Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1999-03-09
2001-01-09
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000
Reexamination Certificate
active
06172503
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance imaging apparatus for obtaining the image of a subject by utilizing NMR (Nuclear Magnetic Resonance) phenomenon, and an RF coil to be used therefor.
Various RF coils have been used for the magnetic resonance imaging apparatus. Among others, a surface coil has been often used which is obtained by annularly forming a conductor to be a coil face. In addition to circular and rectangular surface coils, an 8-shaped coil has been often used.
The 8-shaped coil is shown in FIG.
13
and has been often used for imaging by so-called vertical field MRI in which the direction of a static magnetic field is substantially vertical to the body axis of the subject. The 8-shaped coil described herein indicates a general coil in which a conductor in a central portion (hereinafter referred to as a “central conductor”)
100
has a sensitivity to an MR signal of a subject region and a current return path is provided to bypass the outside of the central conductor
100
. A static magnetic field B
0
direction is a z direction.
FIG. 14
shows another example of the 8-shaped coil according to the prior art. The 8-shaped coil shown in
FIG. 14
does not have a wholly square 8-shape shown in
FIG. 13
but has a smooth 8-shape. A coil having a shape shown in
FIG. 15
is also included in the 8-shaped coil.
FIG. 16
shows the positional relationship between the subject and the 8-shaped coil. In a case where an ordinary circular or rectangular shaped surface coil is put below the medulla spinals of a lying subject to carry out imaging of the medulla spinals of the subject by the vertical field MRI, a sensitivity magnetic field direction is the z direction which is parallel with the direction of a static magnetic field and has no effective MRI signal. In the 8-shaped coil, the direction of the magnetic field where the central conductor has a sensitivity is set to a direction (y direction) orthogonal to a transit direction (x direction) of the central conductor and a region having the sensitivity is placed in the vicinity of the central conductor. Thus, if the 8-shaped coil is provided in such a manner that the transit direction of the central conductor is parallel with the transit direction of the medulla spinals (x direction), a magnetic field sensitivity region in a direction along the medulla spinals can be obtained. In this case, the magnetic field sensitivity direction is orthogonal to the static magnetic field. In this direction, an effective MRI signal is obtained, a long sensitivity region is provided in the direction of the medulla spinals and an RF coil (surface coil) useful for a vertical field MRI apparatus is obtained.
However, the RF coil according to the prior art has a small effective sensitivity region in a transverse direction (y direction) as is apparent from FIG.
16
. For this reason, there is a problem in that the use of the RF coil is restricted to only diagnosis of a slender subject portion such as the medulla spinals.
In a case where an 8-shaped coil having a sensitivity concentrated only in the vicinity of the central conductor is used to carry out imaging of the medulla spinals, for example, a glaring image is obtained in a portion close to the body surface of the subject and the sensitivity is rapidly lowered in a portion greatly apart from the body surface. According to such an uneven sensitivity, there are problems in that S/N tends to be lacking in a certain deep region and a final image is diagnosed with difficulty.
As one of well-known techniques, a plurality of surface coils having sensitivity magnetic field directions different from each other are combined to form a surface QD (Quadrature Detection) coil having higher S/N. In general, this technique has been often used in an MRI apparatus in which the direction of a static magnetic field B
0
is parallel with the direction of the body axis of a subject as shown in FIG.
17
.
In a case where the surface QD coil shown in
FIG. 17
is used for the vertical field MRI apparatus, the following problem arises. More specifically, a rectangular coil
110
shown in
FIG. 17
has no sensitivity to an MRI signal sent from a medulla spinals portion. Therefore, the S/N cannot be enhanced.
Therefore, it is also supposed that a surface QD coil is constituted by overlapping two 8-shaped coils
130
and
140
orthogonally to each other as shown in FIG.
18
.
Also in this case, however, the following problem arises. More specifically, the surface QD coil can have high S/N only in a region
150
in the vicinity of a portion where both central conductors intersect. Consequently, it is impossible to keep a region which is long in the x direction and has high S/N. For this reason, in a case where the position of an affected part is precisely grasped in advance, a part of the medulla spinals can be observed with high quality of image. However, the long region of the medulla spinals cannot be observed. Similarly, high S/N can be obtained in a small visual field in the y-direction. Therefore, the use is limited.
BRIEF SUMMARY OF THE INVENTION
The present invention has been made to cope with the above-mentioned circumstances, and it is an object of the present invention to provide an RF coil of a magnetic resonance imaging apparatus capable of enlarging an effective sensitivity region in the direction of a magnetic field orthogonal to the direction of a static magnetic field, thereby enhancing quality of image in an interest region over the wide range of a subject.
According to the present invention, there is provided an RF coil to be used for a magnetic resonance imaging apparatus, comprising: a wide plate-shaped conductor having an magnetic field sensitivity of interest; and a return path connected to the plate-shaped conductor for bypassing the plate-shaped conductor.
According to the present invention, there is provided an RF coil to be used for a magnetic resonance imaging apparatus, comprising: a conductor portion including a plurality of conductor paths which are split at a predetermined starting point and join at a predetermined end point, and having an magnetic field sensitivity of interest; and a return path connected to the conductor portion for bypassing the conductor portion.
According to the present invention, since the wide plate-shaped conductor or the conductor portion including a plurality of conductor paths which are split at a predetermined starting point and join at a predetermined end point is provided, the effective sensitivity region in the direction of the magnetic field orthogonal to the direction of the static magnetic field can be enlarged so that the quality of image in the interest region can be enhanced over the wide range of the subject.
Furthermore, it is possible to obtain an advantage that the sensitivity is not rapidly reduced in the longitudinal direction of the subject.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.
REFERENCES:
patent: 4721913 (1988-01-01), Hyde et al.
patent: 4878022 (1989-10-01), Carison
patent: 5473251 (1995-12-01), Mori
patent: HO5-49614 (1992-08-01), None
patent: 9-187437 (1997-07-01), None
Fetzner Tiffany A.
Kabushiki Kaisha Toshiba
Nixon & Vanderhye P.C.
Oda Christine
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