JPH042330A - Magnetic resonance imaging apparatus and magnetic resonator - Google Patents

Abstract

PURPOSE:To enable adjustment of an impedance of a high frequency coil by a remote control by connecting the impedance of the high frequency coil to a tuning circuit through an nlambda/2 cable (n: integer). CONSTITUTION:An apparatus 20 has an electrostatic magnetic field generation coil to generate an electrostatic magnetic field applied to an object P to be inspected, an inclined magnetic field generation coil 22 to generate an inclined magnetic field, a high frequency coil 23 to detect a magnetic resonance signal while radiating a high frequency magnetic field and the high frequency coil 23 is connected to a transmitting circuit system 25 and a receiving circuit system 26. Moreover, the apparatus 21 has a computer system 28 to process a magnetic resonance signal while controlling a sequencer 27 and the high frequency coil 23 comprises a coil section 11 and a tuning circuit section 12 composing a resonance circuit together with the coil section 11 and connection to a tuning circuit 14 to tune and match an impedance of the high frequency coil 23 through a lambda/2 cable 13.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to nuclear magnetic resonance (NMR)
The present invention relates to a magnetic resonance imaging apparatus and a magnetic resonance apparatus that apply the (Magnetic Resonance) phenomenon, and particularly relates to a circuit that adjusts the tuning frequency of a high-frequency coil.

(Prior Art) Nuclear magnetic resonance is a phenomenon in which atomic nuclei placed in a magnetic field resonate and absorb electromagnetic wave energy of a specific wavelength, and then emit this energy as electromagnetic waves. Devices that utilize this phenomenon to diagnose living organisms use the above-mentioned atomic nuclei, especially
Electromagnetic waves (echo signals) emitted from protons are detected, and the detected signals are processed to determine the atomic nucleus (proton) density, longitudinal relaxation time T□, transverse relaxation time T2. Diagnostic information such as a tomographic image of the subject reflecting information such as flow and chemical shift can be obtained.

By the way, a magnetic resonance imaging apparatus that utilizes such a nuclear magnetic resonance phenomenon consists of a static magnetic field generating coil that generates a static magnetic field that is applied to a subject, and a high frequency pulse that is applied to a high frequency pulse to induce an echo signal. It includes a high-frequency coil that radiates a magnetic field and detects the induced echo signal, and a gradient magnetic field generating coil that generates a gradient magnetic field for obtaining positional information of the region where the echo signal is induced. The radio frequency coil of a magnetic resonance imaging device is adjusted to tune its tuning frequency to the frequency of use.

This frequency adjustment is performed by adjusting the impedance of the high frequency coil as described below.

That is, the high frequency coil (30) has a coil section (31) and a tuning circuit section (3), as shown in the equivalent circuit diagram of FIG.
2). This tuning circuit section (32) is composed of variable capacitance capacitors Cv1, CVz, and CVI. Therefore, the tuning frequency of the high frequency coil (30) is adjusted by adjusting the capacitances of these capacitors C, CVz, and C.

By the way, this high-frequency coil is divided into a reception-only high-frequency coil that only detects echo signals, and a transmitting/receiving high-frequency coil that detects echo signals and emits high-frequency pulses. Since this reception-only high-frequency coil only receives echo signals, the variable capacitance capacitor used does not need to have large power withstand characteristics for high frequencies, and varicaps are generally used. On the other hand, a high-frequency transmitting/receiving coil also has the function of emitting high-frequency pulses, so it is required to have high power resistance characteristics (several hundred W to several kW) against high frequencies, and a vacuum capacitor is used.

When adjusting the tuning frequency of such a reception-only high-frequency coil, adjust it by varying the voltage applied to the varicap mentioned above.Also, when adjusting the tuning frequency of a transmission-reception high-frequency coil, adjust the tuning frequency of the vacuum capacitor. By rotating the rotating shaft using an ultrasonic motor, the capacity can be varied,
I'm making adjustments.

In the case of a receive-only high-frequency coil, this tuning frequency can be adjusted by simply changing the voltage of the varicap, so it can be done remotely, and there are no special restrictions on the installation position of the high-frequency coil. However, in the case of a high-frequency coil for both transmitting and receiving purposes, the ultrasonic motor must be installed at the location of the high-frequency coil, which severely limits the installation position of the high-frequency coil.

(Problems to be Solved by the Invention) As described above, the high-frequency coils in conventional magnetic resonance imaging apparatuses, especially the transmitting/receiving high-frequency coils using vacuum capacitors having large power-withstanding characteristics, have extremely difficult tuning frequency adjustment. To use a sonic motor,
There is a problem in that the installation position is severely restricted.

An object of the present invention is to provide a magnetic resonance imaging apparatus and a magnetic resonance apparatus in which the tuning frequency of a high-frequency coil can be controlled by remote control.

[Structure of the invention]

(Means for Solving the Problems) The present invention provides a static magnetic field generating coil that generates a static magnetic field to be applied to a subject, a static magnetic field generating coil that emits a high frequency magnetic field for inducing a magnetic resonance signal by applying a high frequency pulse, and A magnetic resonance imaging apparatus comprising a high-frequency coil for detecting an induced magnetic resonance signal, and a gradient magnetic field generating coil for generating a gradient magnetic field for obtaining positional information of a site where the magnetic resonance signal is induced, wherein the high-frequency coil A magnetic resonance imaging apparatus and a magnetic resonance apparatus are characterized in that they include a tuning circuit that is connected to a radio frequency coil via an nλ/2 cable (n is an integer) and adjusts the tuning frequency of a high-frequency coil.

(Function) According to the present invention, the impedance of the high frequency coil is nλ
/2 cable (n is an integer) to connect the impedance of the high-frequency coil to the tuning circuit, and then the tuning circuit performs the final tuning and matching of the impedance of the high-frequency coil away from the high-frequency coil. This can be done remotely at any location. As a result, the impedance of the high-frequency coil can be adjusted by remote control, and the installation location of the high-frequency coil is not restricted.

The principle of the present invention will be explained using the Coe diagram. In Figure 1, the high frequency coil ■ is the nλ/2 cable ■
Connected to the tuning circuit ■ through the shell. This tuning circuit (2) consists of variable capacitors 0M and 0. Letting the impedance of this high frequency coil ■ be Z, and using the impedance z1 of the tuning circuit ■ side frequency coil ■ at the output end (2a) of the nλ/2 cable ■, the characteristic impedance of the cable ■ can be calculated using the following formula: γ=
α+jβ, the specific impedance is Zo (=50Ω), or the length of the cable.

In equation (g), assuming that the cable is lossless and distortionless, βx=n·2π, α=0, and the relationship of equation 0 holds true.

2-engine = Z-engine...■ As can be seen from equation 0, the impedance Z■ of the high-frequency coil O) has been transferred to the output end (2a) of the nλ/2 cable ■.

However, in reality, although the phase shift can be ignored, cable loss still exists. Therefore, in this case, tanh Y x = tanh a x in equation (3), and Z□ is expressed by equation (3).

Otsu=Z0Kari ZL+Zotanh(Ex)/(Zo+Z
ttanh αx )−(3) This equation (3) is transformed into nλ/
2 cables: high frequency coaxial cable RG-55A/
This will be explained using U as an example. The loss of this high frequency coaxial cable RG-55A/U is
-0,08 for 0MHz and 100MHz, respectively
dB/m, -0,152dB/m.

The frequency of the high frequency used is 35 MHz and 85 MHz.
In the case of Hz, the calculation is as follows.

First, if the frequency is 35MHz, the cable tan
hctx =0.74, and applying this to equation 0, the impedance is 2. is the formula (4).

Zi=50X(Zi+37.0)/(50+0.74Z
i) ・mAlso, when the frequency is 85MHz, it becomes similarly, and when this is applied to the 0 formula, the impedance system 2 becomes the 0 formula.

z,=5QX(Zi+36.4)/(50+0.728
Zi) ・-■ In this way, as shown in equations (4) and (c), the impedance zi of the high-frequency coil is nλ/2 cable■
is transferred to the output terminal (2a) of the output terminal (2a), and by adjusting the tuning circuit ■ and performing tuning and matching for the impedance Zi at this output terminal (2a), the high frequency coil can be tuned, and then the tuning circuit ■
We will explain the adjustment in .

The impedance of the output terminal (3a) of the tuning circuit ■ is zout, Zg= X, + j 3/L, ZH=
r;, +j
When r is set, Equation 0 holds true.

1 / Zout” ] / (Zi+ ZcM) +
1/ZcT=1/(rl+j (xf-XM))
+j (1/XT) ・−(6) In the 0 formula, Zou
Since t"" is 50Ω, equation 0 becomes equation 0.

+j/XT...■ By solving this 0 equation assuming that XM and XT are real numbers, the values of the variable capacitor C, 4, 0T, XM and XT, can be obtained from equation (8) and It can be calculated according to the formula 0.

Ru.

x, = x', -, ffi door surface sound' - (c)
=50・'＞/g7−...■) Here, LOP the values of variable capacitors C, CT of the tuning circuit, respectively.
Assuming that the voltage can be varied from F to 125 pF, from the above formula (8) and formula
) at impedance z′.

seek.

First, when the resonant frequency is 35 MHz, the equations (8) and 0) become the equations (]0) and (11).

37≦ml−Ωπ琶可−≦455 =, (10) 37≦
50rQ/! ≦4ss...(11) This (10)
Solving the equation and equation (11), rl of the following equation (12)
, Xl are obtained.

17.7≦ri≦49.4.42.4≦xi≦485・
(12) Next, when the resonant frequency is 85.15MHz, similarly, equations (c) and (9) become (13)
and (14).

15≦X knee, 1□jmu]-;y1〽■knee,;-2-
≦186 − (13) I5≦50 r (' /!
”-≦186-(14) This equation (13) and (j4)
By solving the equation, ri and X in equation (15) below are obtained.

4.1≦ri≦46.6. 16o6 ≤
The impedance Zi transferred by the cable (2) can be adjusted, and as a result, the high frequency coil (2) can be tuned and matched.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings. Third
The figure is a schematic diagram showing the configuration of a nuclear magnetic resonance imaging apparatus according to an embodiment of the present invention.

As shown in Fig. 3, this device [1 (21)
) that generates a static magnetic field in the Z-axis direction, which is the direction of the person's height, and a gradient magnetic field that generates a gradient magnetic field to obtain positional information of the site where the magnetic resonance signal is induced. It has a magnetic field generating coil (22) and a high frequency coil (23) to which a high frequency pulse is applied to emit a high frequency magnetic field for inducing a magnetic resonance signal and to detect the induced magnetic resonance signal.

This gradient magnetic field generating coil (22) has two axes in the height direction of the subject (■), and the axes perpendicular to this Z axis are the X and Y axes, and generates a gradient magnetic field about these axes. It is composed of an X-axis gradient magnetic field generation coil (22a), a Y-axis gradient magnetic field generation coil (22b), and a Z-axis gradient magnetic field generation coil (22c). Each gradient magnetic field generating coil (22a, 22b, 22C) is connected to an X-axis gradient magnetic field power supply (
24a), a Y-axis gradient magnetic field power supply (24b), and a Z-axis gradient magnetic field power supply (24c), respectively. Further, the high frequency coil (23) is connected to a transmitting circuit system (25) and a receiving circuit system (26). This high frequency coil (23).

It is connected to a tuning circuit, which will be described later, so as to be tuned to the high frequency being used.

Furthermore, this device [(2],) includes a sequencer (27) that implements a pulse sequence, and a computer system (28) that controls this sequencer (27) and performs signal processing of detected magnetic resonance signals. The signal processed by this computer system (28) is transferred to disk 1.
Noi (29) is displayed.

A tuning circuit for adjusting the tuning frequency of the high frequency coil (23) in this device (21) will be explained using FIG. 2. As shown in the equivalent circuit diagram of FIG. 2, this high frequency coil (23) consists of a coil section (11) and a tuned circuit section (12) that forms a resonant circuit together with this coil section (11). This product frequency coil (23) is connected to the high frequency coil (23) via the λ/2 cable (13).
3) is connected to a tuning circuit (14) that performs impedance tuning and matching.

This coil section (11) consists of a coil (L), and the tuned circuit section (12) includes fixed capacitors (Cν1, CV2, CV
3). This capacitor (CV, , CV2, C
V3) are 48PF, 50p F, 50p F, respectively.
A vacuum capacitor with a capacity of In this case, it is desirable to use capacitors (CV2, CV,) with the same capacity.

The tuning circuit (14) includes a variable capacitor (C
v□, CVz, CV3), and each capacitor (Cv
□, C,, C1/3) has a variable capacitance range of 5PF to 12
A 5PF vacuum capacitor was used. This capacitor (CV
2. CV3) are preferably linked so that they have the same capacity.

Furthermore, as a λ/2 cable (13), the length is 2.7
A high frequency coaxial cable RG-55A/U was used.

The length of this cable (13) is equivalent to one that acts as a λ/2 cable for a high frequency of 34.5 MHz.

This high frequency coil (23) was used in a 2.0 Stellar magnetic resonance imaging device (21). The frequency used was 34.5 MHz. In a state in which a human body equivalent phantom is placed in this device (21) and in a state in which a human body equivalent phantom is not placed (no load).

Variable capacitor (CV) of tuning circuit (14)
By automatically controlling the capacity of CO7, CV□) by an ultrasonic motor, the impedance of the input terminal (14a) of the tuning circuit (14) is set to 50Ω, and the high frequency coil (23) is tuned. I was able to do that.

In the above embodiment, a fixed capacitor whose capacitance cannot be changed is used as the capacitor in the coil portion of the high-frequency coil, but a variable capacitor can be used. In this case, there is an advantage that the value of the capacitor can be adjusted when adjusting the imaging device at the production factory. After completing this adjustment, you can fix the capacitance of the capacitor or leave it as is, but when you want to adjust the tuning with the frequency used when actually using this device, you can simply adjust the tuning circuit. There is no need to adjust the capacity.

Further, in the above embodiment, the number of capacitors in the coil portion and the tuning circuit is three, but as shown in FIG. 1, the number may be two.

Note that the present invention can also be applied to an analytical magnetic resonance apparatus as shown in FIG. Although this embodiment does not require a gradient magnetic field generating coil, other basic configurations are the same as the embodiment shown in FIG. Numbers 13', 14', 2 are the same as those in Figure 3 with a dash attached.
3', 25' to 29' indicate the same parts as in FIG. Note that 18 indicates a magnet.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to provide a magnetic resonance imaging apparatus in which the tuning frequency of a high-frequency coil can be controlled by remote control.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an equivalent circuit and a tuning circuit of a high-frequency coil according to the present invention, FIG. 2 is a circuit diagram showing an equivalent circuit and a tuning circuit of a high-frequency coil according to an embodiment of the present invention, and FIG. 3 is a circuit diagram showing an equivalent circuit and a tuning circuit of a high-frequency coil according to an embodiment of the present invention. FIG. 4 is a schematic diagram showing the configuration of a magnetic resonance imaging apparatus according to an embodiment of the present invention. FIG. 5 is a circuit diagram showing an equivalent circuit of a conventional high-frequency coil. be. DESCRIPTION OF SYMBOLS 1...High frequency coil, 2...Dλ/2 cable, 3, 14.14'・Tuning circuit, 11...Coil section, 12...Tuning circuit section, 13, 13'...
- λ/2 cable, 21... Magnetic resonance imaging device, 22... Gradient magnetic field generation coil, 23, 23'... High frequency coil, 24a... X-axis gradient magnetic field power supply, 24b... Y-axis gradient magnetic field power supply , 24c... Z-axis gradient magnetic field power supply, 25.25'... Transmission circuit system, 26, 26'... Receiving circuit system, 27.2
7'・Sequencer, 28, 28'...Computer system, 29, 29'...Display Agent Patent attorney Norio Ogo Figure 2c Nono

Claims (2)

[Claims] (1) A static magnetic field generating coil that generates a static magnetic field to be applied to the subject, and a high frequency coil that emits a high frequency magnetic field to induce a magnetic resonance signal by applying a high frequency pulse and detects the induced magnetic resonance signal. A magnetic resonance imaging apparatus comprising a coil and a gradient magnetic field generating coil that generates a gradient magnetic field for obtaining position information of a site where a magnetic resonance signal is induced, the high frequency coil and an nλ/2 cable (n is an integer). A magnetic resonance imaging apparatus, characterized in that the tuning circuit is connected through a tuning circuit and adjusts the tuning frequency of a high-frequency coil. (2) A static magnetic field generating coil that generates a static magnetic field to be applied to the subject, and a high frequency coil that emits a high frequency magnetic field to induce a magnetic resonance signal by applying a high frequency pulse and detects the induced magnetic resonance signal. A magnetic resonance apparatus comprising: a tuning circuit connected to the high-frequency coil via an nλ/2 cable (n is an integer) and adjusting the tuning frequency of the high-frequency coil.