JPH0975317A - Rf coil for mri - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the generation of magnetic field caused by a loop current flowing to a capacitor and a decoupling circuit. SOLUTION: This device is provided with an element 11 formed by cylindrically rounding a sheet-shaped conductor with a short gap (g) between both terminals, plural resonant capacitors 3a and 3b interposed at the plural positions of this gap (g), and active type decoupling circuit 101 interposed at almost the center of plural positions where those capacitors 3a and 3b are interposed. The decoupling circuit 101 is the serial circuit of an inductor 1 for resonance, for which an inductance value is decided so that the parallel resonance frequency of resonant capacitors 3a and 3b can be almost matched with the frequency of RF pulse for MRI, and a diode 2 for switch. Thus, the generation of nonuniformity in sensitivity or the reduction of blocking performance can be prevented.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an MRI (Magnet)
The present invention relates to an RF coil for tic resonance imaging), and more specifically, to an RF coil for MRI improved so as not to disturb the static magnetic field and the gradient magnetic field and to reduce the decoupling performance.

[0002]

2. Description of the Related Art FIG. 8 is a schematic external view showing an example of an RF coil for MRI of a type called a conventional 1 turn solenoid or a loop gap coil. This type of M
There are various types of RI RF coils for neck, knee, thigh, and the like. This MRI RF coil 700 is
An element 51 having a shape such that a sheet-shaped conductor (copper, aluminum, etc.) is rolled into a tubular shape with a short gap g between both ends;
A plurality of capacitors 53 interposed between the both ends at a plurality of positions of the gap g, and a decoupling circuit (also called a blocking circuit) 501 connected between both ends of the element 51 are configured. (The reason for providing the plurality of resonance capacitors 53 is that the high frequency current generally flows to the left and right ends of the cylindrical element 51 due to the skin effect, so that the resonance capacitors are inserted near the left and right ends, respectively. This is because it is preferable to provide the resonance capacitor, and it is even more preferable to provide the resonance capacitor at another place). The decoupling circuit 501
The resonance frequency of the capacitor 53 is the RF for MRI.
It is a series circuit of an inductor 1 whose inductance value is determined so as to substantially match the pulse frequency, and a switching diode 2. Further, the core of the coaxial cable W and the outer conductor are connected to a connection point between one end of the inductor 1 and one end of the switching diode 2 and the other end (cathode) of the switching diode 2, respectively.

The MRI RF coil 700 is placed in a vertical static magnetic field Bo. Then, from another transmitter coil (for example, a body coil) M to excite the subject.
When transmitting the RF pulse for RI, a forward voltage (anode voltage> cathode voltage) is applied to the switching diode 2 through the coaxial cable W and the inductor 1. As a result, the switching diode 2 is turned on, and the capacitor 53 and the inductor 1 cause M
A resonance circuit for the RF pulse for RI is formed. As a result, the element 51 does not resonate with the MRI RF pulse, and the MRI RF coil 700 becomes MRI.
It stops functioning as a coil for the RF pulse for use (it becomes a decoupling state with another transmission coil). As a result, the MRI RF pulse is transmitted to the MRI RF coil 700.
The subject can be effectively excited without being disturbed by the above. Next, the NMR (Nuclear Magnetic R
When receiving a signal, the switching diode 2 is passed through the coaxial cable W and the inductor 1.
A reverse voltage (anode voltage <cathode voltage) is applied to. As a result, the switching diode 2 is turned off, and the resonance circuit including the capacitor 53 and the inductor 1 is not formed. As a result, a resonance circuit for the NMR signal is formed by the element 51 and the capacitor 53, and the NMR signal is transferred to the element 51.
And is transmitted to the MRI apparatus main body (not shown) through the coaxial cable W (at this time, a DC bias voltage is superimposed on the NMR signal on the coaxial cable W, but the bias voltage is generated by the capacitor 53. There is no problem because it will be cut).

The decoupling circuit 501 is called an active decoupling circuit.

FIG. 9 is a schematic external view showing another example of a conventional MRI RF coil. This MRI RF coil 8
00 is the same as the MRI RF coil 700 in that it has a basic configuration as a one-turn solenoid, but the configuration of the decoupling circuit 601 is different. That is, the decoupling circuit 601 includes the inductor 1 and the switching diodes 2a and 2 connected in antiparallel.
It is a series circuit with b (may be a varistor). Further, a connection point between one end of the inductor 1 and one ends of the switching diodes 2a and 2b connected in antiparallel and the other end of the switching diodes 2a and 2b connected in antiparallel are connected to the core wire of the coaxial cable W. The outer conductors are connected to each other.

When an MRI RF pulse is transmitted from another transmission coil (for example, a body coil) to excite a subject, a forward voltage is applied to the switching diodes 2a and 2b by the voltage induced in the element 51, The switching diodes 2a and 2b are turned on, and the capacitor 53 and the inductor 1 form a resonance circuit for the MRI RF pulse. As a result,
The element 51 does not resonate with the MRI RF pulse, and the MRI RF coil 800 becomes MRI RF.
It does not function as a coil for the pulse. As a result, the MRI RF pulse is transmitted to the MRI RF coil 800.
The subject can be effectively excited without being disturbed by the above. Next, when receiving the NMR signal from the subject, the forward voltage is not applied to the switching diodes 2a and 2b because the voltage induced in the element 51 is weak.
The switching diodes 2a and 2b are turned off, and a resonance circuit including the capacitor 53 and the inductor 1 is not formed. As a result, a resonance circuit for an NMR signal is formed by the element 51 and the capacitor 53, and the NMR signal is received by the element 51 and transmitted to the MRI apparatus main body (not shown) through the coaxial cable W.

The decoupling circuit 601 is called a passive decoupling circuit.

[0008]

FIG. 10 shows the conventional MRI RF coil 700 in which the switching diode 2 is turned on, the capacitor 53 and the inductor 1 are turned on.
6A and 6B show a state in which a resonance circuit for the RF pulse for MRI is formed. The high frequency current i generated by the MRI RF pulse flows in a loop as shown by the broken line, so that the magnetic field Bf is generated. The same applies to the conventional MRI RF coil 800. However, when the magnetic field Bf is generated, there is a problem that the static magnetic field Bo and the gradient magnetic field near the gap g are disturbed to cause uneven sensitivity. In addition, there is a problem in that decoupling performance is deteriorated due to electromagnetic coupling with the subject. This problem becomes more remarkable as the width of the element 51 is wider because the magnetic field Bf is generated in a wider range. Therefore, an object of the present invention is to provide an RF coil for MRI which suppresses the generation of the magnetic field Bf, prevents the static magnetic field and the gradient magnetic field from being disturbed, and improves the decoupling performance. is there.

[0009]

According to a first aspect of the present invention, two conductors have a gap portion facing each other with a gap, and capacitors are provided at a plurality of positions in the gap portion.
A decoupling circuit consisting of a series circuit of an inductor and a switch means, the inductance value of which is determined so that the resonance frequency of the capacitor and the frequency of the MRI RF pulse (or the frequency of the NMR signal) is approximately between the two conductor ends. In the MRI RF coil provided in the above, the decoupling circuit is provided at a substantially central position of a plurality of locations where the plurality of capacitors are provided. M of the first aspect
In the RF coil for RI, the directions of the current loops are opposite when a high-frequency current flows when a plurality of capacitors and an inductor resonate through a capacitor on the left side of the inductor and when a high-frequency current flows through a capacitor on the right side of the inductor. . Therefore, the directions of the magnetic fields generated by the currents in these loops are also reversed, and the results cancel each other out. Therefore, it is possible to prevent the magnetic field from disturbing the static magnetic field and the gradient magnetic field. Further, it is possible to prevent the decoupling performance from deteriorating.

In the above structure, "two conductors" means
For example, as an embodiment of the present invention, as will be described in detail later with reference to FIGS. 1 and 3, the concept of an element having a shape like a sheet-shaped conductor is rolled into a tubular shape with a short gap between both ends. Is included.

According to a second aspect of the present invention, in the MRI RF coil having the above-mentioned structure, the element is formed by rolling a sheet-shaped conductor into a substantially cylindrical shape so that both ends thereof face each other with a gap therebetween. M having the shape
An RF coil for RI is provided. MRI of the second aspect
The RF coil for use in MRI can improve the performance of the RF coil for MRI called a one-turn solenoid or a loop gap coil.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail with reference to the embodiments shown in the drawings. It should be noted that the present invention is not limited by this.

First Embodiment FIG. 1 is a schematic external view showing an MRI RF coil according to the first embodiment of the present invention. The MRI RF coil 100 includes an element 11 having a shape like a sheet-shaped conductor (copper, aluminum, etc.) rolled into a tubular shape with a short gap g between both ends, and a plurality of gaps between the both ends. And a plurality of capacitors 3a and 3b interposed therebetween, and an active decoupling circuit 101 interposed substantially at the center of a plurality of locations where the capacitors 3a and 3b are interposed. The decoupling circuit 101 has a resonance frequency with the capacitors 3a and 3b for MRI RF.
It is a series circuit of an inductor 1 whose inductance value is determined to approximately match the pulse frequency and a switching diode 2 (one end of the inductor 1 and one end of the switching diode 2 are connected via a land conductor). Exist).
Further, the core wire of the coaxial cable W and the outer conductor are connected to the land conductor and the other end of the switch diode 2, respectively.

The MRI RF coil 100 is placed in a vertical static magnetic field Bo. Then, from another transmitter coil (for example, a body coil) M to excite the subject.
When transmitting the RF pulse for RI, a forward voltage (anode voltage> cathode voltage) is applied to the switching diode 2 through the coaxial cable W and the inductor 1. As a result, the switching diode 2 is turned on, and the capacitors 3a and 3b and the inductor 1 form a resonance circuit for the MRI RF pulse. As a result, the element 11 does not resonate with the MRI RF pulse, and the MRI RF coil 100
Will not function as a coil for the MRI RF pulse. As a result, the RF pulse for MRI becomes R for MRI.
The subject can be effectively excited without being hindered by the F coil 100. Next, when receiving an NMR signal from the subject, a reverse voltage (anode voltage <cathode voltage) is applied to the switching diode 2 through the coaxial cable W and the inductor 1. As a result, the switch diode 2 is turned off and the capacitors 3a,
A resonance circuit is not formed by 3b and the inductor 1. As a result, a resonance circuit for an NMR signal is formed by the element 11 and the capacitors 3a and 3b, and the NMR signal is received by the element 11,
The data is transmitted to the main body of the MRI apparatus (not shown) through the coaxial cable W (at this time, the coaxial cable W is NM).
A DC bias voltage is superimposed on the R signal, but there is no problem because the bias voltage is cut by the capacitors 3a and 3b).

FIG. 2 shows the RF coil 100 for MRI.
Then, the switching diode 2 is turned on, and the capacitors 3a and 3b and the inductor 1 turn on the R diode for MRI.
It shows a state in which a resonance circuit for the F pulse is formed. High-frequency current i generated by MRI RF pulse
However, since it flows in a loop as shown by the broken line, the magnetic field B
a and Bb are generated. Here, the high frequency current i flows through the capacitor 3a on the left side of the inductor 1 and the high frequency current i flows through the capacitor 3b on the right side of the inductor 1.
The direction of the current loop is reversed. Therefore, the directions of the magnetic fields Ba and Bb generated by the currents in these loops are also reversed, and the results cancel each other out. Therefore, the magnetic field B
It is possible to prevent a and Bb from disturbing the static magnetic field Bo and the gradient magnetic field. Further, it is possible to prevent the decoupling performance from deteriorating.

-Second Embodiment- In FIG. 3, the MRI RF coil 200 uses a passive decoupling circuit 201 instead of the active decoupling circuit 101 of FIG.
This MRI RF coil 200 also enables the MRI of FIG.
The same effect as the RF coil 100 for use can be obtained.

-Third Embodiment- In FIG. 4, an MRI RF coil 300 is a high-pass birdcage coil having a structure in which a large number of elements E are provided between ring conductors R1 and R2 having a relatively wide width. Is. Capacitors 4a and 4b are provided at a plurality of positions of the gap g provided in the ring conductors R1 and R2, and the decoupling circuit 201 (101) is provided substantially at the center of the plurality of places where the capacitors 4a and 4b are provided. But it's OK). Further, a core wire of a coaxial cable W for transmitting an NMR signal and an outer conductor are connected to both ends of one of the capacitors 4a provided in any of the gaps g. With this MRI RF coil 300, the same effect as that of the MRI RF coil 100 of FIG. 1 can be obtained.

Fourth Embodiment In FIG. 5, an MRI RF coil 400 is a low-pass birdcage coil having a structure in which a large number of relatively wide elements Ea are provided between ring conductors R11 and R12. Is. Capacitors 5a and 5b are provided at a plurality of positions of the gap g provided in each element Ea, and a decoupling circuit 201 (101 may be provided at approximately the center of the plurality of places where the capacitors 5a and 5b are provided. ) Is connected. Further, a core wire of a coaxial cable W for transmitting an NMR signal and an external conductor are connected to both ends of one of the capacitors 5a provided in any of the gaps g. With this MRI RF coil 400, the same effect as that of the MRI RF coil 100 of FIG. 1 can be obtained.

-Fifth Embodiment- In FIG. 6, an MRI RF coil 500 is a surface coil in which a relatively wide element Es is formed in a substantially C shape so that a narrow gap g is formed between both ends. Is. Capacitors 6a and 6b are provided at a plurality of locations in the gap g, and a decoupling circuit 201 (or 101) may be connected to the approximate center of the plurality of locations where the capacitors 6a and 6b are provided. . W is a coaxial cable.
This MRI RF coil 500 also enables the MRI of FIG.
The same effect as the RF coil 100 for use can be obtained.

-Sixth Embodiment- In FIG. 7, an MRI RF coil 600 includes elements E1, E2, E3, E4 and arch conductors A1, A2, A.
It is a saddle type coil which has 3 and A4. Arch conductor A
The connection portions of A2 and A4 and the connection portions of the elements E1 and E3 face each other with a narrow gap over a relatively long distance. The capacitors 7a and 7b are provided at a plurality of positions in this gap.
And the decoupling circuit 20 is provided at the approximate center of a plurality of places where the capacitors 7a and 7b are provided.
1 (or 101) may be connected. W is a coaxial cable. With this MRI RF coil 600, the same effect as that of the MRI RF coil 100 of FIG. 1 can be obtained.

[0021]

According to the MRI RF coil of the present invention, it is possible to suppress the generation of a magnetic field due to a loop current flowing through a capacitor and a decoupling circuit. Therefore, the static magnetic field or the gradient magnetic field is not disturbed, and uneven sensitivity is generated. Can be prevented. In addition, it is possible to prevent deterioration of decoupling performance.

[Brief description of drawings]

FIG. 1 is an R for MRI according to a first embodiment of the present invention.
It is a typical external view which shows an F coil.

FIG. 2 is an explanatory diagram of a high frequency current in the RF coil for MRI of FIG.

FIG. 3 is an R for MRI according to a second embodiment of the present invention.
It is a typical external view which shows an F coil.

FIG. 4 is an R for MRI according to a third embodiment of the present invention.
It is a typical external view which shows an F coil.

FIG. 5 is an R for MRI according to a fourth embodiment of the present invention.
It is a typical external view which shows an F coil.

FIG. 6 is an R for MRI according to a fifth embodiment of the present invention.
It is a typical external view which shows an F coil.

FIG. 7 is an R for MRI according to a sixth embodiment of the present invention.
It is a typical external view which shows an F coil.

FIG. 8 is a schematic external view showing an example of a conventional MRI RF coil.

FIG. 9 is a schematic external view showing another example of a conventional MRI RF coil.

10 is an explanatory diagram of a high frequency current in the RF coil for MRI of FIG.

[Explanation of symbols]

100, 200, 300 MRI RF coil 400, 500, 600 MRI RF coil 101, 201 Decoupling circuit 1 Inductor 2, 2a, 2b Switching diode 3a, 3b, 4a, 4b Capacitor 5a, 5b, 6a, 6b, 7a, 7b Capacitor 11, E, Ea, Es, E1-E4 Element W Coaxial cable g Gap

Claims (2)

[Claims] 1. Two conductors have a gap portion facing each other with a gap, and capacitors are provided at a plurality of positions in the gap portion, and the resonance frequency with the capacitors is R for MRI.
The decoupling circuit composed of a series circuit of an inductor and a switch means, the inductance value of which is determined to substantially match the frequency of the F pulse (or the frequency of the NMR signal),
An MRI RF coil provided between two conductor ends, wherein the decoupling circuit is provided at an approximate center of a plurality of locations where the plurality of capacitors are provided. coil. 2. The MRI RF coil according to claim 1, wherein the element has a shape obtained by rolling a sheet-shaped conductor into a substantially cylindrical shape such that both ends thereof face each other with a gap therebetween. The characteristic MRI RF coil.