JP2767311B2 - High frequency coil for magnetic resonance imaging equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a magnetic resonance which obtains a tomographic image of a desired part of a subject (human body) using a nuclear magnetic resonance (hereinafter abbreviated as “NMR”) phenomenon. A high-frequency coil used in a transmission system or a reception system of an imaging apparatus and having two conductive loops formed in a set with their sensitivity directions orthogonal to each other, and in particular, to reduce the coupling between the two conductive loops. The present invention relates to a high-frequency coil of a magnetic resonance imaging apparatus capable of performing the above.

[Conventional technology]

The magnetic resonance imaging apparatus includes a magnetic field generating unit that applies a static magnetic field and a gradient magnetic field in a direction perpendicular to the body axis direction of the subject, and a magnetic field generating unit that causes nuclear magnetic resonance to occur in nuclei of atoms constituting a living tissue of the subject. A transmission system that irradiates a high-frequency signal, a reception system that detects a high-frequency signal emitted by the above-described nuclear magnetic resonance, and a signal processing system that performs an image reconstruction operation using the high-frequency signal detected by the reception system are provided. It is configured. Then, while applying a uniform static magnetic field to the subject by the static magnetic field generating means, a high-frequency signal having a frequency for exciting nuclear magnetic resonance is applied by a high-frequency coil of the transmission system, whereby a nuclear magnetic resonance signal emitted from the subject is emitted. Is detected by the high frequency coil of the receiving system. At this time,
In order to specify the emission position of the nuclear magnetic resonance signal from the subject, imaging is performed by further applying a gradient magnetic field by a gradient magnetic field generating means.

Conventionally, as a high-frequency coil in such a magnetic resonance imaging apparatus, there has been one that uses one conductive loop, for example, a solenoid coil or a saddle coil, and receives a unidirectional nuclear magnetic resonance signal. On the contrary,
In order to improve the S / N ratio, there is a type in which two conductive loops are formed in a set with the sensitivity directions orthogonal to each other to receive nuclear magnetic resonance signals in two directions. The latter high-frequency coil consisting of two conductive loops is combined with a quadrature receiver coil (Quardr
ature Detection Coils (hereinafter abbreviated as "QD coil")
However, as a conventional QD coil, a combination of a saddle coil and a saddle coil has been proposed as, for example, a horizontal magnetic field type. However, when a combination of the saddle-shaped coil and the saddle-shaped coil is used, especially in a vertical magnetic field type magnetic resonance imaging apparatus, the direction of the static magnetic field and the receiving direction coincide with each other, so that it is impossible to receive with high sensitivity. Therefore, recently, for example, a combination of a solenoid coil and a saddle coil has been proposed as a QD coil of the vertical magnetic field type.

[Problems to be solved by the invention]

However, in such a conventional high-frequency coil, particularly, a QD coil of a vertical magnetic field type is a combination of different coils such as a solenoid coil and a saddle-shaped coil. Was. Here, the term "coupling" means that when a high-frequency current flows through one coil, the high-frequency current leaks into the other coil. When such coupling occurs, each coil acts as a load on the other side, acts as a loss on each coil, and the sensitivity of the high-frequency coil as a whole decreases. Therefore, the resulting image
The S / N ratio sometimes deteriorated.

Here, as a cause of the coupling in the high-frequency coil, the interval between the intersections of the two coils is close to several mm, so that a capacitive coupling that forms a stray capacitance therebetween and leaks to the other side, or a coil of one method is generated. An inductive coupling in which the generated magnetic flux causes imbalance with the magnetic flux of the other coil is considered. Coupling by inductive coupling can be reduced by adjusting the imbalance of magnetic flux by arranging a good conductor such as a copper plate near the coil, and there is not much problem. On the other hand, the coupling by the capacitive coupling can reduce the stray capacitance formed between the coils by increasing the distance between the coils at the intersection of the two coils. However, in this case, as the nuclear magnetic resonance frequency becomes higher, the interval between the coils must be increased, which results in an increase in the size of the high-frequency coil as a whole. In addition, at least one of the coils had a greater distance from the subject, so that the sensitivity was further reduced and the S / N ratio was deteriorated.

Therefore, an object of the present invention is to solve such a problem and to provide a high-frequency coil of a magnetic resonance imaging apparatus capable of reducing coupling between two conductive loops (coils).

[Means for solving the problem]

In order to achieve the above object, a high-frequency coil of a magnetic resonance imaging apparatus according to the present invention includes: a magnetic field generating means for applying a static magnetic field and a gradient magnetic field to a subject; A transmission system that irradiates a high-frequency signal to cause resonance, a reception system that detects a high-frequency signal emitted by the above-described nuclear magnetic resonance, and performs an image reconstruction operation using the high-frequency signal detected by the reception system A signal processing system is provided in the transmission system or the reception system of the magnetic resonance imaging apparatus, and two conductive loops are formed in one set with their sensitivity directions orthogonal to each other. In a high-frequency coil in which the sensitivity direction for irradiating or detecting a high-frequency signal emitted by nuclear magnetic resonance is orthogonal to the static magnetic field, The potential of the intersection of the flop is obtained by a ground potential near each conductive loop.

In addition, it is effective to use a solenoid coil and a saddle coil as the two conductive loops and set their respective feeding points to positions where the intersection between the solenoid coil and the saddle coil is near the ground potential. is there.

[Action]

The high-frequency coil thus configured operates so as to lower the operating voltage at the intersection by setting the potential at the intersection of the two conductive loops constituting the coil near the ground potential of each conductive loop. Therefore, even if the intersection of the two conductive loops has the same interval as in the related art, the capacitive coupling can be relaxed and the coupling between the two can be reduced.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a perspective explanatory view showing an embodiment of a high-frequency coil of a magnetic resonance imaging apparatus according to the present invention, and FIG. 2 is a circuit diagram showing the principle and connection of the high-frequency coil.
FIG. 3 is a block diagram showing the overall configuration of a magnetic resonance imaging apparatus to which the high-frequency coil is applied.

The magnetic resonance imaging apparatus is a nuclear magnetic resonance (NM)
R) Obtain a tomographic image of the subject using the phenomenon.
As shown in the figure, a static magnetic field generating magnet 2, a magnetic field gradient generating system 3, a transmitting system 4, a receiving system 5, a signal processing system 6, a sequencer 7, and a central processing unit (CPU) 8 are provided. Consisting of

The static magnetic field generating magnet 2 generates a uniform static magnetic field around the subject 1 in the body axis direction or in a direction perpendicular to the body axis. A permanent magnet type, normal conduction type or superconducting type magnetic field generating means is arranged. Magnetic field gradient generation system 3
Is a gradient magnetic field coil 9 wound in three directions of X, Y and Z,
And a gradient magnetic field power supply 10 for driving each coil. By driving the gradient magnetic field power supply 10 for each coil in accordance with an instruction from the sequencer 7, gradient magnetic fields Gx, Gy in the three axial directions of X, Y, Z are provided. , Gz are applied to the subject 1. Depending on how the gradient magnetic field is applied, the subject 1
Can be set for the slice plane.

The transmission system 4 irradiates a high-frequency signal (electromagnetic wave) to cause nuclear magnetic resonance to the nuclei of the atoms constituting the living tissue of the subject 1, and transmits the high-frequency oscillator 11, the modulator 12, the high-frequency amplifier 13 and Side high-frequency coil 14a,
A high-frequency pulse output from the high-frequency oscillator 11 is amplitude-modulated by a modulator 12 in accordance with a command from the sequencer 7, and the high-frequency pulse subjected to the amplitude modulation is amplified by a high-frequency amplifier 13, and thereafter, a high-frequency The electromagnetic wave is applied to the subject 1 by supplying the coil 1a to the coil 14a. The receiving system 5 includes a high-frequency signal (NM) emitted by nuclear magnetic resonance of nuclei of living tissue of the subject 1.
R signal) and includes a high-frequency coil 14b on the receiving side, an amplifier 15, a quadrature detector 16 and an A / D converter 17, and the electromagnetic wave emitted from the high-frequency coil 14a on the transmitting side The high-frequency signal (NMR signal) of the response of the subject 1 due to the above is detected by the high-frequency coil 14b arranged close to the subject 1, and is input to the A / D converter 17 via the amplifier 15 and the quadrature phase detector 16. Is converted into a digital quantity, and further, at a timing according to an instruction from the sequencer 7, the quadrature phase detector 16
It is two series of collected data sampled by
The signal is sent to the signal processing system 6.

The signal processing system 6 includes a CPU 8, a recording device such as a magnetic disk 18 and a magnetic tape 19, and a display 20 such as a CRT. The CPU 8 performs processes such as Fourier transform and correction coefficient calculation image reconstruction. A signal intensity distribution of an arbitrary section or a distribution obtained by performing an appropriate operation on a plurality of signals is imaged and displayed on the display 20 as a tomographic image. The sequencer 7 operates under the control of the CPU 8,
Various commands necessary for data collection of a tomographic image of the subject 1 are sent to the transmission system 4, the magnetic field gradient generation system 3 and the reception system 5 to serve as a means for generating the sequence for measuring the NMR signal. In FIG. 3, the high-frequency coil 14a on the transmitting side, the high-frequency coil 14b on the receiving side, and the gradient magnetic field coils 9, 9 are arranged in the magnetic field space of the static magnetic field generating magnet 2 arranged in the space around the subject 1. Are located.

Here, in the present invention, for example, the above-described high-frequency coil 14b on the receiving side is formed by forming two conductive loops into one set with their sensitivity directions orthogonal to each other and emitted from the subject 1 by nuclear magnetic resonance. The receiving direction for detecting the high-frequency signal is orthogonal to the static magnetic field generated by the static magnetic field generating magnet 2, and the potential at the intersection of the two conductive loops is near the ground potential of each conductive loop.

That is, for example, in the case of a QD coil of a vertical magnetic field type, as shown in FIG. 1, a solenoid coil 22 is wound around the outer peripheral surface of a cylindrical resin bobbin 21 as one conductive loop in the circumferential direction, and the other side. A saddle-shaped coil 23 is arranged so that its receiving direction is orthogonal to the receiving direction of the solenoid coil 22, and each coil 22, 23 is divided by a capacitor 24 to lower the operating voltage. The saddle-shaped coil 23 is a coil member in which one side end of the resin bobbin 21 is located aside in order to improve the receiving sensitivity of a portion corresponding to the top of the subject 1 inserted into the high-frequency coil 14b. The resin bobbin 21 is deformed by deforming 23a, 23b.
Projecting outward in the longitudinal direction. Further, the crossing portion 25 between the solenoid coil 22 and the saddle-shaped coil 23 is, for example, about
There is an interval of about 6mm. However, even with such an interval, this alone does not reduce the capacitive coupling between the two coils to a practical level. Therefore,
The feeding point of the solenoid coil 22 and the feeding point of the saddle-shaped coil 23 are set at a position where the intersection 25 between the solenoid coil 22 and the saddle-shaped coil 23 is near the ground potential, that is, in FIG. , The solenoid coil 22 is set at point A, and the saddle coil 23 is set at point I. In the illustrated embodiment, these points A and I are connected to the ground. This allows each coil
The operating voltages at 22, 23 are reduced, and the stray capacitance at the intersection 25 between the solenoid coil 22 and the saddle coil 23 is reduced.

FIG. 2 is a circuit diagram showing the principle and connection of the high-frequency coil 14b thus configured. In the figure, a coil tuning circuit and the like are omitted for simplification of the description. In the figure, the direction of the static magnetic field is indicated by an arrow S, and the magnetization vector rotating in one plane is the same signal with a phase difference of 90 degrees between the solenoid coil 22 and the saddle coil 23 constituting the high-frequency coil 14b. Is induced. Here, since the solenoid coil 22 and the saddle coil 23 are arranged so that their axial directions are orthogonal to each other, a high-frequency signal (NMR signal) is detected with random noise independent of each other. Possible sources of this noise are the resistance of each coil 22 and 23 and these coils 2
This is an equivalent resistance from the subject 1 due to magnetic coupling, electric coupling, and the like of 2,23.

The signals from the solenoid coil 22 and the saddle coil 23 are supplied to the first amplifier 15a or the second amplifier 1a in the amplifier 15.
After being amplified in 5b, they are input to the shifter 26.
This shifter 26 includes a phase shifter 27 and an attenuator 28
And an adder 29. Then, the phase of the signal from the solenoid coil 22 is shifted by 90 degrees by the phase shifter 27 so that the phase of the signal from the saddle-shaped coil 23 is matched. On the other hand, the sensitivity of the saddle coil 23 and the solenoid coil 22 are not equal. For example, when the sensitivity of the former is “1”, that of the latter is “1.4”. Therefore, in this case, the adder
Unless the addition ratio of the signal at 29 is changed, a high S / N ratio cannot be obtained. The optimum addition ratio at this time is I ² ÷ 1.
4 ² = 0.51. Therefore, the attenuator 28 is inserted in the middle of the signal path from the saddle-shaped coil 23 so that when the signal from the solenoid coil 22 is set to “1”, the signal from the saddle-shaped coil 23 becomes “0.51”. I am adjusting. After adjusting the signal intensities from the two coils 22 and 23 in this way, the adder 29 adds the two signals and outputs the result from the shifter 26. The output signal from the shifter 26 is
The signal is transmitted to the quadrature detector 16 shown in the figure.

As described above, when the phases of the signals from both the coils 22 and 23 are adjusted by the phase shifter 27 and added by the adder 29, the noise is slightly increased, but the detection signal is considerably increased.
As a result, the S / N ratio increases. For example, one coil 2
If the size and shape of 2 and the other coil 23 are equal and the equivalent resistance from the subject 1 is equal, the detection signal is doubled and noise is reduced. As a result, the S / N ratio is To improve.

FIG. 4 is a graph showing the operating voltage of the high-frequency coil 14b configured as described above. FIG. 4 (a) shows the operating voltage of the solenoid coil 22, and FIG. 4 (b) shows the saddle coil 23. Are shown. In FIG. 4A, the point A on the solenoid coil 22 is connected to the ground as shown in FIG. Between the points A and B on the solenoid coil 22, the voltage drops as shown in the figure due to the resistance of the coil member. Next, the portion from point B to point C is divided by the capacitor 24 as shown in FIG. 1, so that the voltage rises as shown and returns to the ground potential again. Next, between point C and point D, the voltage drops again due to the resistance of the coil member. Further, since the portion from point D to point E is divided by the capacitor 24, the voltage increases and returns to the ground potential again. Thereafter, at the point H at which a signal is taken out by repeating this, the voltage is in a reduced state. At this time, as shown in FIG.
Since the point is connected to the ground, the potential of the point A 'located at the intersection 25 with the saddle coil 23 near the point A is near the ground potential of the solenoid coil 22. Similarly, other intersections on the solenoid coil 22
The potentials at points C ', E', and G 'located at 25 are also near the ground potential, as shown in FIG.

In FIG. 4 (b), the point I on the saddle-shaped coil 23 is connected to the ground as shown in FIG.
Ground potential. Between the points I and J on the saddle coil 23, the voltage drops as shown in the figure due to the resistance of the coil member. Next, since the portion from point J to point K is divided by the capacitor 24 as shown in FIG. 1, the voltage rises as shown in the figure and returns to the ground potential again. Thereafter, the voltage rises and falls in the same manner as in the case of FIG. 4 (a), and at point P where the signal is taken out, the voltage has fallen. At this time, as shown in FIG. 1, the point I of the saddle-shaped coil 23 is connected to the ground, so that the point I 'located at the intersection 25 with the solenoid coil 22 near the point I is connected. The potential is near the ground potential of the saddle coil 23. Similarly, the potentials of the points K ′, M ′, and O ′ located at the other intersections 25 on the saddle coil 23 are
As shown in FIG. 4 (b), it is near the ground potential.

As described above, since the potential at the point located at the intersection 25 between the solenoid coil 22 and the saddle coil 23 is near the ground potential, the stray capacitance between the two coils 22, 23 is reduced, and the capacitive The result will be mitigated.

FIG. 5 is a partially omitted perspective view showing another embodiment of the high-frequency coil 14b according to the present invention. This embodiment is based on the first
In the figure, a saddle coil 23 to be combined with a solenoid coil 22 is a coil member 23a '
And 23b 'have a normal shape without being deformed as shown in FIG. In FIG. 5,
The components other than the saddle-shaped coil 23 are not shown.

In the above description, the combination of the solenoid coil 22 and the saddle coil 23 as the vertical magnetic field type QD coil has been described. However, the present invention is not limited to this. Regarding the combination of the coil 23 and other saddle-shaped coils 23 or the combination of various other types of coils, the potential of the intersection of the two coils is set to be near the ground potential, so that Capacitive coupling can be reduced. FIG. 6 shows the connection of the high-frequency coil 14b in the case where the saddle-shaped coils 23, 23 having the same shape are combined. At this time, the two coils constituting the high-frequency coil 14b are saddle-shaped coils 23, 23 having the same sensitivity.
Since these are combinations, the addition ratio of the signals in the adder 29 may be 1: 1. Therefore, it is not necessary to insert an attenuator 28 for changing the addition ratio of the signals from the two coils as shown in FIG. 2 inside the shifter 26 '.

In the above description, an example in which the present invention is applied to the high-frequency coil 14b on the receiving side in FIG. 3 has been described.
The present invention is not limited to this, and may be applied to the high frequency coil 14a on the transmission side. Various resonance circuits have been proposed for the high-frequency coil 14a on the transmission side and the high-frequency coil 14b on the reception side.
In any of the circuits, the potential at the intersection of the two coils can be set near the ground potential, and the coupling between the two coils can be reduced.

〔The invention's effect〕

Since the present invention is configured as described above, the high-frequency coil
By setting the potential of the intersection 25 of the two conductive loops (22, 23) constituting 14a or 14b near the ground potential of each conductive loop (22, 23), the operating voltage of the intersection 25 can be reduced. it can. From this, even if the intersection 25 of the two conductive loops (22, 23) has the same interval as before,
The coupling between them can be reduced by relaxing the capacitive coupling. Therefore, the high-frequency coils 14a, 14b
Can be improved as a whole, and a decrease in the S / N ratio of the obtained image can be prevented. Further, it is not necessary to increase the interval between the intersections 25 of the two conductive loops (22, 23), or it can be made smaller than before, so that the entire high-frequency coils 14a, 14b can be downsized.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an embodiment of a high-frequency coil of a magnetic resonance imaging apparatus according to the present invention, FIG. 2 is a circuit diagram showing the principle and connection of the high-frequency coil, and FIG. FIG. 4 is a block diagram showing the overall configuration of the magnetic resonance imaging apparatus, FIG. 4 is a graph showing the operating voltage of the high-frequency coil, FIG. 5 is a partially omitted perspective view showing another embodiment of the high-frequency coil according to the present invention, FIG. 6 is a circuit diagram showing connection of a high-frequency coil according to still another embodiment. DESCRIPTION OF SYMBOLS 1 ... Subject, 2 ... Static magnetic field generation magnet, 3 ... Magnetic field gradient generation system, 4 ... Transmission system, 5 ... Reception system, 6 ... Signal processing system, 7 ... Sequencer, 8 ... CPU , 14a: high-frequency coil on the transmitting side, 14b: high-frequency coil on the receiving side, 15: amplifier, 22: solenoid coil, 23: saddle coil, 24
…… condenser, 25 …… intersection, 26 …… shifter.

Claims (2)

(57) [Claims] 1. A magnetic field generating means for applying a static magnetic field and a gradient magnetic field to a subject, a transmitting system for irradiating a high frequency signal to cause nuclear magnetic resonance in nuclei of atoms constituting the living tissue of the subject, The transmission of the magnetic resonance imaging apparatus, comprising: a receiving system for detecting a high-frequency signal emitted by the above-described nuclear magnetic resonance; and a signal processing system for performing an image reconstruction operation using the high-frequency signal detected by the receiving system. Provided in a system or a receiving system, two conductive loops are formed as a set with their sensitivity directions orthogonal to each other, and irradiate the subject with a high-frequency signal or detect a high-frequency signal emitted by nuclear magnetic resonance In a high-frequency coil whose sensitivity direction is orthogonal to the static magnetic field, the potential at the intersection of the two conductive loops is set near the ground potential of each conductive loop. RF coil of a magnetic resonance imaging apparatus that. 2. A solenoid coil and a saddle-shaped coil are used as the two conductive loops, and respective feeding points are set at positions where the intersection of the solenoid coil and the saddle-shaped coil is near the ground potential. The high-frequency coil of the magnetic resonance imaging apparatus according to claim 1, wherein: