JPH05309079A - Magnetic resonance imaging device - Google Patents

Abstract

PURPOSE:To provide a refrigerating machine which can function effectively even in the vicinity of the magnetic resonance imaging device in which a superconductive coil for generating a strong magnetic field is contained. CONSTITUTION:Refrigerant gas of helium, etc., compressed by a compressor is fed into a refrigerating machine 7. An expansion engine is provided with a cylinder 13 having the center shaft in the vertical direction, and a piston 14 which can move vertically in its inside, the piston 14 divides a space in the cylinder 13 into an expansion chamber 15 of the lower side and an ordinary temperature chamber 16 of the upper side, and the lower part of the expansion chamber 15 becomes a cooling stage 17. To the upper end of the piston 14, a rod 18 is attached, and to the cylinder 13, an ultrasonic motor 20 having an output shaft 19 in the horizontal direction is attached as a driving means of the piston 14. By a scotch yoke 11 for converting a rotating motion to a reciprocating motion, a rotation of the ultrasonic motor 20 is converted to the reciprocating motion of the piston 14.

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 MRI apparatus) having a built-in superconducting coil, and more particularly to improvement of a refrigerator for reducing the evaporation amount of a refrigerant for cooling the superconducting coil.

[0002]

2. Description of the Related Art FIG. 1 shows a schematic diagram of the internal structure of an MRI apparatus. The living body 1 is placed in a static magnetic field, a gradient magnetic field is applied,
Irradiation with a high frequency pulse causes a magnetic resonance phenomenon in the living body. An MRI apparatus is widely used in which a magnetic resonance signal generated from the inside of a living body is detected by a coil 2 and a magnetic resonance image of the living body is obtained using this signal. Image S / N
Since the ratio is proportional to the strength of the static magnetic field, a static magnetic field as strong as possible is required, and in order to achieve high resolution,
High uniformity and stability are required over a wide area. Superconducting magnets are widely used as magnets that meet this demand. However, achieving a superconducting state
A cryogenic temperature is required to maintain the temperature. For example, the superconducting coil 3 is immersed in liquid helium 4 to make a vacuum state 5 on the outer side thereof, and the outer side thereof is filled with liquid nitrogen 6. It takes the form of multiple thermos, which uses a refrigerator 7 to reduce the evaporation of 6. Conventionally, an electric motor such as a synchronous motor has been used as a driving means for a refrigerator, as disclosed in Japanese Patent Laid-Open No. 3-152353.

[0003]

With the sophistication of MRI systems, stronger static magnetic field strength tends to be adopted.
In an MRI apparatus having a static magnetic field strength of about T (Tesla) or more, the leakage magnetic flux becomes considerably strong. With conventional electric motors, normal operation is difficult unless a large amount of iron shield is applied in order to eliminate the effects of such leakage magnetic flux.
There was a problem that the iron shield disturbed the uniformity of the magnetic field.

An object of the present invention is to provide a refrigerator that can effectively function without magnetic shielding even in the vicinity of a magnetic resonance imaging apparatus which requires a strong magnetic field.

[0005]

In order to achieve the above object, the present invention uses an ultrasonic motor as a drive source of a refrigerator.

[0006]

[Operation] Since the ultrasonic motor is made of non-magnetic material and its driving method does not include a magnetic mechanism,
Not affected by magnetic field. Therefore, it is possible to provide a refrigerator that can effectively function even in the vicinity of a magnetic resonance imaging apparatus that requires a strong magnetic field. Further, since the ultrasonic motor has a smaller rotation sound than the electric motor, it is possible to reduce the operation sound of the refrigerator.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 shows an MR to which the present invention is applied.
It is a block diagram of the refrigerator for I apparatuses. Compressor 10
The refrigerant gas such as helium compressed by is sent to the refrigerator 7. The refrigerator 7 includes a cylinder 13 having a vertical center axis and a piston 14 capable of moving vertically in the cylinder 13.
The piston 14 divides the space inside the cylinder 13 into a lower expansion chamber 15 and an upper room temperature chamber 16, and the lower part of the expansion chamber 15 serves as a cooling stage 17. A rod 18 is attached to the upper end of the piston 14, and an ultrasonic motor 20 having a horizontal output shaft 19 as a drive means of the piston 14 is attached to the upper part of the cylinder 13. The output shaft 19 and the upper end of the rod 18 are provided with a Scotch yoke 11 that converts rotational movement into reciprocating movement.
The piston 14 is reciprocated by the rotation of the ultrasonic motor 20.

The compressor 10 and the refrigerator 7 are connected by a refrigerant pipe 8. The refrigerant pipe 8 is composed of a high-pressure pipe 8a connected to the discharge side of the compressor 10 and a low-pressure pipe 8b connected to the suction side. A high-pressure switching valve 21a is provided in the middle of the high-pressure pipe 8a, and a low-pressure pipe 8b is provided in the middle of the low-pressure pipe 8b. The low pressure switching valves 21b are installed respectively. The downstream end of the high-pressure pipe 8a and the upstream end of the low-pressure pipe 8b join together, and the joining part and the expansion chamber 15 communicate with each other via the regenerator 12. Further, one end of a pipe 8d is connected to the pipe 8c between the merging portion and the regenerator 12, and the other end of this pipe communicates with the room temperature chamber 16. Then, the ultrasonic motor 20 is operated to reciprocate the piston 14, and the high pressure switching valve 21a and the low pressure switching valve 21b are alternately opened and closed in synchronization with the operation of the piston 14, whereby the high pressure refrigerant gas from the compressor 10 is discharged. Is supplied to and discharged from the refrigerator 7.

When the piston 14 is raised and the high pressure switching valve 21a is opened and the low pressure switching valve 21b is closed, the high pressure refrigerant gas is supplied to the expansion chamber 15 via the regenerator 12. On the other hand, when the piston 14 is lowered, the high pressure switching valve 21a is closed, and the low pressure switching valve 21b is opened, the refrigerant gas in the expansion chamber 15 is returned to the compressor 10 through the regenerator 12. Adiabatic expansion of the refrigerant gas during this time causes the cooling stage 17 to generate cold.

As an example of an electric motor used in a conventional MRI apparatus, for example, the volume of a liquid nitrogen tank is 2
00l, nitrogen evaporation 0.8l / hr (when using refrigerator)
In the case of the above, a synchronous motor having a torque of 18 kg · cm and a rotation speed of about 60 rpm is used. To date, as an ultrasonic motor, a part of the longitudinal vibration of the piezoelectric element is converted into a torsional vibration by a torsion coupler, and by combining the longitudinal vibration and the torsional vibration, a rotational speed of 120 rpm, a torque of 13 kg · cm, an efficiency of 80%. Has been devised,
It can meet the performance required for refrigerators for MRI equipment.

In the embodiment, an example of using an ultrasonic motor that makes a rotary motion is shown, but by using an ultrasonic motor that makes a linear motion, the same effect can be obtained when the piston is directly moved up and down. ..

Although the cooling of nitrogen is described in the embodiment,
It is clear that a similar structure holds for helium.

[0013]

According to the present invention, it is possible to provide a refrigerator which can effectively function even near a magnetic resonance imaging apparatus which requires a strong magnetic field.

[Brief description of drawings]

FIG. 1 is a block diagram showing the internal structure of a magnetic resonance imaging apparatus.

FIG. 2 is a block diagram of a refrigerator showing an embodiment of the present invention.

[Explanation of symbols]

7 ... Refrigerator, 8a ... High-pressure piping, 8b ... Low-pressure piping, 10 ...
Compressor, 11 ... Scotch yoke, 12 ... Regenerator,
13 ... Cylinder, 14 ... Piston, 15 ... Expansion chamber, 17
... cooling stage, 18 ... rod, 19 ... output shaft, 20 ...
Ultrasonic motor, 21a ... High pressure valve, 21b ... Low pressure valve.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location 8203-2G G01R 33/22 S

Claims (1)

[Claims] 1. A magnetic resonance imaging apparatus having a built-in superconducting coil for generating a static magnetic field, wherein the refrigerator is used for maintaining a low temperature state of a refrigerant for cooling the superconducting coil. A magnetic resonance imaging apparatus using a sound wave motor.