Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2001-10-13
2002-06-25
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S309000, C324S322000
Reexamination Certificate
active
06411091
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a coil for an MRI apparatus which forms at least two loops, and particularly to a coil for an MRI apparatus which is capable of enhancing the coupling characteristics of the reception coil by having a reduced coupling capacitance at the crossing section of the loops.
An MRI apparatus have been designed to implement the imaging process by detecting with a reception coil a magnetic signal which is created by the nuclear magnetic resonance.
FIG. 7
is a diagram showing a developed view of a conventional saddle-type reception coil. In
FIG. 7
, a coil
101
forms a pair of loop coils
201
and
202
on the right and left, and the loop coils
201
and
202
are connected in series. The loop coils
201
and
202
have conductor patterns
105
and
106
which form loop conductor patterns
107
and a pattern crossing section
111
. Disposed between the conductor pattern
106
and conductor pattern
107
is a resonance capacitor C
1
, which is connected to a cable section
103
for leading out a signal received by the coil
101
. A balance/unbalance converting circuit such as an impedance matching circuit and balun is provided between the resonance capacitor C
1
and the cable section
103
.
The conductor patterns
105
and
106
cross each other at the pattern crossing section
111
.
FIG. 8
is a diagram showing the detailed structure of the pattern crossing section
111
. In
FIG. 8
, the conductor patterns
105
and
106
cross each other by being interposed by a glass-epoxy substrate
121
which is an insulator. The conductor patterns
105
and
106
cross each other at right angles in order to reduce their magnetic coupling.
Based on this structure, there exists at the pattern crossing section
111
a coupling capacitance C, which is expressed in terms of the crossing area S of the conductor patterns
105
and
106
, the thickness d of the glass-epoxy substrate
121
, and the dielectric constant &egr; of the glass-epoxy substrate
121
as in the following formula (1).
C=&egr;S/d
(1)
The conductor patterns
105
and
106
have a width D, and the formula (1) is reformed as in the following formula (2).
C=&egr;·
(
D×D
)/
d
(2)
The conductor patterns
105
and
106
have their width D set large in order to reduce the resistance component of the coil. Consequently, the crossing area S is large. The glass-epoxy substrate
121
has its thickness d set small due to the limited layout space and cost of the coil
101
. On this account, the coupling capacitance C of the pattern crossing section
111
is nonnegligible with respect to the resonance capacitor C
1
.
FIG. 9
is a diagram showing an equivalent circuit of the coil
101
. This equivalent circuit forms a parallel resonance circuit. The impedance characteristic of this equivalent circuit is represented by a resonance curve which has a large impedance value at the resonant frequency fc as shown in FIG.
10
. Generally, a coil has its Q value expressed in terms of the inductance L of the coil, the resistance component r of the coil, and the resonant frequency &ohgr; as in the following formula (3).
Q=&ohgr;L/r=fc/&Dgr;f
(3)
By setting a 3-dB band width &Dgr;f of the peak value on the resonance curve of
FIG. 10
, the Q value is evaluated by the formula (3). The resonant frequency fc relates to &ohgr; as &ohgr;=2&pgr;fc, and the S/N factor (signal to noise ratio), which is a crucial parameter indicative of the quality of the tomographic image produced by the MRI apparatus, relates to the Q value as in the following formula (4).
S/N∝
(
Q
) (4)
As described above, the resistance component r increases with the increase of the coupling capacitance C, which results in a decreased Q value as suggested by the formula (3). The smaller Q value of the coil deteriorates the S/N factor as suggested by the formula (4), which results in a degraded quality of tomographic image. Namely, an increase of coupling capacitance C of the pattern crossing section
111
reduces the Q value of the coil
101
, which gives rise to a problem of a degraded quality of tomographic image.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide a coil for an MRI apparatus which is designed to reduce the coupling capacitance C of the pattern crossing section
111
based on a simple structure so that the Q value of the coil
101
does not decrease, thereby producing a high-quality tomographic image.
In order to achieve the above objective, the coil for an MRI apparatus according to the first aspect resides in a coil for an MRI apparatus which forms a plurality of loops and has an insulated crossing section, and is characterized by including a first conductor pattern which forms a first loop and has its one end at the crossing section branching into a set of a prescribed number of first partial conductor patterns, and a second conductor pattern which forms a second loop and has its one end at the crossing section branching into a set of the prescribed number of second partial conductor patterns, and is further characterized in that each confronting pair of the first and second partial conductor pattern sets cross each other by being insulated from each other at the crossing section, and the adjacent first partial conductor patterns and adjacent second partial conductor patterns have their ends beyond the crossing section each connected together to other ends of the second conductor pattern and first conductor pattern by conductors which are spaced out from the second partial conductor patterns and first partial conductor patterns, respectively, by a prescribed distance or more.
The coil for an MRI apparatus according to the first aspect is designed to reduce the coupling capacitance of the crossing section based on the structure in which each confronting pair of the first and second partial conductor patterns each formed in a prescribed number of branches cross each other by being insulated from each other at the crossing section, and the first and second partial conductor patterns each have their ends beyond the crossing section connected together by conductors which are spaced out from the second and first partial conductor patterns by a prescribed distance or more.
The coil for an MRI apparatus according to the second aspect resides in a coil for an MRI apparatus which forms a plurality of loops and has an insulated crossing section, and is characterized by including a first conductor pattern which forms a first loop and has its one end at the crossing section branching into first partial conductor patterns of two in number, and a second conductor pattern which forms a second loop and has its one end at the crossing section branching into second partial conductor patterns of two in number, and is further characterized in that each confronting pair of the first and second partial conductor patterns cross each other by being insulated from each other at the crossing section, and the first partial conductor patterns and second partial conductor patterns have their ends beyond the crossing section each connected together to other ends of the second conductor pattern and first conductor pattern by conductors which are spaced out from the second partial conductor patterns and first partial conductor patterns, respectively, by a prescribed distance or more.
The coil for an MRI apparatus according to the second aspect is designed to reduce the coupling capacitance of the crossing section based on the structure in which each confronting pair of the first and second partial conductor patterns each formed in two branches cross each other by being insulated from each other at the crossing section, and the first and second partial conductor patterns each have their ends beyond the crossing section connected together to another end of the second and first conductor patterns by conductors which are spaced out from the second and first partial conductor patterns by a prescribed distance or more.
The c
Sato Yoshiya
Sugiura Sunao
GE Medical Systems Global Technology Company LLC
Kojima Moonray
Lefkowitz Edward
Shrivastav Brij B.
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