JP3170771B2 - Receiving coil for magnetic resonance imaging equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a receiving coil in a magnetic resonance imaging apparatus for imaging a desired portion of a subject using magnetic resonance.

[0002]

2. Description of the Related Art Magnetic resonance imaging apparatus (hereinafter referred to as MR)
The device I) measures the nuclear spin density distribution, relaxation time distribution, and the like at a desired inspection site in a subject by using a nuclear magnetic resonance phenomenon, and displays an image of a cross section of the subject from the measurement data. Things.

A nuclear spin of a subject placed in a uniform and strong static magnetic field generator precesses around a direction of the static magnetic field at a frequency (Larmor frequency) determined by the strength of the static magnetic field. Therefore, when a high-frequency pulse having a frequency equal to the Larmor frequency is irradiated from the outside, the spins are excited and transit to a high energy state (nuclear magnetic resonance (NMR) phenomenon). When the irradiation is stopped, the spin returns to the original low energy state with a time constant corresponding to each state, and at this time, an electromagnetic wave (magnetic resonance signal = NMR signal) is emitted to the outside. This is detected by a high-frequency receiving coil tuned to that frequency. At this time, for the purpose of adding positional information in the space, gradient magnetic fields in the three axial directions of X, Y, and Z are applied to the static magnetic field space. As a result, position information in space can be captured as frequency information.

FIG. 2 is a block diagram showing the overall configuration of such an MRI apparatus. As shown in FIG. 2, the MRI apparatus comprises a static magnetic field generating magnet 10 and a central processing unit (hereinafter referred to as a CP).
U) 11, a sequencer 12, a transmission system 13,
Gradient magnetic field generation system 14, reception system 15, signal processing system 16
Consists of

[0005] The static magnetic field generating magnet 10 generates a strong and uniform static magnetic field in the subject 6, and a permanent magnet system, a normal conducting system, or a superconducting system is provided in a certain space around the subject 6. Magnetic field generating means are arranged. The sequencer 12 operates under the control of the CPU 11 and sends various commands necessary for data collection of tomographic images of the subject 6 to the transmission system 13, the gradient magnetic field generation system 14, and the reception system 1.
5 to send.

The transmission system 13 includes a high-frequency generator 17, a modulator 18, a power amplifier 19, and an irradiation coil 20 on the transmission side.
A high-frequency pulse output from the high-frequency generator 17 is modulated by a modulator 18 in accordance with a command from the sequencer 12, and the modulated irradiation pulse is converted into a power amplifier 19.
The electromagnetic wave is applied to the irradiation coil 20 disposed in close proximity to the subject 6 after the amplification by the electromagnetic wave, so that the subject 6 is irradiated with the electromagnetic wave.

The gradient magnetic field generating system 14 comprises a gradient magnetic field coil 21 wound in three directions of X, Y and Z, and a gradient magnetic field power supply 22 for driving each coil. By driving the gradient magnetic field power supply 22 of each coil in accordance with
An axial gradient magnetic field GX, GY, GZ is applied to the subject 6. The slice plane for the subject 6 can be set by the method of applying the gradient magnetic field.

The receiving system 15 includes a receiving coil 2, a preamplifier 23, a quadrature detector 24, and an A / D converter 25. The response of the subject 6 by the electromagnetic wave radiated from the radiating coil 20 on the transmitting side. Of the electromagnetic wave (NMR signal) is detected by the receiving coil 2 arranged close to the subject 6,
A / D via preamplifier 23 and quadrature detector 24
The data is input to the converter 25, converted into a digital quantity, and further obtained as two sets of collected data sampled by the quadrature phase detector 24 at a timing according to a command from the sequencer 12, and the signal is sent to the signal processing system 16. It has become. The signal processing system 16 includes a CPU 11, a recording device such as a magnetic disk 26 and an optical disk 27,
And a display 28 such as an RT.
In step 1, processing such as Fourier transform, correction coefficient calculation, and image reconstruction is performed, and a signal intensity distribution of an arbitrary cross section or a distribution (signal distribution) obtained by performing an appropriate operation on a plurality of signals is imaged and displayed on the display 28. It is displayed.
In FIG. 2, the irradiation coil 20, the reception coil 2, and the gradient magnetic field coil 21 are arranged in the magnetic field space of the static magnetic field generating magnet 10 arranged in the space around the subject 6.

In such an MRI apparatus, in addition to imaging the anatomical structure of the subject, by using different pulse sequences for pulse irradiation, an image whose emphasized state has changed according to the properties of the tissue can be obtained. You can also get. This requires control of the timing of the applied pulse and the irradiation level, and the accuracy of these parameters is important.

FIG. 3 is a diagram schematically showing the state of irradiation of a high-frequency pulse and reception of an NMR signal in the MRI apparatus. As shown in FIG. An appropriate irradiation output is applied to the subject 6.

Next, at the time of reception, a weak NMR signal obtained from the subject 6 is detected by the receiving coil 2, and the preamplifier 2
Enter 3 Each of the coils 20, 2 comprises a conductor loop 4 and a tuning circuit, mainly a resonance capacitor 3, and both are tuned to the Larmor frequency. Irradiation and reception are performed at different times and are not performed simultaneously.

As described above, the irradiation and reception coils 20 and 2 are a resonance circuit tuned to the same frequency and are disposed relatively close to each other, so that their coupling becomes a problem. In particular, if a strong irradiation output from the irradiation coil 20 is directly mixed into the receiving system, the elements constituting the receiving coil 2 and the preamplifier 23 will be damaged.

Therefore, conventionally, a tuning circuit of the receiving coil 2 has
In some cases, a coupling blocking circuit including an LC parallel resonance circuit connected in series with the conductor loop 4 and tuned to the irradiation frequency of the irradiation coil 20 and a cross diode connected between its L and C is provided. JP-A-62-298344
Reference).

According to this, the cross diode is turned on by the voltage of the irradiation output, the LC parallel resonance circuit resonates, the connection between the conductor loop 4 and the resonance capacitor 3 is cut off, and the receiving coil 2 is separated from the irradiation frequency. The circuit components of the receiving system can be prevented from being damaged.

[0015]

The prior art coupling prevention works on the basis of the supply of irradiation power, and it is assumed that sufficient input can be obtained. That is, it is necessary for the receiving coil 2 to generate a voltage sufficient to make the cross diode conductive.

Generally, a diode that performs high-speed switching has different characteristics depending on the current capacity. A diode having a relatively large capacity has a large turn-on voltage (forward voltage drop VF) of about 1 V as shown in FIG. The ON resistance in the conductive state has a small characteristic. On the other hand, a diode having a small current capacity has a resistance of 0.1 mm as shown in FIG.
Although the transistor is turned on at a low VF of about 3 V, the ON resistance in the conductive state is large, and the slope of the VI characteristic becomes gentle.

Conventionally, the cross diode used for blocking the coupling is a large-capacity diode having a small on-resistance (a diode having the characteristics of FIG. 4A) in order to withstand a large current during irradiation and to increase the blocking efficiency. ) Was selected and used.

As described above, such a diode has a large VF and is satisfactory in protecting the receiving coil 2 against a high-level irradiation output. However, subject 6
Depending on the pulse sequence of the irradiation pulse to the laser, some use a relatively low level irradiation output. In this case,
The voltage generated in the receiving coil 2 becomes low, so that a sufficient coupling blocking effect cannot be expected, and the excitation power changes due to the coupling radiation from the receiving coil 2, so that there is a problem that the control cannot be performed with high accuracy.

In the MRI apparatus, the accuracy of the excitation power level is important. If the excitation power level is insufficient, problems such as a decrease in the S / N ratio, a deterioration in the slice characteristics, and a change in the image enhancement state occur. There was a request for improvement.

An object of the present invention is to improve the incompleteness of the operation at the time of low-level irradiation of the coupling prevention in the prior art, and to improve the operation even at the time of low-level irradiation, that is, from the time of low-level irradiation to the time of high-level irradiation. An object of the present invention is to provide a receiving coil for an MRI apparatus capable of obtaining an image.

[0021]

The above problem at the time of low level irradiation is caused by a large VF of the cross diode for preventing coupling. Therefore, in the present invention, this V
The characteristics were improved by connecting another set of cross diodes composed of a diode having a small VF to a cross diode having a large F to improve the characteristics.

That is, the above object is to detect a magnetic resonance signal emitted from an object by applying an irradiation pulse from an irradiation coil to the object. The object comprises a conductor loop and a tuning circuit. LC connected in series with a conductor loop and tuned to the irradiation frequency of the irradiation coil
In a receiving coil for a magnetic resonance imaging apparatus having a coupling blocking circuit including a parallel resonance circuit and a cross diode connected between L and C, two pairs of the cross diodes of the coupling blocking circuit are connected in parallel. The pair of cross diodes consist of a pair of diodes having a characteristic that the turn-on voltage is large and the on-resistance in the conductive state is small, and the cross diode in the other set is both a small turn-on voltage and the on-resistance in the conductive state. This is achieved by having a pair of diodes with large characteristics.

[0023]

The combined characteristics when two types of diodes shown in FIGS. 4 (a) and 4 (b) are connected in parallel are such that the VF is small, the current capacity is large and the on-state is high as shown by the solid line in FIG. 4 (c). The ideal characteristic is that the resistance is small.

Of the two sets of cross diodes connected in parallel in the coupling blocking circuit, one set has a characteristic that the turn-on voltage (VF) is large and the on-resistance in the conductive state is small, and the other set is By giving the characteristics that the turn-on voltage (VF) is small and the on-resistance in the conductive state is large, the characteristics as a whole are shown by the solid line in FIG. 4C. As a result, it is possible to protect the circuit components of the receiving system and to obtain a good image from low level irradiation to high level irradiation.

[0025]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of a receiving coil for an MRI apparatus according to the present invention. As shown in the figure, the receiving coil 2 of the present invention basically includes a conductor loop 4 forming a coil and a resonance capacitor 3 (3a, 3b, 3) forming a tuning circuit.
c). The inductance of the conductor loop 4 and the capacitance of the resonance capacitor 3 form a resonance circuit tuned to the NMR signal frequency. Here, the resonance capacitor 3 is composed of three resonance capacitors 3a, 3b, and 3c. One of the resonance capacitors 3b is connected to a variable capacitance diode 7, and the tuning frequency is adjusted by a DC voltage applied to the reception coil 2. Can be controlled. The variable capacitance diode 7 has a property that the capacitance between junctions greatly changes according to the reverse bias voltage. The higher the voltage, the lower the capacitance. Therefore, by adjusting this, it is possible to make the resonance frequency having the maximum sensitivity coincide with the signal frequency f0.

The other resonance capacitor 3a is connected in series to the conductor loop 4, and a coil 5 is connected in parallel to the resonance capacitor 3a so as to tune to the irradiation frequency of the MRI apparatus. A circuit is formed. In this case, on one side of the connection between the resonance capacitor 3a and the coil 5,
Two sets of cross diodes 1 (1a, 1
b) are inserted, and the coupling prevention circuit 1
00 is configured. The LC parallel resonance circuit formed by the inductance L of the coil 5 and the resonance capacitance 3a (C) is M
The LC is tuned to the irradiation frequency of the RI device, and when the cross diode 1 is turned on by the irradiation output mixed into the receiving coil 2, the LC enters a parallel resonance state and acts to cut off the receiving coil 2 with high impedance.

Thus, the circuit components of the receiving coil 2 and the preamplifier 2
For example, circuit components such as 3 can be prevented from being damaged. Although the received NMR signal is very weak, the coupling prevention circuit 100 is cut off by the cross diode 1 at the time of receiving the NMR signal, and does not adversely affect the reception of the NMR signal.

Here, in the present invention, one of the two sets of cross diodes 1a connected in parallel has a large current capacity, that is, a large turn-on voltage and a small on-resistance in a conductive state. The cross diode 1b of the other set is composed of a pair of diodes having a small current capacity, that is, a small turn-on voltage and a large on-resistance in a conductive state.

Therefore, the characteristic of the cross diode 1 is an ideal characteristic that the forward voltage drop VF is small, the current capacity is large, and the on-resistance is small (FIG. 4C).
The characteristic shown by the solid line in the figure). As a result, the coupling preventing circuit 100 performs a reliable coupling preventing operation at any irradiation level.

[0030]

As described above, according to the present invention, it is possible to protect the circuit components of the receiving system at the time of irradiation and to solve the problem of the combined radiation at the time of the low irradiation level. It is possible to apply the excitation power to the subject accurately. As a result, the S / N ratio in various pulse sequences can be improved, the slice characteristics can be improved, a desired enhanced state can be accurately realized, and a good MRI image can be obtained. There is an effect that it is sufficient to add only

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a receiving coil for an MRI apparatus according to the present invention.

FIG. 2 is a block diagram illustrating an overall configuration of the MRI apparatus.

FIG. 3 shows irradiation of a high-frequency pulse in an MRI apparatus, NM
FIG. 3 is a diagram illustrating a state of reception of an R signal.

FIG. 4 is an explanatory diagram of characteristics of a diode.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 2 sets of cross diodes 1a, 1b Cross diode 2 Receiving coil 3, 3a, 3b, 3c Resonance capacity 4 Conductor loop 5 Coil 6 Subject 7 Variable capacitance diode 20 Irradiation coil 100 Coupling prevention circuit

Claims (1)

(57) [Claims] The present invention detects a magnetic resonance signal emitted from an object by applying an irradiation pulse from an irradiation coil to the object, and comprises a conductor loop and a tuning circuit. A receiving coil for a magnetic resonance imaging apparatus, comprising: a coupling blocking circuit comprising an LC parallel resonance circuit connected in series and tuned to an irradiation frequency of the irradiation coil, and a cross diode connected between L and C thereof. Two sets of cross diodes of the blocking circuit are connected in parallel, and one of the sets of cross diodes is composed of a pair of diodes having characteristics of a large turn-on voltage and low on-resistance in a conductive state, and the other set of cross diodes. Are composed of a pair of diodes that both have low turn-on voltage and high on-resistance in the conductive state. A reception coil for a magnetic resonance imaging apparatus.