Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Superconductive type
Patent
1997-09-05
1999-08-10
Donovan, Lincoln
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Superconductive type
335299, 324318, H01F 100
Patent
active
059364980
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a superconducting magnet apparatus that is suitable for use in a magnetic resonance imaging system (hereunder referred to as "MRI system"). More particularly, the present invention relates to a superconducting magnet apparatus having a large opening to thereby prevent a subject from feeling claustrophobic, to thereby allow an operator to have easy access to a subject, to thereby reduce magnetic field leakage and to thereby cut down the manufacturing cost, and also relates to an MRI system using such a superconducting magnet apparatus.
BACKGROUND ART
FIG. 11 illustrates an example of a conventional superconducting magnet apparatus for use in MRI system. This example is a superconducting magnet apparatus of the horizontal magnetic field type. This superconducting magnet apparatus is composed of small-diameter main coils 13, 14, 15, 16, 17 and 18 and large-diameter shield coils 19 and 20 and is adapted to produce a horizontal (namely, Z-axis direction) magnetic field. In this example, the main coils 13 to 18 are placed to produce a magnetic field along the center axis 22 of a magnet, while the shield coils 19 and 20 are placed to shield magnetic field leakage to the surroundings thereof. With such a configuration of the magnet, a uniform magnetic field region 21 of magnetic homogeneity of about 10 ppm or less is formed in a magnetic field space. Magnetic resonance imaging is performed in this uniform magnetic field 21.
These coils are made by using superconducting wires, and thus are required to be cooled to a predetermined temperature (for example, liquid-helium temperature (namely, 4.2 K) in the case of alloy superconductors; and liquid-nitrogen temperature (namely, 77 K) in the case of oxide superconductors). The coils are, therefore, held in a cooling vessel consisting of a vacuum enclosure, a thermal shield and a coolant container (which contains liquid helium or the like). In the case of the example of FIG. 11, the main coils 13 to 18 and the shield coils 19 and 20 are placed in the coolant container 11 containing coolant 12, such as liquid helium, for superconductivity, and are supported by means of supporting elements (not shown). Further, the coolant container 11 is held in the vacuum enclosure 10.
Moreover, to keep each of the coils at a low temperature, the thermal shield is maintained at a constant temperature by using a refrigerator (not shown), or the evaporation of coolant 12 for superconductivity is reduced. Recently, the performance of refrigerators has been increased, so that the superconductor coils are sometimes, cooled directly by the refrigerator without using the coolant 12 for superconductivity.
However, in the case of the superconducting magnet apparatus illustrated in FIG. 11, an opening, in which a subject is accommodated and images of the subject are taken, is narrow and moreover, a measuring space is surrounded, so that subjects sometimes feel claustrophobic. Thus, occasionally, subjects refuse to enter the opening of the apparatus for examination. Furthermore, it is difficult for an operator to get access to a subject from the outside of the superconducting magnet apparatus.
FIG. 12 illustrates a second example of a conventional superconducting magnet apparatus for use in MRI system. This example is a superconducting magnet apparatus of the vertical magnetic field type. FIG. 12(a) is a schematic external view of this apparatus. FIG. 12(b) is a sectional view taken in the direction of an arrow A of FIG. 12(a). This example of the conventional superconducting magnet apparatus is disclosed in the U.S. Pat. No. 5,194,810. In this magnet, a magnetic field is produced by two sets of superconducting coils 23 and 23, which are placed vertically in a line in such a manner as to face each other. Further, iron shimming means 24 are provided inside each of the aforesaid superconducting coils 23 and 23 so as to obtain favorable magnetic homogeneity. Thereby, this magnet enhances the magnetic homogeneity of the uniform magnetic field region
REFERENCES:
patent: 5402094 (1995-03-01), Enge
patent: 5410287 (1995-04-01), Laskaris
patent: 5545997 (1996-08-01), Westphal et al.
Kakugawa Shigeru
Kawano Hajime
Takeshima Hirotaka
Donovan Lincoln
Hitachi Medical Corporation
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