JP2000175880A - Rf coil for mri - Google Patents

Abstract

PROBLEM TO BE SOLVED: To change direction of sensitivity depending on resonance frequency. SOLUTION: An RF coil 100 for MRI is provided with ring coils 1 and 2, element conductors 3, 4, 5 and 6 placed between the ring coils 1 and 2, a first capacitor Ca placed between connecting points between the element conductors 3 and 4 and the ring coil 1, and a second capacitor placed between connecting points between the element conductors 3 and 4 and the ring coil 2, a third capacitor Cb placed on the element conductor 3 and a fourth capacitor Cb placed on the element conductor 4. Capacities Ca and Cb are adjusted to values in such a way that mutual inductance between the ring coils 1 and 2 functioning as an RF coil to a resonance frequency ω1 can be substantially ignored and mutual inductance between the first saddle type coil and the second saddle type coil functioning as an RF coil to a resonance frequency ω2 can be substantially ignored.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an MRI (Magnetic
More particularly, the present invention relates to an MRI RF coil having a different sensitivity direction depending on a resonance frequency.

[0002]

2. Description of the Related Art NMR spectroscopy (Nuclear Ma
In gnetic resonance spectroscopy, shimming to increase the spatial uniformity of the magnetic field and decoupling to cut off the coupling between different nuclides (decouplin)
From the viewpoint of performing g) properly, a dual tune coil is known, which enables one RF coil to transmit RF pulses for excitation at different frequencies or to receive NMR signals at different frequencies. I have.

[0003]

However, there is no known MRI RF coil whose sensitivity direction varies depending on the resonance frequency. Therefore, an object of the present invention is to provide an MRI RF coil having different sensitivity directions depending on the resonance frequency.

[0004]

According to a first aspect of the present invention, there is provided an MRI RF coil having a plurality of element conductors interposed between two ring coils, the RF coil having a first resonance frequency. For the two ring coils
An F coil is formed, and an RF coil is formed in a saddle shape by the element conductor adjacent to each other with respect to an RF signal of a second resonance frequency and a ring coil portion sandwiched between connection points of the element conductor and the ring coil. An MRI RF coil is provided. The first
In the MRI RF coil according to the aspect of the present invention, two ring coils can be formed as RF coils for an RF signal of a first resonance frequency (generally an excitation RF pulse or NMR signal), and an RF signal of a second resonance frequency can be formed. For signals,
A saddle-shaped coil composed of an element conductor and a part of a ring coil can be formed as an RF coil, and an RF coil whose sensitivity direction differs by 90 ° depending on the resonance frequency can be formed.

[0005] In a second aspect, the present invention provides a first ring coil, a second ring coil, and first to third coils interposed between the first and second ring coils.
A fourth element conductor, a first connection point between the first element conductor and the first ring coil, and a first connection point provided between a connection point between the second element conductor and the first ring coil. A capacitor, a second capacitor interposed between a connection point between the first element conductor and the second ring coil and a connection point between the second element conductor and the second ring coil; A third capacitor interposed in the first element conductor, a fourth capacitor interposed in the second element conductor, a connection point between the third element conductor and the first ring coil, A first power supply capacitor interposed between a connection point of the fourth element conductor and the first ring coil, and a series circuit of a parallel resonance circuit that resonates in parallel with the second resonance frequency. A second power supply capacitor interposed between a connection point between the third element conductor and the second ring coil and a connection point between the fourth element conductor and the second ring coil; A series circuit of a parallel resonance circuit that performs parallel resonance with respect to the second resonance frequency,
A third power supply capacitor interposed in the third element conductor, a series circuit of a parallel resonance circuit that resonates in parallel with the first resonance frequency, and a third circuit interposed in the fourth element conductor. And a series circuit of a parallel resonance circuit that resonates in parallel with respect to the first resonance frequency, wherein the first to fourth capacitors are configured such that, with respect to the first resonance frequency, The first and third element conductors and their elements such that the mutual inductance between the first ring coil and the second ring coil is substantially negligible and the second resonance frequency is A first saddle-shaped coil formed by a ring coil portion sandwiched between a conductor and a connection point between the first and second ring coils, the second and fourth element conductors, and their elements; The capacitance is adjusted so that the mutual inductance between the second saddle-shaped coil formed by the ring conductor portion and the ring coil portion sandwiched between the connection points of the first and second ring coils can be substantially ignored. An MRI RF coil is provided. In the above configuration, “power supply” means one or both of transmission of an RF pulse for excitation and supply and extraction of power for reception of an NMR signal. MRI R according to the second aspect
In the F coil, with respect to the first resonance frequency, the parallel resonance circuit provided between the third and fourth element conductors is in a parallel resonance state, and the parallel resonance portion is substantially cut off. The ring coil and the second ring coil function as an RF coil. Here, the mutual inductance between the first ring coil and the second ring coil can be substantially neglected from the condition of the capacitance of the first to fourth capacitors. The mutual interference between the coils is substantially eliminated, and the SNR (Signal to Noise)
Ratio) can be sufficiently increased. On the other hand, for the second resonance frequency, the parallel resonance circuit provided between the connection points between the third and fourth element conductors and the first ring coil, and the third and fourth element conductors The parallel resonance circuit provided between the connection points with the second ring coil is in a parallel resonance state, the parallel resonance portion is substantially separated, and the first and third element conductors and the first and second element conductors are separated. A first saddle-shaped coil formed by a part of the ring coil and a second saddle-shaped coil formed by a part of the second and fourth element conductors and the first and second ring coils , R
Functions as an F coil. Here, the mutual inductance between the first saddle coil and the second saddle coil can be substantially neglected from the condition of the capacitance of the first to fourth capacitors. Mutual interference between the second saddle coils can be substantially eliminated, and the SNR can be sufficiently increased. From the above, it functions as an RF coil in which the sensitivity direction differs by 90 ° between the first resonance frequency and the second resonance frequency.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail with reference to the embodiments of the invention shown in the drawings. Note that the present invention is not limited by this.

Before describing the embodiment of the present invention, the basic principle of the MRI RF coil according to the present invention will be described. FIG. 1 is a perspective view showing a basic circuit of an MRI RF coil 100 'according to the present invention. This MR
The RF coil for I 100 ′ includes a ring coil 1, a ring coil 2, the ring coil 1 and the ring coil 2.
Element conductors 3, 4, 5, and 6, interposed between the element conductors 3 and the ring coil 1, and interposed between the element conductors 4 and the ring coil 1. A first capacitor Ca, a second capacitor Ca interposed between a connection point between the element conductor 3 and the ring coil 2 and a connection point between the element conductor 4 and the ring coil 2, And a fourth capacitor Cb provided on the element conductor 4. A switch S1 is interposed between a connection point between the element conductor 5 and the ring coil 1 and a connection point between the element conductor 6 and the ring coil 1 to connect a connection point between the element conductor 5 and the ring coil 2 and the element. A switch S2 is provided between the connection point of the conductor 6 and the ring coil 2.
The element conductor 5 is provided with a switch S3, and the element conductor 6 is provided with a switch S4. The first and second capacitors Ca and the third and fourth capacitors Cb have a first resonance angular frequency ω
1 so that the mutual inductance between the ring coils 1 and 2 (M1 in FIG. 2) is substantially negligible.
For the second resonance angular frequency ω2, a saddle-shaped coil formed by the element conductors 3, 5 and the ring coil portion sandwiched between the connection points of the element conductors 3, 5 and the ring coils 1, 2 (FIG. 3 K1) and a saddle coil (K2 in FIG. 3) formed by the element conductors 4, 6 and the ring coil portion sandwiched between the connection points of the element conductors 4, 6 and the ring coils 1, 2. To make the mutual inductance (M2 in FIG. 3) substantially negligible,
The capacity has been adjusted. The principle of determining the capacity will be described in detail below.

First, as shown in FIG.
1, S2 are closed, switches S3 and S4 are opened, and an RF voltage source Vr is applied to the ring coil 1 (this corresponds to an applied voltage of an RF pulse for excitation or an induced voltage by an NMR signal).
It is assumed that the current I having the resonance angular frequency ω1 is caused to flow. In the figure, a loop ABDE (third capacitor Ca → second capacitor Ca)
Assuming that a current flowing through the path from the capacitor Ca to the fourth capacitor Cb) is Im, a voltage VBA appearing between AB in FIG.

On the other hand, the mutual inductance between the ring coil 1 and the ring coil 2 is represented by M1, and the positive direction of the current I in the ring coil 1 and the positive direction of the current I2 in the ring coil 2 as shown in FIG. Decide in the same direction, M1
If it is defined as> 0, the voltage VBA induced between AB in FIG.

The voltage VBA expressed by the above equation (1) and the voltage VBA
When the voltages VBA expressed by the equation (2) become equal, an induced current is prevented from flowing through the ring coil 2 due to the influence of mutual interference.

On the other hand, according to Kirchhoff's law in the loop ABDE,

From the above equations (3) and (4), the conditions for canceling the mutual interference between the ring coil 1 and the ring coil 2 are as follows:

Next, as shown in FIG.
1 and S2 are opened, the switches S3 and S4 are closed, and a current I having a resonance angular frequency ω2 flows through the element conductor 6 by the RF voltage source Vr ′. In this case, the mutual inductance between the saddle coil K1 and the saddle coil K2 is M
2, as shown in FIG. 3, the positive direction of the current I5 in the element conductor 5 and the positive direction of the current I in the element conductor 6 are determined in the same direction, and when M2> 0 is defined, the saddle coil K1 and Conditions for canceling mutual interference between the saddle coils K2 are as follows:

By substituting the above equation (5) into the above equation (6), Ca
To summarize,

From the above equation (5), the condition for Cb to have a solution of Cb> 0 is M1> 0.

[1] When ω1 ² · M1−ω2 ² · M2 = 0, Therefore, there is a physically meaningful solution that satisfies the above equations (5) and (6) simultaneously.

[2] When ω1 ² · M1−ω2 ² · M2 ≠ 0, After all, Therefore, f (Ca) = 0 is , And there is a physically meaningful solution.

From the above [1] and [2], the capacitances Ca of the first and second capacitors and the capacitances Cb of the third and fourth capacitors are obtained.
Has been proved to be able to be adjusted so as to satisfy the above equations (5) and (6). At this time, for the resonance angular frequency ω1, as shown in FIG. 2, the mutual inductance M1 between the ring coils 1 and 2 can be substantially neglected, and the ring coils 1 and 2 have R
It can be used as an F coil. For the resonance angular frequency ω2, as shown in FIG. 3, the mutual inductance M2 between the saddle coil K1 and the saddle coil K2.
Can be substantially ignored, and the saddle coils K1 and K2 can be used as RF coils having no mutual interference.

FIG. 4 shows an MR according to an embodiment of the present invention.
FIG. 4 is a configuration diagram of a transmission / reception circuit 101 including an I RF coil. In the figure, Bz indicates a vertical static magnetic field. In the transmitting / receiving circuit 101, the MRI RF coil 100
A ring coil 1, a ring coil 2, element conductors 3, 4, 5, 6 interposed between the ring coil 1 and the ring coil 2, and a connection point between the element conductor 3 and the ring coil 1. A first capacitor Ca interposed between a connection point between the element conductor 4 and the ring coil 1, a connection point between the element conductor 3 and the ring coil 2, and a connection point between the element conductor 4 and the ring coil 2; A second capacitor Ca interposed therebetween, a third capacitor Cb interposed between the element conductors 3, and a fourth capacitor Cb interposed between the element conductors 4. However, the first and second capacitors C
a and the third and fourth capacitors Cb are adjusted so as to satisfy the above equations (5) and (6). Also, a parallel connection between the element conductor 5 and the ring coil 1 and a power supply capacitor C5 interposed between the connection point between the element conductor 6 and the ring coil 1 and the resonance angular frequency ω2. A resonance circuit (a parallel circuit of the capacitor C1 and the inductor L1), a connection point between the element conductor 5 and the ring coil 2, and a power supply capacitor C6 provided between the connection point between the element conductor 6 and the ring coil 2; Parallel resonance circuit (capacitor C
2 and a parallel circuit of an inductor L2). That is, the capacitances C1 to C4 of the capacitors constituting the parallel resonance circuit and the inductances L1 to L4 of the inductors.
4 satisfies the following equation. Further, two DC blocking capacitors C are provided in the ring coil 1. In addition, the ring coil 2
, Two DC blocking capacitors C are interposed.

Next, the operation of the transmitting / receiving circuit 101 will be described. [1] To excite the first nuclide (for example, proton),
RF for excitation of resonance angular frequency (= Larmor angular frequency) ω1
When transmitting a pulse, the power of the RF pulse for excitation is amplified by the power amplifier 10a, and the power of the hybrid 11
a, the power is equally divided and the transmission / reception switch 1
Input to 2-1 and 12-2. The transmission / reception switch 12-1 transmits the excitation RF pulse to the balun 13-.
Then, the excitation RF pulse is applied to both ends of the power supply capacitor C5 via the balun 13-1. On the other hand, the transmission / reception switch 12
-2 is a switching state in which the excitation RF pulse is output to the balun 13-2 side, and the excitation RF pulse is applied to both ends of the power supply capacitor C6 via the balun 13-2. The length of the transmission line connecting the transmission / reception changeover switch 12-1 and the balun 13-1 and the transmission / reception changeover switch 12-2 and the balun 13-2
Is λ 伝 送, where λ1 is the wavelength corresponding to the resonance angular frequency ω1. At this time, as shown in FIG. 5, the capacitor C3 and the inductor L3 are in a parallel resonance state (= infinite impedance), the capacitor C4 and the inductor L4 are in a parallel resonance state, and the parallel resonance part is substantially Are separated. As a result, the ring coils 1 and 2 function as a set of transmission coils having no mutual interference, and generate a horizontal oscillating magnetic field bx (in the illustrated direction). Note that the parallel circuit of the capacitor C1 and the inductor L1 and the parallel circuit of the capacitor C2 and the inductor L2 show inductive or capacitive with respect to the resonance angular frequency ω1 depending on the magnitude of the resonance angular frequency ω2. Power supply capacitor C
By adjusting the capacitances of C5 and C6, the resonance frequency of the ring coils 1 and 2 can be tuned to the resonance angular frequency ω1. [2] When receiving the NMR signal of the resonance angular frequency ω1,
The transmission / reception switch 12-1 outputs the NMR signal extracted from both ends of the power supply capacitor C5 via the balun 13-1 to the preamplifier 14-1. The preamplifier 14-1 amplifies the NMR signal and sends it to a receiver. On the other hand, the transmission / reception switch 12-2
Outputs the NMR signal extracted from both ends of the power supply capacitor C6 via the balun 13-2 to the preamplifier 14-2 side. At this time, as in the case of transmission, the MRI RF coil 100 is in the state shown in FIG. 5, and the ring coils 1 and 2 function as a set of receiving coils without mutual interference, and the horizontal oscillating magnetic field is generated. bx is detected. By processing the NMR signal sent to each receiver,
The ring coils 1 and 2 are phased array coils (ph
ased array coil).

[3] When transmitting an excitation RF pulse having a resonance angular frequency ω2 to excite a second nuclide (eg, carbon), the power of the excitation RF pulse is changed to the power amplifier 1
The power is amplified by 0b, the power is equally divided by the hybrid 11b, and input to the transmission / reception changeover switches 12-3 and 12-4. The transmission / reception switch 12-3 is in a switching state in which the excitation RF pulse is output to the balun 13-3 side, and the excitation RF pulse is supplied to both ends of the power supply capacitor C7 via the balun 13-3. Apply. on the other hand,
The transmission / reception switch 12-4 is connected to the excitation R
A switching state for outputting the F pulse to the balun 13-4 side,
The excitation RF pulse is applied to both ends of the power supply capacitor C8 via the balun 13-4. The length of a transmission line connecting the transmission / reception changeover switch 12-3 and the balun 13-3 and the length of a transmission line connecting the transmission / reception changeover switch 12-4 and the balun 13-4. The length is λ2 / 4, where λ2 is the wavelength corresponding to the resonance angular frequency ω2. At this time, as shown in FIG. 6, the capacitor C1 and the inductor L1 are in a parallel resonance state, and the capacitor C2 and the inductor L2 are in a parallel resonance state, and the parallel resonance portion is substantially separated. A coil K1 and a saddle coil K2 are formed. Thereby, the saddle type coils K1 and K2 are
It functions as a set of transmission coils without mutual interference, and generates a horizontal oscillating magnetic field by (direction shown in the figure). Note that the parallel circuit of the capacitor C3 and the inductor L3 and the parallel circuit of the capacitor C4 and the inductor L4 show inductive or capacitive with respect to the resonance angular frequency ω2 depending on the magnitude of the resonance angular frequency ω1. By adjusting the capacitance of the power supply capacitors C7 and C8, the saddle type coil K
1 and K2 can be tuned to the resonance angular frequency ω2. [4] When receiving the NMR signal of the resonance angular frequency ω2,
The transmission / reception switch 12-3 outputs the NMR signal extracted from both ends of the power supply capacitor C5 via the balun 13-3 to the preamplifier 14-3. On the other hand, the transmission / reception switch 12-3 outputs the NMR signal extracted from both ends of the power supply capacitor C8 via the balun 13-4 to the preamplifier 14-4.
At this time, as in the transmission, the MRI RF coil 1 is used.
00 is in the state shown in FIG. 6, and the saddle-shaped coil K1 and the saddle-shaped coil K2 function as a pair of receiving coils without mutual interference, and detect a horizontal oscillating magnetic field by. In addition,
The saddle coils K1 and K2 may be used as phased array coils by processing the NMR signals sent to each receiver. In addition, from the viewpoint of increasing the excitation efficiency at the time of transmission, points p, p ′, q, q ′, r, r ′ in FIG.
It is preferable that the power of the RF pulse for excitation be supplied in parallel to the power supply capacitors C5 to C8 so that the potentials and phases of s and s' become equal to each other. Further, from the viewpoint of improving the detection efficiency at the time of reception, points p and p ′ in FIG.
The MRI RF coil 1 is set such that the potentials and phases of q, q ′, r, r ′, and s, s ′ are equal.
It is preferred to increase the symmetry of the 00 geometry.
According to the above-described MRI RF coil 100, the ring coils 1 and 2 can be used as RF coils having no mutual interference for the resonance angular frequency ω1, and the element conductors 3 to 6 can be used for the resonance angular frequency ω2. And ring coils 1 and 2
Can be used as RF coils having no mutual interference.

-Other Embodiments- In the above embodiment, the MRI RF coil 100 is used alone, but two MRI RF coils 100 in which the sizes (diameters) of the ring coil 1 and the ring coil 2 are slightly changed.
A and 100B may be nested and quadratured at the time of transmission or reception. That is, MR
Regarding the I RF coil 100A, the ring coils 1 and 2 function as transmission / reception coils for the resonance angular frequency ω1, the saddle type coils K1 and K2 function as transmission / reception coils for the resonance angular frequency ω2, and the MRI RF coil. For the coil 100B, the saddle coils K1 and K2 function as transmission / reception coils for the resonance angular frequency ω1, and the ring coils 1 and 2 function as transmission / reception coils for the resonance angular frequency ω2. In this case, upon reception, the MRI
NMR signals extracted from the RF coil 100A for MRI and 2 NMR signals extracted from the RF coil 100B for MRI.
By receiving and processing two NMR signals with separate receivers, it can also be used as a 4-channel phased array coil.

[0023]

According to the RF coil for MRI according to the present invention, one set of ring coils is made to function as an RF coil for a specific resonance frequency (or a resonance angular frequency), and for another resonance frequency. In addition, since a set of saddle coils composed of the element conductor and a part of the ring coil can function as an RF coil, a phased array coil having different sensitivity directions depending on the resonance frequency can be easily configured. In particular, it is useful for a vertical magnetic field type MRI apparatus.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a basic circuit of an MRI RF coil according to the present invention.

FIG. 2 shows a ring coil of the RF coil for MRI of FIG.
It is explanatory drawing which shows the state at the time of functioning as an F coil.

FIG. 3 is an explanatory view showing a state when a saddle-shaped coil is formed on the MRI RF coil of FIG. 1;

FIG. 4 is a configuration diagram of a transmission / reception circuit including an MRI RF coil according to an embodiment of the present invention.

FIG. 5 is an explanatory diagram showing a state in which a ring coil of the MRI RF coil according to the embodiment of the present invention functions as an RF coil.

FIG. 6 is an explanatory diagram showing a state when a saddle-shaped coil is formed in the MRI RF coil according to one embodiment of the present invention.

[Explanation of symbols]

 100 RF coil for MRI 1,2 Ring coil 3,4,5,6 Element conductor Ca 1st, 2nd capacitor Cb 3rd, 4th capacitor C1-C4 capacitor C5-C8 Power supply capacitor K1, K2 Saddle coil L1 ~ L4 Inductor

Claims (2)

[Claims] 1. An MRI RF coil having a plurality of element conductors interposed between two ring coils, wherein the RF coil is formed by the two ring coils for an RF signal of a first resonance frequency. An RF coil is formed in a saddle shape by the element conductor adjacent to the RF signal of the second resonance frequency and a ring coil portion sandwiched between connection points of the element conductor and the ring coil. RF coil for MRI. 2. A first ring coil, a second ring coil, and first to fourth element conductors interposed between the first ring coil and the second ring coil.
A first capacitor interposed between a connection point between the first element conductor and the first ring coil and a connection point between the second element conductor and the first ring coil; A connection point between an element conductor and the second ring coil, and a connection point between the second element conductor and the second ring coil;
A second capacitor interposed between the connection points of the ring coils, a third capacitor interposed in the first element conductor, and a fourth capacitor interposed in the second element conductor. A capacitor, a first power supply capacitor interposed between a connection point between the third element conductor and the first ring coil and a connection point between the fourth element conductor and the first ring coil; and A series circuit of a parallel resonance circuit that resonates in parallel with the second resonance frequency, a connection point between the third element conductor and the second ring coil, and the fourth element conductor and the second ring coil And a series circuit of a second power supply capacitor interposed between the connection points and a parallel resonance circuit which resonates in parallel with the second resonance frequency, and a third circuit interposed in the third element conductor. A power supply capacitor and a series circuit of a parallel resonance circuit that resonates in parallel with the first resonance frequency; and a fourth power supply capacitor provided in the fourth element conductor and a series circuit with respect to the first resonance frequency. And a series circuit of a parallel resonance circuit that resonates in parallel with each other, wherein the first to fourth capacitors are disposed between the first ring coil and the second ring coil with respect to the first resonance frequency. So that the mutual inductance of the first and second resonance frequencies is substantially negligible.
A third saddle-shaped coil formed by a third element conductor and a ring coil portion sandwiched between connection points of the element conductor and the first and second ring coils; and the second and fourth element conductors In addition, the capacitance is adjusted so that mutual inductance between the element conductors and the second saddle coil formed by the ring coil portion sandwiched between the connection points of the first and second ring coils can be substantially ignored. An RF coil for MRI characterized by being performed.