Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2001-04-06
2003-04-01
Vo, Peter (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S602100, C029S605000, C336S212000, C336S213000
Reexamination Certificate
active
06539611
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a gradient coil manufacturing method, a gradient coil and a magnetic resonance imaging system, and particularly to a gradient coil provided on a polar surface of a static magnetic field magnet, a method of manufacturing the same, and a magnetic resonance imaging system having such a gradient coil.
In a magnetic resonance imaging (MRI: Magnetic Resonance Imaging) system, a target to be shot or imaged is carried in an internal bore of a magnet system, i.e., a bore or space in which a static magnetic field is formed. A gradient magnetic field and a high-frequency magnetic field are applied to produce a magnetic resonance signal within the target. A tomogram is produced (reconstructed) based on its received signal.
In a magnet system using permanent magnets for generating static magnetic fields, pole piece for uniformizing a magnetic flux distribution in a static magnetic field space are respectively provided at leading ends of a pair of the permanent magnets opposite to each other. Further, gradient coils for generating gradient magnetic fields are provided on their corresponding polar surfaces of the pole pieces.
In the above-described magnet system, the pole pieces are magnetized by the gradient magnetic fields since the gradient coils are respectively close to the pole pieces. Due to the residual gradient magnetic fields formed by their remanent magnetization, the phase of a spin is subjected to such an influence as though eddy currents extremely long in time constant had existed. Therefore, it would interfere with imaging made by, for example, a fast spin echo (FSE) method or the like which needs accurate phase control.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to implement a gradient coil low in magnetizing force with respect to each pole piece, a method of manufacturing the same, and a magnetic resonance imaging system having such a gradient coil.
(1) The invention according to one aspect for solving the above problems is a gradient coil manufacturing method comprising the step of, upon manufacturing a pair of gradient coils which is respectively provided along the surfaces of bottom plate portions lying inside peripheral edge portions of a pair of pole pieces having the bottom plate portions and the peripheral edge portions protruding in the direction orthogonal to the surfaces of the bottom plate portions, the pole pieces being opposed to each other with the protruded peripheral edge portions formed with a space defined therebetween, and which produces gradient magnetic fields in the space by currents that flow through a plurality of concentric passes, setting the maximum radius of one of the passes for each of the gradient coils to the minimum value within a range for producing a gradient magnetic field having a magnetic field error lying within a predetermined allowable range.
In the invention according to this aspect, the maximum radius of one pass for each gradient coil is set to the minimum value at which a gradient magnetic field having a magnetic field error lying within a predetermined allowable range can be produced. Thus, the distance between the outermost pass and a protruded peripheral edge portion of each pole piece increases. Therefore, the magnetizing force with respect to the protruded peripheral edge portion is low and the residual magnetization is low.
(2) The invention according to another aspect for solving the above problems is the gradient coil manufacturing method described in (1), wherein the radii of the plurality of passes are determined according to the following procedures.
Note
(a) setting measurement points Pi (where i=1−N) onto the maximum spherical surface supposed in an imaging area.
(b) calculating magnetic fields Bit (where i=1−N) at the measurement points, to be produced by the gradient coils.
(c) setting a tolerance at for an error with respect to each magnetic field.
(d) setting an allowable value r
0
of the maximum radius of the pass for each gradient coil within a range not exceeding a limit value r
00
.
(e) defining the radii of the plurality of passes as r
1
, r
2
, . . . , rM.
max (
r
1
,
r
2
, . . . ,
rM
)<
r
0
,
under the above restricted condition, and
{right arrow over (r)}=(
r
1
,
r
2
, . . . ,
r
M
)
with the above as a parameter,
min
[
∑
i
=
1
N
{
B
it
-
B
^
i
(
r
⇀
)
}
2
]
determining the optimum values of rj (where j=1−M) using quadratic programming so that the above equation 18 is established. Incidentally,
{circumflex over (B)}
i
(
{right arrow over (r)}
)
the above is calculated using the Biot-Savart's law.
(f) calculating an error in magnetic field at each measurement point Pi according to the following equation.
α
i
=
B
^
i
(
r
⇀
)
-
B
it
B
it
×
100
(g) determining rj when &agr;i≦&agr;t is satisfied.
(h) when &agr;i≦&agr;t is not satisfied, increasing the allowable value r
0
within a range not exceeding the limit value r
00
and (i) repeating the procedures subsequent to (e).
In the invention according to this aspect, the maximum radius of one pass for each gradient coil is set to the minimum value at which a gradient magnetic field having a magnetic field error lying within a predetermined allowable range can be produced. Thus, the distance between the outermost pass and a protruded peripheral edge portion of each pole piece increases. Therefore, the magnetizing force with respect to the protruded peripheral edge portion is low and the residual magnetization is low.
(3) The invention according to a further aspect for solving the above problems is a pair of gradient coils which is respectively provided along the surfaces of bottom plate portions lying inside peripheral edge portions of a pair of pole pieces having the bottom plate portions and the peripheral edge portions protruding in the direction orthogonal to the surfaces of the bottom plate portions, the pole pieces being opposed to each other with the protruded peripheral edge portions formed with a space defined therebetween, and which produces gradient magnetic fields in the space by currents that flow through a plurality of concentric passes, wherein the maximum radius of one of the passes for each of the gradient coils is set to the minimum value within a range for producing a gradient magnetic field having a magnetic field error lying within a predetermined allowable range.
In the invention according to this aspect, the maximum radius of one pass for each gradient coil is set to the minimum value at which a gradient magnetic field having a magnetic field error lying within a predetermined allowable range can be produced. Thus, the distance between the outermost pass and a protruded peripheral edge port of each pole piece increases. Therefore, the magnetizing force with respect to the protruded peripheral edge portion is low and the residual magnetization is low.
(4) The invention according to a still further aspect for solving the above problems is the pair of gradient coils described in (3), wherein the plurality of passes respectively have radii determined according to the following procedures.
Note
(a) setting measurement points Pi (where i=1−N) onto the maximum spherical surface supposed in an imaging area.
(b) calculating magnetic fields Bit (where i=1−N) at the measurement points, to be produced by the gradient coils.
(c) setting a tolerance &agr;t for an error with respect to each magnetic field.
(d) setting an allowable value r
0
of the maximum radius of the pass for each gradient coil within a range not exceeding a limit value r
00
.
(e) defining the radii of the plurality of passes as r
1
, r
2
, . . . , rM.
max (
r
1
,
r
2
, . . . ,
rM
)<
r
0
,
under the above restricted condition, and
{right arrow over (r)}
=(
r
1
,
r
2
, . . . ,
r
M
)
with the above as a parameter,
min
[
∑
i
=
1
N
{
B
it
-
B
^
i
(
r
&RightV
GE Medical Systems Global Technology Company LLC
Kim Paul D
Kojima Moonray
Vo Peter
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