includes the static magnetic field generating magnet 2
, A magnet environment (magnetic field fluctuation) F in the static magnetic field direction (direction of arrow A) is detected. The A / D converter 22 converts a detection signal from the magnet sensor 21 and sends it to the sequencer 7.

The magnetic resonance frequency depends on the static magnetic field generating magnet 2.
When the magnetic field environment F changes, the magnetic field strength in the measurement space of the static magnetic field generating magnet 2 also changes, and the resonance frequency also changes. Therefore, if the magnetic field environment F changes during measurement, the imaging position shifts or the imaging position is not determined, resulting in image blur.

Accordingly, in the present invention, as shown in FIG. 2, a change in the magnetic field environment F is detected by the magnetic field sensor 21 and input to the sequencer 7 via the A / D converter. Sequencer 7
The CPU 8 or its own MPU calculates a change in resonance frequency from a change in the magnetic field environment F, and calculates a frequency of an electromagnetic wave for causing magnetic resonance. A signal corresponding to the obtained resonance frequency is output from the sequencer 7, and a signal output from the high-frequency oscillator 11 is modulated by the modulator 12 based on the output signal of the sequencer 7.
The subject 1 is irradiated with an electromagnetic wave (RF pulse) by amplifying it at 3 and applying it to the transmission system high-frequency coil 14a.

On the other hand, the receiving system high-frequency coil 14b detects an NMR signal from the subject 1. The detection signal (NMR signal) is amplified by the preamplifier 23 and the amplifier 24 and then input to the quadrature detector 16.

The NMR signal used as image information is
The frequency is converted by the mixer 25 in the quadrature phase detector 16, and the signal of the resonance frequency determined by the sequencer 7 is input to the mixer 25. As a result, the frequency of the transmission signal (RF pulse) and the frequency of detection become the magnetic field environment F
Will be the same even if the position changes,
Image blur does not occur.

The resonance frequency input to the transmission system and the reception system is best corrected every time a pulse sequence of NMR signal collection is executed. However, it is about once every several pulse sequences. By doing so, the effect of improving the image can be obtained.

Further, in this embodiment, the detection direction of the magnetic field sensor 21 is the same as the direction in which the static magnetic field is generated. However, the same result can be obtained by detecting a direction perpendicular to the static magnetic field direction.

If a circuit for varying the response speed is provided in the sequencer 7 (the rising speed is made faster or slower), the blur of the image quality can be further suppressed.

[0021]

As described above, the present invention is configured as described above, so that the magnetic field sensor provided near the static magnetic field generating means detects the fluctuation of the magnetic field environment around the magnetic field generating means, and the variation of the resonance frequency is detected. Accordingly, the sequencer controls the irradiation electromagnetic wave and the frequency of the detector in the receiving system to be the same, so that the influence of the fluctuation of the surrounding magnetic field environment can be reduced, and the image quality can be prevented from deteriorating.

Further, unlike the related art, the correction magnetic field coil and the correction magnetic field are not required, so that the installation property and the cost can be greatly improved.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an entire magnetic resonance imaging apparatus according to the present invention.

FIG. 2 is a configuration diagram of frequency control in the embodiment.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 subject 2 static magnetic field generating magnet 4 transmission system 5 reception system 6 signal processing system 7 sequencer 8 CPU 9 gradient magnetic field coil 14a high-frequency coil on transmission side 14b high-frequency coil on reception side 21 magnetic field sensor 22 ADC 25 mixer

Claims (1)

[Claims] 1. A magnetic field generating means for applying a static magnetic field and a gradient magnetic field to a subject, a transmission system for irradiating an electromagnetic wave to cause nuclear magnetic resonance in nuclei of atoms constituting a living tissue of the subject, A receiving system for detecting electromagnetic waves emitted by nuclear magnetic resonance, a signal processing system for performing image reconstruction operations using signals of the electromagnetic waves detected by the receiving system, and a magnetic field environment fluctuation around the magnetic field generating means. Based on the magnetic field sensor to be detected and the detection signal from the magnetic field sensor when the magnetic field environment changes, the frequency of the radiated electromagnetic wave output from the transmission system and the detection frequency of the reception system are the same. A magnetic resonance imaging apparatus, comprising: means for controlling the magnetic resonance imaging apparatus.