JP2005021325A - Qd coil for magnetic resonance imaging equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a QD (Quadrature Detection) coil for magnetic resonance imaging equipment in which unstabilization of the impedance characteristic of a coil and lowering of a sharpness Q value of resonance due to influence of coaxial cables connected to the feeding points of the QD coil, multi-element coils in particular is prevented. <P>SOLUTION: Between two feeding points of the QD coil, the multi-element coils in particular and baluns provided on the coaxial cables connected there, inductance which connects the ground lines of the two coaxial cables is arranged. Thus, a circuit formed by the inductance and the capacitor of a coil in a parallel relation becomes a parallel resonance circuit to a signal with the nuclear magnetic resonance frequency of the MRI equipment for preventing the signal from being leaked from one of the feeding points to the other. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a QD coil used in a magnetic resonance imaging apparatus that takes a tomographic image of a desired part of a human body that is a subject using a nuclear magnetic resonance phenomenon, and in particular, the performance of the coil due to the influence of a power supply line in the QD coil. The present invention relates to technology for preventing deterioration.
[0002]
[Prior art]
A magnetic resonance imaging apparatus includes a transmission coil for causing a magnetic resonance to occur by irradiating a high-frequency magnetic field to a nucleus constituting a biological tissue of a subject, and a reception coil for receiving a nuclear magnetic resonance signal generated by the nuclear magnetic resonance. In some cases, one coil can be used for both transmission and reception.
Various types of transmitting and receiving coils are known. Among them, a multi-element coil, which is a distributed constant type coil, generates a standing wave in the coil when power is supplied at the resonance frequency of the coil. For this reason, another feeding point is provided at a position corresponding to the standing wave node, and transmission or reception can be performed simultaneously without affecting each other, so that transmission / reception in the QD (Quadrature Detection) system is possible.
[0003]
However, since a coaxial cable having a capacity of about 100 pF / m is used between the signal line and the ground line for power supply to the multi-element coil, a standing wave current is also carried on the power supply line. Then, the resonance condition of the standing wave on the feeder line easily changes due to the stray capacitance between the feeder line and the surrounding environment (for example, other cables and metal structures). In some cases, the impedance characteristic becomes very unstable, and the sharpness Q value of resonance is lowered to deteriorate SN.
[0004]
This problem is not limited to a multi-element coil, but also applies to general QD coils. In other words, even if a general QD coil has capacitive coupling between the two coils constituting the QD, the impedance characteristic of the QD coil becomes very unstable for the same reason as in the case of the multi-element coil. There is a case. In other words, a standing wave current rides on the power supply line (coaxial cable having a capacity of about 100 pF / m) from the power supply point of the QD coil to the balun, so that the power supply is caused by the influence of stray capacitance between the power supply line and the surrounding environment. The resonance condition of the standing wave on the line easily changes.
[0005]
In order to prevent the above destabilization and the like, as described in [Patent Literature 1] and [Patent Literature 2], a balun is provided on the feeding line as close to the multi-element coil as possible, so that The influence is cut off.
[0006]
[Patent Document 1]
JP-A-8-280652 [Patent Document 2]
Japanese Patent Laid-Open No. 10-127600
[Problems to be solved by the invention]
Even if a balun is used as described above, a clear effect cannot be obtained unless the balun is sufficiently placed close to the multi-element coil. This is because a standing wave current is carried on the coaxial cable between the multi-element coil and the balun, and the longer this is, the more susceptible to stray capacitance.
[0008]
However, the balun cannot always be arranged near the multi-element coil because of the demands on the appearance and operability of the MRI apparatus. This point is not considered in [Patent Document 1] and [Patent Document 2].
[0009]
Accordingly, an object of the present invention is to prevent instability of impedance characteristics of the QD coil and decrease in sharpness Q of resonance due to the influence of a feeding line connected to two feeding points of the QD coil in combination with the balun. It is to provide a QD coil, in particular a multi-element coil, for a magnetic resonance imaging apparatus.
[0010]
[Means for Solving the Problems]
In order to solve the above-described problems, the present invention is configured as follows.
In a QD coil for a magnetic resonance imaging apparatus in which a coupling capacitance is generated between two element coils by arranging two element coils provided with feeding points orthogonally,
A balun is provided on each of the power supply lines connected to each of the power supply points, and between the power supply point and the balun, between the ground lines of the two power supply lines is desired between the coupling capacitors having a parallel relationship. It is connected by the inductor adjusted so that it may resonate in parallel with the frequency of.
[0011]
This prevents a signal from leaking from one feeding point to the other, and even if the balun cannot be placed closest to the feeding point of the QD coil, the influence of the feeding lines connected to the two feeding points. It is possible to prevent the impedance characteristics of the QD coil from becoming unstable and the resonance sharpness Q from being lowered.
[0012]
Further, in a multi-element coil for a magnetic resonance imaging apparatus in which a plurality of concentric rings, a plurality of elements that radiate the rings, and a resonance capacitor disposed between connection points of the elements in each ring , Two feeding points orthogonal to each other, each provided with a balun on a feeding line connected to each feeding point, and between the feeding point and the balun, between the ground lines of the two feeding lines in parallel An inductor adjusted so as to resonate in parallel at a desired frequency is connected to the resonance capacitor concerned.
[0013]
This prevents the signal from leaking from one feeding point to the other, and even when the balun cannot be placed closest to the feeding point of the multi-element coil, the feeding lines connected to the two feeding points It is possible to prevent the impedance characteristics of the multi-element coil from being unstable and the resonance sharp Q value from being lowered due to the influence.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiment of the invention, and the repetitive description thereof is omitted.
[0015]
FIG. 1 is a block diagram showing the entire MRI apparatus according to the present invention. The magnetic resonance imaging apparatus according to the present invention includes a central processing unit (CPU) 1, a sequencer 2, a transmission system 3, a gradient magnetic field generation system 4, a reception system 5, a signal processing system 6, and a static magnetic field generation region 22. Configured. The central processing unit 1 controls each of the sequencer 2, the transmission system 3, the reception system 5, and the signal processing system 6 in accordance with a predetermined program.
[0016]
The sequencer 2 operates based on a control command from the central processing unit 1 and sends various commands necessary for collecting tomographic image data of the subject 11 to the transmission system 3, the gradient magnetic field generation system 4, and the reception system 5. .
[0017]
The transmission system 3 includes a high-frequency oscillator 7, a modulator 8, and transmission coils 10a and 10b, and a high-frequency pulse from the high-frequency oscillator 7 modulated by the modulator 8 according to a command from the sequencer 2 is amplified by high-frequency amplifiers 9a to 9d. Then, by supplying the transmission coils 10a and 10b to the transmitting coil 10a and 10b, the subject 11 is irradiated with a predetermined pulsed electromagnetic wave.
[0018]
The gradient magnetic field generating system 4 includes gradient magnetic field coils 13a and 13b having a configuration capable of independently applying gradient magnetic fields in Cartesian coordinate axis directions orthogonal to each other, that is, in the X axis direction, the Y axis direction, and the Z axis direction, and the gradient magnetic field coil 13a. , 13b and a gradient magnetic field power source 12 for supplying current to the magnetic field generator 13b and a sequencer 2 for controlling the gradient magnetic field power source 12.
[0019]
The receiving system 5 includes the receiving coil 14, a preamplifier 15, a quadrature detector 16 and an A / D converter 17, and when the receiving coil 14 detects a nuclear magnetic resonance signal from the subject 11, The signal is converted into a digital quantity by an A / D converter 17 via a preamplifier 15 and a quadrature detector 16 and sent to a central processing unit (CPU) 1.
[0020]
The signal processing system 6 includes an external storage device such as a magnetic disk 18 and an optical disk 19, a CRT 20, a keyboard 21, and the like.
[0021]
When data from the receiving system 5 is input to the central processing unit (CPU) 1, the central processing unit CPU1 executes signal processing, image reconstruction processing, and the like, and a desired cross-sectional image of the subject 11 as a result. Is displayed on the CRT 20 and stored in, for example, the magnetic disk 18 of the external storage device.
[0022]
The static magnetic field generation region 22 is for generating a uniform static magnetic field around the subject 11 in a predetermined direction.
Inside the static magnetic field generation region 22, gradient magnetic field coils 13a and 13b for generating a gradient magnetic field, transmission coils 10a and 10b, and a reception coil 14 are installed.
[0023]
Hereinafter, the case of a multi-element coil will be described.
FIG. 2 shows a schematic configuration of a power supply line for the transmission coil 10a. The transmission coil 10a in FIG. 2 is a multi-element type having two feeding points. Resonant capacitive elements 25a to 25l in series with each other on the first ring, resonant capacitive elements 26a to 26l in series with each other on the second ring, and resonant capacitive elements 27a to 27l in series with each other on the third ring. A transmission coil 10a having the elements 25a and 25j as feed points is configured. Further, baluns 28a and 28b are arranged in series with the cables connected to the feeding points 25a and 25j.
[0024]
Now, it is assumed that the two feeding points 25a and 25j on the transmission coil 10a in FIG. 2 are completely isolated, and there is no signal leakage. FIG. 6 is a diagram showing the positional relationship between the antinodes and nodes of the standing wave created by the power feeding to the feeding point 25a and the feeding point 25j on the first ring of the transmission coil 10a of FIG.
[0025]
When the multi-element coil is resonating, a standing wave is standing on the coil, and antinodes and nodes of the standing wave are alternately arranged every ¼ wavelength. The feeding point 25j is located at a node of a standing wave created by feeding to the feeding point 25a. Similarly, the feeding point 25a is located at the node of the standing wave created by the feeding to the feeding point 25j.
[0026]
However, the state of FIG. 6 is an ideal state in which the two feeding points 25a and 25j are completely isolated, and in fact, the feeding points 25a and 25j are deviated from the positions of the standing wave nodes created by each other. There is. This is because the feeding line for the feeding points 25a and 25j is a coaxial cable, and the coaxial cable itself has a capacity of about 100 pF / m between the signal line and the ground line, and therefore, from the feeding points 25a and 25j to the baluns 28a and 28b. The coaxial cable also becomes a part of the resonance capacity of the coil, and the standing wave current rides on it. In such a situation, the resonance condition of the standing wave on the cable easily changes due to the effect of stray capacitance existing between the cable and the surrounding environment (for example, another cable or metal structure). End up.
[0027]
FIG. 7 shows the antinodes and nodes of the standing wave created by the power supply to the power supply point 25a on the first ring of the transmission coil 10a of FIG. 2 under the influence of such an unstable stray capacitance. It is the figure which showed the positional relationship of the feeding point 25j. In FIG. 7, the node of the standing wave created by the power supply to the power supply point 25a and the power supply point 25j are shifted from the same position. Therefore, the feeding point 25j receives the voltage of the standing wave current generated by the feeding point 25a. Similarly, it is conceivable that the feeding point 25a receives a voltage of a standing wave current generated by feeding to the feeding point 25j.
[0028]
In such a situation, signal leakage occurs between the power supply lines connected to the power supply points 25a and 25j in FIG. 2, and the impedance characteristics of the coil as viewed from the RF power amplifiers 9a and 9b become unstable. To do. FIG. 3 is a diagram illustrating a state in which signal leakage I1 occurs between the ground lines due to the stray capacitance existing in the power supply line and the potential difference V1 generated between the two ground lines.
[0029]
In order to stabilize the impedance characteristics of the coil viewed from the RF Power Amps 9a and 9b in FIG. 3, the signal leakage I1 between the ground lines must be cut off. FIG. 4 is a schematic configuration diagram of a power supply line for the transmission coil 10a according to the first embodiment of the present invention. The difference from the configuration of FIG. 2 is that between the power supply points 25a and 25j and the baluns 28a and 28b. This is that an inductance 29 for connecting the ground lines of the two power supply lines (coaxial cables) is arranged.
[0030]
FIG. 5 is an equivalent circuit diagram between the ground lines in FIG. The circuit in FIG. 5 is an LC parallel resonance circuit, and by determining the inductance so that the resonance frequency matches the nuclear magnetic resonance frequency of the MRI apparatus, the impedance between the ground lines at that frequency can be made very large. .
[0031]
That is, in FIG. 4, an inductance 29 having an optimum value for connecting the ground lines of the power supply line is arranged between the power supply points 25 a and 25 j and the baluns 28 a and 28 b, thereby connecting to the power supply points 25 a and 25 j. It is possible to prevent signal leakage between the power supply lines. The optimum inductance is an inductance that causes the LC parallel resonance circuit of FIG. 5 to resonate at the nuclear magnetic resonance frequency of the MRI apparatus.
[0032]
As a result, the resonance condition of the standing wave riding on the coaxial cable changes due to the influence of stray capacitance existing between the feeding line of the transmission coil and the surrounding environment (for example, another cable or metal structure). However, it is possible to prevent the impedance characteristics of the coil from being unstable and the sharpness Q of the resonance from being lowered as viewed from the RF Power Amp.
[0033]
The above description is a case where a multi-element coil is used as a transmission coil, but the same applies to the case where it is used as a reception coil. When used as a receiving coil, the RF Power Amp is replaced with a preamplifier for signal amplification. By applying the same as the above to the receiving coil, it is possible to prevent the impedance characteristics of the coil from becoming unstable and the sharpness Q of the resonance from decreasing as seen from the preamplifier.
[0034]
Next, a second embodiment in which the present invention is applied to a general QD coil will be described. FIG. 8 is a simple example of a QD coil in which two loop coils 30a (solid line) and 30b (dotted line) are orthogonally stacked (in FIG. 8, element coils are shown in different sizes for the sake of clarity). But may be approximately the same if necessary). Each element coil uses the resonance capacitive elements 31a and 31b as feeding points. Ideally, these two element coils need to be electrically independent, but in reality, a slight coupling capacitance (capacitor) 31c remains.
[0035]
The ground lines of the two power supply lines (coaxial cables) are connected by an inductance 29, and the inductance 29 is adjusted so as to resonate in parallel with the coupling capacitor 31c at the nuclear magnetic resonance frequency of the MRI apparatus. This can prevent the impedance characteristics of the QD coil from becoming unstable and the sharpness Q of the resonance from decreasing. The case where such a QD coil is used as a receiving coil is the same as the case of the multi-element coil.
[0036]
【The invention's effect】
As described above, according to the present invention, in a QD coil, particularly a multi-element coil, two lines (coaxial cables) are provided between two feeding points and a balun provided on each feeding line connected thereto. By arranging the inductance connecting between the ground lines, the circuit formed by the coil capacitor in parallel with the inductance becomes a parallel resonant circuit for signals having the nuclear magnetic resonance frequency of the MRI apparatus. The signal can be prevented from leaking from one feeding point to the other.
[0037]
As a result, even when the balun cannot be arranged closest to the feeding point of the coil, instability of the impedance characteristic of the coil due to the influence of the feeding line connected to the two feeding points and the decrease in the sharpness Q value of the resonance are prevented. It is possible to prevent a high-frequency pulse irradiation or a decrease in reception efficiency.
[Brief description of the drawings]
FIG. 1 is a block diagram of an embodiment of an MRI apparatus according to the present invention.
FIG. 2 is a schematic configuration diagram of a transmission coil 10a and a power supply line.
FIG. 3 is a diagram showing a state in which signal leakage I1 occurs between ground lines due to stray capacitance existing in the power supply line of the transmission coil 10a and a potential difference V1 generated between the ground lines.
FIG. 4 is a diagram in which an inductance 29 for connecting ground lines of a power feeding line is disposed between power feeding points 25a and 25j of the transmission coil 10a and baluns 28a and 28b (first embodiment).
5 is an equivalent circuit diagram between the ground lines in FIG. 4;
FIG. 6 is a diagram showing a positional relationship between antinodes and nodes of a standing wave standing on a transmission coil 10a and a feeding point. However, when the isolation between the feeding points 25a and 25j is sufficient.
FIG. 7 is a diagram showing a positional relationship between antinodes, nodes, and feeding points of standing waves standing on the transmission coil 10a. However, when the isolation between the feeding points 25a and 25j is insufficient.
FIG. 8 is a diagram in which an inductance 29 for connecting ground lines of a power feeding line is arranged (second embodiment) between power feeding points 31a and 31b of the QD coil and a balun.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Central processing unit (CPU), 2 Sequencer, 3 Transmission system, 4 Gradient magnetic field system, 5 Reception system, 6 Signal processing system, 7 High frequency transmitter, 8 Modulator, 9a-d High frequency amplifier (RF Power Amp), 10 Transmitting coil, 11 Subject, 12 Gradient magnetic field power supply, 13a-b Gradient magnetic field coil, 14 Receiving coil, 15 Pre Amp, 16 Detection circuit, 17 A / D Converter, 18 Magnetic disk, 19 Optical disk, 20 CRT, 21 Keyboard , 22 Static magnetic field generation region by magnetic circuit, 25a-l capacitor, 26a-l capacitor, 27a-l capacitor, 28a-b balun, 29 inductance, 30a-b Element coil constituting QD coil, 31a-b Capacitor

Claims (2)

In a QD coil for a magnetic resonance imaging apparatus in which a coupling capacitance is generated between two element coils by arranging two element coils provided with feeding points orthogonally,
A balun is provided on each of the power supply lines connected to each of the power supply points, and between the power supply point and the balun, between the ground lines of the two power supply lines is desired between the coupling capacitors having a parallel relationship. A QD coil for a magnetic resonance imaging apparatus, wherein the QD coil is connected by an inductor adjusted so as to resonate in parallel at a frequency of. In a multi-element coil for a magnetic resonance imaging apparatus in which a plurality of concentric rings, a plurality of elements that radiate the rings, and a resonance capacitor disposed between connection points of the elements in the rings,
Two feed points that are orthogonal to each other, each provided with a balun on a feed line connected to each feed point, and a parallel relationship between the ground lines of the two feed lines between the feed point and the balun A multi-element coil for a magnetic resonance imaging apparatus, wherein the multi-element coil is connected to an inductor adjusted so as to resonate in parallel at a desired frequency.

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