JPH08229019A - Neutral circuit and magnetic resonance imaging device - Google Patents

Abstract

PURPOSE: To reduce the interference between RF coils by magnetically interlinking multiple RF coils of a neutral circuit in the electrically noncontact state, and making the reactance of a closed loop made of a good conductor adjustable. CONSTITUTION: RF coils 30a, 30b tuned to nearly the same frequency interfere with each other via mutual induction, and the magnetic field generated by the current flowing in one RF coil is interlined with the other RF coil in this neutral circuit. A closed loop 14 provided in parallel with the RF coils 30a, 30b is magnetically linked with the RF coil 30a and the RF coil 30b via mutual induction. The closed loop 14 is made of a good conductor, and the impedance of the closed loop 14 in the highfrequency region is dominated by inductance rather than resistance. A variable-capacity adjusting capacitor 12 is inserted in the closed loop 14 in series to the closed loop 14. When the adjusting capacitor 12 is properly adjusted, the interference between the RF coils 30a, 30b can be effectively removed without adjusting the position and size of the closed loop 14 and changing the conductor of the closed loop 14.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an RF coil neutralizing circuit and a magnetic resonance imaging apparatus using the same.

[0002]

2. Description of the Related Art In a magnetic resonance imaging apparatus, R
The F coil transmits a high frequency magnetic field having a radio frequency to excite the magnetized spins of specific nuclei, and after completion of the transmission, transmits a magnetic resonance signal generated in the process of relaxation of the excited magnetized spins to the initial state. The same RF coil used for the above or another RF coil dedicated to reception is used for reception.

A plurality of RF coils used for this reception are prepared, and a plurality of RF coils may be simultaneously driven.
For example, a technique known as a QD coil that combines two RF coils that create orthogonal high-frequency magnetic fields, or a phased-type that simultaneously receives a large number of RF coils.
There are techniques known as phased array coils, which are useful techniques for obtaining magnetic resonance imaging images with high signal-to-noise ratio.

When using a plurality of these RF coils, the RF
Since interference occurs mainly between the coils due to magnetic mutual induction, a technique for eliminating this interference is required. The following techniques are known as such interference removal techniques.

(1) T. R. There is a neutralizing circuit with the capacitor bridge of the Fox invention. This neutralization circuit 10
As shown in FIG. 9, four capacitors 11 and 1 are provided between the capacitors of the two RF coils 30a and 30b.
Bridged by a capacitor bridge including two variable-capacitance adjusting capacitors 12, a voltage opposite in phase to the induced electromotive force generated by mutual induction is supplied by the capacitor bridge to cancel interference.

(2) There is an interference removal technique as described in Roemer's paper in which two RF coils are arranged so as to overlap each other (see FIG. 10). This technology
The interference is eliminated by utilizing the fact that, depending on the degree of overlap, there is no magnetic mutual induction between the two RF coils 30a and the RF coil 30b having the adjustment capacitors 21a and 21b, respectively.

(3) Furthermore, there is an interference cancellation technique using a low input impedance preamplifier described in Roemer's paper (FIG. 11). With this technique, the inductor 32 connecting the preamplifier 13 having a low input impedance and the RF coil 30 and the output end capacitor 21 of the RF coil 30 are set to resonate at the Larmor frequency. Lar flowing in
The mor frequency current is reduced. Therefore, even with this technique, magnetic mutual induction between the RF coils can be reduced.

However, the above technique has the following problems. In the method (1), the wiring becomes complicated as the number of RF coils increases. Further, a closed loop is formed via the capacitor bridge, and the magnetic field formed by the high-frequency current flowing in the RF coil is linked to the closed loop, which may cause a new interference problem.

The method (2) has no degree of freedom in the arrangement of the RF coil, and is not necessarily the optimum arrangement for the sensitivity distribution of the RF coil and the signal noise ratio. Further, for example, when it is desired to arrange three or more RF coils in a straight line, it can be used only for removing interference from adjacent RF coils.

In the method (3), the loss of the inductor connecting the preamplifier and the RF coil may lower the signal-noise ratio, and the inductance becomes a small value as the frequency of the RF coil increases. , Difficult to make.
Further, in this technique, it is desirable to adjust the value of the output capacitor so as to eliminate the deviation of the tuning frequency of the RF coil for each subject in order to obtain the maximum signal noise ratio, but the value of the output capacitor is adjusted. In this case, the value of the inductor connecting the preamplifier and the RF coil also has to be adjusted according to the value of the output capacitor. Adjusting the value of this inductor is practically difficult.

[0011]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a neutralization circuit effective for reducing interference between RF coils and a magnetic resonance imaging apparatus using the neutralization circuit.

[0012]

The present invention has taken the following means in order to solve the above problems. The neutralization circuit of the present invention is magnetically coupled to at least two RF coils in an electrically non-contact state, is composed of a closed loop made of a good conductor, and the reactance of this closed loop is adjustable and variable.

In another neutralization circuit of the present invention, at least two first closed loops made of a good conductor that are magnetically coupled to at least two RF coils in an electrically non-contact state, respectively, and the two closed loops. And a transmission line that connects the transmission lines, and the at least two first closed loops and the transmission line form one second closed loop, and the reactance of the second closed loop is adjustable to adjust the two RF lines. The coil interference was removed.

The magnetic resonance imaging apparatus of the present invention is provided with the neutralization circuit capable of adjusting the reactance of the closed loop, which is magnetically coupled to the RF coil in a state of being electrically non-contact with the RF coil as described above.

[0015]

As a result of taking the above-mentioned means, the following effects occur. Since the interference between the RF coils can be removed without being electrically connected to the RF coil (that is, as an electrical non-contact), new interference due to the high frequency magnetic field caused by the neutralization circuit is not generated, and the RF is not generated. Interference between coils can be eliminated.

Since the RF coils can be arranged with a high degree of freedom, it is possible to eliminate interference between the RF coils while keeping the optimum arrangement for the sensitivity distribution of the RF coils and the signal noise ratio. In addition, it is not restricted to circuit constants with extreme values. Therefore, the present invention facilitates manufacturing.

Further, since the tuning of the RF coils and the adjustment of the neutralization circuit of the present invention can be performed independently, even when the tuning of the RF coils is adjusted for each subject, the interference between the RF coils is reduced. There is no effect on the effect.

Further, according to the present invention, since the degree of freedom of arrangement of the RF coils is high as described above, it becomes easy to install a large number of RF coils. Therefore, QD coils and phased
The array coil becomes easier and the S / N ratio is improved.

Further, since the neutralization circuit of the present invention has a simple structure and is an interference elimination technique for effectively eliminating interference, when the neutralization circuit of the present invention is applied to a magnetic resonance imaging apparatus. The reliability of the magnetic resonance imaging apparatus is improved, and the configuration of the magnetic resonance imaging apparatus is simplified.

[0020]

Embodiments of the present invention will be described with reference to the drawings. In the following description, only the neutralization circuit will be described, and an example applied to a magnetic resonance imaging apparatus will be omitted. In addition,
In the following embodiments, an example of removing interference from two RF coils is shown, but the present invention is not limited to this, and the present invention can of course be applied to removing interference from two or more RF coils.

A first embodiment of the neutralization circuit of the present invention will be described with reference to FIG. FIG. 1 is a diagram showing a schematic configuration of a neutralization circuit according to a first embodiment of the present invention. The first embodiment is characterized in that a closed loop including a variable capacitance adjusting capacitor is arranged between two RF coils so as to overlap the two RF coils.

When there are two RF coils 30a and 30b tuned to almost the same frequency as shown in FIG. 1, the RF coil 30a and the RF coil 30b are provided.
And shall be interfering with each other by mutual induction. That is, the magnetic field created by the current flowing in one RF coil is linked to the other RF coil.

The RF coil 30a and the RF coil 30b are provided together with a closed loop 14, and the closed loop 14 is R
Magnetic coupling by mutual induction is provided between the F coil 30a and the RF coil 30b. Since the closed loop 14 is made of a good conductor, the impedance of the closed loop 14 is dominated by the inductance rather than the resistance in the high frequency region. In addition, in the middle of the closed loop 14, a variable capacitance adjusting capacitor 12 is inserted in series with the closed loop 14.

For example, a part (referred to as φ1) of the high frequency magnetic field generated by the current flowing through one RF coil 30a interlinks with the other RF coil 30b, but at the same time, also partially interlinks with the closed loop 14. The magnetic field interlinking the closed loop 14 generates an induced electromotive force in the closed loop 14. The resulting current flowing in the closed loop 14 is
The phase and magnitude are determined by the impedance of 4. Then, the current flowing through the closed loop 14 further creates a high frequency magnetic field, and a part of the high frequency magnetic field is linked to the other RF coil 30b (φ2).

At this time, if the condition of φ2 = −φ1 is satisfied, mutual induction from one coil to the other is canceled.
In order to satisfy the condition of φ2 = −φ1, in principle, the reactance is adjusted by adjusting the position and size of the closed loop 14 and by changing the conductor width of the closed loop 14, and the like. Adjust the amount of current. However, the above method is impractical because it means a trial and error remake of the closed loop 14.

Therefore, in the present invention, as shown in FIG.
A variable-capacitance adjusting capacitor 12 is placed in series in a closed loop 14. According to the invention, this adjusting capacitor 12
By appropriately adjusting, the condition of φ2 = −φ1 can be satisfied without adjusting the position and size of the closed loop 14 and without changing the conductor of the closed loop 14, so that the interference between the RF coils can be effectively achieved. Can be removed. The reason for this will be described below.

The impedance Z of the closed loop 14 is a series reactance of the inductance L and the capacitance C of the adjusting capacitor 12, because the resistance component is negligible at high frequencies. In the following description, the resistance component is ignored as the impedance, and only the reactance component is described.
At the frequency ω, the reactance Z is given by the equation (1). Z = j (ωL-1 / (ωC)) (1) where j is an imaginary unit.

From the equation (1), it is apparent that the absolute value of the reactance Z can be freely changed only by adjusting the capacitance C. Therefore, according to the present invention, it is possible to easily adjust the reactance Z of the closed loop 14 to a desired reactance by adjusting only the capacitance C of the adjustment capacitor 12. Is effective for.

Further, according to the present invention, the inductance L
The sign of the reactance Z is inverted at the frequency at which the capacitor C and the capacitor C resonate. That is, since the current flowing in the closed loop 14 and the direction of the magnetic field generated by the current are inverted, this can be used to deal with whether the polarity of mutual induction with the RF coil 30a and the RF coil 30b is positive or negative.

2 to 5, the first embodiment of the first embodiment
-The 4th modification is explained. 2 to 5, since the positional relationship between the RF coil and the neutralization circuit is almost the same as that in FIG. 1, the RF coil is omitted and only the configuration of the neutralization circuit (closed loop) is shown.

FIG. 2 is a diagram showing a first modification of the first embodiment. In the first embodiment shown in FIG. 1, the closed loop 14 is
Although it is composed of two large loops, the operation is exactly the same even if the closed loop 14 has a constricted part as shown in FIG. Actually, in the constricted shape as shown in FIG. 2, the high-frequency magnetic field created by the closed loop 14 is the RF for which interference is to be removed.
This is preferable because it reduces the problem of reaching another RF coil (not shown) other than the coil 30a and the RF coil 30b to generate new interference.

FIG. 3 is a diagram showing a second modification of the first embodiment. The second modification has a twisted structure so that the intermediate portion of the closed loop 14 is crossed. The sign of the reactance of the closed loop 14 is inverted by adjusting the adjusting capacitor 12 of FIG.
As described above, it is possible to deal with whether the polarity of the mutual induction with b is positive or negative, but instead of inverting the sign of the reactance of the closed loop 14 by adjusting the adjusting capacitor 12, as in this modification, By twisting the closed loop 14 in the middle, the polarity of mutual induction between the closed loop 14 and the RF coil 30a or 30b can be reversed. Therefore,
Both positive and negative mutual induction can be dealt with by twisting the closed loop 14 midway or not.

FIG. 4 is a diagram showing a third modification of the first embodiment. This modification shows an example in which the adjustment capacitor 12 is replaced with a variable inductor. As shown in FIG. 4, instead of connecting the variable adjustment capacitor 12 in series in the closed loop 14, it is possible to control the amount of current induced in the closed loop 14 by connecting the variable inductor 15 in series. The modified example is effective when the impedance of the closed loop 14 is too small to satisfy the condition of φ2 = −φ1.

FIG. 5 is a diagram showing a fourth modification of the first embodiment. In this modification, an additional inductor 16 is added to FIG. In the fourth modification, when the impedance of the closed loop 14 is too small, it is not always easy to adjust the value of the inductor. Therefore, the additional inductor 16 having a sufficient value or more is added to the adjustment capacitor 12 connected in series. The interference removal adjustment is performed only by adjusting the capacity of.

According to the first embodiment of the present invention, the interference between the RF coils can be removed without being electrically connected to the RF coils (that is, as an electrical non-contact). The interference between the RF coils can be removed without generating new interference due to the resulting high-frequency magnetic field.

Since the RF coils can be arranged with a high degree of freedom, the interference between the RF coils can be removed while maintaining the optimum arrangement for the sensitivity distribution of the RF coils and the signal-noise ratio. In addition, it is not restricted to circuit constants with extreme values. Therefore, the present invention facilitates manufacturing.

Further, since the tuning of the RF coils and the adjustment of the neutralization circuit of the present invention can be performed independently, even when the tuning of the RF coils is adjusted for each subject, the interference between the RF coils is reduced. There is no effect on the effect.

Further, according to the present invention, since the degree of freedom in arranging the RF coils is high as described above, it becomes easy to install a large number of RF coils side by side. Therefore, QD coils and phased
The array coil becomes easier and the S / N ratio is improved.

Further, since the neutralization circuit of the present invention has a simple structure and is an interference elimination technique for effectively eliminating interference, the neutralization circuit according to the above-mentioned first embodiment is applied to the magnetic resonance imaging apparatus. When is applied, the reliability of the magnetic resonance imaging apparatus is improved and the configuration of the magnetic resonance imaging apparatus is simplified.

A second embodiment of the neutralization circuit of the present invention will be described with reference to FIG. FIG. 6 is a diagram showing a schematic configuration of the neutralization circuit according to the second embodiment of the present invention. The second embodiment is RF
This is an embodiment for removing interference when the coil 30a and the RF coil 30b are separated from each other.

When the RF coil 30a to be subjected to interference removal and the RF coil 30b are separated from each other, in order to prevent the closed loop 14 for interference removal from interfering with other RF coils not shown, as shown in FIG. It is not enough to have a constricted shape in the middle.

In this embodiment, the closed loop 14a and the closed loop 14b are installed at both ends of the transmission line 17a and the transmission line 17b (for example, described as a coaxial cable in this embodiment) magnetically shielded from the outside. . The closed loop 14a and the closed loop 14b at both ends are respectively the RF coil 3
0a and the RF coil 30b are magnetically coupled. The closed loop 14a and the closed loop 14b at both ends of the transmission line 17a and the transmission line 17b are connected to the central conductor 41 of the coaxial cable and the other to the outer conductor 42 of the coaxial cable, respectively. It forms one closed loop. Then, in the central portion of the two coaxial cables (that is, between the transmission line 5a and the transmission line 5b), the fixed adjustment capacitor 18 is connected in series to the central conductor 41, and the variable capacitance adjustment capacitor 12 is connected in series to the outer conductor 42. Has been done.

Similar to the first embodiment, this embodiment achieves the condition of φ2 = −φ1 by adjusting the reactances of the closed loop 14a and the closed loop 14b with the adjusting capacitor 12. In the configuration of the second embodiment, the operation is the same even if the variable adjustment capacitor 12 is connected to the center conductor 41 and the fixed adjustment capacitor 18 is connected to the outer conductor 42.
Further, although it may be useful to insert the fixed adjustment capacitor 18 for the convenience of adjustment of the variable adjustment capacitor 12,
Generally, it is not necessary. Further, in the present embodiment, the variable adjustment capacitor 12 for adjustment is arranged in the central portion of the coaxial cable, but when the frequency is not high, the length of the coaxial cable does not matter, so the adjustment capacitor 12 is ,
For example, one closed loop 14a and one closed loop 14b may be placed anywhere, for example. Further, similarly to the first embodiment, as the reactance adjusting means, a variable inductor or an additional fixed inductor may be used instead of or in addition to the adjusting capacitor 12.

FIG. 7 is a diagram showing a first modification of the second embodiment. In FIG. 7, the positional relationship between the RF coil and the neutralization circuit is the same as in FIG. 6, so the RF coil is omitted and only the configuration of the neutralization circuit is shown. In this modification, a correction inductor 19 is added to FIG. Transmission line 17
When the lengths of a and the transmission line 17b are not sufficiently short with respect to the wavelength, the transmission line 17a and the transmission line 17b affect the operation of the closed loop 14a and the closed loop 14b, and even if the capacitance of the adjustment capacitor 12 is minimized, the closed loop 14a and The reactance of the closed loop 14b may not be a desired value. In this case, an inductor is inserted between the center conductor 41 and the outer conductor 42 in order to reduce the influence of the cable length. Therefore, according to this modification, since the effect of the cable length is mitigated by adding the correction inductor 19, the adjustment capacitor 12 can easily adjust the value of the inductor. Further, the reactance inserted between the central conductor 41 and the outer conductor 42 may be the adjustment capacitor 12 depending on the situation even if it is not an inductor.

FIG. 8 is a diagram showing a second modification of the second embodiment. In FIG. 8, the positional relationship between the RF coil and the neutralization circuit is the same as in FIG. 6, so the RF coil is omitted and only the configuration of the neutralization circuit is shown. In this modification, the transmission line 17a
And the closed loops 14a at both ends of the transmission line 17b
In this example, the correction capacitor 20a and the correction capacitor 20b are connected in series to the closed loop 14b and the closed loop 14b, respectively. In the case where the closed loop 14a and the closed loop 14b at the respective ends of the transmission line 17a and the transmission line 17b have inductors that are too large and the value of the adjustment capacitor 12 cannot be made smaller than the value necessary to achieve the adjustment condition, the closed loop 14a. By inserting the correction capacitor 20a and the correction capacitor 20b in series in the closed loop 14b and the closed loop 14b, respectively, the inductances of the closed loop 14a and the closed loop 14b can be effectively reduced. Therefore, it becomes easy to adjust the reactance by the adjusting capacitor 12.

As described above, according to the second embodiment of the present invention, the same effect as that of the first embodiment can be obtained. As described above, if the neutralization circuit according to the present invention is used, any two
Since the interference between two RF coils can be easily removed, it is easy to install a large number of RF coils together, which makes it easy to realize a so-called phased array coil that can obtain an image with a wide field of view with a high signal-noise ratio. become. Of course,
The interference removal technique of the present invention can achieve a more excellent interference removal effect when used in combination with a known interference removal technique.

The present invention is summarized as follows. (1) A neutralization circuit that is a closed loop made of a good conductor that is magnetically coupled to at least two RF coils, and that has a means for adjusting the reactance of the closed loop. (2) In (1), the adjustable means is a neutralization circuit that is a variable capacitor inserted in series in the closed loop. (3) In (1), the adjusting means is a neutralizing circuit which is a variable inductor inserted in series in the closed loop. (4) For each of the two RF coils, there is a closed loop made of a good conductor that is magnetically coupled to each one, and there is a transmission line connecting the two closed loops. The two closed loops and the transmission line form one closed loop. The neutralization circuit is configured so that the reactance of the one closed loop can be adjusted and adjusted, thereby eliminating the interference of the two RF coils. (5) In (4), the transmission line is a neutralization circuit that is a coaxial cable. (6) In (4), the adjustment variable means is a neutralization circuit which is a variable capacitor inserted in series in the closed loop. (7) In (4), the adjustment variable means is a neutralization circuit that is a variable inductor inserted in series in the closed loop. (8) In the neutralization circuit according to (4), the adjustment variable means is arranged near the center of the transmission line.

(9) In the neutralization circuit according to (8), a reactance is further arranged near the center of the transmission line, the reactance extending over two conductors of the transmission line. (10) A magnetic resonance imaging apparatus including the neutralization circuit according to (1) or (4).

The present invention is not limited to the above embodiment. For example, in the above embodiment, the variable capacitance capacitor or inductor is used for adjusting the reactance, but the present invention is not limited to this, and any other one can be used as long as it can adjust the reactance of the neutralization circuit such as a varactor diode. May be used. Of course, various modifications can be made without departing from the scope of the present invention.

[0050]

According to the present invention, the following effects can be obtained. Since the interference between the RF coils can be removed without being electrically connected to the RF coil (that is, as an electrical non-contact), new interference due to the high frequency magnetic field caused by the neutralization circuit is not generated, and the RF is not generated. Interference between coils can be eliminated.

Since the RF coils can be arranged with a high degree of freedom, the interference between the RF coils can be removed while maintaining the optimum arrangement for the sensitivity distribution of the RF coils and the signal noise ratio. In addition, it is not restricted to circuit constants with extreme values. Therefore, the present invention facilitates manufacturing.

Further, since the tuning of the RF coils and the adjustment of the neutralization circuit of the present invention can be performed independently, even when the tuning of the RF coils is adjusted for each subject, the interference between the RF coils is reduced. There is no effect on the effect.

Further, according to the present invention, since the degree of freedom of arrangement of the RF coils is high as described above, it becomes easy to install a large number of RF coils side by side. Therefore, QD coils and phased
The array coil becomes easier and the S / N ratio is improved.

Further, since the neutralization circuit of the present invention has a simple structure and is an interference elimination technique for effectively eliminating interference, when the neutralization circuit of the present invention is applied to a magnetic resonance imaging apparatus. The reliability of the magnetic resonance imaging apparatus is improved, and the configuration of the magnetic resonance imaging apparatus is simplified.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic configuration of a neutralization circuit according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a first modification of the first embodiment.

FIG. 3 is a diagram showing a second modification of the first embodiment.

FIG. 4 is a diagram showing a third modification of the first embodiment.

FIG. 5 is a diagram showing a fourth modification of the first embodiment.

FIG. 6 is a diagram showing a schematic configuration of a neutralization circuit according to a second embodiment of the present invention.

FIG. 7 is a diagram showing a first modification of the second embodiment.

FIG. 8 is a diagram showing a second modification of the second embodiment.

FIG. 9 is a diagram showing an example of a conventional neutralization circuit.

FIG. 10 is a diagram showing an example of a conventional neutralization circuit.

FIG. 11 is a diagram showing an example of a conventional neutralization circuit.

[Explanation of symbols]

10 ... Neutralizing circuit 11 ... Capacitor 12 ... Adjusting capacitor 13 ... Preamplifier 14, 14a, 14b ... Closed loop 15 ... Variable inductor 16 ... Additional inductor 17a, 17b ...
Transmission line 18 ... Fixed adjustment capacitor 19 ... Correction inductors 20a, 20b ... Correction capacitors 30, 30a, 3
0b ... RF coils 31a, 31b ... Tuning capacitor 32 ... Inductor 41 ... Center conductor 42 ... Outer conductor

Claims (3)

[Claims] 1. A neutralization circuit, which is magnetically coupled to at least two RF coils in an electrically non-contact state, and comprises a closed loop of good conductors, wherein the reactance of the closed loop is adjustable. And the neutralization circuit. 2. At least two first closed loops made of good conductors that are magnetically coupled to at least two RF coils in a non-contact state with each other, and a transmission line connecting the two closed loops. In the summing circuit, the at least two first closed loops and the transmission line form one second closed loop, and the reactance of the second closed loop is adjustable and variable. 3. A magnetic resonance imaging apparatus comprising the neutralization circuit according to claim 1 or 2.