JPH0719919B2 - Cryostat - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a cryostat for a superconducting magnet such as an MRI (magnetic resonance imaging apparatus).

(Prior Art) Conventionally, a superconducting magnet such as an MRI is kept at a low temperature and
There is a cryostat as shown in FIG. 3 that is configured to reduce the evaporation amount of the refrigerant as much as possible.

This cryostat has at least one heat shield 2 between a cryogenic refrigerant container 3 for accommodating the superconducting coil 5 and a vacuum container 7 in order to reduce heat entering the refrigerant by conduction and radiation. A method of actively cooling with a small refrigerator 1 or the like is adopted.

Further, the gradient coil 4 is excited in a pulsed manner in order to make a uniform magnetic field generated by the superconducting coil 5 into a gradient magnetic field and obtain an image signal by nuclear magnetic resonance.

(Problems to be Solved by the Invention) However, in the above-described structure, although the intrusion heat due to radiation and conduction can be reduced, the fluctuating magnetic field generated by the gradient coil 4 causes an eddy current in the refrigerant container 3,
This has the disadvantage that this loss is very large.

It is an object of the present invention to provide a cryostat that can reduce not only the heat of invasion due to radiation and conduction but also the loss due to the eddy current generated in the refrigerant container by the gradient coil.

[Structure of Invention]

(Means for Solving the Problem) In the cryostat of the present invention, a portion of the heat shield that is strongly influenced by the gradient coil, that is, a portion that exists between the superconducting coil and the gradient coil is provided with a cylindrical electrical insulating layer. Then, it is composed of two or more shield materials that are thermally coupled to each other and are made of materials having high electric conductivity.

When having a plurality of heat shields, some or all of them are constructed as described above.

(Operation) With this configuration, the multiple shield portions reduce the amount of the variable magnetic field generated by the gradient coil that penetrates the heat shield and reaches the refrigerant container, and reduces the eddy current loss in the refrigerant container.

(Embodiment) An embodiment of the cryostat according to the present invention will be described below with reference to FIG.

FIG. 1 shows the gradient coil 4 of the heat shield 2.
2 is a schematic configuration diagram in the case where a portion existing between the superconducting coil 5 and the superconducting coil 5 is doubled, in which 1 is a single-stage refrigerator, but the same applies to a refrigerator having two or more stages. . In other words, the same applies when the heat shield 2 has two or more stages. The heat shield 2 is made of a copper or aluminum plate. By making the above-mentioned parts of the heat shield 2 multiple, the leakage magnetic flux to the refrigerant container 3 can be reduced as compared with the conventional heat shield.

That is, when the magnetic flux generated by the gradient coil 4 escapes to the portion of the heat shield 2 that is arranged on the gradient coil 4 side, an eddy current that cancels the change in the magnetic flux is generated. Due to this eddy current, a heat load due to Joule heat generation is applied to the heat shield 2. At the same time, due to this eddy current, the magnetic flux passing through the portion of the heat shield 2 arranged on the gradient coil 4 side is reduced to some extent. The magnetic flux passing through the portion of the heat shield 2 further generates an eddy current in the portion of the heat shield 2 arranged on the superconducting coil 5 side, and is further weakened. The Joule heat generated in each part of the heat shield 2 is commonly cooled by the cold head of the refrigerator 1 because each part of the heat shield 2 is thermally coupled. This means that the number of heat shields can be selected regardless of the number of stages of the refrigerator 1, and the number can be selected so that the magnetic flux reaching the refrigerant container 3 is equal to or less than the allowable value.

(Other Embodiments) Almost the same effect can be obtained when the multiple shields 2 shown in FIG. 1 are replaced with multilayer shields having electrically insulated layers as shown in FIG.

〔The invention's effect〕

As is clear from the above description, according to the present invention, MRI
In a cryostat such as the one described above, not only the steady heat of invasion such as conduction and radiation, but also the eddy current loss generated in the refrigerant container when a gradient coil is applied can be suppressed to a small amount. It is possible to provide a cryostat with a small amount.

[Brief description of drawings]

FIG. 1 is a sectional view of a cryostat according to an embodiment of the present invention, FIG. 2 is a sectional view of a main portion of another embodiment, and FIG. 3 is a schematic configuration diagram of a conventional cryostat. 1 ... Small refrigerator, 2 ... Heat shield 3 ... Refrigerant container, 4 ... Gradient coil 5 ... Superconducting coil, 9 ... Insulating layer

Claims (1)

[Claims] 1. A cryostat in which a cryogenic refrigerant container for accommodating a superconducting coil, a heat shield and a vacuum container arranged so as to surround the container are formed into a cylindrical shape and a gradient coil is arranged in the cylinder. , A heat shield composed of two or more shield materials made of highly electrically conductive materials that are thermally coupled to each other with a cylindrical electric insulation layer interposed between the gradient coil and the superconducting coil A cryostat that is characterized by that.