Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Superconductive type
Reexamination Certificate
2002-07-10
2004-08-24
Barrera, Ramon M. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Superconductive type
C324S319000, C324S320000, C335S296000, C335S297000, C335S298000, C335S301000
Reexamination Certificate
active
06781492
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a static magnetic field generating apparatus used in a magnetic resonance imaging apparatus (hereafter referred to as MRI apparatus) and the like, and in particular to a magnetic field correction means for improving the field homogeneity of the static magnetic field generated by a static magnetic field generating apparatus.
Furthermore, the present invention relates to a superconducting magnet apparatus having a wide opening to provide a subject with feeling of openness and facilitate access to the subject.
Furthermore, the present invention relates to a superconducting magnet apparatus which can be constructed with ease and high precision after it has been carried in a site of use such as a hospital or a clinic.
BACKGROUND ART
In the case of MRI apparatuses, characteristics of the static magnetic field are the most important characteristics exert directly upon the image distortion, image blurredness, and signal-to-noise ratio, and the characteristics of the static magnetic field are the most important characteristics determining the fundamental performance in the MRI apparatus. Therefore, the static magnetic field in an imaging space (also called measurement space) is required to have high homogeneity and high stability of several ppm order. In order to generate a highly homogeneous and highly stable static magnetic field, a static magnetic field generating apparatus is used.
In the static magnetic field generating apparatus, there occurs a problem that a lot of magnetic fields leak outside the apparatus because a high static magnetic field is generated in the imaging space.
In order to reduce the leakage magnetic field, paths of lines of magnetic flux (magnetic path) generated from a magnetic field generating source are formed and the lines of magnetic flux are converged to the magnetic paths. By doing so, it is attempted to reduce the leakage of the magentic field to the outside of the apparatus.
An example of a conventional static magnetic field generating apparatus is shown in FIG.
21
.
FIG. 21
is an oblique view of the static magnetic field generating apparatus as a whole. In a static magnetic field generating apparatus
1
shown in
FIG. 21
, a high static magnetic field is generated in an imaging space
2
by static magnetic generating sources
3
disposed above and below the imaging space
2
so as to have the imaging space
2
between. Each of the static magnetic field generating sources
3
is formed by a superconducting coil
4
and a cooling vessel
5
for cooling the superconducting coil to a superconducting state. Magnetic circuits are formed so that lines Bz of magnetic flux generated in the vertical direction by the upper and lower superconducting coils
4
will be caught by platelike ferromagnetic substances
6
disposed above and below the static magnetic field generating sources
3
and returned to the superconducting coils
4
by using pillarlike ferromagnetic substances
7
disposed at the sides of the static magnetic field generating sources
3
as return paths. The pillarlike ferromagnetic substances
7
not only serve as the return paths of the lines of magnetic flux but also function to mechanically support structural components disposed above and below the imaging space
2
. As materials of the platelike ferromagnetic substances
6
and the pillarlike ferromagnetic substances
7
, iron is typically used from the viewpoint of the mechanical strength and cost price.
In the conventional static magnetic field generating apparatus of the horizontal magnetic field scheme, the magnetic circuits take the shape of a solenoid and the return paths are disposed symmetrically. Therefore, magnetic moments of the pillarlike portions are also uniform in the circumferential direction. In the case of the magnetic circuits having the return paths formed by the pillarlike ferromagnetic substances
7
, however, distribution of the lines of magnetic flux in directions having the pillarlike ferromagnetic substance
7
differs from that in directions having no pillarlike ferromagnetic substance
7
. Therefore, the magnetic field distribution in the circumferential direction varies. As a result, the static magnetic field in the imaging space
2
becomes inhomogeneous because of this variation of the magnetic field distribution in the circumferential direction. From the stage of design, therefore, correction of the inhomogeneous magnetic field components is considered. In the example of the conventional technique shown in
FIG. 21
, ferromagnetic substance pieces
8
are disposed on the surface of each of the cooling vessels
5
to conduct the magnetic field correction.
As a method for evaluating the magnetic field homogeneity, there is well known a method of determining a measuring point on a spherical surface to be evaluated in the static magnetic field, expanding the value of the magnetic field at the measuring point with the Legendre functions P
n
m
(cos &thgr;) as shown in equation (1):
B
Z
=
∑
n
=
0
∞
∑
m
=
0
n
r
n
P
n
m
(
cos
θ
)
&LeftBracketingBar;
A
(
m
,
n
)
cos
m
φ
+
B
(
m
,
n
)
sin
m
φ
&RightBracketingBar;
(
1
)
where (r, &thgr;, &phgr;) means a spherical coordinate system, r a distance from the coordinate center, &thgr; an angle formed with respect to the Z axis, and &phgr; a rotation angle taken with center on the Z axis. Bz is a Z direction component magnetic flux density.
The smaller the value of the term A(m, n) in the equation (1) becomes as compared with the value of the static magnetic field term A(0, 0), the better the magnetic field homogeneity becomes. Hereafter, “the ratio of the coefficient of each expansion term to the term A(0, 0)” is referred to as expansion coefficient ratio.
Components generated from the equation (1) can be broadly classified into “axisymmetrical components” and “unaxisymmetrical components.” The axisymmetrical components are magnetic field components which are symmetrical with respect to the Z axis (the static magnetic field direction). The axisymmetrical components depend on only the coordinates of (r, &thgr;). Mathematically, the axisymmetrical components are terms of the Legendre functions with m=0. The unaxisymmetrical components depend on all coordinates of (r, &thgr;, &phgr;). The unaxisymmetrical components are terms of the Legendre functions with m≠0.
Typically as means for correcting the magnetic field homogeneity of the static magnetic field, ferromagnetic substance pieces (hereafter referred to as iron pieces because of use of iron)
8
are disposed on an opposed surface of opposed cooling vessels. The positions of the iron pieces
8
are determined so as to cancel the inhomogeneous magnetic field components. The greater the inhomogeneous magnetic field components are, therefore, the more amount of the iron pieces
8
, i.e., the wider region for accommodating the iron pieces
8
is needed. As a result, the region (clear bore) for accommodating gradient magnetic coils (GCs) and a high frequency coil (RF coil) cannot be utilized effectively.
As a countermeasure against this, it is also conceivable to increase the region accommodating the iron pieces
8
by widening the distance between the opposed cooling vessels
5
. If the distance between the opposed cooling vessels
5
is widened, however, the distance between opposed superconducting coils
4
is also increased. If the distance between opposed superconducting coils
4
is increased, magnetomotive force for generating a desired static magnetic field intensity increases in proportion to the third to fifth power of the distance. The increase in the magnetomotive force causes a complicated mechanical structure and an increased cost, resulting in a serious problem.
As heretofore described, the conventional magnetic field generating apparatus had a problem that an attempt to correct the inhomogeneous magnetic field in the static magnetic field caused an
Hommei Takao
Kawano Hajime
Kuroda Shigenori
Takeshima Hirotaka
Tanabe Hajime
Antonelli Terry Stout & Kraus LLP
Barrera Ramon M.
Hitachi Medical Corporation
LandOfFree
Superconducting magnetic apparatus does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Superconducting magnetic apparatus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Superconducting magnetic apparatus will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3322650