JP2000030937A - Magnetic field generator for mri - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control the temperatures of permanent magnets with high accuracy by improving the heat efficiency by reducing temperature unevenness by arranging a plurality of heater means near a supporting yoke on the outsides of platy yokes between which the permanent magnets are arranged. SOLUTION: To the outsides of a pair of upper and lower platy yokes 3 made of pure iron on the side opposite to a void 8, sheet-like heaters 10 and 11 are stuck near a supporting yoke 4 and the void-opened surface side of the yoke 4 on the opposite side. Temperature sensors 12 are respectively arranged on the side faces of a pair of permanent magnets 5 on the opposite side of the yoke 4 and on the yoke 4 side. Here, heat control is performed very efficiently by means of a temperature controller in such a way that the controller maintains the temperature of a magnetic circuit by making controlled currents to flow to the heaters 10 and 11 through a solid-state relay(SSR) by sending a control signal to the SSR in accordance with the difference between the temperature of the magnetic circuit detected by means of the sensors 12 and a set temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a medical magnetic resonance tomography apparatus (hereinafter, referred to as an MRI apparatus).
By measuring the temperature distribution of the permanent magnet and controlling the temperature of the heater means for temperature control installed at multiple locations of the yoke, the asymmetry of the temperature distribution of the permanent magnet has been reduced without impairing the uniformity of the generated magnetic field. The present invention relates to a magnetic field generator for MRI.

[0002]

2. Description of the Related Art In an MRI apparatus, a part or the whole of a subject is inserted into a gap of a magnetic field generator for generating a strong magnetic field, and a tomographic image of an object is obtained and the properties of the tissue are drawn. A device that can

In the magnetic field generator for the above-mentioned MRI apparatus, the gap needs to be large enough to allow a part or the whole of a subject to be inserted, and in order to obtain a clear tomographic image, imaging is usually performed in the gap. Within the field of view, 0.02-2.0
It is required to form a stable strong and uniform magnetic field having T and an accuracy of 1 × 10 ⁻⁴ or less.

As a magnetic field generator used in an MRI apparatus,
As shown in FIGS. 5A and 5B, R-
A pair of permanent magnet structures 30, 3 using Fe-B magnets
The pole pieces 31, 31 are fixed to and opposed to one end of each of the pole pieces 0, and the other ends are connected by plate yokes 35, 35.
A configuration for generating a static magnetic field in the gap 33 between the first and the first 31 has been known (Japanese Patent Publication No. 2-231010). In addition,
The illustrated yoke 34 is assembled by connecting a columnar yoke 36 between a pair of plate yoke 35, 35, and 37 in the figure is a gradient magnetic field coil.

[0005] The pole piece 31 is usually made of a plate-like bulk (integral body) obtained by shaving a magnetic material such as electromagnetic soft iron or pure iron. In order to improve the uniformity of the magnetic field distribution in the air gap 33, A configuration in which an annular projection 32 is provided in a peripheral portion and a convex projection (not shown) is further provided in a central portion (actual
No. 37446).

Further, a magnetic field generator for MRI having a configuration capable of reducing a feeling of oppression given to a subject at the time of diagnosis,
A configuration employing a so-called C-shaped yoke as shown in FIG. 6 has been proposed. That is, the MRI magnetic field generator shown in FIG. 6 supports and connects one end of the pair of plate yokes 42, 42 so that the pair of plate yokes 42, 42 face each other with a predetermined gap 46 formed therebetween. It has a configuration in which it is connected and supported by iron 47, and further has a configuration in which pole pieces 41, 41 are arranged together with permanent magnets 40, 40 on the air gap-facing surfaces of a pair of plate yokes 42, 42, respectively.

In order to form a static magnetic field, permanent magnets are generally relatively inexpensive to maintain, and are increasingly used from the viewpoint of miniaturization of the apparatus. However, this permanent magnet has a disadvantage that the magnetic field strength is easily changed by a change in temperature due to the characteristics of the magnet itself.

Since the stability of the magnetic field strength of the static magnetic field is important in the MRI apparatus, the magnetic field generator is insulated in the ordinary MRI apparatus so that the magnetic circuit is maintained at a constant temperature in order to stabilize the magnetic field strength. A heater is provided inside the magnetic circuit while being covered with a material.

A technique for reducing the influence of a temperature change on a static magnetic field in an MRI apparatus is disclosed in, for example, Japanese Patent Application Laid-Open No. 63-43.
No. 649. As shown in FIG. 7, reference numeral 40 denotes a permanent magnet, 41 denotes a pole piece provided on the gap-facing surface side of the permanent magnet, 42 denotes a plate-like yoke arranged on the side opposite to the gap-facing surface of the permanent magnet, and 43 denotes a yoke. A heater is arranged on a plate-shaped yoke 42, and the entire yoke is covered with a heat insulating material 45. A heater current is supplied from a power supply (not shown) to control the temperature of the magnetic circuit. JP-A-63-278310 discloses the above-mentioned JP-A-63-4.
MRI with start-up heater installed in 3649 configuration
An apparatus is disclosed.

[0010]

However, in the configuration shown in FIG. 7, since the heater 44 is installed on the side opposite to the gap facing surface of the permanent magnet 40, that is, on the outer surface of the yoke, heat escapes from the apparatus to the outside. That is, thermal efficiency is poor. Moreover, the plate yoke 42
Is surrounded by a heat insulating material 45, an air flow passage is formed between the plate-shaped yoke 42 and the heat insulating material 45, air is forcibly circulated by a fan, and the temperature is adjusted by a temperature adjusting heater and a temperature sensor. Since the yoke is heated via air, the apparatus is complicated and has poor thermal efficiency.

Further, as an improved structure of the invention described in Japanese Patent Application Laid-Open No. 63-278310, Japanese Patent Application Laid-Open No.
No. 6 is disclosed. Japanese Patent Application Laid-Open No. 8-266506 discloses a configuration in which a lower base yoke having a lower permanent magnet mounted on an upper surface thereof, a column yoke erected from an edge of the lower base yoke, and a top yoke supported by the column yoke. An upper base yoke having a magnet attached to the lower surface thereof facing the lower permanent magnet, temperature sensor means for detecting a temperature related to the temperature of the lower and upper permanent magnets, and heater means for heating the lower and upper permanent magnets And a temperature control means for controlling the heater means based on the temperature detected by the temperature sensor means, wherein the heater means is provided directly on the side surfaces of the upper base yoke and the lower base yoke. Or through a non-gas heat transfer material.

Japanese Patent Application Laid-Open No. Hei 8-266506 discloses that this configuration prevents heat from escaping upward, which causes a large loss, and has high thermal efficiency. Further, since no heater is attached to the lower surface of the lower base yoke, workability is improved. It is stated to improve. In addition, since the lower base yoke and the upper base yoke are covered with a heat insulating material together with the AC sheet heater and the DC sheet heater, it is possible to prevent the occurrence of a temperature difference between the upper and lower sides due to convection of air, and to use the fixing bake plate for the AC sheet heater. Further, it is described that the heat transfer efficiency is good because the DC sheet heater is completely adhered to the lower base yoke and the upper base yoke.

However, in the configuration of Japanese Patent Application Laid-Open No. 8-266506, since the sheet heater is mounted on the side surface of the yoke, a large amount of heat is radiated from such a planar heater to the side opposite to the surface in contact with the yoke, and the thermal efficiency is increased. Is bad. In addition, since these are one control system, there is a problem that temperature unevenness is large and magnetic field uniformity is impaired.

In particular, in a magnetic circuit employing a C-shaped yoke with emphasis on reducing the feeling of oppression exerted on the subject, the yoke is asymmetrical with respect to the subject and supports the upper and lower magnetic poles. There is a problem in that heat dissipation from the yoke is large, and the temperature distribution in the yoke becomes non-uniform, and as a result, the uniformity of the magnetic field deteriorates.

[0015] Also, for a pair of permanent magnets disposed above and below the magnetic circuit, heat is dissipated from the legs of the magnetic circuit to the floor, and there is a temperature difference between the upper and lower parts of the room due to convection of room air. Therefore, the temperature of the permanent magnets disposed below becomes low and the stability of the magnetic field deteriorates. Therefore, it is necessary to keep the temperatures of the upper and lower permanent magnets constant and maintain the uniformity and stability of the magnetic field.

The present invention solves the conventional problem of temperature control of permanent magnets in a magnetic field generator for MRI, reduces temperature unevenness and enhances thermal efficiency without impairing magnetic field uniformity, and controls temperature of permanent magnets. The purpose of the present invention is to provide a magnetic field generator for MRI having a configuration capable of performing the method with high precision.

[0017]

The inventor of the present invention has conducted various investigations with a view to solving the above-mentioned problems and aiming at a configuration capable of performing the temperature control of the permanent magnet with high accuracy.
By disposing a plurality of heater means near the support yoke on the outer surface of the plate yoke where the permanent magnets are arranged and on the opposite side, the asymmetry of the temperature distribution of the pair of upper and lower permanent magnets due to the asymmetry of the magnetic circuit is reduced. Then, they found that a good captured image could be obtained by maintaining the uniformity and stability of the magnetic field, and completed the present invention.

That is, the present invention includes a pair of plate yokes which are opposed to each other while forming an imaging space, and a support yoke which connects and supports one end of the pair of plate yokes. A permanent magnet is arranged on each gap-facing surface of the plate-shaped yoke, and a magnetic circuit generator for MRI comprising a magnetic circuit having three open surfaces for the gap, the magnetic circuit comprising a temperature sensor means and a heater means, The magnetic field generator for MRI is characterized in that the heater means is arranged at least on the outer surface of the plate yoke in the vicinity of the support yoke and on the side opposite to the open surface.

The present invention also provides an MRI magnetic field generator having the above-mentioned structure, wherein an MRI magnetic field generator is provided on the plate yoke side of the outer surface of the support yoke in addition to the heater means. It is a magnetic field generator.

In addition, in the MRI magnetic field generator having the above-mentioned structure, the present invention provides a structure in which the temperature sensor means is disposed at each of the opposed magnetic poles, and the heater based on the temperature detected by the temperature sensor means. The present invention also proposes a magnetic field generator for MRI characterized by having a temperature control means for controlling the means.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The magnetic field generator for MRI according to the present invention has the problem that the temperature distribution of the permanent magnets of the open type magnetic circuit is asymmetric, because the permanent magnet has an asymmetrical temperature distribution. By arranging the heater means in at least two places on the opposite side, the reduction is achieved, and the uniformity and stability of the magnetic field can be maintained, and a high-quality image can be obtained.

Further, by arranging a plurality of temperature sensor means and heater means not only in the vicinity of the support yoke on the outer surface of the plate-like yoke and on the opposite side thereof, but also on the upper and lower opposed magnetic poles, the control system is further improved. By providing a plurality, a more uniform and stable magnetic field can be obtained. Besides that part,
The side and the center of the yoke and the side and the center of the permanent magnet may be appropriately arranged as necessary. When the heater is installed on the surface of the permanent magnet, it may be installed directly, or may be installed via a heat conducting member such as aluminum.

In the case where the heater means is configured to be embedded inside the yoke, heat can be very efficiently controlled without heat being dissipated to the outside and no loss occurs.
Further, by arranging a plurality of temperature sensors, partial temperature control is possible, and there is an advantage that asymmetry of the magnetic field uniformity hardly occurs.

An MRI magnetic field generator according to the present invention includes a pair of plate yokes which are opposed to each other while forming an imaging space, and a support yoke which connects and supports one end of the pair of plate yokes. The present invention is not limited to the embodiments described below, and can be applied to any configuration as long as the configuration includes permanent magnets disposed on the gap-facing surfaces of the pair of plate-like yokes. Furthermore, the shape and size of the plate yoke and the support yoke for forming the magnetic path may be appropriately selected according to various characteristics such as the required size of the air gap, the magnetic field strength, and the uniformity of the magnetic field.

As a magnet constituting a magnetic field source, a normal conducting magnet, a superconducting magnet, a permanent magnet, or the like can be used. When a permanent magnet is used, a known magnet material such as a ferrite magnet or a rare earth cobalt-based magnet is used. Can be used. In particular, by using a resource-rich light rare earth element such as Nd or Pr as R, and using an Fe-BR permanent magnet having B and Fe as main components and exhibiting an extremely high energy product of 30 MGOe or more. , Can be significantly reduced in size. In addition, by arranging these known permanent magnets in combination, it is possible to provide an economically excellent magnetic field generator without significantly impairing the miniaturization of the device.

As a material of the yoke of the present invention, soft magnetic iron,
A conventionally known material such as pure iron can be used. The plate yoke and the support yoke ensure the mechanical strength of the entire pole piece together with the equalization of the magnetic field strength, and improve the workability of assembling the magnetic field generator.

The material constituting the pole piece is not limited to the material of the embodiment. Lamination of various soft ferrite or silicon steel sheets such as pure iron or soft magnetic powder molded from an electrically insulating material, such as Mn-Zn or Ni-Zn, with low coercive force and high electric resistance By employing a body or a combination thereof, eddy current and residual magnetism generated in the magnetic material base when a pulsed magnetic field is applied can be reduced.

In particular, since the laminated silicon steel sheet is inexpensive as compared with the soft ferrite, the economical advantage is great. As shown in FIG. 4, when the pole piece 20 is made of the above-mentioned silicon steel plate, a plurality of and various blocks 23 and 24 of laminated silicon steel plates are arranged on the magnetic material base 21 and the blocks are further laminated. With such a configuration, the effect of reducing eddy currents and remanent phenomena is great, and mounting can be performed with good workability. By optimizing the thickness of the pole piece and the thickness ratio of the plate yoke, the mechanical strength of the pole piece is secured, the magnetic field strength required for the pole piece is equalized, and the eddy current and remanence phenomenon are reduced. The prevention effect is obtained.

Further, in the present invention, in order to improve the uniformity of the magnetic field in the air gap, it is desirable to form an annular projection made of a magnetic material ring such as electromagnetic soft iron or pure iron on the periphery of the pole piece. In particular, as shown in FIG.
If one or more slits are provided in the circumferential direction of 2 and divided,
The effect of reducing the eddy current is further obtained. The cross section of the annular projection is appropriately selected from a rectangle, a substantially triangle, a trapezoid, and the like. It is also effective to form a uniform magnetic field by arranging a convex protrusion inside the annular protrusion of the pole piece.

In the present invention, the heater for controlling the temperature of the heater means disposed on the yoke is not particularly limited. For the start-up, an AC heater having a large heat value is used, and the heater for controlling the temperature is controlled. Can use a DC heater having good tracking performance. Further, the present invention is not limited to the sheet-shaped heater shown in FIG. 1A, and an existing heater such as a sheathed heater, an embedded heater, and a ribbon heater may be used.

Further, a tubular heater in which a generator is held in a metal pipe and the inside of the pipe is filled with an insulator such as MgO is formed, and as shown in FIG. The temperature of the permanent magnet can be efficiently controlled without dissipating heat to the outside air.

As the temperature sensor means disposed for controlling the temperature, a known sensor such as a thermocouple, a resistance temperature detector, a thermistor or the like can be used as appropriate according to the configuration of the control system. The position where the temperature sensor is disposed is, in addition to being disposed on the side of the pair of permanent magnets 5 shown in the embodiment on the opposite side of the supporting yoke and on the opposite side thereof, a heater mounting portion of the yoke, the side of the yoke,
The permanent magnet may be arranged as appropriate according to the configuration of the magnetic circuit, such as the central portion and side surfaces.

As the temperature control means, any electric control means other than the circuit configuration shown in the embodiment can be adopted, and a single control system or a plurality of control systems can be used as required. When it is necessary to raise the temperature of the magnetic circuit from a relatively low temperature state to a certain temperature, a heater having a large capacity can be used in combination to shorten the heating time. In this case, it is desirable to use a temperature controller having two systems of outputs for accelerating temperature rise and fine adjustment for maintaining the set temperature. The temperature controller may be a plurality of independent circuits or a multi-channel (multipoint) temperature controller.

[0034]

Embodiment 1 The features of the present invention will be described below with reference to an embodiment shown in FIG. 1A. The yoke of the magnetic field generator has a configuration in which a pair of plate-shaped yoke 3 and 3 ends are connected by a plate-shaped support yoke 4, and legs 2 are arranged on the floor 1 via rubber for heat insulation. is there. A pair of permanent magnet structures 5 and 5 using R-Fe-B magnets as magnetic field sources are mounted on opposing surfaces of the plate-shaped yoke 3 and magnetic pole pieces 6 and 6 are fixed to one end of each. Then, a static magnetic field is generated in the gap 8 between the pole pieces 6, 6, and in particular, a uniform magnetic field is generated in the imaging space 9. Further, the pole pieces 6, 6 have a configuration having an annular projection 7, and are formed by a block of laminated silicon steel plates in FIG.

Sheet heaters 10 and 11 are provided on the outer surfaces of the pair of upper and lower plate yoke 3 and 3 made of pure iron opposite to the gap 8.
Are attached to a portion close to the support yoke 4 and the opposite side of the gap opening surface side, and are connected to a temperature controller via a lead and a relay (not shown).

On the side surfaces of the pair of permanent magnets 5, temperature sensors 12 are arranged on the opposite side of the supporting yoke 4 and on the opposite side.

Here, two temperature control systems 13 and 14 having the structure shown in FIG.
Based on the difference between the temperature of the magnetic circuit and the set temperature detected by the control unit 2, control signals are sent from the temperature controllers 16 to SSRs (solid state relays) 15, 15. SS
The controlled current is supplied through the heaters 10 and 15 through R15 and R15.
11, the temperature of the magnetic circuit is kept constant.

That is, in a magnetic circuit in which the magnetic poles face up and down as shown in FIG. 1, the temperature of the upper magnet tends to increase.
Since the temperature of the lower magnet tends to be lower, the upper and lower plate yokes 3, 3 separate the temperature control system 1 shown in FIG.
3 and 14, and the heater 10 of the upper plate-shaped yoke 3
And a temperature sensor 12 attached near the magnet of the upper magnetic pole.
Has an electric circuit as one control system.

A plurality of heaters 10 and 11 are connected to each of the control systems 13 and 14 in order to prevent local heating of the magnetic circuit and to uniformly heat the entire magnetic circuit. Although not shown in the figure, a heat insulating material for thermally isolating the surrounding air from the magnetic circuit can be appropriately arranged.

With the temperature control means having the above configuration,
When the target temperature of the upper and lower permanent magnet members 5 was set to 32 ° C., it was possible to keep the temperature difference between the upper and lower magnets in the range of 0.1 ° C.

Embodiment 2 The magnetic field generator shown in FIG. 1B has the same configuration as that of Embodiment 1, but differs in the configuration of the temperature sensor and the heater means.
That is, the heaters 10a and 10b of the bar-shaped heating elements are provided on the outer surfaces opposite to the gap 8 of the pair of upper and lower plate yoke 3 and 3 at the location close to the support yoke 4 and on the opposite side of the gap opening surface. Are embedded in the upper and lower ends of the outer surface of the supporting yoke 4, and heaters 11a and 11b of rod-like heating elements are embedded, and are connected to a temperature controller via unillustrated leads and relays, respectively.

Heaters 10a, 10b, 11a, 11b
Are two temperature control systems 16 having the configuration shown in FIG.
a, 16b, and here a pair of permanent magnets 5
Temperature sensors 12a, 1
2b, a control signal is sent from the temperature controllers 16a and 16b through the SSR according to the difference between the temperature of the magnetic circuit detected by the temperature sensors 12a and 12b and the set temperature.
The controlled current is supplied to the heaters 10a, 10b, 11a, 11
b, so that the temperature of the magnetic circuit is kept constant.

With the temperature control means having the above configuration,
When the target temperature of the upper and lower permanent magnet structures 5 was set to 32 ° C., the temperature difference between the upper and lower magnets could be kept in the range of 0.1 ° C., and the power consumption was 600 W. Compared to the case where a seat heater is placed outside the plate yoke,
Significant reduction in power consumption was possible.

[0044]

According to the C-shaped magnetic circuit to which the present invention is applied, since the supporting yoke is connected to only one end of the plate yoke, the yoke is asymmetric in the lateral direction of the subject. Therefore, the heat radiation from the supporting yoke has been large, and the temperature distribution in the magnetic circuit has not been uniform, so that the magnetic field has been uneven.

The magnetic field generator for MRI according to the present invention comprises:
This problem is solved by arranging a plurality of heater means near the support yoke on the outer surface of the plate yoke where the permanent magnet is arranged and on the opposite side thereof. When the control heater is heated by the temperature controller, the permanent magnet disposed near the yoke is efficiently heated, and the control followability is good.

Further, when the temperature control heater is buried inside the yoke, the heat generated from the heater is conducted through the yoke and directly reaches the permanent magnet, so that the heat is dissipated to the outside without loss. It is possible to perform heat control very efficiently. Further, by arranging a plurality of temperature sensors on the permanent magnet, partial temperature control is possible, and there is an advantage that asymmetry of the magnetic field uniformity hardly occurs.

[Brief description of the drawings]

FIGS. 1A and 1B are explanatory views showing the configuration of a magnetic field generator for MRI of the present invention.

FIG. 2 is a circuit diagram for controlling the temperature of the magnetic field generator for MRI of the present invention.

FIG. 3 is a circuit diagram for controlling the temperature of the magnetic field generator for MRI of the present invention.

4A and 4B show the configuration of a pole piece of the magnetic field generator for MRI of the present invention, wherein FIG. 4A is a top view and FIG.

5A and 5B show the configuration of a conventional MRI magnetic field generator, wherein FIG. 5A is a partially vertical front view and FIG. 5B is a cross-sectional top view.

FIG. 6 is an explanatory perspective view showing a configuration of a magnetic field generator for MRI of a C-shaped magnetic circuit.

FIG. 7 is a partially cutaway perspective view showing another configuration of a conventional magnetic field generator for MRI.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Floor 2 Leg part 3,35,42 Plate yoke 4,47 Support yoke 36,43 Column yoke 5,40 Permanent magnet 6,20,31,41 Magnetic pole piece 7,22,32 Annular protrusion 8,33 , 46 void 9 imaging space _{10,10a, 10a 1, 10a 2,} 10b, 10b 1,
10b ₂ , 11, 11a, 11b, 44 Heater 12 Temperature sensor 13, 14, Temperature control system 15 SSR 16, 16, a, 16b Temperature controller 21 Magnetic material base 23, 24 Block 30 Permanent magnet structure 34 Yoke 37 Gradient magnetic field coil 45 Insulation

Claims (4)

[Claims] 1. A pair of plate-like yokes, each of which has a pair of plate-like yokes that are opposed to each other while forming an imaging space, and a support yoke that connects and supports one end of the pair of plate-like yokes. In a magnetic field generator for MRI comprising a magnetic circuit having a permanent magnet on each of the gap facing surfaces of iron and having three open surfaces for the gap, the magnetic circuit includes a temperature sensor means and a heater means, and the heater means MRI arranged at least on the outer surface of the plate yoke in the vicinity of the support yoke and on the opposite open surface side
Magnetic field generator. 2. The MRI magnetic field generator according to claim 1, wherein the MRI magnetic field generator is provided on the plate yoke side of the outer surface of the support yoke in addition to the heater means. 3. The MRI magnetic field generator according to claim 1, wherein said temperature sensor means is disposed at each of the opposed magnetic poles. 4. The MRI magnetic field generator according to claim 1, further comprising a temperature controller for controlling the heater based on the temperature detected by the temperature sensor.