JP2638419B2 - Orthogonal RF coil for MRI equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MRI apparatus,
In particular, the present invention relates to an improvement in a quadrature RF coil used for exciting an object or detecting an NMR signal.

[0002]

2. Description of the Related Art An orthogonal RF coil used in an MRI apparatus has two electrically independent coil loops arranged at right angles to each other. / 2, and an S / N ratio of √2 times is obtained when receiving an NMR signal. In order to approach the above theoretical value in the orthogonal RF coil, it is important to increase the independence between the two coil loops, that is, how to increase the orthogonality.

FIG. 3 shows an example of a conventional generalized orthogonal RF coil. In FIG. 3, four straight conductors 11-1
4 are connected at both ends to ring-shaped conductors 15 and 16 via capacitors 31 to 34 and 35 to 38, and capacitors 21 to 24 and 25 to 2 are connected to the ring-shaped conductors 15 and 16.
8 are inserted in series. The capacitors 21 to 28, 31 to 38 and the inductance formed by the linear conductors 11 to 14 form a resonance circuit.
Power is supplied to the points B and C by the 90 ° phase combiner 51, and the capacitors 39 and 40 are used for impedance matching.

In this example, the capacitors 21 to 28 have a relatively large capacity, and stable characteristics can be obtained mainly by adjusting the resonance frequency by the capacitors 31 to 38. Further, capacitors 21 to 24, 25 to 28, and 31
, 34, and 35 to 38 have substantially the same capacitance.

In such an orthogonal RF coil, a resonance current flows as shown in FIGS. In the case of FIG. 4A, a resonance current indicated by a solid line and a resonance current indicated by a dotted line flow independently. These independent currents generate RF magnetic fields that are orthogonal to each other. Therefore, if a resonance mode in which such a current flows at the resonance frequency of NMR is established, it is useful for MR imaging (imaging mode).

On the other hand, there is also a resonance mode (differential mode) in which a resonance current flows as shown in FIG. At this time, currents flow through the linear conductors 11 to 14 mutually, and there is no independence between them. At this time, since the sum of the RF magnetic field at the center of the RF coil becomes zero, it cannot be generally used as an imaging mode.

[0007]

However, the conventional orthogonal RF coil has a problem that the orthogonality of the RF magnetic field is not good. That is, the frequency at which the imaging mode is established and the frequency at which the differential mode is established are RF
Depending on factors such as the balance of the capacitance or inductance in the coil, the value is often close in some cases. In that case, the resonance current in the differential mode is combined with the resonance current in the imaging mode, and the orthogonality of the RF magnetic field in the imaging mode is deteriorated.

SUMMARY OF THE INVENTION In view of the above, the present invention provides an orthogonal RF coil for an MRI apparatus in which current in an undesired non-imaging mode is suppressed, thereby improving the orthogonality of an RF magnetic field in an imaging mode. The purpose is to provide.

[0009]

In order to achieve the above object, in an orthogonal RF coil of an MRI apparatus according to the present invention, two ring-shaped conductors are provided between the two ring-shaped conductors. First to fourth linear conductors which are arranged at substantially 90 ° intervals in the circumferential direction and are electrostatically coupled to the two ring-shaped conductors, and each of these four linear conductors and one of the ring-shaped conductors The first provided between
To a fourth feeding point, a first differential feeding circuit connected to the first and third feeding points of the first and third straight conductors facing each other, and a second and fourth A second differential power supply circuit connected to the second and fourth power supply points of the linear conductor, and a 90 ° phase combiner connected to the first and second differential power supply circuits. , The first and second differential feed circuits are respectively connected to two feed points.
It consists of two capacitors having substantially the same capacitance C and two inductances having substantially the same value L formed between the two terminals, and is connected to a 90 ° phase combiner at the midpoint. A parallel resonance circuit in which a capacitor having a capacitance Ca and a inductance of a value La are connected in parallel and a switch is inserted, and a capacitor connected in series to the parallel resonance circuit,
A series circuit of the parallel resonance circuit and the capacitor is connected to the two terminals on the feed point side in parallel with the bridge circuit, and the capacitances C and Ca and the inductance values L and La of the capacitor are set to the differential values. The resonance frequency of the mode is f,
Assuming that the impedance on the feeding point side is Zo and the impedance on the 90 ° phase combiner side is Zi, (1 / 2π) √ (CL) = f L / C = ZoZi (1 / 2π) √ (Ca ・ La) ) = F.

[0010]

The first to fourth linear conductors are arranged at intervals of approximately 90 degrees in the circumferential direction of the two ring-shaped conductors between the two ring-shaped conductors. , So that one coil loop including the opposing first and third linear conductors and the opposing second and fourth
Is formed with another coil loop including the linear conductor of the above, and these two coil loops are arranged at a shift of 90 °, so that two coils having sensitivity to each of two axes orthogonal to each other are formed.
This means that one coil loop has been formed. These first
Feed points are provided between each of the fourth to fourth linear conductors and one of the ring-shaped conductors, so that two feed points are provided in each of the two coil loops. Two feed points of each coil loop are connected to differential feed circuits whose RF phases are shifted by 180 °, respectively, and these differential feed circuits are connected to a 90 ° phase synthesizer. Connected. These differential feed circuits consist of a bridge circuit consisting of a capacitor and an inductor, and a series connection of a capacitor and a parallel resonant circuit consisting of a capacitor, an inductor, and a capacitor connected to two terminals connected to two feed points. Connected in parallel, and the resonance frequency of the differential mode is 18
Since each constant is determined so as to shift the phase by 0 °, the resonance current in the differential mode can be canceled, and the orthogonality of the RF magnetic field in the imaging mode can be improved.
Furthermore, since the constants are determined so that the impedance at the frequency f of the parallel resonance circuit becomes high when the switch inserted in the parallel resonance circuit is turned on, the capacitor connected in series to the parallel resonance circuit Is adjusted, only the frequency fo of the imaging mode can be adjusted without affecting the frequency f. Thereby, the frequency of the imaging mode can be adjusted without breaking the balance by adjusting at one point for each of the loop coils having a phase difference of 90 °. Further, the frequency adjustment range of the capacitor is wide because the bridge circuit has an impedance conversion function. Further, since the displacement of the frequency fo when the switch is turned off increases, the decoupling effect can be enhanced.

[0011]

An embodiment of the present invention will be described below in detail with reference to the drawings. As shown in FIG. 1, the orthogonal RF coil according to one embodiment of the present invention has four linear conductors 11 to 14, which are arranged in a direction parallel to the axis of the cylinder. ° are evenly spaced. One end of each of the linear conductors 11 to 14 is connected to one of the ring-shaped conductors 15 via a capacitor 31 to 34, and the other end is connected to a capacitor 35 to 38, respectively.
Is connected to the other ring-shaped conductor 16. The ring-shaped conductors 15 and 16 have capacitors 21 and 2 respectively.
4, 25 to 28 are inserted in series.

This configuration is the same as that of FIG. 3. The capacitors 21 to 28 have a relatively large capacity, and the resonance frequency is mainly adjusted by the capacitors 31 to 38. The capacitors 21 to 24, 25 to 28, 31 to 34 , 35-3
The same applies to the case where the capacitance of each of the four capacitors 8 is substantially the same.

According to the present invention, the four straight conductors 11 to 11
14 and the point A, the point B,
Power supply is different at points C and D. That is, by connecting the differential power supply circuit 52 to the points A and C and connecting the differential power supply circuit 53 to the points B and D, the phases at the points A and C are different from each other by 180 °. At point D, 18
The phases are different by 0 °. The 90 ° phase combiner 51 supplies the two differential feed circuits 52 and 53 with 90 °.
° Supply with different phases.

These differential power supply circuits 52 and 53 are configured as shown in FIG. In FIG. 2, the capacitors 61,
62 and the inductances 71 and 72 form a bridge circuit, and two opposing points 81 and 82 are connected to the A of the RF coil.
The points (B) and C (D) are connected to each other, and the other two opposing points 83 and 84 are connected to the transmission line to the 90 ° phase combiner 51. Usually, the capacitors 61 and 62 have the same capacitance C, the inductances 71 and 72 have the same value L, and satisfy (1 / 2π) √ (CL) = f. Here, f is the resonance frequency in the differential mode. Zo is the impedance of the RF coil,
When Zi is the impedance of the transmission line to the 90 ° phase combiner 51, the condition of L / C = Zo · Zi is satisfied. This eliminates the need for a capacitor for impedance matching of the RF coil.

Thus, at the frequency f, the RF currents having phases inverted by 180 ° from each other can flow at the points A and C, and the RF currents having phases inverted by 180 ° at the points B and D can be supplied. , It is possible to cancel the current in the differential mode (the common mode current at the points A and C and the common mode current at the points B and D) shown in FIG. 4B.
As a result, the influence on the current in the imaging mode can be removed, and the orthogonality of the RF magnetic field in the imaging mode can be increased. In order to eliminate the current in the differential mode with higher accuracy, it is sufficient if the capacitance of the capacitor 61 can be finely adjusted.

Further, as shown in FIG. 2, the differential feed circuits 52 and 53 include variable capacitors 63 and 64, an inductance 73, and a switch 74. here,
Let the capacitance of the capacitor 64 be Ca and the inductance 73 be
Is defined as La, and each value is determined so that (1 / 2π) √ (Ca · La) = f. Then, the switch 74
Is on, the parallel resonance circuit of the capacitor 64 and the inductance 73 has a high impedance with respect to the frequency f, and this parallel resonance circuit is equivalent to not being connected, and only the above-described bridge circuit operates. Assuming that the frequency of the imaging mode is fo, at this frequency fo, the above-described bridge circuit and parallel resonance circuit have a constant having a certain value, and are equivalent to those obtained by adding this constant to the capacitors 31, 33 or the capacitors 32, 34. You can think. Therefore, by making the capacitor 63 variable, it is possible to adjust only the frequency fo without affecting the frequency f. That is, for each of the loop coils having a phase difference of 90 °, the frequency of the imaging mode can be adjusted without breaking the balance by adjusting at one place. Further, since the bridge circuit has the impedance conversion function as described above, the frequency adjustment range by the capacitor 63 can be widened. Further, since the displacement of the frequency fo when the switch 74 is turned off increases, the decoupling effect can be enhanced.

[0017]

As described above, the MRI of the present invention
According to the orthogonal RF coil of the device, the current in the differential mode can be eliminated, so that the orthogonality of the RF magnetic field in the imaging mode can be increased, and the performance as an RF coil can be improved. Further, for each of the loop coils having a phase difference of 90 °, by adjusting at one point, the frequency of the imaging mode can be adjusted without breaking the balance, and the frequency adjustment range can be widened. Further, the decoupling effect can be enhanced.

[Brief description of the drawings]

FIG. 1 is a schematic view of an embodiment of the present invention.

FIG. 2 is a circuit diagram of a differential power supply circuit in the embodiment.

FIG. 3 is a schematic view of a conventional example.

FIG. 4 is a current distribution diagram in each mode in a conventional example.

[Explanation of symbols]

11 to 14 linear conductors 15, 16 ring-shaped conductors 21 to 28, 31 to 40, 61 to 64 capacitors 51 90 ° phase synthesizer 52, 53 differential power supply circuit 71 to 73 inductance 74 switch

Claims (1)

(57) [Claims] 1. Two ring-shaped conductors are disposed between the two ring-shaped conductors at intervals of approximately 90 ° in a circumferential direction of the ring-shaped conductors, and are electrostatically connected to the two ring-shaped conductors. First to fourth linear conductors to be coupled;
The first to fourth feeding points provided between each of the two straight conductors and one of the ring-shaped conductors, and the first and third feeding points of the first and third straight conductors facing each other. A first differential power supply circuit to be connected, a second differential power supply circuit to be connected to the second and fourth power supply points of the second and fourth linear conductors facing each other, A 90 ° phase combiner connected to the second differential feed circuit, and the first and second differential feed circuits each have two terminals connected to two feed points respectively. It consists of two capacitors having substantially the same capacitance C and two inductances having substantially the same value L formed between them.
ブ リ ッ ジ A bridge circuit provided with a terminal connected to the phase synthesizer, a parallel resonance circuit in which a capacitor of capacitance Ca and an inductance of value La are connected in parallel and a switch is inserted, and a series connection to the parallel resonance circuit A series circuit of the parallel resonance circuit and the capacitor is connected to the two terminals on the feed point side in parallel with the bridge circuit, and the capacitances C and Ca of the capacitor and the inductance values L and La is represented by f = (共振 π) の (CL) = f L / C = Zo, where f is the resonance frequency of the differential mode, Zo is the impedance on the feed point side, and Zi is the impedance on the 90 ° phase combiner side. MRI characterized in that Zi (1 / 2π) √ (Ca · La) = f is satisfied.
The orthogonal RF coil of the device.