Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Superconductive type
Reexamination Certificate
1997-07-16
2003-06-17
Barrera, Ramon M. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Superconductive type
C335S299000, C324S319000, C324S320000
Reexamination Certificate
active
06580346
ABSTRACT:
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”) and, more particularly, to a superconducting magnet apparatus that has a large opening to thereby prevent a subject from feeling claustrophobic and to thereby allow an operator to have easy access to a subject.
BACKGROUND
FIG. 7
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 pictures are taken in this uniform magnetic filed
21
.
These coils are generally made by using superconducting wires, and thus need cooling 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 (containing liquid helium or the like). In the case of the example of
FIG. 7
, the main coils
13
to
18
and the shield coils
19
and
20
are placed in a coolant container
11
, which contains coolant
12
, such as liquid helium, for superconductivity by supported by means of supporting elements (not shown). Further, the coolant container
11
is held in a vacuum enclosure
10
.
Moreover, to hold 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 the refrigerator has been increased, so that the superconductor coils are sometimes cooled directly by the refrigerator without using the coolant container
11
.
However, in the case of the superconducting magnet apparatus illustrated in
FIG. 7
, 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 outside the superconducting magnet apparatus.
FIG. 8
illustrates another example of a conventional superconducting magnet apparatus for use in MRI system. This example is an open superconducting magnet apparatus of the horizontal magnetic field type. This example of the conventional superconducting magnet apparatus has been disclosed in the U.S. Pat. No. 5,410,287 and remedies the drawbacks of the aforementioned example of the conventional superconducting magnet apparatus of
FIG. 7
in that the measuring space causes a subject to feel claustrophobic and in that there is the difficulty in getting access to a subject by an operator. FIG.
8
(
a
) shows a sectional view of this example. FIG.
8
(
b
) shows an external view thereof. As shown in FIG.
8
(
a
), a set of three coils
23
A,
24
A and
25
A and another set of three coils
23
B,
24
B and
25
B are spaced apart from each other by a predetermined distance in such a manner as to be coaxial with the center axis
22
of a magnet. Further, a uniform magnetic field region
21
is generated at the halfway position between the sets of the coils. Coils of each of the sets are supported by supporting elements (not shown) and are directly cooled by a refrigerator. All of the coils of each of the sets are surrounded with thermal shields
9
A and
9
B that are held in vacuum enclosures
10
A and
10
B, respectively.
Coils
23
A,
23
B,
24
A and
24
B are main coils, through which electric currents flow in a same direction. Coils
25
A and
25
B are auxiliary coils, through which electric currents flow in a direction opposite to the direction of the current flow in the main coils. In the magnet having this configuration, the main coils
23
A,
23
B,
24
A and
24
B produce a magnetic field along the center axis
22
. Further, the auxiliary coils
25
A and
25
B enhance the magnetic homogeneity of the uniform magnetic field region
21
. Incidentally, this magnet does not use shield coils. However, a room, in which the superconducting magnet apparatus is installed, is magnetically shielded.
Further, as illustrated in FIG.
8
(
b
), the vacuum enclosures
10
A and
10
B facing each other in the lateral direction are shaped like doughnuts and are supported by two supporting posts
26
interposed therebetween. Thus, there is provided an open space between the vacuum enclosures
10
A and
10
B. A subject is inserted into the uniform magnetic field region
21
along the center axis
22
, which is illustrated in FIG.
8
(
a
), through the central bores of the vacuum enclosures
10
A and
10
B. Then, images of the subject are taken there.
In accordance with such a configuration, outward side surfaces of the uniform magnetic field region
21
serving as an imaging region are opened. Thus, a subject can avoid feeling claustrophobic. Moreover, an operator can easily get access to the subject from a side of the apparatus and further can use the images displayed on the screen of a monitor during an operation.
However, in the case of the superconducting magnet apparatus illustrated in
FIG. 8
, each of the sets of coils
23
A,
24
A and
25
A and coils
23
B,
24
B and
25
B and the vacuum enclosures
10
A and
10
B is shaped like a doughnut. Thus, a space between the doughnut-like vacuum enclosures
10
A and
10
B facing each other is not used as a region used for performing improvement in magnetic homogeneity. Therefore, it has been difficult to obtain favorable magnetic homogeneity over a large space. Further, magnetic fluxes generated by the superconducting coils return through the external space of the superconducting magnet apparatus, so that a leakage magnetic field becomes large. Thus, a large area is needed for installing the superconducting magnet apparatus. Alternatively, strong magnetic shielding should be performed.
FIG. 9
illustrates a third 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. This example of the conventional superconducting magnet apparatus has been disclosed in the U.S. Pat. No. 5,194,810. This magnet enhances the magnetic homogeneity of a uniform magnetic field region
21
by generating a magnetic field by the use of two sets of superconducting coils
31
and
31
, the respective sets of which are placed vertically in such a way as to face each other, and by providing iron shimming means
32
on the inner surfaces of the aforesaid superconducting coils
31
and
31
so as to obtain favorable magnetic field homogeneity. Moreover, this magnet has a structure in that upper and lower magnetic-field generating sources are mechanically supported by iron yokes
33
,
33
, . . . that further serve as return paths for magnetic fields generated by the upper and lower superconducting coils
31
and
31
.
In the case of this example of the conventional superconducting magnet apparatus, the uniform magnetic field region
21
is opened in all directions, a subject can avoid feeling claustrophobic. Moreover, an operator can easily get access to the subject. Further, magneti
Hino Noriaki
Kakugawa Shigeru
Kawano Hajime
Takeshima Hirotaka
Barrera Ramon M.
Hitachi Medical Corporation
Laubscher, Jr. Lawrence E.
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