JPH10179539A - Receiving rf coil for magnetic resonance tomograph - Google Patents

Abstract

PROBLEM TO BE SOLVED: To protect the electromagnetic coupling restraining function of a decoupling circuit against deterioration due to the adjustment of the resonance frequency of a coil loop. SOLUTION: A variable capacitor 10 for resonance frequency adjustment, and a decoupling circuit 11 are arranged on a coil loop in such a state as connected in series, thereby providing a circuit configuration where the variable capacitor 10 becomes independent of the decoupling circuit 11. As a result, the resonance frequency of the decoupling circuit 11 does not change, regardless of the capacity change of the variable capacitor 10, and the electromagnetic coupling restraining function of the decoupling circuit 11 can be maintained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reception-only RF coil (RF probe) for detecting an NMR signal of a magnetic resonance tomography apparatus.
The present invention relates to a technique for suppressing (decoupling) electromagnetic coupling between a transmission RF coil to which an F signal is applied.

[0002]

2. Description of the Related Art Magnetic resonance tomography apparatuses (hereinafter referred to as "M
In an "RI device", an RF signal (excitation magnetic field pulse) is applied to an object carried into the gantry by an RF coil, and an NM generated at an imaging region of the object is called.
An R signal (magnetic resonance signal) is detected by the RF coil,
Image reconstruction is performed based on the detected NMR signal. Although the application of the RF signal and the detection of the NMR signal may be performed by the same RF coil, the RF signal and the NMR signal have significantly different signal levels. An RF coil dedicated to transmission and an RF coil dedicated to NMR signal detection (hereinafter abbreviated to RF coil dedicated to reception as appropriate) are used to separate the application of the RF signal and the detection of the NMR signal. This is often done by a coil.

The conventional receiving-only RF coil shown in FIG.
This is a birdcage type coil which is one of the cylindrical coils. This reception-only RF coil is constituted by four coil loops. That is, in the receiving-only RF coil,
Eight parallel line circuits 52 to 59 are provided to the two ring-shaped circuit paths 50 and 51 arranged in opposition, and the parallel line circuits 52 and 56, the parallel line circuits 53 and 57, and the parallel lines which are 180 ° apart from each other Electrical circuits 54, 58, and
As shown in the equivalent circuit diagram of FIG. 8, the four sets of parallel line electric circuits 55 and 59 are electrically connected to each other at ring electric circuits 50 and 51 to form a coil loop. It is composed.

A decoupling circuit 61 is provided in one of the parallel line circuits 52 to 55 of each coil loop, and the other parallel line circuits 56 to 5 are provided in each of the coil loops.
8 is provided with a fixed capacitor 62. further,
The decoupling circuit section 61 has a configuration in which an inductance 64 and two diodes 65 and 66 in an anti-parallel connection form are connected in series, and further connected in parallel to a variable capacitor 63.

[0005] Each coil loop of the RF coil for reception only is
As shown in FIG. 3, it has the characteristic of resonating at the frequency Fa, and can detect a weak NMR signal with high sensitivity. This reception-only RF coil should naturally detect (receive) an RF signal much larger than the NMR signal, but the RF signal emitted from the transmission-only RF coil is received by the reception-only RF coil and absorbed. If done,
The application of the RF signal is not uniform. If the application of the RF signal is not uniform, the tomographic image will be uneven. Therefore, the decoupling circuit unit 61 is provided to suppress (detune) the electromagnetic coupling between the coils to prevent reception and absorption of the RF signal by the reception-only RF coil.

That is, as shown in FIG. 3, the decoupling circuit section 61 is provided with a characteristic of resonating at a frequency Fb away from the resonance frequency Fa of the coil loop, and
At the signal frequency Fa, the electromagnetic coupling between the coils is forcibly suppressed by setting the decoupling circuit section 61 to a high impedance (open state). Note that, for a weak NMR signal, since the diodes 65 and 66 of the decoupling circuit unit 61 are non-conductive, this is equivalent to the case where the decoupling circuit unit 61 is not connected. Is maintained as it is, and the NMR signal is detected without any trouble.

[0007]

However, in the above-mentioned conventional receiving-only RF coil, it often happens that the decoupling circuit does not sufficiently suppress the electromagnetic coupling between the coils. In the reception-only RF coil, the coil loop does not always have the resonance characteristic of the frequency Fa due to manufacturing variations or the like. When the resonance frequency of the coil loop is deviated, the capacitance of the variable capacitor 63 of the decoupling circuit unit 61 is changed so that the coil loop resonates at the frequency Fa. However, this variable capacitor 6
As shown by the dashed line in FIG.
As a result, the resonance characteristics of the decoupling circuit unit 61 shift and overlap with the resonance characteristics of the coil loop. As a result, the electromagnetic coupling between the coils cannot be sufficiently suppressed.

SUMMARY OF THE INVENTION In view of the above-mentioned problems, the present invention provides a magnetic coupling device in which the electromagnetic coupling suppressing function of the decoupling circuit for suppressing electromagnetic coupling between coils is not impaired by adjusting the resonance frequency of the coil loop. It is an object to provide a receiving RF coil of a resonance tomography apparatus.

[0009]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention relates to a magnetic resonance tomography apparatus for performing tomography of a subject by utilizing a nuclear magnetic resonance phenomenon (NMR phenomenon). Emitted from the subject with the application of an RF signal (excitation magnetic field pulse) to the NM
A receiving RF coil used to detect an R signal (magnetic resonance signal), wherein a decoupling circuit unit including a capacitor and a coil and a variable capacitor for adjusting a resonance frequency of the RF coil itself are connected in series. It is arranged in the form of a coil loop.

[Operation] Next, the operation of the reception-only RF coil for the MRI apparatus of the present invention at the time of executing the imaging, and
An operation at the time of adjusting the resonance frequency of the coil loop of the reception-only RF coil will be described. When imaging is performed, an RF signal is applied to the subject by the transmission-only RF coil. However, since the electromagnetic coupling between the transmission-only RF coil and the reception-only RF coil is cut off by the decoupling circuit unit, the RF signal is applied. The signals are all applied to the subject without being absorbed by the reception-only RF coil, and the application of the RF signal is made uniform. On the other hand, the decoupling circuit unit does not function at all for a weak NMR signal emitted from the subject, and the NMR signal is detected without any trouble by the reception-only RF coil.

When adjusting the resonance frequency of the coil loop, the capacitance of the variable capacitor for adjusting the resonance frequency is changed. In the case of the reception-only RF coil of the present invention, the variable capacitor for adjusting the resonance frequency is connected in series with the decoupling circuit unit, and the variable capacitor is outside the decoupling circuit unit and is independent. ,
A change in the resonance frequency of the decoupling circuit due to a change in the capacitance of the variable capacitor does not cause a change in the resonance frequency. In the conventional case, a variable capacitor for adjusting the resonance frequency is incorporated in a part of the decoupling circuit section, so the resonance characteristic of the decoupling circuit section shifts due to a change in the capacitance of the variable capacitor, and the resonance frequency changes. The inconvenience of having occurred.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a receiving-only RF coil for an MRI apparatus according to the present invention will be described below with reference to the drawings. FIG. 1 is a schematic perspective view showing the entire configuration of a receiving-only RF coil according to an embodiment, and FIG. 2 is an equivalent circuit diagram of the RF coil of the embodiment.

As shown in FIG. 1, the receiving-only RF coil of the embodiment is a birdcage coil which is one of cylindrical coils. This reception-only RF coil is constituted by four coil loops. That is, in the receiving-only RF coil, eight parallel line circuits 2 to 9 are passed to the two ring-shaped circuit paths 1A and 1B arranged in opposition, and the parallel line circuits 2 and 6, which are 180 ° apart from each other, are parallel. As shown in FIG. 2, each of the four sets of parallel line circuits, that is, the line circuits 3 and 7, the parallel line circuits 4 and 8, and the parallel line circuits 5 and 9 are connected to each other at ring-shaped circuits 1 </ b> A and 1 </ b> B.
By being electrically connected, a total of four coil loops are formed. These four coil loops are arranged at an interval of 45 ° from each other.

A variable capacitor 10 for adjusting the resonance frequency and a decoupling circuit section 11 are respectively connected in series to one of the parallel line circuits 2 to 5 of each coil loop, and the other is connected to the other. Parallel line circuit 6
9 are provided with a fixed capacitor 12, and as shown in FIG. 3, each coil loop has a resonance characteristic of a resonance frequency Fa. The specific value of the resonance frequency Fa of the coil loop varies depending on the strength of the static magnetic field in the MRI apparatus. 42.6 MHz for 2.0 T (terrace), 63.8 M for 1.5 T (terrace)
Hz.

Each of the decoupling circuit units 11 includes a series connection of an inductance 13 and two diodes 14 and 15 connected in parallel in opposite directions to each other, and is further connected in parallel to a fixed capacitor 16. The decoupling circuit section 11 has a resonance characteristic of a resonance frequency Fb as shown in FIG.

Next, the operation of the receiving-only RF coil of the embodiment having the above-described configuration will be described. First, the operation of detecting the reception of an NMR signal by the reception-only RF coil during tomographic image capturing will be described. At the time of imaging, both the reception-only RF coil and the transmission-only RF coil are placed in a gantry (not shown) of the MRI apparatus, and an RF signal is applied to the subject by the transmission-only RF coil. The RF signal is a high power signal that allows the diodes 14 and 15 of the decoupling circuit unit 11 to conduct, but the RF signal is
Since the first resonance frequency Fb is different from the frequency of the RF signal, the decoupling circuit unit 11 becomes high impedance (open state) for the RF signal, and as a result, the RF signal is received and absorbed by the reception-only RF coil. Never.

On the other hand, since the weak NMR signal is a low power signal that does not allow the diodes 14 and 15 of the decoupling circuit section 11 to conduct, the decoupling circuit section 11 is opened at the diodes 14 and 15. . As a result, only the fixed capacitor 16 of the decoupling circuit unit 11 is connected to the coil loop, so that the decoupling circuit unit 11 does not hinder anything,
The NMR signal can be received and detected by the reception-only RF coil.

Next, the operation of adjusting the resonance frequency of the coil loop of the reception-only RF coil will be described. When adjusting the resonance frequency of the coil loop, the capacitance of the variable capacitor 10 is changed. The value of the variable capacitor 10 of the embodiment is C
a, the value of the fixed capacitor 16 is Cb, and the inductance is 1.
If the value of 3 is L, the variable capacitor 10 of the embodiment
Is adjusted by the combined value Ct of the variable capacitor 10 and the fixed capacitor 16, that is, [(Ca · Cb) / (Ca + C
b) = Ct] corresponds to the resonance frequency to be set.

The resonance frequency in the decoupling circuit section 11 of the RF coil of the embodiment is [1 / 2π√ (L
Cb)]. On the other hand, if the value of the variable capacitor 63 of the conventional example of FIG.
The resonance frequency of the conventional decoupling circuit section 61 is [1
/ 2π√ (LC)]. For these reasons, in the conventional reception-only RF coil, if the value of the variable capacitor 63 is adjusted, the resonance frequency of the decoupling circuit unit 11 changes, but in the case of the reception-only RF coil of the embodiment, the variable capacitor It can be seen that the resonance frequency of the decoupling circuit section 11 does not change even if the value of 10 is adjusted. That is, in the receiving-only RF coil of the embodiment, the variable capacitor 10 for adjusting the resonance frequency is moved outside the decoupling circuit unit 11, and the variable capacitor 10 has a circuit configuration independent of the decoupling circuit unit 11. The adjustment of the variable capacitor 10 no longer affects the resonance frequency of the decoupling circuit unit 11.

The present invention is not limited to the above embodiment, but can be modified as follows. (1) In the previous embodiment, the birdcage-type reception-only RF coil was used.
A modified example is a non-loop shaped receiving-only RF coil. In the case of a single-loop type reception-only RF coil, as shown in FIG. 4, the configuration is such that only one coil loop 21 is provided. In addition, an R-shaped R for exclusive use of 8-shaped loop
As shown in FIG. 5, the F coil has a configuration in which the coil loop 22 crosses in the middle to form an 8-shaped figure.

Further, as shown in FIG. 6, a modification in which a single-loop type RF coil for exclusive use of reception and an 8-noise loop type RF coil for exclusive use of reception are combined can be mentioned. In any of the modifications, the configurations and functions of the variable capacitor 10 and the decoupling circuit unit 11 are the same as those in the above embodiment.

(2) In the RF coil of the above embodiment, the fixed capacitor 16 of the decoupling circuit section 11 cannot change the capacitance, but a configuration in which the fixed capacitor 16 is also a variable capacitor is mentioned as a modification. . Also in this modification, the adjustment of the resonance frequency of the coil loop does not affect the resonance frequency of the decoupling circuit unit 11.

[0023]

According to the receiving RF coil of the magnetic resonance tomography apparatus of the present invention, the variable capacitor for adjusting the frequency is outside the decoupling circuit and has an independent circuit configuration. As a result, the resonance frequency of the decoupling circuit section does not change due to the change in capacitance, and the adjustment of the resonance frequency of the coil loop impairs the electromagnetic coupling suppression function of the decoupling circuit section that suppresses electromagnetic coupling between the coils. Never.

[Brief description of the drawings]

FIG. 1 is a schematic perspective view illustrating an entire configuration of a reception-only RF coil according to an embodiment.

FIG. 2 is an equivalent circuit diagram of a reception-only RF coil of the embodiment.

FIG. 3 is a graph showing resonance characteristics of a coil loop and a decoupling circuit unit.

FIG. 4 is a schematic plan view showing an entire configuration of a receiving-only RF coil of a modified example.

FIG. 5 is a schematic plan view showing the entire configuration of a receiving-only RF coil of another modification.

FIG. 6 is a schematic plan view showing the overall configuration of a receiving-only RF coil according to another modification.

FIG. 7 is a schematic perspective view showing the entire configuration of a conventional reception-only RF coil.

FIG. 8 is an equivalent circuit diagram of a conventional reception-only RF coil.

[Explanation of symbols]

 1A, 1B: ring-shaped electric circuit 2-9: parallel line electric circuit 10: variable capacitor for adjusting resonance frequency 11: decoupling circuit section 13: inductance 16: fixed capacitor

Claims (1)

[Claims] In accordance with application of an RF signal (excitation magnetic field pulse) to a subject in a gantry of a magnetic resonance tomography apparatus for performing tomography of the subject using a nuclear magnetic resonance phenomenon (NMR phenomenon). NMR emitted from the subject
A receiving RF coil used for detecting a signal (magnetic resonance signal), wherein a decoupling circuit unit including a capacitor and a coil and a variable capacitor for adjusting a resonance frequency of the RF coil itself are connected in series. A receiving RF coil for a magnetic resonance tomography apparatus, wherein the RF coil is arranged in a coil loop in a form.