Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
2002-03-15
2003-03-25
Barrera, Ramon M. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S297000, C335S301000, C324S320000
Reexamination Certificate
active
06538545
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to magnets which are suitable for use in magnetic resonance imaging apparatus and more particular, to an open magnet which has a wide opening and generates a uniform magnetic field.
Since a magnetic resonance imaging apparatus (which will be referred to as the MRI apparatus, hereinafter) utilizes a nuclear magnetic resonance phenomenon caused when an electromagnetic wave is irradiated on an object to be inspected placed in a uniform static magnetic field space to obtain an image indicative of its chemical property, the MRI apparatus is used especially in medical fields.
Because the MRI apparatus includes, as its main components, a means for applying a uniform static magnetic field in an imaging volume, an RF coil system for irradiating and receiving an electromagnetic wave, and a means for applying a gradient magnetic field to give position information about a resonance phenomenon.
An MRI apparatus is roughly divided into two types, that is, a horizontal magnetic field type wherein an imaging volume is provided in an interior space of a group of coils made in the form of a coaxial multilayer, and a vertical magnetic field type (open type) wherein a group of coils is provided so as to receive an imaging volume therein and to be opposed thereto. The latter is predominantly used nowadays because it can lighten mental burden to a person to be inspected due to its open structure and can remarkably improve inspector's access ability to a patient to be inspected.
FIG. 2
shows, in cross sectional view, an example of an arrangement of an open MRI apparatus. The apparatus includes, as its main components, magnetic poles
1
a
and
1
b
for applying a uniform magnetic field to an imaging volume
10
, superconducting coils
5
a
and
5
b
, cryostats
6
a
and
6
b
, gradient magnetic field coils
7
a
and
7
b
for providing position information of a resonance phenomenon, RF coil systems
8
a
and
8
b
for irradiating and receiving electromagnetic wave, and uniformity adjusters
9
a
and
9
b
for adjusting the uniformity of a magnetic field in an imaging volume, these components being disposed to surround the imaging volume (measuring space)
10
and to be opposed thereto.
The MRI apparatus is being made higher in magnetic field, since an increase in the intensity of static magnetic field enables increase of the intensity of a resonance signal, reduction of an imaging time and a higher level of imaging function. In an open magnet, shift is being made from a conventional type using a permanent magnet to a type using superconducting coils and ferromagnetic material (magnetic poles).
In the MRI apparatus, an magnetic field is required to have a uniformity of 10 ppm or less in a predetermined imaging volume. As prior art methods for generating a uniform magnetic field in a magnet for use in an open MRI apparatus having magnetic poles, there exist techniques wherein ring-shaped magnetic materials or magnets or contour-formed magnetic materials are positioned in magnetic pole portions arranged in an opposed manner across an imaging volume to control a flow of magnetic flux between opposing magnetic poles.
In these techniques of generating a uniform static magnetic field, since magnetic poles are formed or arranged to be symmetric with respect to an axis as its major means for generating a uniform magnetic field, a uniform magnetic field is generated by canceling each other or compensating for magnetic fields of inhomogeneous field components distributed axi-symmetrically. In such a system that the presence of an inhomogeneous magnetic field distributed non-axi-symmetrically cannot be ignored, it is difficult to compensate for it. Accordingly in these methods, a separate means or device for compensating for the non-axi-symmetric inhomogeneous magnetic field is required. As a conventional method for correcting such a non-axi-symmetric inhomogeneous magnetic field, there exists a technique wherein a region having a discrete iron piece positioned between an imaging volume and surfaces of magnetic poles is secured, and an iron for correcting non-axi-symmetric inhomogeneous magnetic field is provided in the region. In this method, however, the region for provision of the magnetic-field correcting iron is required, but is limited in its iron positioning from restrictions demanded by the openness of the magnet and system. In a magnet having non-axi-symmetric passive or active magnetic shield and flux returning means, the magnet has a large non-axi-symmetric inhomogeneous magnetic field. Thus this method has a limit in correcting the non-axi-symmetric inhomogeneous magnetic field and it is impossible to obtain a predetermined field uniformity demanded by the MRI apparatus.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a magnet including a plurality of magnetic-field generating means arranged as nearly opposed to each other and an imaging volume defined by the opposing magnetic-field generating means, wherein the magnetic-field generating means include magnetic poles and coils, a surface of the magnetic pole includes grooves or projections arranged nearly concentrically and formed continuous in a circumferential direction of the magnetic pole surfaces and also includes grooves or projections arranged nearly concentrically and discontinuous in the circumferential direction of the magnetic pole surface.
In the MRI apparatus, a highly uniform static magnetic field in the imaging volume is required. A magnetic field distribution in the imaging volume is determined by a superposition of magnetic fields generated by arrangement of a current, a permanent magnet and all magnetic sources including a magnetic dipole moment in the space. In order to obtain a uniform magnetic field as a magnet for use in the MRI apparatus, it is necessary to arrange the magnetic sources in such a manner as to eventually cancel an inhomogeneous magnetic field generated by the respective magnetic sources. The magnetic sources arranged axi-symmetrically will generate an axi-symmetric magnetic field, and the magnetic sources arranged non-axi-symmetrically will generate a mainly non-axi-symmetric magnetic field. Accordingly compensation of an axi-symmetric magnetic field inhomogenity is carried out by combining with the axi-symmetric arrangement of the magnetic sources mainly; whereas, compensation of a non-axi-symmetric magnetic field inhomogenity is carried out by combining the magnetic sources arranged non-axi-symmetrically.
In the present invention, generation of a uniform magnetic field is realized by arranging a magnetic dipole moment as one of the magnetic sources in a three-dimensional manner, that is, by providing a three-dimensional shape to the magnetic pole surface. Since the coil as the major magnetic source has a ring shape, magnetic dipole moment for compensating for an inhomogeneous magnetic field generated by the coil is arranged in an almost ring-shaped form, whereas non-axi-symmetric arrangement of magnetic dipole moment is necessary to compensate for non-axi-symmetric inhomogeneous magnetic fields caused by an asymmetric magnet structure, that is, an asymmetrically arranged return yoke and passive or active shield. Accordingly the magnetic pole has grooves or projections arranged nearly concentrically and continuous in the circumferential direction and discontinuous grooves or projections in the circumferential direction.
That is, in a magnet including magnetic poles and coils arranged in an opposed manner across an imaging volume for generating a static magnetic field for an MRI apparatus in accordance with the present invention, the magnetic pole facing the imaging volume is arranged to have a non-axi-symmetric shape. As the shape of the above non-axi-symmetric magnetic pole surface, the magnetic pole surface has a projected and recessed surface corresponding to a superposition of ring-shaped recesses and projections arranged axi-symmetrically for compensating for the axi-symmetric inhomogeneous magnetic field a
Kakugawa Shigeru
Wadayama Yoshihide
Wakuda Tsuyoshi
Barrera Ramon M.
Hogan & Hartson LLP
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