JPH0356005Y2 - - Google Patents

Description

[Detailed description of the invention] Industrial field of application This invention is a permanent magnet used in medical nuclear magnetic resonance tomography (hereinafter referred to as NMR-CT), which can obtain cross-sectional images of objects and depict the properties of tissues. The present invention relates to a magnetic field generating device using a magnet, and relates to a magnetic field generating device that generates a strong, highly accurate, and uniform static magnetic field in a large space.

BACKGROUND ART
In order to obtain the desired tomographic image by inserting the device into the air, which forms a strong magnetic field of 10 kG, this magnetic field must be strong and uniformly stable with an accuracy of 10 ^-4 or less. As magnetic field generators for use in the field, there are two types of known magnetic field generators: normal conducting magnets made of conductive wires made of copper or aluminum wrapped around them in a cylindrical shape, and superconducting magnets that use special conducting wires cooled to a temperature close to absolute zero. There is.

The former is structurally inexpensive, but it requires a huge amount of electricity and cooling water to generate a sufficiently strong magnetic field, and has problems such as high running costs.
On the other hand, the latter type of superconducting magnet has the advantage of consuming less power and being able to generate a strong magnetic field in a small size, but it is essential to use expensive liquid helium as a coolant, which increases the running cost as well as the so-called initial cost. There are significant problems.

The present applicant has previously proposed a magnetic field circuit for NMR-CT that uses permanent magnets with magnetic field strength equal to or higher than the above-mentioned normal conducting magnets, consumes no power, and has low leakage magnetic field. A magnetic field generating device for magnetically coupling a magnetic pole piece and at least one permanent magnet with a yoke to generate a magnetic field in the air, characterized in that at least the permanent magnet or the magnetic pole piece is surrounded by a heat insulating material. proposed a magnetic field generator (Utility Model Application No. 152956/1983).

With the above magnetic field generator, it was possible to improve the uniformity of the magnetic field generated in the air and to reduce changes in the magnetic field strength due to changes in the ambient temperature of the entire device.

However, in order to obtain stable and clear images during diagnosis with NMR-CT, magnetic field uniformity of several ppm/hr is required in response to changes in the ambient temperature of the device. There was a problem in that it could not adequately meet the requirements for a highly stable uniform magnetic field.

Purpose of the invention In view of the current situation, this invention was developed to create a magnetic field generator using permanent magnets that can generate a strong magnetic field.
An object of the present invention is to provide a magnetic field generating device that can minimize changes in magnetic field strength due to changes in the ambient temperature of the device and can generate a highly accurate, uniform, and stable magnetic field.

Summary of the invention The inventor of this invention aimed to reduce the amount of change in ambient temperature by blocking changes in ambient temperature using heat insulating materials, etc.
After cooling the device to a temperature sufficiently lower than the ambient temperature (for example, about 10°C lower than the ambient temperature),
Furthermore, as a result of repeated various experiments from the viewpoint that by maintaining this temperature range with high precision, a highly stable uniform magnetic field can be achieved with high precision, we have developed a cooling device with a thermoelectric cooling element that utilizes the Peltier effect. We have discovered that a highly accurate, uniform, and stable magnetic field can be obtained by using a configuration in which a cooling device and other heating devices are used together.

That is, this idea forms a space and magnetically couples a pair of opposing permanent magnets with a yoke, fixes a magnetic pole piece to the opposing surface of each permanent magnet, and generates a magnetic field in the space. In the generator, at least one cooling device having an electronic cooling element utilizing the Peltier effect is disposed at a position where the device can be cooled, and the permanent magnet, magnetic pole piece, and yoke are surrounded by a heat insulating material. This is a magnetic field generating device characterized by the following.

Furthermore, by disposing at least one heating device in the magnetic field generating device having the above configuration, even more precise temperature control becomes possible.

Structure of the invention The permanent magnet used in the magnetic field generator of this invention is
Ferrite magnets, alnico magnets, and rare earth cobalt magnets can be used, but the applicant has previously proposed a new high-performance permanent magnet that does not require expensive Sm or Co. By using a permanent magnet (Japanese Patent Application No. 57-145072) containing at least one rare earth element, the device can be miniaturized.

Various configurations and shapes can be used for the yoke constituting the magnetic field generator. For example, a configuration in which a pair of plate-shaped yokes are arranged facing each other and connected by a plurality of column-shaped yokes, and a disc-shaped yoke. A structure in which the two are arranged facing each other and connected by a cylindrical yoke, or a square cylindrical yoke with the opening direction horizontal is used.

In addition, the magnetic pole piece that is fixed to the opposite surface of the permanent magnet has an annular projection with a predetermined inner diameter and height protruding from the periphery of the opposite surface, and a predetermined outer diameter and a predetermined outer diameter in the center of the pole piece. It is desirable to select an appropriate structure according to the required magnetic field strength and magnetic field uniformity, such as a structure in which a convex projection consisting of a height is provided, as in the case of a permanent magnet or a yoke.

The insulating material surrounding the permanent magnet, magnetic pole piece, and yoke is preferably made of a material that is non-magnetic, lightweight, thin, and adaptable to various shapes, and includes foamed plastics such as urea, phenol, and urethane, and rubber. Various insulation materials can be applied, including fibers such as felt, natural materials such as carbonized cork, asbestos, rock wool, glass fiber, refractories such as bricks and plastic refractories, and other gas insulation materials. Various materials may be selected depending on the shape of the magnet etc.

The cooling device, which is the main feature of this invention, can be configured as long as it has an electronic cooling element that utilizes the Peltier effect. In addition to placing a plate, etc., or placing a heat dissipating fin in the heat dissipating part of the electronic cooling element, the heat dissipating fin may be cooled by air, forced air cooling, water cooling, etc., and the Peltier effect may be used. As long as there is a configuration for effectively operating the electronic cooling element, known heat exchanger technology can be appropriately employed.

In this idea, the entire magnetic field generating device is cooled to a temperature range of 10 to 50 degrees Celsius lower than the ambient temperature, and subsequent changes in the ambient temperature are mitigated by insulation throughout the device, and a cooling device consisting of an electronic cooling element is used. The main focus is to make fine adjustments and maintain a constant level.

The Peltier effect is a phenomenon in which when a current is passed through a contact area between a metal and a semiconductor, heat other than Joule heat is generated or absorbed at the contact area, and when the direction of the current is reversed, the heat absorption area becomes the heat generation area. In addition, since the heat generating part is changed to a heat absorbing part, in the device of the present invention, it is possible to cool the device and then heat it only with the cooling device, but if even more stability is required, heating It is desirable to use a heating device equipped with a heater or the like.

The heat capacity and number of these cooling and heating devices should be selected depending on the size and shape of the magnetic field generator, and the locations should be placed at locations where temperature control can be performed most efficiently, that is, cooling and heating. It is desirable to appropriately select a position in consideration of transmission efficiency, etc., depending on the configuration of the magnetic field generator.

In addition, for temperature control once the temperature has been cooled to the required temperature range, a known temperature sensor is placed on a permanent magnet or magnetic pole piece, and the voltage applied to the cooling device or heating device is adjusted according to the detection output of the sensor. By controlling the above, the entire magnetic field generating device can be controlled to have a temperature fluctuation of ±2° C. or less, and extremely stable and highly accurate magnetic field adjustment is possible.

Disclosure of the invention based on drawings FIG. 1 is a longitudinal sectional front view of a magnetic circuit used in NMR-CT according to this invention.

The magnetic circuit shown in FIG. 1 is constructed by attaching a pair of Fe-B-R permanent magnets 1, 1 to the inner circumferential surface of the yoke 3 (the inner circumferential surface of the upper and lower yoke in the figure), and 4 is formed.

A pair of magnetic pole pieces 2, 2 disposed on opposing surfaces of the permanent magnets 1, 1 are provided with an annular protrusion 5 having a substantially trapezoidal cross section and having a predetermined inner diameter and height protruding from inside the periphery of the opposing surfaces. be.

Further, rock wool 6 is applied to the outer circumferential surfaces of the permanent magnet 1, the magnetic pole piece 2, and the yoke 3, so that they are integrally encapsulated.

The cooling device 7 is arranged at a position facing each permanent magnet 1 via the yoke 3, that is, on the yoke 3 closest to the permanent magnet 1.

The cooling device 7 includes a cooling plate 7b that comes into contact with the yoke 3 and a heat radiation fin 7c that is close to the cooling fan 7d.
An electronic cooling element 7 having a Peltier effect is provided between
It consists of a configuration in which a.

Function/Effect In the above configuration, the cooling device 7 cools the entire device once to about 10° C. lower than the ambient temperature (room temperature).

Once the device has been cooled, the effect of changes in ambient temperature is alleviated by the heat insulating rock wool 6, and the voltage applied to the cooling device 7 is controlled by a temperature sensor (not shown) attached to the permanent magnet 1. By controlling according to the detection output, it becomes possible to control the temperature change of the entire device to within ±2°C.

That is, even if a temperature change occurs around the NMR-CT, the current temperature of the permanent magnet 1 can be maintained, and the uniform magnetic field strength in the air can be stabilized.

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional front view of a magnetic circuit used in a magnetic field generating device according to this invention. 1... Fe-BR-based permanent magnet, 2... Magnetic pole piece,
3... Yoke, 4... Sky, 5... Annular projection, 6...
...Rock wool, 7...Cooling device, 7a...Electronic cooling element, 7b...Cooling plate, 7c...Radiation fin, 7d...Cooling fan.

Claims (1)

[Claims for Utility Model Registration] 1. A pair of opposing permanent magnets are magnetically coupled with a yoke to form a space, and a magnetic pole piece is fixed to the surface of each permanent magnet facing the space, and a magnetic field is applied to the space. In a magnetic field generating device that generates a A magnetic field generating device characterized by being surrounded by a material. 2. The magnetic field generating device according to claim 1, wherein the magnetic field generating device is provided with at least one heating device.