JPH0654819A - Magnetic resonance imaging device - Google Patents

Abstract

PURPOSE:To stabilize the temperature of a magnetic circuit and also to uniformly improve a generated static magnetic field by providing a groove for embedding a coil along a magnetic field generation pattern and a cooling liquid passage for cooling the coil on the surface of a base member of the inclined magnetic field coil and on the reverse side of its groove respectively. CONSTITUTION:The X direction coil 17x of an inclined magnetic field coil is constituted by forming a groove 12a along an inclined magnetic field generation pattern on the surface of a base member 12 which is non-magnetic and into which a cooling liquid does not infiltrate and also which has an electric insulation property and embedding a coil wire rod therein. On the reverse side of the groove 12a, a cooling liquid passage 13 for cooling heat generated from the coil is formed. The groove 12a is formed by such a pattern as shown in full lines, and can be drawn by one stroke writing extending from one end part 31 to the other end part 32. The cooling liquid passage 13 is also formed so that it can be drawn by one stroke writing and heat generated from the X direction coil 17x is cooled uniformly. On the other hand, a Y direction coil 17y is also formed in the same manner as the X direction coil 17x on the reverse side of the base member 12.

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 (hereinafter referred to as an MRI apparatus) for obtaining a tomographic image of an examination region of a subject by using a nuclear magnetic resonance (NMR) phenomenon using a permanent magnet, In particular, the present invention relates to an MRI apparatus suitable for stabilizing the temperature of a static magnetic field generating magnetic circuit and improving the uniformity of the static magnetic field generated.

[0002]

2. Description of the Related Art An MRI apparatus uses the above-mentioned NMR phenomenon to measure the density distribution, relaxation time distribution, etc. of nuclear spins at a desired examination site in a subject and arithmetically processes the measurement signal to obtain the above-mentioned examination site. The image is displayed as a tomographic image. Here, in the case of measuring a spatially wide range such as a human body, 0.05-2T (Tesla; 1 Tesla is 10 in a measurement space consisting of a spherical space with a diameter of 30-50 cm).
A magnetic field generator that can generate a static magnetic field of about 000 gauss) with a uniformity of several tens of ppm or less is required.

As shown in FIG. 2, a conventional MRI apparatus using a permanent magnet as the magnetic field generator has a pair of permanent magnets 1 facing each other with a space A into which a subject can enter.
a, 1b, yokes 2a, 2b magnetically coupling the permanent magnets 1a, 1b, and the permanent magnets 1a, 1b.
b, a static magnetic field generating magnetic circuit having magnetic pole pieces 3a, 3b fixed to the opposing surfaces on the side of the air gap A for generating a uniform magnetic field, and the magnetic pole pieces 3a, 3b arranged close to each other inside the magnetic pole pieces 3a, 3b. Gradient magnetic field coils 4a and 4b that generate a gradient magnetic field to be added to the uniform magnetic field generated by 3b, an irradiation coil 5 that applies an electromagnetic wave to the subject inside the air gap A inside the gradient magnetic field coils 4a and 4b, and the target magnetic field. The receiving coil 6 for receiving the nuclear magnetic resonance signal emitted from the specimen and the temperature adjusting means 7 for keeping the temperature of the static magnetic field generating magnetic circuit constant were provided.

Here, in the static magnetic field generating magnetic circuit using the permanent magnets 1a and 1b in the MRI apparatus, the intensity of the static magnetic field tends to change due to the change in ambient temperature.
Generally, the temperature coefficient is -1000 ppm / ° C, and the magnetic field strength is weakened by 1000 ppm when the temperature rises by 1 ° C. In this way, when the magnitude of the static magnetic field changes under the influence of temperature, a gradient magnetic field is added to the static magnetic field by the gradient magnetic field coil so that the detection position corresponds to the magnitude of the magnetic field, and the resonance frequency corresponding to that position is set. An error occurs in the operation of generating and detecting the NMR signal having this resonance frequency to specify the position. Then, the deviation of the position detection causes distortion and blurring of the image. In general, the limit value that affects the image by the change in static magnetic field is 5 ppm / hour. Based on this standard, it is necessary to suppress the change in ambient temperature of the static magnetic field generating magnetic circuit within 5/1000 ° C. per hour.

However, in recent MRI apparatuses, high-speed imaging and improvement in patient throughput are required,
Along with this, new high-speed sequences such as the gradient echo method have been used. Unlike the conventional spin echo method, this sequence of the gradient echo method does not use a 180-degree high frequency pulse for spin imaging, but uses the inversion of a gradient magnetic field. Therefore, the current applied to the gradient magnetic field coils 4a and 4b shown in FIG. 2 increases, and the frequency of use (duty) thereof also increases. Here, the gradient magnetic field coils 4a, 4
Since b is basically composed of copper wire, it has a finite electric resistance value. When an electric current is applied to form a gradient magnetic field, heat is generated in proportion to the square of the electric current value. , Which is transmitted to the magnetic pole pieces 3a, 3b and the permanent magnet 1a,
It was also transmitted to 1b and had the above influence.

[0006]

In the conventional MRI apparatus shown in FIG. 2 described above, the gradient magnetic field coils 4a and 4b are provided.
Is disposed inside the magnetic pole pieces 3a, 3b inwardly of the annular projection in the horizontal direction with substantially no space therebetween, so that the heat generated in the gradient magnetic field coils 4a, 4b is generated by the magnetic pole pieces 3a, 3b.
There was a case where the temperature was immediately transmitted to and the temperature rose. In this regard, the temperature adjusting means 7 including the heat insulating material 8 and the planar heater 9 provided around the static magnetic field generating magnetic circuit including the permanent magnets 1a and 1b and the magnetic pole pieces 3a and 3b causes the static magnetic field generating magnetic circuit to operate. The temperature is controlled so as to be kept constant, but the gradient magnetic field coils 4a and 4b depend on the type of sequence.
Of the static magnetic field generating magnetic field, since the magnetic pole piece 3b located above the air gap A and the magnetic pole piece 3a located below the air gap A have different effects due to the heat generation of the upper and lower gradient magnetic field coils 4a and 4b. The temperature of the circuit was not constant and became unstable. Therefore, the intensity of the static magnetic field generated by the static magnetic field generating magnetic circuit changes, and its uniformity decreases. As a result, there is a case where a position detection shift occurs, and the obtained tomographic image is distorted or blurred.

Therefore, the present invention solves such a problem, stabilizes the temperature of the static magnetic field generating magnetic circuit, and improves the uniformity of the static magnetic field generated.
An object is to provide an RI device.

[0008]

In order to achieve the above object, the present invention provides a pair of permanent magnets opposed to each other with a gap into which a subject can enter, and magnetically coupling these permanent magnets. A static magnetic field generating magnetic circuit having yokes and pole pieces for generating a uniform magnetic field, which are fixed to the facing surfaces of the permanent magnet on the air gap side, respectively, and a uniform magnetic field generated by the pole pieces arranged close to the inside of the pole pieces. Gradient magnetic field coil for generating a gradient magnetic field, an irradiation coil for applying an electromagnetic wave to the subject in the air gap inside the gradient magnetic field coil, and a nuclear magnetic resonance signal emitted from the subject. In a magnetic resonance imaging apparatus including a receiving coil and a temperature adjusting means for keeping the temperature of the static magnetic field generating magnetic circuit constant, a non-magnetic and electrically insulating magnetic field generating gradient magnetic field generating surface is provided. A base member of each of the gradient magnetic field coils, in which a groove for burying the gradient magnetic field coil is formed along the groove, and a passage for a cooling liquid for cooling the gradient magnetic field coil is formed on the back surface side of the groove; It is configured to include a pair of cooling devices that radiate the heat of the cooling liquid passing through the passage formed in the base member.

[0009]

With the above structure, the heat generated in each of the upper and lower gradient magnetic field coils is supplied to the coil cooling liquid passage formed on the back side of the groove in which the gradient magnetic field coil of each base member is embedded. Is transmitted to the cooling liquid and is radiated by circulating the cooling liquid with a cooling device,
It can be removed without being transmitted to the pole pieces.

Since a pair of cooling devices are provided to control the cooling separately for each gradient magnetic field coil, it is possible to make the thermal influence on each magnetic pole piece and the permanent magnet the same, and to reduce the static magnetic field. It is possible to stabilize the temperature of the generated magnetic circuit and improve the homogeneity of the generated static magnetic field.

[0011]

Embodiments of the present invention will be described below with reference to FIGS. 1 is a perspective view showing an embodiment of an MRI apparatus according to the present invention, FIG. 2 is a central longitudinal sectional view of FIG. 1, FIG. 3 is a plan view showing the shape and structure of a gradient magnetic field coil, and FIG. 4 is IV- of FIG. FIG. 5 is a cross-sectional view taken along the arrow IV, FIG. 5 is a plan view showing a coil formed in a groove along the gradient magnetic field generation pattern, and FIG. 6 is a groove for embedding a coil formed in a base member of the gradient magnetic field coil and a coolant. FIG. 7 is a diagram showing a part of the passage, and FIG. 7 is a block diagram showing an example of a cooling device of the gradient magnetic field coil. In the figure,
3, FIG. 4, FIG. 5, and FIG. 6 all show only the lower half of the configuration of the static magnetic field generating magnetic circuit shown in FIG.

First, in FIGS. 1 and 2, the pair of permanent magnets 1a and 1b are vertically opposed to each other with a space A between them, into which a subject can enter. These permanent magnets 1
Reference characters a and 1b are for generating a static magnetic field in the space A, and are formed, for example, in a disk shape, and are supported by upper and lower yokes 2a and 2b, respectively. Yoke 2a, 2
b is permanent magnets 1a and 1b and magnetic pole pieces 3a and 3b described later.
Are formed to face each other with a predetermined gap therebetween and form a magnetic path, and are formed in, for example, a rectangular shape having a shorter depth than the lateral width. The upper and lower yokes 2a and 2b are opposed to each other and supported by a plurality of vertical yokes 2c. These vertical yokes 2c are formed by arranging the upper and lower yokes 2a and 2b so as to face each other at a predetermined interval and closing the magnetic path by the permanent magnets 1a and 1b, and are made of a member that easily allows magnetic flux to pass through inside. For example, a total of four pieces are erected, one at each of the four corners of the upper and lower yokes 2a and 2b, and each forms a magnetic flux return circuit that passes through the measurement space set in the air gap A.
Magnetic pole pieces 3a and 3b are magnetically and mechanically fixed to the facing surfaces of the permanent magnets 1a and 1b on the side of the air gap A, respectively. The magnetic pole pieces 3a and 3b are set in a predetermined area in the air gap A and enhance the uniformity of the static magnetic field in the measurement space in which the examination site of the subject enters. Is provided with an annular protrusion. And the permanent magnets 1a, 1b, the yokes 2a, 2b, 2
c and the magnetic pole pieces 3a and 3b constitute a static magnetic field generating magnetic circuit for generating a uniform static magnetic field in the measurement space into which the subject is inserted.

As shown in FIG. 2, gradient magnetic field coils 4a, 4 are provided on the inner sides of the magnetic pole pieces 3a, 3b facing the air gap A side.
b are arranged close to each other. Gradient coil 4
a and 4b generate a gradient magnetic field that is added to the uniform static magnetic field generated by the magnetic pole pieces 3a and 3b. In addition to this gradient magnetic field, the position is made to correspond to the magnitude of the magnetic field, and the resonance frequency corresponding to the position is set. It is supposed to be generated. Then, the gradient magnetic field coils 4a and 4b, as shown in FIGS. 3 and 4 (only the pole piece 3a side is shown in FIGS. 3 and 4),
Three types of coils, an X-direction coil 17x, a Y-direction coil 17y, and a Z-direction coil 17z, are arranged on the front surface and the back surface of a circular plate that can enter the annular protrusion 3b.

An irradiation coil 5 is provided inside the gradient magnetic field coils 4a and 4b. The irradiation coil 5 is
An electromagnetic wave for causing nuclear magnetic resonance is applied to a subject located in the void A, and is formed in a cylindrical shape surrounding the void A. The receiving coil 6 is arranged inside the irradiation coil 5. The receiving coil 6 is
It receives a nuclear magnetic resonance signal emitted from the subject, and is formed so as to cover the subject located in the void A.

A temperature adjusting means 7 is provided around the static magnetic field generating magnetic circuit. Temperature adjusting means 7
Is for keeping the temperature of the static magnetic field generating magnetic circuit constant and covers the entire permanent magnets 1a, 1b, yokes 2a, 2b, 2c and magnetic pole pieces 3a, 3b, for example, a heat insulating material 8 made of styrene foam or sponge. And a planar heater 9 fixed to the inside of an aluminum plate 10 attached to the inner surface of the heat insulating material 8. The heat insulating material 8 has a void A
The center part is open so that it can be placed inside. The static magnetic field generating magnetic circuit is covered with a gantry cover 11a for the front surface, a gantry cover 11b for the rear surface, a gantry cover 11c for the side surfaces, and a gantry cover 11d for the upper surface, as shown in FIG.

As shown in FIGS. 5 and 6, the X-direction coil 17x is non-magnetic, does not penetrate the cooling liquid, and generates a gradient magnetic field on the surface of the base member 12 made of an electrically insulating material. Forming grooves 12a along the pattern,
A coil wire is embedded in the formed groove 12a. A passage 13 for a cooling liquid for cooling the heat generated by the X-direction coil 17x is formed on the back side of the groove 12a. The groove 12a is formed in a pattern substantially indicated by a solid line in FIG. 5, and the one end portion 31 to the other end portion 32 can be drawn with one stroke. Similarly, the cooling liquid passage 13 is also formed so that it can be drawn with a single stroke, and the heat generated from the X-direction coil 17x can be uniformly cooled in the entire X-direction coil 17x.

On the other hand, the Y-direction coil 17y is formed by forming a groove (not shown) on the back side surface of the base member 12 like the X-direction coil 17x, and burying a coil wire material in the formed groove. A passage for a cooling liquid for cooling the heat generated by the Y-direction coil 17y is formed on the back side of the groove, like the passage 13. The groove and the passage in this case are formed in a pattern as shown by the dotted line in FIG. 5, and are formed so that one end can be drawn with one stroke like the case of the X-direction coil 17x. Therefore, the heat generated from the Y-direction coil 17y can be uniformly cooled in the entire Y-direction coil 17y. The Y-direction coil 17y cooling liquid passage is configured such that the end thereof communicates with the end of the X-direction coil 17x cooling liquid passage 13.

The gradient magnetic field coils 4a and 4b are magnetic pole pieces 3.
An a-side (lower side) installation and a pole piece 3b-side (upper side) installation are connected in series, and each of the X, Y, and Z direction coils 17 is connected.
x, 17y, and 17z are electrically insulated. Therefore, the amount of heat generated by each coil is the same for both the one installed on the side of the magnetic pole piece 3a and the one installed on the side of the magnetic pole piece 3b, but there is a difference in the amount of heat transfer between the two. The temperature becomes higher than that of the magnetic pole pieces 3a side (lower side), and the magnetic pole pieces 3a and 3b
And the influence on the permanent magnets 1a and 1b appears differently. Therefore, it is necessary to separately control the cooling of the gradient magnetic field coils 4a and 4b.

FIG. 7 shows an example of a cooling device for a gradient magnetic field coil. In the figure, 12 'is a base member installed on the side of the magnetic pole piece 3b, 14 is a Y-direction coil 17y coolant passage, 21 is a coolant circulation pump, 22 is a temperature sensor, and 23 is coolant replenishment. And an auxiliary tank for inspecting liquid amount, etc., 24 is a heat dissipation portion provided with a large number of heat dissipation fins, 25
Is a fan for promoting heat dissipation, 26 is a pipe, a non-magnetic copper tube,
Vinyl pipes and plastic pipes are used. Reference numeral 30 denotes a static magnetic field generating magnetic circuit. In this way, the base member 12,
In 12 ', an independent cooling device having the same structure is provided outside the static magnetic field generating magnetic circuit 30.

As shown in FIG. 7, in both the base member 12 installed on the side of the magnetic pole piece 3a and the base member 12 'installed on the side of the magnetic pole piece 3b, the X-direction coil 17x and the cooling liquid passage 13 and Y are provided. Directional coil 17y Coolant passage 14
Is connected in series, the cooling liquid supplied by the pump 21 passes through the passages 13 and 14 to absorb the heat generated from the X-direction coil 17x and the Y-direction coil 17y, and the cooling liquid is supplied to the heat radiating portion 24. After radiating the heat sent and absorbed, it is circulated and supplied again to the passages 13 and 14 via the pump 21. In this case, the heat absorbed in the passages 13 and 14 is not constant in time and the apparatus may be stopped, so that the temperature sensor 22 controls the liquid amount.

In this way, the upper and lower gradient magnetic field coils 4a,
The heat generated in 4b is transferred to and absorbed by the cooling liquid that is circulated and supplied to the passages 13 and 14, and the cooling liquid that has absorbed the heat is radiated by the cooling device and removed without being transferred to the magnetic pole pieces 3a and 3b. Will be possible. Then, the gradient magnetic field coil 4a on the side of the magnetic pole piece 3a and the gradient magnetic field coil 4b on the side of the magnetic pole piece 3b are individually controlled to be cooled by independent cooling devices, and the magnetic pole pieces 3a and 3b and the permanent magnet 1a,
It is possible to prevent a difference in thermal influence on 1b.
Therefore, by stabilizing the temperature of the static magnetic field generating magnetic circuit,
The homogeneity of the static magnetic field generated can be improved.

[0022]

As described above, according to the present invention, in a magnetic resonance imaging apparatus, a groove having a non-magnetic property and an electrical insulation property, in which a gradient magnetic field coil is embedded along a gradient magnetic field generating pattern is formed on the surface side. In addition, the heat of the cooling liquid passing through the base member of each gradient magnetic field coil having a passage for the cooling liquid for cooling the gradient magnetic field coil on the back surface side of the groove and the heat of the cooling liquid passing through the passage formed in each base member is provided. Since it is configured to include a pair of cooling devices for radiating heat, heat generated in the upper and lower gradient magnetic field coils is supplied to the coil cooling liquid passage formed on the back surface side of the groove in which the gradient magnetic field coils are embedded. The heat of the cooling liquid can be removed without being transferred to the magnetic pole pieces by transmitting the heat to the cooling liquid and radiating the heat of the cooling liquid through the cooling device. Moreover, since a pair of cooling devices are provided to control cooling separately for each gradient magnetic field coil, it is possible to make the thermal influence on each magnetic pole piece and the permanent magnet the same, and the static magnetic field generating magnetic circuit. The temperature can be stabilized, the uniformity of the generated static magnetic field can be improved, the image quality of the tomographic image can be improved, and good image diagnosis can be performed.

[Brief description of drawings]

FIG. 1 is a perspective view showing an embodiment of an MRI apparatus of the present invention.

FIG. 2 is a central vertical sectional view of FIG.

FIG. 3 is a plan view showing the shape and structure of a gradient magnetic field coil.

4 is a sectional view taken along the line IV-IV in FIG.

FIG. 5 is a plan view showing a coil formed in a groove along a gradient magnetic field generation pattern.

FIG. 6 is a diagram showing a part of a groove for embedding a coil and a passage for a coolant formed in a base member of a gradient magnetic field coil.

FIG. 7 is a block diagram showing an example of a cooling device for a gradient magnetic field coil.

[Explanation of symbols]

 1a, 1b Permanent magnets 2a, 2b Yoke 3a, 3b Magnetic pole pieces 4a, 4b Gradient field coil 7 Temperature adjusting means 12, 12 'Base member 12a Groove 13,14 Coolant passage 17x X-direction coil 17y Y-direction coil 17z Z-direction coil 21 Coolant circulation pump 24 Radiator 25 Fan 26 Piping

Claims (1)

[Claims] 1. A pair of permanent magnets, which are opposed to each other to form a void into which a subject can enter, a yoke for magnetically coupling these permanent magnets, and a void-side facing surface of the permanent magnet, respectively. A static magnetic field generating magnetic circuit having a magnetic pole piece that is fixed and generates a uniform magnetic field, a gradient magnetic field coil that is disposed close to the inside of the magnetic pole piece and that generates a gradient magnetic field that adds to the uniform magnetic field generated by the magnetic pole piece, and this gradient An irradiation coil for applying electromagnetic waves to the subject in the air gap inside the magnetic field coil, a receiving coil for receiving a nuclear magnetic resonance signal emitted from the subject, and a constant temperature of the static magnetic field generating magnetic circuit. In a magnetic resonance imaging apparatus provided with a temperature adjusting means for holding, a groove which is non-magnetic and has an electrical insulation property and in which a gradient magnetic field coil is embedded is formed on a surface side along a gradient magnetic field generation pattern. At the same time, the heat of the cooling liquid passing through the base member of each of the gradient magnetic field coils, in which a passage for the cooling liquid for cooling the gradient magnetic field coil is formed on the back surface side of the groove, and the heat of the cooling liquid passing through the passage formed in each of the base members are A magnetic resonance imaging apparatus comprising a pair of cooling devices for radiating heat.