Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Superconductive type
Reexamination Certificate
2002-04-12
2004-03-16
Barrera, Ramon M. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Superconductive type
C335S301000, C324S320000
Reexamination Certificate
active
06707359
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a superconductive magnet device to improve uniformity of a uniform magnetic field space between a pair of superconductive magnet bodies.
2. Background Art
Recently superconductive magnet devices have come to be widely used as a generating source of a static magnetic field that has a high intensity and a temporal stability. In particular, superconductive magnet devices have been popularly employed as a generating source of a static magnetic field in medical tomography equipment (magnetic resonance imaging equipment) or silicon single crystal pickup devices. Most of those conventional superconductive magnet devices were of cylindrical solenoid type, while more popularly used recently are such devices provided with two superconductive magnets horizontally or vertically opposed each other with a large clearance therebetween in order to utilize their large opening and spacious magnetic field area.
In the field of superconductive magnet devices for MRI equipment, for example, a vertically opposed type superconductive magnet device provided with a wide opening is rapidly increasing, which brings about a comfort of roominess to a patient while providing a better accessibility to the patient for an examination staff.
FIG. 18
is a perspective view showing an appearance of, for example, a vertically opposed type superconductive magnet device used in MRI equipment, and
FIG. 19
is a schematic sectional drawing and
FIG. 20
is a schematic circuit diagram thereof, respectively. Referring to
FIGS. 18
to
20
, (1) high intensity of magnetic field, (2) temporal stability of magnetic field, and (3) uniformity of magnetic field are required as a performance of magnetic field in image pickup area
1
of a superconductive magnet device for MRI.
Moreover, it is also required to reduce the amount of evaporation of liquid helium and to reduce leak magnetic field. For the purpose of attaining a high magnetic field intensity satisfying a required performance of a magnetic field of a vertically opposed type superconductive magnet device for MRI, current density of annular superconductive coils
2
,
3
is increased so that an intense magnetic field is generated. Also, for the purpose of attaining temporal stability of the magnetic field, a permanent current switch
4
is employed for permanent current mode operation so that a magnetic field becomes temporally super-stable. Further, for the purpose of attaining uniformity of the magnetic field, a plurality of annular superconductive coils
2
,
3
are provided so that a higher uniformity is achieved. Then the respective annular superconductive coils
2
,
3
are connected in series, through which an identical current is applied. A permanent current switch
4
is connected in parallel to these annular superconductive coils
2
,
3
, and ON/OFF of the permanent current switch
4
is conducted by applying or interrupting a current to a heater
5
for the permanent current switch for magnetization or demagnetization, thereby a permanent current mode being achieved. Coil protecting elements
6
,
7
are provided at appropriate points for protection against a high voltage generated at the time of normal conduction transition (quench) in the magnetization or demagnetization. Current leads between coils, and a part of leads for the permanent current switch
4
or the coil protecting elements
6
,
7
are connected through connecting tubes
8
,
9
.
For the purpose of reducing evaporation amount of liquid helium, cryogenic containers
10
,
11
are entirely covered with vacuum adiabatic containers
12
,
13
and, further, one or two thermal shield baths (not shown) or superinsulation materials (not shown) are provided between the cryogenic containers
10
,
11
and the vacuum adiabatic containers
12
,
13
. In addition, the annular superconductive coils
2
,
3
, the cryogenic containers
10
,
11
and the vacuum adiabatic containers
12
,
13
form superconductive magnet bodies
14
,
15
as a whole. Also, the thermal shield baths are cooled by a refrigerator not shown. □ For the purpose of lowering leak magnet field, upper and lower magnetic shield plates
16
,
17
made of a ferromagnetic member are provided outside the respective vacuum adiabatic containers
12
,
13
, and yokes of a ferromagnetic body
18
,
19
are placed between the magnetic shield plates
16
,
17
to secure them.
In designing a superconductive magnet device, number of units of annular superconductive coils is determined, and also dimensions, positioning, number of windings, current density, etc. of the annular superconductive coils
2
,
3
are strictly determined taking into consideration a magnetic field generated in the image pickup area
1
by magnetic moment of yokes
18
,
19
made of a ferromagnetic material, in such a manner that all the error magnetic field components become substantially zero. In the case of a superconductive magnet device for MRI, in general, a deviation of 1 mm in dimensions or positioning will result in an influence mounting to several tens ppm in overall error magnetic field components.
Accordingly, in designing the superconductive magnet device, the dimensions, positioning, number of windings or current density have to be determined through a strict optimization so that all the error magnetic field components become substantially zero, while it is usual that uniformity of a magnetic field has a range of several hundreds ppm when actually magnetized, because of dimension tolerance in the manufacture or magnetism existing in the employed materials, etc. Particularly in the design of a vertically split type superconductive magnet device, the uniformity tends to be inferior, as compare with the conventional cylindrical solenoid type superconductive magnet device, to such an extent that a positioning error between upper and lower superconductive magnet bodies
14
,
15
is added.
For the purpose of compensating the uniformity deteriorated to over hundreds ppm, as well as circumstantial influences due to magnetism of structural steel of a room in which a superconductive magnet device is installed or that of peripheral equipments, thereby improving the uniformity in the state of practical use, iron shims in the form of fine chips have been conventionally used.
Referring to
FIGS. 18 and 19
, reference numerals
20
,
21
are iron shim chips mounted on the gap side surface of the superconductive magnet bodies
14
,
15
, numerals
22
,
23
are iron shim chips mounted on bore portions
14
a
,
15
a
respectively provided at the center of superconductive magnet bodies
14
,
15
. In mounting the iron shim chips
20
to
23
, from the viewpoint of adjusting the uniformity in the image pickup area
1
, a smaller amount of iron shim chips can perform a greater compensation effect in the central region of the gap side surface closer to the image pickup area
1
, and in a region closer to the gap within the bore portions
14
a
,
15
a.
In this manner, the uniformity of magnetic field can be improved by placing a plurality of iron shim chips
20
to
23
on the gap side surface of the upper and lower superconductive magnet bodies
14
,
15
and in central bore portions
14
a
,
15
a
, as well as by adjusting number of the iron shim chips
20
to
23
. Practically, magnetic moment of the iron shim chips
20
to
23
and magnetic field components generated by the magnetic moment in the image pickup area
1
must be precisely analyzed in advance based on the magnetic field intensity applied to the iron shim chips
20
to
23
of the respective positions, and then compensation amount of each of the magnetic field components must be determined based on the analysis of error magnetic field components that generate a uniformity of several hundreds ppm, thereby optimizing the positions and quantity of the iron shim chips
20
to
23
to be mounted. Usually, it is difficult to attain a desired uniformity in just one execution of works, and therefore the process has to be repeated
Kuroda Shigenori
Tanabe Hajime
Yoshida Naoji
Mitsubishi Denki & Kabushiki Kaisha
Sughrue & Mion, PLLC
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