JP3185153B2 - Magnetic resonance imaging equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to nuclear magnetic resonance (NMR).
A magnetic resonance imaging apparatus (hereinafter referred to as an "MRI apparatus") for obtaining a tomographic image of an examination part of a subject by using a phenomenon, and particularly, an apparatus having a static magnetic field generating means of a permanent magnet type, which generates noise due to extraneous electromagnetic waves around the apparatus. The present invention relates to an MRI apparatus capable of easily reducing the influence and preventing image quality deterioration of an obtained tomographic image.

[0002]

2. Description of the Related Art An MRI apparatus uses an NMR phenomenon to measure a nuclear spin density distribution, a relaxation time distribution, and the like at a desired inspection site in a subject, and images an arbitrary cross section of the subject from the measurement data. To display. Then, in order to obtain this tomographic image, in a state where a static magnetic field and a gradient magnetic field are applied to the subject, an electromagnetic wave is generated from the transmission system to cause nuclear magnetic resonance in the nuclei of the atoms constituting the living tissue of the subject. Irradiation and electromagnetic waves emitted by the above nuclear magnetic resonance are detected by a receiving system. Therefore, in a normal state, the image quality of the obtained tomographic image may be degraded due to the influence of noise due to external electromagnetic waves around the apparatus.

In order to cope with this, the conventional MRI apparatus is designed to prevent external electromagnetic waves such as electronic equipment and radio communication equipment installed in the vicinity from being mixed as noise into the receiving system. Aluminum around the entire six sides
It was electromagnetically shielded by surrounding it with an electromagnetic wave shielding material such as copper or steel plate. Such a room is referred to as a shield room, and has a shielding performance of, for example, about 60 dB, 80 dB, and 100 dB, and can exhibit a desired electromagnetic shielding effect against an external electromagnetic wave measured at the time of design.

[0004]

However, such a measure for preventing external electromagnetic waves from being mixed by the shield room is effective for the external electromagnetic waves at the level at the time of the initial measurement. If the level of the extraneous electromagnetic wave becomes higher than the initial level after installation and use, the extraneous electromagnetic wave cannot be shielded, and the image quality of the obtained tomographic image may deteriorate.
On the other hand, to prevent the image quality degradation of the tomographic image,
There was no other way but to improve the shielding performance of the shield room. In this case, it must be modified to reinforce the shield effect of the above shield room, or a new shield room must be constructed according to the level of recent external electromagnetic waves, which requires a large amount of cost and a long construction period Met. Even after taking measures in this way, when the level of extraneous electromagnetic waves becomes higher thereafter, the same countermeasures must be taken again, and further costs and construction time are required. In the above shield room, the effect of shielding against external electromagnetic waves cannot be said to be perfect, and there is a problem that deterioration in image quality of tomographic images cannot be sufficiently prevented.

In view of the above, the present invention addresses the above problems and provides an MRI apparatus which has a static magnetic field generating means of a permanent magnet type and which can easily reduce the influence of noise due to extraneous electromagnetic waves around the apparatus. The purpose is to provide.

[0006]

In order to achieve the above-mentioned object, an MRI apparatus according to the present invention comprises: a static magnetic field generating means using a permanent magnet for applying a static magnetic field to a subject; a gradient magnetic field generating means for applying a gradient magnetic field; A transmission system that irradiates an electromagnetic wave to cause nuclear magnetic resonance in atomic nuclei of constituent atoms of the living tissue of the subject, a reception system that detects an electromagnetic wave emitted by the nuclear magnetic resonance, and the detected electromagnetic wave A signal processing system for performing an image reconstruction operation using the same; a temperature generator for heating or cooling the static magnetic field generating means; a temperature detector for detecting the temperature of the heated or cooled static magnetic field generating means; the magnetic resonance imaging apparatus which receives the temperature detection signal which is composed of a heat insulating circuit to maintain the set temperature of the static magnetic field generating means to control the temperature generator, the temperature detection
The output device has holes in one or more locations of the static magnetic field generating means.
It is arranged in it .

[0007]

In the MRI apparatus thus constructed, the temperature detector is disposed in one or a plurality of holes formed in the static magnetic field generating means, so that the heat retaining temperature of the static magnetic field generating means can be directly determined. It operates to detect and detect the temperature with high accuracy. Accordingly, temperature control for shifting the resonance frequency of the MRI apparatus from the noise frequency due to the external electromagnetic wave can be performed with high accuracy.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to the accompanying 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 apparatus obtains a tomographic image of a subject by utilizing a nuclear magnetic resonance (NMR) phenomenon. As shown in FIG.
, A magnetic field gradient generation system (2, 3), a transmission system 4, a reception system 5, a signal processing system 6, a sequencer 7, a central processing unit (CPU) 8, a temperature generator 9, a temperature detector Vessel 10;
And a heat retaining circuit 11.

The static magnetic field generating magnet 1 serves as a means for generating a uniform static magnetic field around the subject 12 in the body axis direction (horizontal direction) or the direction perpendicular to the body axis (vertical direction). A permanent magnet type magnetic field generating means is arranged in a certain space around the subject 12. 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 comprises a gradient magnetic field coil 2 wound in three directions of X, Y and Z, and a gradient magnetic field power supply 3 for driving each coil. By driving the gradient magnetic field power supply 3 of the coil, the gradient magnetic fields Gx, Gy and Gz in the three axes of X, Y and Z are applied to the subject 12. The slice plane with respect to the subject 12 can be set by the method of applying the gradient magnetic field.

The transmitting system 4 irradiates an electromagnetic wave to cause nuclear magnetic resonance in the nuclei of the atoms constituting the living tissue of the subject 12, and includes a high-frequency oscillator 13, a modulator 14, a high-frequency amplifier 15, and a transmitting side. The high-frequency pulse output from the high-frequency oscillator 13 is amplitude-modulated by the modulator 14 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 in step 5, the electromagnetic wave is applied to the high-frequency coil 16a disposed close to the subject 12, so that the subject 12 is irradiated with electromagnetic waves.

The receiving system 5 is an electromagnetic wave (NMR signal) emitted by nuclear magnetic resonance of nuclei of living tissue of the subject 12.
Which comprises a high-frequency coil 16b on the receiving side, an amplifier 17, a quadrature phase detector 18 and an A / D converter 19, and is provided by an object irradiated by electromagnetic waves emitted from the high-frequency coil 16a on the transmitting side. The electromagnetic wave (NMR signal) of the response 12 is a high-frequency coil 1 arranged close to the subject 12.
6b, input to an A / D converter 19 via an amplifier 17 and a quadrature detector 18 and converted into a digital quantity, and further sampled by the quadrature detector 18 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 20 and a magnetic tape 21, a CRT,
And the like, and the CPU 8 performs processing such as Fourier transform, correction coefficient calculation, image reconstruction, and the like.
The signal intensity distribution of 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 22.
Is displayed as a tomographic image.

The sequencer 7 operates under the control of the CPU 8 and sends various commands necessary for data collection of tomographic images of the subject 12 to the transmission system 4, the magnetic field gradient generation systems (2, 3) and the reception system 5. This is a means for sending and generating a sequence for measuring the NMR signal. In FIG. 1, the high-frequency coil 16a on the transmitting side, the high-frequency coil 16b on the receiving side, and the gradient coils 2
Are arranged in the magnetic field space of the static magnetic field generating magnet 1 arranged in the space around the.

The temperature generator 9 heats or cools the static magnetic field generating magnet 1, and is composed of, for example, a planar heater. As shown in FIG. Is fixed to an inner surface of a heat insulating material 25 made of, for example, styrene foam or sponge, which covers the entirety of the yoke 23 for supporting the magnetic pole piece 24 for improving the uniformity of the static magnetic field. Note that the number of the temperature generators 9 and the total amount of power thereof are determined by the room temperature of the place where the MRI apparatus is installed, the heat retention set temperature, the heat insulation capacity of the heat insulator 25, and the like. In FIG. 2, reference numeral 26 denotes an aluminum plate that reflects heat generated from the planar heater as the temperature generator 9.

Further, the temperature detector 10 detects the temperature of the static magnetic field generating magnet 1 heated or cooled by the temperature generator 9, for example, from a resistance whose resistance changes linearly with a change in temperature. As shown in FIG. 2, for example, a hole is formed in one or a plurality of places on the plate surface of the static magnetic field generating magnet 1, and each is installed therein. The heat retaining circuit 11 receives the detection signal output from the temperature detector 10 and controls the temperature generator 9 to thereby keep the temperature of the static magnetic field generating magnet 1 constant. Is provided with a power supply unit 27 for inputting a detection signal from the temperature detector 10 and supplying a current to the temperature generator 9.

In the MRI apparatus configured as described above, the resonance frequency at which nuclear magnetic resonance occurs is determined by the intensity of the static magnetic field generated by the static magnetic field generating magnet 1, for example, 0.2 T
For a magnet with a static magnetic field strength of (Tesla), the frequency is 8.515 MHz.
In this case, the static magnetic field generating magnet 1 using a permanent magnet is usually
It has a certain temperature coefficient and shows a temperature characteristic that the resonance frequency changes with a change in temperature. The above temperature coefficient is, for example, -1000 ppm / ° C for a neodymium-based permanent magnet.
When this is applied to a magnet having a static magnetic field strength of 0.2 T, the result is −2 G (Gauss) / ° C. The change in the resonance frequency due to the change in temperature at this time is, for example, -8.5 KHz / ° C.
Becomes FIG. 4 shows an example of such a temperature characteristic.

Here, in the present invention, the heat retaining circuit 11 includes a temperature setting circuit 28 therein. The temperature setting circuit 28 arbitrarily sets the heat retaining temperature of the static magnetic field generating magnet 1 and sends a control signal to the temperature generator 9 so as to reach the set temperature, as shown in FIG. , the plurality of resistors R _1, R _2, each resistance value corresponds to the temperature to be set is connected in parallel to the supply line of the voltage Vcc is different, ..., and Rn, the resistors R _1,
R _2, ..., set to select the temperature to be set are connected in series to Rn switches S _1, S _2, ..., Sn and, these setting switch S _1, S _2, ..., signal and the temperature of the Sn consisting comparator 29 for comparing to input detection signals from the detector 10 sends a control signal Sg _1. For example, the first resistor R ₁ has a resistance value corresponding to the set temperature of 30 ° C., the second resistor R ₂ has a resistance value corresponding to the set temperature of 31 ° C.,. Resistance Rn is set temperature
It shall have a resistance value corresponding to 50 ° C. Also, for example, the first setting switch S ₁ selects a set temperature of 30 ° C.,
The second setting switch S ₂ selects the set temperature of 31 ° C.,.
Further, the n-th setting switch Sn selects the set temperature of 50 ° C. Then, each of the setting switches S _{1 to} Sn
In the resistance value of each resistor R ₁ ~Rn selected, the relationship between the resistance value at the detected temperature of the temperature detector 10, when a voltage inputted to the comparator 29 exceeds the Vcc / 2, the comparator and it is configured to output a control signal Sg ₁ to 29. The above comparator 2
The control signal Sg ₁ output from the power supply 9
To be entered.

Next, the operation of the thus configured heat retention circuit 11 of the present invention will be described. First, the resistance of the temperature detector 10 increases as the detected temperature decreases, and decreases as the detected temperature increases. In this state, the temperature and, for example, 31 ° C. to be set, and second setting switches S ₂ only ON applicable. At this time, when the temperature of the static magnetic field generating magnet 1 is lower than the set temperature, this is detected by the temperature detector 10 and the resistance value corresponding to the detected temperature is indicated. Then, over the a second setting the resistance value of a resistor R ₂ selected by the switch S _2, the relationship between the resistance value at that time of the temperature detector 10, the voltage input to the comparator 29 is the Vcc / 2 becomes possible, the control signal Sg ₁ from the comparator 29 is output. Thereafter, the control signal S
g ₁ is input to the power supply unit 27, current is supplied as the final control signal Sg ₂ to a temperature generator 9. Thus, the temperature generator 9 generates heat and heats the static magnetic field generating magnet 1. Therefore, the temperature of the static magnetic field generating magnet 1 gradually increases.

Conversely, when the temperature of the static magnetic field generating magnet 1 is higher than the temperature of 31 ° C. set by the second setting switch S ₂ , this is detected by the temperature detector 10 and the resistance value corresponding to the temperature is detected. Is shown. At this time, from the relationship between the resistance value of the second resistor R _{2 and} the resistance value of the temperature detector 10 at that time,
Voltage input to the comparator 29 becomes smaller than Vcc / 2, the control signal Sg ₁ from the comparator 29 is not output. Therefore, no current is supplied to the temperature generator 9,
No fever. At this time, the temperature of the static magnetic field generating magnet 1 gradually decreases.

[0021] In this way, the static magnetic field generating magnet 1 is made to the second set temperature that is set on the switch S _2, stabilized. Then, it is assumed that the resonance frequency of nuclear magnetic resonance is 8.515 MHz at a temperature t ° C., for example, at a temperature t ° C. in a state where the temperature is stable, and that there is no influence of noise due to an external electromagnetic wave.

However, while the MRI apparatus is being installed and used, the environment of the surrounding external electromagnetic waves changes, and electromagnetic waves from outside such as new surrounding electronic devices and wireless communication devices become noise, and the above-mentioned resonance frequency is changed. Of about 8.515 MHz. In this case, the thermal insulation circuit 1 is used by utilizing the temperature characteristics of the static magnetic field generating magnet 1 by the permanent magnet.
One of the setting switches S _{1 to} Sn of the temperature setting circuit 28 is turned ON, and the set temperature for the static magnetic field generating magnet 1 is set, for example, from t ° C. to (t−1) ° C. or (t) in FIG.
+1) It shifts like ℃. Then, by the same operation as described above, the temperature of the static magnetic field generating magnet 1 is controlled, and is stabilized at (t−1) ° C. or (t + 1) ° C. after a certain time. As a result, as apparent from the temperature characteristic graph shown in FIG. 4, the resonance frequency of nuclear magnetic resonance is 8.506 MHz or 8.56 MHz, for example.
It shifts like 24MHz. Therefore, it is possible to prevent reception of noise due to extraneous electromagnetic waves mixed in with the resonance frequency of 8.515 MHz at the initial set temperature.

In the above description, the temperature generator 9 is
Although mainly described as a heating element such as a planar heater, the present invention is not limited to this, and a cooling element may be used when the set temperature for the static magnetic field generating magnet 1 is lower than the ambient temperature such as room temperature. At this time, the static magnetic field generating magnet 1 is cooled by the cooling body as the temperature generating body 9 according to the above set temperature. Alternatively, both the heating element and the cooling element may be used as the temperature generator 9. In this case, the operation can be performed with the temperature set for the static magnetic field generating magnet 1 being equal to room temperature.

[0024]

Further, in FIG. 3, the temperature setting circuit 28
Represents a plurality of resistors R ₁ , R ₂ ,
, Rn and the same number of setting switches S ₁ , S ₂ ,.
Although Sn is used in combination, the present invention is not limited to this, and different resistance values may be arbitrarily set according to the set temperature using a variable resistor.

In FIG. 1, only one system is provided for the temperature generator 9, the temperature detector 10, and the heat retaining circuit 11. However, two or more systems may be provided depending on the performance and scale of the MRI apparatus. By doing so, the temperature stability of the static magnetic field generating magnet 1 can be increased.

[0027]

The present invention has been configured as described above.
By arranging the temperature detector 10 in a hole formed at one or a plurality of positions of the static magnetic field generating means (1), the temperature keeping temperature of the static magnetic field generating means (1) is directly detected, and the temperature is detected. It can be detected with high accuracy. Therefore, temperature control for shifting the resonance frequency of the MRI apparatus from the noise frequency due to the external electromagnetic wave can be performed with high accuracy. this
This prevents extraneous electromagnetic waves from entering as noise
And easily reduce the effects of noise from extraneous electromagnetic waves
be able to. Therefore, it is possible to prevent the image quality of the tomographic image from deteriorating.
Thus, a good diagnostic image can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram of an overall configuration showing an embodiment of an MRI apparatus according to the present invention;

FIG. 2 is a central longitudinal sectional view showing a main part of the MRI apparatus,

FIG. 3 is a circuit diagram showing an internal configuration of a temperature setting circuit;

FIG. 4 is a graph showing an example of temperature characteristics of a static magnetic field generating magnet using a permanent magnet.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Static magnetic field generation magnet, 2 ... Gradient magnetic field coil, 3 ... Gradient magnetic field power supply, 4 ... Transmission system, 5 ... Reception system, 6 ... Signal processing system, 7 ... Sequencer, 8 ... CPU, 9 ... Temperature generator, 10 ... Temperature detector, 11 ... Insulation circuit, 12
... subject, 25 ... insulation material, 27 ... power supply unit, 28
... Temperature setting circuit, 29 ... Comparator, R _{1 to} Rn
... resistance, S ₁ ~Sn ... setting switch.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) A61B 5/055

Claims (1)

(57) [Claims] 1. A static magnetic field generating means by a permanent magnet for applying a static magnetic field to a subject, a gradient magnetic field generating means for applying a gradient magnetic field, and a nuclear magnetic resonance in atomic nuclei of constituent atoms of a living tissue of the subject. A transmission system that irradiates electromagnetic waves to the electromagnetic wave, a reception system that detects the electromagnetic waves emitted by the nuclear magnetic resonance, a signal processing system that performs an image reconstruction operation using the detected electromagnetic waves, and the static magnetic field generation unit. A temperature generator for heating or cooling, a temperature detector for detecting the temperature of the heated or cooled static magnetic field generating means, and a detection signal of the detected temperature inputted to control the temperature generator and control the static magnetic field. A magnetic resonance imaging apparatus comprising: a heat retaining circuit for keeping the generating means at a set temperature; wherein the temperature detector comprises one or more static magnetic field generating means.
A magnetic resonance imaging apparatus characterized in that a hole is formed in a place and arranged therein .