Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2000-04-10
2002-03-26
Williams, Hezron (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S320000, C324S319000
Reexamination Certificate
active
06362623
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a gradient coil for an MRI (magnetic resonance imaging) apparatus, a method of manufacturing a gradient coil for an MRI apparatus, and an MRI apparatus, and more particularly to a gradient coil for an MRI apparatus having a low electric power loss and a low heat release in its shielding coil, a method of manufacturing such a gradient coil, and an MRI apparatus.
FIG. 11
illustrates an exemplary magnet assembly in a conventional MRI apparatus.
The magnet assembly
51
comprises yokes
20
, a pair of opposing permanent magnets
1
Mt and
1
Mb attached to the yokes
20
for generating a static magnetic field, magnetic field conditioning plates
24
and
25
disposed on the opposing surfaces of the permanent magnets
1
Mt and
1
Mb, respectively, for improving homogeneity of the static magnetic field, upper and lower Z-axis main gradient coils
1
Zt and
1
Zb disposed on the opposing surfaces of the magnetic field conditioning plates
24
and
25
, respectively, for generating a Z-axis gradient magnetic field, an upper Z-axis shielding coil
51
Zts for preventing magnetic flux generated by the upper Z-axis main gradient coil
1
Zt from affecting the magnetic field conditioning plate
24
, and a lower Z-axis shielding coil
51
Zbs for preventing magnetic flux generated by the lower Z-axis main gradient coil
1
Zb from affecting the magnetic field conditioning plate
25
.
A combination of the upper Z-axis shielding coil
51
Zts, the upper Z-axis main gradient coil
1
Zt, the lower Z-axis main gradient coil
1
Zb and the lower Z-axis shielding coil
51
Zbs constitutes a Z-axis gradient coil
51
Z.
Although omitted in the drawings, X- and Y-axis gradient coils are also disposed on the opposing surfaces of the magnetic field conditioning plates
24
and
25
.
FIG. 12
is a schematic perspective view of the Z-axis gradient coil
51
Z.
Windings of the upper Z-axis shielding coil
51
Zts are disposed corresponding to the entire winding area of the upper Z-axis main gradient coil
1
Zt. However, the number of the windings of the upper Z-axis shielding coil
51
Zts is less than that of the upper Z-axis main gradient coil
1
Zt.
Although the number of windings of the conventional Z-axis shielding coils
51
Zts and
51
Zbs is less than that of the Z-axis main gradient coils
1
Zt and
1
Zb, it is significantly large because the windings of the Z-axis shielding coils
51
Zts and
51
Zbs are positioned corresponding to the entire winding area of the Z-axis main gradient coils
1
Zt and
1
Zb. Thus, the Z-axis shielding coils
51
Zts and
51
Zbs have a large electric power loss.
Specifically, the conventional gradient coil for the MRI apparatus gives rise to a problem of generating a large electric power loss, and hence a large heat release in its shielding coil.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a gradient coil for an MRI apparatus having a low electric power loss and a low heat release in its shielding coil, a method of manufacturing such a gradient coil, and an MRI apparatus.
In accordance with a first aspect of the present invention, there is provided a gradient coil for an MRI apparatus comprising a partially shielding coil interposed between a main gradient coil and a magnetic component, the partially shielding coil having its windings positioned only in a high winding density zone containing a portion in which a winding density of the main gradient coil is highest.
In the gradient coil for an MRI apparatus of the first aspect, the windings of the shielding coil are positioned corresponding only to a high winding density zone, rather than corresponding to the entire winding area, of the main gradient coil. (For this reason, the coil is called a “partially shielding coil”.) Accordingly, the number of the windings can be reduced, thereby reducing the electric power loss and the heat release. In addition, since the shielding performance required can still be attained, magnetic flux generated by the main gradient coil can be prevented from affecting a magnetic component (such as a magnetic field conditioning plate), thereby avoiding an adverse effect of remanence in the magnetic component.
In accordance with a second aspect of the invention, there is provided a method of manufacturing a gradient coil for an MRI apparatus, comprising the steps of applying an image electric current method only to a high winding density zone containing a portion in which a winding density of a main gradient coil is highest and applying a boundary condition only to the same zone to determine the position of windings of a partially shielding coil having its windings positioned only in the high winding density zone.
In the method of manufacturing a gradient coil for an MRI apparatus of the second aspect, the image electric current method is applied only to a high winding density zone and the boundary condition is applied only to the same zone, rather than to the entire winding area of the main gradient coil. Accordingly, the windings of the shielding coil can be suitably positioned corresponding only to the high winding density zone. (For this reason, the coil is called a “partially shielding coil”.)
In accordance with a third aspect of the invention, there is provided a method of manufacturing a gradient coil for an MRI apparatus, comprising the steps of defining an optimization plane between a partially shielding coil and a magnetic component lying near the partially shielding coil, the partially shielding coil having its windings positioned only in a high winding density zone containing a portion in which a winding density of a main gradient coil is highest, and optimizing the position of the windings of the partially shielding coil by a least squares technique so that a magnetic field is minimized within the optimization plane.
In the method of manufacturing a gradient coil for an MRI apparatus of the third aspect, the windings of the shielding coil are positioned only in a high winding density zone, rather than over the entire winding area, of the main gradient coil, and the position of the windings is optimized by a least squares technique. Accordingly, the windings of the shielding coil can be suitably positioned corresponding only to the high winding density zone. (For this reason, the coil is called a “partially shielding coil”.)
In accordance with a fourth aspect of the invention, there is provided a method of manufacturing a gradient coil for an MRI apparatus, comprising the steps of applying an image electric current method only to a high winding density zone containing a portion in which a winding density of a main gradient coil is highest and applying a boundary condition only to the same zone to determine the position of windings of a partially shielding coil having its windings positioned only in the high winding density zone, and then defining an optimization plane between the partially shielding coil and a magnetic component lying near the partially shielding coil, and optimizing the position of the windings of the partially shielding coil by a least squares technique so that a magnetic field is minimized within the optimization plane.
In the method of manufacturing a gradient coil for an MRI apparatus of the fourth aspect, the image electric current method is applied only to a high winding density zone and the boundary condition is applied only to the same zone, rather than to the entire winding area of the main gradient coil. Accordingly, the windings of the shielding coil can be suitably positioned corresponding only to the high winding density zone. (For this reason, the coil is called a “partially shielding coil”.) Moreover, the position of the windings of the partially shielding coil is optimized by a least squares technique. Accordingly, the windings of the shielding coil can be more suitably positioned.
In accordance with a fifth aspect of the invention, there is provided an MRI apparatus comprising a gradient coil including a main gradient coil, and a partially shielding coil having its
GE Yokogawa Medical Systems Limited
Kojima Moonray
Vargas Dixomara
Williams Hezron
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