JP2865375B2 - Quadrature coil device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to a quadrature coil device for transmitting a high-frequency pulse and receiving a magnetic resonance signal used in a magnetic resonance imaging (MRI) device. About.

(Prior art) In general, if a quadrature coil is used for transmitting high-frequency pulses with an MRI device, perfect circularly polarized waves can be generated, so it is theoretically possible to reduce power for transmission by about half. Is known. Also, if used for receiving magnetic resonance signals, the S / N ratio It is also known to improve. It is also known that transmission and reception using a quadrature coil can improve sensitivity unevenness of an image due to penetration of a high-frequency pulse. As a conventional example of such a quadrature coil device, there is a Journal of Magnetic Resonance 69,236-242.
(1986) Alderman Grant
Grant) type adult head quadrature coil. First
FIG. 0 (a) shows a perspective view thereof, and FIG. 0 (b) shows an equivalent circuit diagram. This conventional example comprises a slotted tube resonator 2 composed of two guard rings 4a, 4b and four electrodes 2a, 2b, 2c, 2d provided outside thereof. Each electrode comprises two arc-shaped wings along the guard ring and a conductor band arranged along the Z direction and connecting the two wings. A tuning capacitor 6 for adjusting the resonance frequency is connected between the wings of the adjacent electrodes.

In the equivalent circuit, _GT is a tuning capacitor. Feed point X for X channel to every other coil 2a, 2c
_1, X ₂ are connected, the remaining coil 2b, the feed point Y _1, Y ₂ for Y channel is connected to 2d. The inner conductor of the coaxial cable connected to the feeding point _{X 1, Y 1, X 2} , Y 2 is matched to 50Ω through a matching capacitor C _M, is connected to the wings of each electrode. Outer conductor of the coaxial cable connected to the feeding point, only as illustrated for the feeding point X _2, is connected to one of the guard rings. Also, the matching capacitor C _M
Wing and the connection point via a trimming capacitor C _TR for fine tuning the resonant frequency of the X-channel is connected to the other of the guard ring and.

In this conventional example, when operated at 60 MHz, the isolation between the X and Y channels is as good as 38 dB, but the deviation of the resonance frequency cannot be reduced to about 1% or less even if the trimming capacitor _CTR is varied. Has been reported.
This is because even if the drive circuit is simply connected to the 90 ° hybrid at the feeding points X ₁ , X ₂ , Y ₁ , and Y ₂ , the magnetic mutual coupling due to the mutual inductance between the conductor bands of the X and Y channels occurs. , X, and Y channels cannot resonate independently. For this reason, this conventional example cannot realize a complete quadrature coil.

(Problems to be Solved by the Invention) An object of the present invention is to cancel magnetic mutual coupling between electrodes of two orthogonal channels, to operate resonance circuits of both channels independently, and to realize resonance between two channels. An object of the present invention is to provide a quadrature coil device capable of matching frequencies.

[Configuration of the Invention] (Means for Solving the Problems) A quadrature coil device according to the present invention includes two guard rings and four electrodes each arranged along the guard ring outside the guard ring. Then, in the quadrature coil device in which the feeding points of the first and second channels are connected to any two adjacent electrodes, respectively, a resonance frequency adjusting device is provided between the guard ring and the electrode to which the feeding point is connected. A tuning capacitor is provided, and a neutralizing capacitor for canceling mutual inductance between the first and second channels is provided between the electrodes.

(Operation) According to the present invention, the magnetic mutual coupling between the electrodes of the two channels can be canceled, and the resonance circuits of both channels can be operated independently, so that the resonance frequencies of both channels can be matched. .

Embodiment An embodiment of a quadrature coil device according to the present invention will be described below with reference to the drawings. 1 (a) and 1 (b) are a perspective view and an equivalent circuit diagram of the first embodiment. In FIG. 1A, one guard ring 10 has four portions 10a and 10a, respectively.
b, 10c, and 10d, and neutralizing capacitors C _N and C _{N ′} are connected therebetween. Here, neutralization means that the circuits of both channels resonate independently without being magnetically coupled to each other, and are also called orthogonalization. Note that, for convenience of adjustment, the neutralizing capacitor has a constant C _{N ′} and a variable capacitor C _N. However, all the neutralizing capacitors may be variable capacitors.

Tuning capacitors C _TA and C _TB for adjusting the resonance frequency are connected between the guard rings 10a and 10b and the conductor bands 14a and 14b. Electrode 14a and guard ring 10
a center conductor of the coaxial cable connected to the matching capacitor C _MA, the feeding point 16 of the X-channel via the C _MA respectively, are connected to the outer conductor. Conductor band 14b and guard ring 10b are matched capacitors C _MB and C _MB respectively.
It is connected to the center conductor and the outer conductor of the coaxial cable connected to the feeding point 18 of the Y channel via the _MB . C _OA and C _OB are fixed capacitors used for tuning.

First, the process of obtaining the equivalent circuit shown in FIG.
This will be described with reference to FIGS.

FIG. 2 is an equivalent circuit diagram rewritten from the conventional example shown in FIG. 10 (b) by omitting the matching capacitor C _M and the trimming capacitor _CTR which are not directly related to neutralization. The circle consisting of only the inner inductance of the upper and lower double circles is the guard ring, the circle consisting of the outer inductance and the tuning capacitor C _T is the wing, and the inductances L _A , L _B , L _A ′, L connecting between the upper and lower wings _The portion consisting of _B 'is the conductor band. A capacitor connected between the wing and the guard ring as shown by a broken line is a parasitic capacitance. In FIG. 2, channels X and Y are replaced with channels A and B for convenience. Z _{1 to} Z ₈ , Z _A , Z _A ′, Z _B ,
Z _B ′ indicates impedance.

To simplify the circuit of FIG. 2, the properties of the quadrature coil are used. That is, the currents flowing in the conductor bands Z _A and Z _A ′ of the A channel are equal to each other, and I ₁
Similarly, it is assumed that the currents flowing in the conductor bands Z _B and Z _B ′ of the B channel are also equal to each other and are I ₂ . When FIG. 2 is rewritten using these conditions, an equivalent circuit diagram shown in FIG. 3 is obtained. Here, E ⁰ is an independent power supply.

In general, mutual inductance is reversible, and M _ij
= M _ji holds, the following is a summary of the dependent power supplies on the A channel side in FIG.

Total A channel side sub power supply _{= jωM AA 'I 1 -jω (} M AB' -M AB) I 2 + jω (M A'B '-M A'B) I 2 -jωM A'A I 1 = jω ( M _AB −M _{AB ′} −M _{AB ′} + _{MA′B ′} ) I ₂ (1) Similarly, the dependent power supplies on the B channel side in FIG. 3 are summarized as follows.

Total B channel side dependent power supply = jωM _B′B I ₂ −jω ( _MB′A − _{MB′A ′} ) I ₁ + jω (M _BA −M _{BA ′} ) I ₁ −jωM _{BB ′} I ₂ = jω ( M _{AB −MA′B} −M _{AB ′} + _{MA′B ′} ) I ₁ (2) In the equations (1) and (2), M≡M _{AB −MA A′B} −M
_{AB '} + M _A'B' (mutual inductance resulting from an error in the geometrical arrangement of the coils), the equations (1) and (2) can be rewritten as follows.

Total A-channel dependent power supply = jωMI ₂ (3) Total B-channel dependent power supply = jωMI ₁ (4) By using equations (3) and (4), FIG. 3 is rewritten as shown in FIG. be able to. In FIG. 4, nodes N ₁ and N ₂
Focusing on the voltage between nodes and the voltage between nodes N ₅ and N ₆ , the following equation is obtained.

E ₁₂ = Z ₁ (I ₂ + I ₃ ) + Z ₃ I ₃ = Z ₂ (I ₁ −I ₂ −I ₃ ) + Z ₄ (I ₁ −I ₃ )… (5) E ₅₆ = Z ₅ (I ₂ + I ₄ ) + Z ₇ I ₄ = Z ₆ (I ₁ −I ₂ −I ₄ ) + Z ₈ (I ₁ −I ₄ ) (6) When focusing on the current flowing through the A channel and the B channel, the following equation is obtained.

_{Z A I 1 + Z 1 (} I 2 + I 3) + Z 3 I 3 + A 'I 1 + Z 5 (I 2 + I 4 + Z 7 I 4 = E 0 + jωMI 2 ... (7) Z B' I 2 + Z 4 ( I ₁ −I ₃ ) −Z ₃ I ₃ + Z ₃ I ₂ + Z _B (I ₁ −I ₄ ) −Z ₇ I ₄ = jωMI ₁ (8) From the independent power supply E ⁰ to the node N ₁ → N ₃ (or N ₄ ) → N ₂ → N ₅ → N ₇
Focusing on the loop on the A channel side which returns to the independent power source E ⁰ again via (or N ₈ ) → N ₆ , the state where only the A channel resonates and the B channel does not resonate (that is, neutralization is possible and I ₂ = 0 It can be seen that there is a situation where Therefore, in order to find the conditions for neutralization, (5)
Substituting I ₂ = 0 into equations (8) to (8) gives equations (5) to (8)
The formula is transformed as follows.

Z ₁ I ₃ + Z ₃ I ₃ = Z ₂ (I ₁ −I ₃ ) + Z ₄ (I ₁ −I ₃ ) ∴ (Z ₂ + Z ₄ ) I ₁ − (Z ₁ + Z ₂ + Z ₃ + Z ₄ ) I ₃ = _{0 ... (5 ') Z 5} I 4 + Z 7 I 4 = Z 6 (I 1 -I 4) + Z 8 (I 1 -I 4) ∴ (Z 6 + Z 8) I 1 - (Z 5 + Z 6 + Z 7 + Z ₈ ) I ₄ = 0 (6 ′) (Z _A + Z _{A ′} ) I ₁ + Z ₁ I ₃ + Z ₃ I ₃ + Z ₅ I ₄ + Z ₇ I ₄ = E ₀ ∴ (Z _A + Z _{A ′} ) I ₁ + (Z ₁ + Z ₃ ) I ₃ + (Z ₅ + Z ₇ ) I ₄ = E ₀
… (7 ′) Z ₄ (I ₁ −I ₃ ) −Z ₃ I ₃ + Z ₈ (I ₁ −I ₄ ) −Z ₇ I ₄ = jωMI ₁ ∴ (Z ₄ + Z ₈ −jωM) I ₁ − (Z _{3 + Z 4) I 3 -} (Z 7 + Z 8) I 4 = 0 ... (8 ') (5'), is substituted into (6 ') equation (7'), I _3, I
Eliminating ₄ gives the following relationship:

｛Z _A + Z _{A ′} + (Z ₁ + Z ₃ ) · (Z ₂ + Z ₄ ) / (Z ₁ + Z ₂ + Z ₃ + Z ₄ ) + (Z ₅ + Z ₇ ) · (Z ₆ + Z ₈ ) / (Z ₅ + Z) ₆ + Z ₇ + Z ₈ )｝ I ₁ = E ₀ Similarly, substituting equations (5 ′) and (6 ′) into equation (8 ′) and eliminating I ₃ and I ₄ , the following relationship is obtained. can get.

｛Z ₄ + Z ₈ -jωM- (Z ₂ + Z ₄ ) · (Z ₃ + Z ₄ ) / (Z ₁ + Z ₂ + Z ₃ + Z ₄ )-(Z ₆ + Z ₈ ) · (Z ₇ + Z ₈ ) / (Z ₅ + Z ₆ + Z ₇ + Z ₈ )｝ I ₁ = 0 In the ideal case (when the impedance in FIG. 2 does not include any resistance component), I ₁ ≠ 0 and the current I ₁ becomes infinite at resonance. , E ₀ / I ₁ are expected to be 0, and the neutralization conditions in an ideal case are as follows.

_{Z A + Z A '+ (} Z 1 + Z 3) · (Z 2 + Z 4) / (Z 1 + Z 2 + Z 3 + Z 4) + (Z 5 + Z 7) · (Z 6 + Z 8) / (Z 5 + Z 6 + Z ₇ + Z ₈ ) = 0 (9) and Z ₄ + Z ₈ −jωM− (Z ₂ + Z ₄ ) · (Z ₃ + Z ₄ ) / (Z ₁ + Z ₂ + Z ₃ + Z ₄ ) − (Z ₆ + Z ₈ ) · (Z ₇ + Z ₈ ) / (Z ₅ + Z ₆ + Z ₇ + Z ₈ ) = 0 (10) Therefore, the distance between the wings of each electrode in FIG. 2 is satisfied so that the equations (9) and (10) are simultaneously satisfied. Adjusting the tuning capacitors C _{T1 to} C _T8 satisfies the neutralization condition.
The coupling between the conductor bands of both B channels can be channeled, both cancellations can be resonated independently, and the resonance frequency between both channels can be matched.

With the equations (9) and (10), ten parameters Z _A ,
Z _{A ',} there is a Z ₁ ~Z _8, it takes a long time to adjust too there is a degree of freedom. Therefore, simplification of equations (9) and (10) will be considered. _{First, Z 1 = Z 4; Z} 2 = Z 3; Assuming _{Z 5 = Z 6 = Z 7} = Z 8, (9), (10) equation can be modified as follows.

_{Z A + Z A '+ (} Z 1 + Z 2) 2/2 (Z 1 + Z 2) + 4Z 5 2 / 4Z 5 = 0 Z A + Z A' + (Z 1 + Z 2) / 2 + Z 5 = 0 ∴Z 5 = _{- {Z A + Z A '} + (Z 1 + Z 2) / 2} ... (11) Z 1 + Z 5 + jωM- (Z 1 + Z 2) 2/2 (Z 1 + Z 2) -4Z 5 2 / 4Z 5 = 0 2Z ₁ −j2ωM− (Z ₁ + Z ₂ ) = 0 ∴Z ₂ = Z ₁ −j2ωM (12) By substituting equation (12) into equation (11), the following equation is obtained.

Z ₅ = − (Z _A + Z _{A ′} + Z ₁ −jωM) Therefore, when the above is summarized, the following condition is obtained as a neutralization condition in a more restricted case.

Z ₁ = Z ₄ Z ₂ = Z ₃ (= Z ₁ −j ₂ ωM) Z ₅ = Z ₆ = Z ₇ = Z ₈ Z _A ′ = −Z _A −Z ₁ −Z ₅ + jωM The limited neutralization conditions described above. In addition, since it is necessary to simultaneously establish reciprocity with respect to B channel → A channel together with A channel → B channel, neutralization is realized even when mutual inductance exists, that is, orthogonalization is achieved. This is illustrated in FIG. In FIG. 5, when Z ₁ = Z ₄ ; Z ₅ = Z ₆ = Z ₇ = Z ₈ , the coupling between the A and B channels caused by the mutual inductance is Z ₂ = Z ₃ = Z ₁ −j2ωM, _{Z a '= -Z a -Z 1} -Z 5 + jωM, Z B' be adjusted such that _{= -Z B -Z 1 -Z 5 +} jωM, it can be seen that cancellation.

Here, in order to explain that if the impedance of each element is adjusted so as to meet the above-described limited neutralization condition, the A channel and the B channel are neutralized.
Consider a ladder-shaped circuit in the equivalent circuit shown in FIG. 5 which is equivalently converted using an admittance matrix of a two-terminal network shown in FIG. The amide-mittance matrix [Y] of the two-terminal network shown in FIGS. Next, when these set equal to _{identically, ∴Y a = Y 2, Y} b = Y 1 -Y 2 becomes, when Y ₁ = Y _2, a _{Y a = Y 2, Y b} = 0 . Y _b =
Applying 0 to FIG. 5 results in FIG. Thus, it is easily assumed that the A channel and the B channel are neutralized.

That is, quadrature coils seen to be equal to those of connecting the resonant circuit of the A channel and B channel ^{_{(jZ 1 2 / 2ωM) +}} Z 1 impedance. In addition, the fifth
From FIG. 7 and FIG. 7, the mutual inductance j between both channels is shown.
To cancel and neutralize ωM, Z ₁ −jωM (= Z ₂ =
It can be seen that Z ₃ ), Z _{A '} and Z _B' should be adjusted.

FIG. 8 shows an equivalent circuit obtained by rewriting FIG. 7 as shown in FIG. FIG. 1 (b) is a rewrite of FIG. 8 according to the perspective view of FIG.

Therefore, according to the first embodiment, the guard rings 10a, 10b
During, 10c, by adjusting the neutralization capacitor C _N provided between 10d, it is possible to satisfy a limited neutralized conditions described above.

9 (a) and 9 (b) show a perspective view and an equivalent circuit diagram of the second embodiment. In the first embodiment, as shown in FIG. 7, the resonance circuits of both channels may be connected with an appropriate impedance that satisfies the above-described second neutralization condition, and the neutralization capacitors C _N , The position of CN _' is not limited to within the guard ring. In the second embodiment, the neutralizing capacitors C _N and C
_{N '} was connected between the electrodes.

In the second embodiment, the same effects as in the first embodiment can be obtained.

The present invention is not limited to the embodiments described above, but can be variously modified without departing from the gist of the invention.

[Effects of the Invention] As described above, according to the present invention, a neutralizing capacitor is connected between the resonance circuits of both channels, and by adjusting this, coupling between the conductor bands of both channels is achieved. Since it can be canceled and both channels can resonate, the resonance frequency between both channels can be matched, and if it is used as a transmission / reception coil of an MRI apparatus, the S / N ratio can be improved and the power for transmitting high-frequency pulses can be improved. And a quadrature coil device capable of improving the sensitivity unevenness of the image caused by the penetration of the high-frequency pulse.

[Brief description of the drawings]

1 (a) and 1 (b) are perspective views and an equivalent circuit diagram of a first embodiment of a quadrature coil device according to the present invention, and FIGS. 2 to 8 are equivalent circuit diagrams of FIG. 1 (b). FIGS. 9 (a) and 9 (b) are perspective views of a second embodiment of the quadrature coil device according to the present invention.
FIGS. 10 (a) and 10 (b) are a perspective view and an equivalent circuit diagram of a conventional quadrature coil device. 10a, 10b, 10c, 10d, 12 ... Guard ring, 14a, 14b, 14c, 14
d… Electrode, 16,18… Feeding point, C _N , C _{N ′} … Neutralization capacitor, C _MA , C _MB … Matching capacitor, C _TA , C _TB ……
Tuning capacitor.

Claims (1)

(57) [Claims] 1. A semiconductor device comprising: two guard rings; and four electrodes, each of which is disposed along the guard ring outside the guard ring, wherein any two adjacent electrodes have a first channel and a second channel, respectively. In the quadrature coil device to which the feeding point is connected, a tuning capacitor for adjusting the resonance frequency is provided between the electrode to which the feeding point is connected and the guard ring, and the first and second channels are provided between the electrodes. And a neutralizing capacitor for canceling the mutual inductance of the quadrature coil device.