JPH0866382A - Rf coil for mri - Google Patents

Abstract

PURPOSE: To provide an RF coil for MRI, capable of a sufficient blocking function. CONSTITUTION: This coil has a distributed constant type capacitor C used for the resonance of an RF coil for MRI, and an inductance L1 electrically laid away from each capacitor electrode, so as to constitute a blocking circuit together with the capacitor C.

Description

Detailed Description of the Invention

[0001]

The present invention relates to magnetic resonance imaging (MR).
I) An MRI RF coil used in an apparatus, more specifically, an MRI RF coil in consideration of blocking.

[0002]

2. Description of the Related Art When an atomic nucleus is placed in a static magnetic field, the atomic nucleus precesses at an angular velocity proportional to a constant that varies depending on the strength of the magnetic field and the type of atomic nucleus. Magnetic resonance occurs when a high-frequency rotating magnetic field of the above-mentioned frequency is applied to the axis perpendicular to this static magnetic field, and a group of specific nuclei having the above constant causes a transition between levels due to the high-frequency magnetic field satisfying the resonance condition, Transition to the higher energy level. After the resonance, the nuclei excited to the high level return to the low level and radiate energy. MRI is an apparatus for observing a nuclear magnetic resonance phenomenon caused by the specific atomic nuclei and photographing a tomographic image of a subject.

In this MRI apparatus, there is an RF coil having a shape called a birdcage type as an RF coil which accommodates a subject therein and applies a high frequency current to the subject in order to apply a high frequency rotating magnetic field to the subject.

An example of this birdcage type RF coil is shown in FIG. In FIG. 9, reference numeral 1 is a birdcage coil for accommodating and transmitting / receiving a subject therein, 2a and 2b form a conductive loop, and are substantially circular ring portions, and two ring portions 2a and 2b are The planes are arranged so that they are parallel to each other. 3 is a point in which the two ring portions 2a, 2b are spaced at equal intervals along the periphery of each ring portion 2a, 2b.
It is an element connected orthogonally to the plane of 2b. Four
Is a capacitor inserted in series between the ring portions 2a and 2b between the connection points of the elements 3 of the ring portions 2a and 2b.

By the way, using a birdcage coil, N
When imaging with an MR (nuclear magnetic resonance) signal, a subject is housed in a birdcage coil for the whole body called a body coil, and transmission / reception is performed to acquire an NMR signal. However, when it is necessary to perform partial tomography such as the knee, it is possible to better observe the necessary portion by receiving only the NMR signal from the knee portion.

In this case, if only the small coil for knee is used for transmission / reception or reception, an image having a better SN ratio than that of the whole body RF coil can be obtained. For example, consider the case of an RF coil that performs only reception. If a small receiving RF coil is present in the body coil when transmitting with the body coil, the small RF coil resonates with the transmission signal, which disturbs the RF magnetic field generated by the body coil.

In order to enable transmission with a small birdcage coil, a transmission cable is prepared outside the cable that supplies the body coil, and it is transmitted to the body coil and the small birdcage coil. It needed a means to switch from the body coil to do it separately.

Therefore, when one birdcage coil is transmitting, it is necessary to prevent the other birdcage coil from being in a resonance state. In such a case, a small parallel resonance circuit called a blocking circuit is inserted to increase the impedance of this parallel resonance circuit so that the birdcage coil does not enter into a resonance state.

FIG. 10 is a circuit diagram showing an example in which the blocking circuit A is added to the birdcage coil.
The birdcage coil circuit is shown expanded.
Here, the inductance L1 is arranged in parallel with the capacitor C of the ring portion 2a together with the switch SW. The switch SW is composed of a diode or the like, and when the switch SW is closed by a control signal from the outside (not shown), the L1 and C are in a resonance state, and the impedance Z increases due to the parallel resonance circuit. As a result, the circuit including the ring portion 2a, the element 3, and the ring portion 2b is blocked.

[0010]

Therefore, when the blocking circuit A is in the resonance state, it is desirable that the sharpness (or selectivity) Q is high and the impedance Z is also high.

By the way, a distributed constant type capacitor using a double-sided substrate as shown in FIG. 11 may be used as the birdcage coil capacitor. Such a distributed constant type capacitor can be represented by an equivalent circuit as shown in FIG. This is because inductance (for example, La to Ld ') is generated on the surface of the substrate.

FIG. 13 is an actual wiring diagram showing a configuration example of the blocking circuit A described above. A blocking circuit A is configured by connecting an inductance to the portions on both sides (electrodes) of a distributed constant type capacitor that are close to each other. In the case of such a blocking circuit A, considering the inductance component contained in the above-mentioned capacitor, it can be shown by the equivalent circuit of FIG. That is, according to the equivalent circuit of the blocking circuit A, there are a plurality of current paths like the currents i1 to i5. Since the values of the inductances (La to Ld ') combined in each current path are different, the resonance frequency is also slightly different.

Therefore, if the Q of each current path is a solid line as shown in FIG. 15, the Q of the entire blocking circuit A is as shown by a broken line in FIG. Therefore, the characteristics become wide and a high Q cannot be obtained, and a satisfactory blocking performance cannot be obtained.

The present invention has been made in view of the above points, and an object thereof is to realize an MRI RF coil capable of obtaining sufficient blocking performance.

[0015]

A first means for solving the above-mentioned problems is to provide a distributed constant type capacitor used for resonance of an MRI RF coil, and to dispose each electrode of the capacitor at positions separated from each other. An RF coil for MRI, comprising: an electrically arranged inductor that constitutes a blocking circuit together with the capacitor.

The second means for solving the above-mentioned problems is MR.
A distributed constant type capacitor used for resonance of the RF coil for I, and an inductance electrically arranged at the farthest position of each electrode of the capacitor and forming a blocking circuit together with the capacitor. Is an RF coil for MRI.

A third means for solving the above problems is MR.
A capacitor used for resonance of the RF coil for I and a blocking circuit together with this capacitor,
An RF coil for MRI, comprising: a distributed constant type inductor.

The fourth means for solving the above-mentioned problems is the first
A main loop that forms a first tuning circuit with the inductance of the first capacitor and the first capacitor, and is connected in series with the first capacitor and the first capacitor, and is activated by an ON signal from the outside. An RF coil for MRI, comprising: a second capacitor; and a blocking circuit that constitutes a second tuning circuit by a second inductance.

[0019]

The RF for MRI which is the first means for solving the problems
In the coil, a distributed constant type capacitor used for resonance of the MRI RF coil, and an inductance are electrically arranged at mutually separated positions of respective electrodes of the distributed constant type capacitor, and blocking is performed together with the capacitor. Since the circuit is configured, the values of the capacitor and the inductance are the same regardless of the current path, and the Q at the resonance frequency of the blocking circuit is high.

In the MRI RF coil which is the second means for solving the problem, a distributed constant type capacitor used for resonance of the MRI RF coil, and the farthest distance between the respective electrodes of the distributed constant type capacitor Since the inductance is electrically arranged at the position and the blocking circuit is configured together with the capacitor, the values of the capacitor and the inductance are the same regardless of the current path, and the Q at the resonance frequency of the blocking circuit is high.

In the MRI RF coil which is the third means for solving the problem, the inductance constituting the blocking circuit is of the distributed constant type and the direct current resistance is low. Therefore, Q can be increased and blocking impedance can also be increased.

In the RF coil for MRI which is the fourth means for solving the problem, the first is applied when the blocking circuit is operating.
Since the second capacitor and the second capacitor are connected in series, the capacitance of the capacitor becomes small, and thus the value of the second inductance becomes large. Therefore, the Q of the blocking circuit also increases in proportion to the increase in the inductance value.

[0023]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is an actual wiring diagram showing the configuration of a blocking circuit used in an MRI RF coil according to an embodiment of the present invention. In the configuration shown in FIG. 1, a blocking circuit is configured by connecting the inductance L1 to the separated positions on both surfaces (electrodes) of the distributed constant type capacitor C. As described above, it is possible to use a diode that is turned on / off by a control signal from the outside as the switch SW. In the case of such a blocking circuit, when the inductance component contained in the capacitor C is taken into consideration, it can be shown as an equivalent circuit of FIG. 2, for example. As a result, the current paths (i1 to i5) flowing through this blocking circuit are as shown in FIG.

Here, the inductance component generated in the capacitor C is La to Ld on one electrode surface and La 'to Ld on the other electrode surface. And, since the inductance values at the opposite positions of both electrode surfaces are the same,
The resonance frequency of each current path is the same. Therefore, it becomes possible to resonate at a single frequency and a high blocking impedance can be obtained despite the existence of a plurality of current paths.

As described in detail above, the RF for MRI
An MRI RF coil including a distributed constant type capacitor used for resonance of a coil, and an inductance electrically arranged at positions separated from each other of the capacitor and forming a blocking circuit together with the capacitor. According to this, the values of the capacitor and the inductance are the same regardless of the current path, and each current path resonates at the same resonance frequency, so that the Q at the resonance frequency of the entire blocking circuit becomes high.

In addition to the position shown in FIG. 1 described above, the connection point of the inductance L1 can be set at both ends of the diagonal line of the capacitor C, and the same effect can be obtained in this case as well.

Therefore, a distributed constant type capacitor used for resonance of the MRI RF coil, and an inductance which is electrically arranged at the farthest position of each electrode of the capacitor and constitutes a blocking circuit together with the capacitor. According to the RF coil for MRI provided with, since the values of the capacitor and the inductance are the same regardless of the current path and each current path resonates at the same resonance frequency, Q at the resonance frequency of the blocking circuit is Get higher

Next, an embodiment in which the blocking impedance is increased by improving the inductance of the blocking circuit of the MRI RF coil will be described. The general inductance is the lumped constant type winding coil (hereinafter,
Simply called a coil). FIG. 4 is a circuit diagram showing a circuit configuration of a blocking circuit in which the inductance L1 and the diode D as a switch are attached to the capacitor C of the ring portion 2a. When such a coil is used, the coil has a direct current resistance, a connecting portion between the coil and the ring portion of the birdcage coil, and a connecting portion between the coil and the diode (soldering or the like).

Q when the blocking circuit resonates is
It can be expressed as Q = ωL / r. Here, ω is the angular frequency, L is the value of the inductance, and r is the DC resistance of the entire blocking circuit. That is, the value of the blocking impedance is determined by how small the DC resistance r is.
In particular, the DC resistance r of the blocking circuit occupies most of the DC resistance of the coil and its connecting portion.

Therefore, as shown in FIG. 5, the inductance provided at both ends of the capacitor in the ring portion is of a distributed constant type. For example, by using the same member as the substrate that constitutes the ring part, the ring part (both ends of the capacitor)
It is possible to reduce the resistance of the connection portion to and to almost zero. Further, the DC resistance of the inductance itself can be greatly reduced by using the distributed constant type pattern instead of the winding. As a result, it is possible to reduce the DC resistance r in the above equation, increase Q, and increase the blocking impedance.

When an MRI RF coil was constructed using a blocking circuit having such an inductance and an experiment was conducted, a Q that was 10 times or more that of the conventional one could be obtained.

As described in detail above, the RF for MRI
By replacing the winding coil with the inductance that forms the coil blocking circuit and using a distributed constant inductance, the DC resistance component can be kept low,
It becomes possible to improve the blocking performance.

FIG. 6 is a circuit diagram showing the configuration of another embodiment of the present invention for improving the blocking performance of the MRI RF coil. In this figure, a capacitor C1 is connected in series with a capacitor C0 of the MRI RF coil, and a switching diode D2 is connected in parallel with the capacitor C1. An inductance L1 'for the blocking circuit and a diode D1 for a switch connected in series with this inductance L1' are connected to both ends of the capacitors C0 and C1.

In such a structure, D1 is off and D2 is on when normal blocking is not performed. Therefore, the capacitor C1 does not work, and according to the normal operation, the inductance L0 and the capacitor C0
It resonates with. In this case, the values of C0 and L0 are set to the conventional values.

When blocking is performed, D1 is on and D2 is off. Therefore, the blocking circuit becomes active and the capacitor C1 becomes effective. Since this capacitor C1 is activated, C
Since 0 and C1 are connected in series, the combined capacitance C01 is (C0 × C1) / (C0 + C1). Therefore, the combined capacitance C01 has a value that is at least smaller than C0. Therefore, to resonate at the same frequency as before,
It is necessary to increase the value of the inductance L1 '. Here, since Q in the resonance state is ωL (or 1 / jωC), the capacitor C01 is made small and the inductance L
As 1'is increased, Q can be increased. As a result, the impedance Z of the blocking circuit can be increased, and effective blocking can be performed.

That is, the capacitor C1 is connected in series to the resonance capacitor C0 of the MRI RF coil,
Normally, by shorting C1 with a switch and opening the switch at the time of blocking to connect C0 and C1 in series, the combined capacitance C01 becomes small and the value of the blocking inductance L1 ′ becomes relatively large. You will be able to.

As described in detail above, the main loop forming the first tuning circuit by the first inductance and the first capacitor, the first capacitor, and the first capacitor are connected in series. According to the MRI RF coil including the second capacitor which is made effective by the ON signal from the outside and the blocking circuit which constitutes the second tuning circuit by the second inductance, when the blocking circuit is in operation. Since the first and second capacitors are connected in series, the capacitance of the capacitor is reduced, and therefore the value of the second inductance can be increased. Therefore, the Q of the blocking circuit also increases in proportion to the increase in the inductance value.

[0038]

As described above, the distributed constant type capacitor used for resonance of the RF coil for MRI and the respective electrodes of this capacitor are electrically arranged at mutually distant positions and together with the capacitor. R for MRI equipped with an inductor forming a blocking circuit
According to the invention of the F coil, the values of the capacitor and the inductance are the same regardless of the current path, and each current path resonates at the same resonance frequency, so that the Q of the entire blocking circuit becomes high. Therefore, it is possible to realize an MRI RF coil capable of obtaining sufficient blocking performance.

Further, a distributed constant type capacitor used for resonance of the MRI RF coil, and an inductance which is electrically arranged at the farthest position of each electrode of the capacitor and constitutes a blocking circuit together with the capacitor. According to the invention of the RF coil for MRI including the above, since the values of the capacitor and the inductance are the same regardless of the current path, and each current path resonates at the same resonance frequency, the Q of the blocking circuit is high. Become. Therefore, MRI that can obtain sufficient blocking performance.
RF coil can be realized.

Further, by making the inductance forming the blocking circuit a distributed constant type, the direct current resistance becomes low, and as a result, the Q of the blocking circuit becomes high, and sufficient blocking performance can be obtained.

Further, the main loop which constitutes the first tuning circuit by the first inductance and the first capacitor, the first capacitor, and the first capacitor are connected in series and are connected from the outside. According to the invention of the RF coil for MRI provided with the second capacitor which becomes effective by the ON signal, and the blocking circuit which constitutes the second tuning circuit by the second inductance, the first and Since the second capacitor is connected in series, the capacitance of the capacitor becomes small, and thus the value of the second inductance can be increased. Therefore, the Q of the blocking circuit also increases in proportion to the increase in the inductance value. Therefore, it is possible to realize an MRI RF coil capable of obtaining sufficient blocking performance.

[Brief description of drawings]

FIG. 1 is an actual wiring diagram in the vicinity of a blocking circuit of an MRI RF coil according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing an equivalent circuit of a blocking circuit of an MRI RF coil according to an embodiment of the present invention.

FIG. 3 is a circuit diagram showing a state of a current path in an equivalent circuit of a blocking circuit of an MRI RF coil according to an example of the present invention.

FIG. 4 is an actual wiring diagram in the vicinity of a blocking circuit of a general MRI RF coil.

FIG. 5 is an actual wiring diagram in the vicinity of a blocking circuit of an MRI RF coil according to another embodiment of the present invention.

FIG. 6 is a circuit diagram showing an equivalent circuit of a blocking circuit of an MRI RF coil according to another embodiment of the present invention.

FIG. 7 is a circuit diagram showing an equivalent circuit (normal time) of a blocking circuit of an MRI RF coil according to another embodiment of the present invention.

FIG. 8 is a circuit diagram showing an equivalent circuit (at the time of blocking) of a blocking circuit of an MRI RF coil according to another embodiment of the present invention.

FIG. 9 is a configuration diagram showing a configuration of a birdcage coil.

FIG. 10 is a circuit diagram showing an equivalent circuit of a blocking circuit of a conventional MRI RF coil.

FIG. 11 is a configuration diagram showing a configuration example of a capacitor.

FIG. 12 is a circuit diagram showing an equivalent circuit of a capacitor.

FIG. 13 is an actual wiring diagram around a blocking circuit of a conventional MRI RF coil.

FIG. 14 is a circuit diagram showing a state of a current path in an equivalent circuit of a blocking circuit of a conventional MRI RF coil.

FIG. 15 is a characteristic diagram showing the appearance of Q in a conventional blocking circuit.

[Explanation of symbols]

L1 inductance (inductance for blocking circuit) C capacitor (capacitor for birdcage coil) 1 birdcage coil 2a, 2b ring part 3 element 4 capacitor

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location G01N 24/04 520 A 9307-2G G01R 33/22

Claims (4)

[Claims] 1. A distributed constant type capacitor used for resonance of an MRI RF coil, and inductors which are electrically arranged at positions separated from each other of electrodes of the capacitor and which form a blocking circuit together with the capacitor. And an RF coil for MRI. 2. A distributed constant type capacitor used for resonance of an MRI RF coil, and an inductance which is electrically arranged at the farthest position of each electrode of the capacitor and constitutes a blocking circuit together with the capacitor. MR characterized by having
RF coil for I. 3. An M which is provided with a capacitor used for resonance of an MRI RF coil, and a distributed constant type inductance which constitutes a blocking circuit together with the capacitor.
RF coil for RI. 4. A main loop forming a first tuning circuit with a first inductance and a first capacitor; a first loop connected to the first capacitor and the first capacitor in series; An RF coil for MRI, comprising: a second capacitor that is made effective by an ON signal; and a blocking circuit that constitutes a second tuning circuit by a second inductance.