Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
2000-08-01
2004-09-21
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S216000
Reexamination Certificate
active
06794973
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a magnetic field generator used in a medical-use magnetic resonance imaging (hereinafter referred to as MRI) device, and more particularly to an MRI magnetic field generator with reduced residual magnetism and eddy current generated by the effect of the pulse current flowing through Gradient magnetic field coils.
BACKGROUND ART
FIGS.
13
(
a
) and (
b
) illustrate a known structure of an MRI magnetic field generator. In this structure, each of a pair of pole pieces
2
is fastened, with the pole pieces
2
facing each other, at one end of each of a pair of permanent magnet structures
1
comprising a plurality of block-shaped R-Fe-B-based magnets that have been integrated as the field generation source the other ends of the permanent magnet structures are connected to a yoke
3
, and a static magnetic field is generated within the air gap
4
between the pole pieces
2
.
In the figure,
5
is an annular protrusion formed in order to increase the uniformity of magnetic field distribution within the air gap
4
, and another known structure is one in which a tiered protrusion (not shown) is formed on the inside of the annular protrusion in an effort to further increase the uniformity of the field distribution.
In the figure,
6
is a tilt field coil, which is disposed in order to obtain information about positioning within the air gap
4
. These Gradient magnetic field coils
6
usually comprise a group of three coils corresponding to the three directions X, Y, and Z within the air gap
4
, but are shown in simplified form in the figure.
With a structure such as this, the air gap
4
must be large enough for all or part of a patient's body to be inserted therein, and a static magnetic field having a high uniformity of 1×10
−4
or less at 0.02 to 2.0 T must be formed within a specified image field of view within the air gap
4
.
With the structure shown in FIGS.
13
(
a
) and (
b
), a so-called four-column yoke consisting of a pair of yoke plates
3
a
and
3
b
and four yoke columns
3
c
is used as the yoke
3
, but as shown in FIGS.
14
(
a
) and (
b
), variously structured yokes can be used according to the required characteristics, such as a so-called C yoke consisting of a pair of yoke plates
3
a
and
3
b
and a supporting yoke plate
3
d.
With the structure shown in FIGS.
13
(
a
) and (
b
), permanent magnets such as R-Fe-B-based magnets are employed as the field generation source, but other structures can also be used, such as one in which an electromagnetic coil is wound around the periphery of an iron core.
Regardless of which of these structures is used, the air gap
4
is formed by the pair of pole pieces
2
, and Gradient magnetic field coils
6
are disposed in the vicinity of the pole pieces
2
, as shown in FIGS.
13
(
a
) and (
b
).
Usually, the pole pieces
2
are made from electromagnetic soft iron, pure iron, or another such bulk material (integrated), so when a pulse current is passed through the Gradient magnetic field coils
6
and a pulse-form tilt field is generated in the desired direction in order to obtain information about positioning within the air gap
4
, the effect of this tilt field generates an eddy current in the pole pieces
2
and decreases the rise characteristics of the tilt field, and even after the flow of the pulse current has been halted, the uniformity of the field distribution in the air gap
4
is decreased by the residual magnetism generated in the pole pieces
2
.
As a means for solving this problem, MRI magnetic field generators characterized in that the main portion of the pole pieces is formed from laminated silicon steel sheets have already been proposed by the inventors (Japanese Patent No. 2,649,436, Japanese Patent No. 2,649,437, U.S. Pat. No. 5,283,544, and European Patent No. 0479514).
The MRI magnetic field generators previously proposed by the inventors are chiefly characterized by the use of pole pieces structured as shown in
FIGS. 16
to
18
.
The structure of the pole piece
10
shown in FIGS.
15
(
a
) and (
b
) comprises a soft iron magnetic ring having a rectangular cross-sectional shape and constituting an annular protrusion
12
on the air gap-facing side of a magnetic base member
11
composed of pure iron or another bulk material, and a plurality of laminated blocks
13
produced by laminating a plurality of silicon steel sheets in the facing direction of the pole pieces and integrating these with an insulating adhesive agent or the like.
In the figure,
14
is a tiered protrusion formed on the inside of the annular protrusion
12
for the purpose of enhancing the uniformity of the field distribution. Just as discussed above, a plurality of silicon steel sheets are laminated in the facing direction of the pole pieces and integrated with an insulating adhesive agent or the like, and the resulting plurality of laminated blocks are laminated in the required number.
15
in the figure is a soft iron core used for mounting the field generation coil.
16
in the figure is a slit formed in the radial direction for the purpose of dividing the soft iron magnetic ring having a rectangular cross-sectional shape and constituting the annular protrusion
12
into a plurality of sections in the circumferential direction and reducing the eddy current that is generated at the annular protrusion
12
.
If the silicon steel sheets used in the above-mentioned laminated blocks
13
are directional silicon steel sheets (JIS C 2553, etc.), then from the standpoint of field distribution uniformity, it is preferable for them to be laminated and integrated such that the readily magnetizable axis direction (calendering direction) is rotated by 90 degrees every specific number of small blocks
13
a
and
13
b
as shown in FIG.
16
(
a
). If the sheets are non-directional silicon steel sheets (JIS C 2552, etc.), then lamination and integration are performed merely in the thickness direction, without taking directionality into account, as shown in FIG.
16
(
b
).
The structure of the pole pieces
20
shown in FIGS.
17
(
a
) and (
b
) comprises a soft iron magnetic ring having a rectangular cross-sectional shape and constituting an annular protrusion
22
on the void-facing side of a magnetic base member
21
composed of pure iron or another bulk material, and a plurality of laminated blocks
23
produced by laminating a plurality of non-directional silicon steel sheets in the direction perpendicular to the facing direction of the pole pieces and integrating these with an insulating adhesive agent or the like.
In the figure,
24
is a tiered protrusion formed on the inside of the annular protrusion
22
for the purpose of enhancing the uniformity of the field distribution,
25
is a soft iron core used for mounting the field generation coil, and
26
is a slit that divides the soft iron magnetic ring having a rectangular cross-sectional shape and constituting the annular protrusion
22
into a plurality of'sections in the circumferential direction.
It is preferable for the above-mentioned laminated blocks
23
to be laminated and integrated with an insulating adhesive agent or the like such that the lamination direction is rotated by 90 degrees for every one of the small blocks
23
a
and
23
b
laminated in the void-facing direction, as shown in FIG.
17
(
c
).
The structure of the pole pieces
30
shown in FIGS.
18
(
a
) and (
b
) is quite different from that of the pole pieces
10
and
20
shown in
FIGS. 15
(
a
) and (
b
) and FIGS.
17
(
a
) and (
b
), respectively, in that the magnetic base members
11
and
21
composed of a bulk material are not used. Specifically, this structure is such that, instead of the magnetic base members
11
and
21
composed of a bulk material, laminated rods
33
, produced by laminating a plurality of non-directional silicon steel sheets, as shown in FIG.
18
(
c
), in the direction perpendicular to the facing direction of the pole pieces and laminating these with an insulating adhesive agent or the like, are supported by an annu
Aoki Masaaki
Hashimoto Shiego
Donovan Lincoln
Dykema Gossett PLLC
Sumitomo Special Metals Co. Ltd.
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