JPH0584228A - Receiving coil for magnetic resonance imaging device - Google Patents

Abstract

(57) [Abstract] [Purpose] In a magnetic resonance imaging apparatus, coupling of a receiving coil to an irradiation coil is blocked for all irradiation levels, and accurate excitation power can be applied to a subject so that low-level irradiation can be performed. Good images are obtained from time to high level illumination. Constitution: An LC parallel resonance circuit, which is connected in series with a conductor loop and tuned to the irradiation frequency of an irradiation coil, and a cross diode connected between L and C of the LC parallel resonance circuit, are turned on together as the cross diode of the coupling blocking circuit. It has the characteristics that the voltage is large and the on-resistance in the conductive state is small
Two sets of a cross diode composed of a pair of diodes and a cross diode composed of a pair of diodes each having a small turn-on voltage and a large ON resistance in a conductive state were connected in parallel.

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 by utilizing magnetic resonance.

[0002]

2. Description of the Related Art A magnetic resonance imaging apparatus (hereinafter referred to as MR
I device) measures the density distribution and relaxation time distribution of the nuclear spins at a desired inspection site in the subject by utilizing the nuclear magnetic resonance phenomenon, and displays the cross section of the subject as an image from the measured data. It is a thing.

Nuclear spins of a subject placed in a uniform and strong static magnetic field generator perform precession about the 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 this Larmor frequency is irradiated from the outside, the spins are excited and transit to a high energy state (nuclear magnetic resonance (NMR) phenomenon). When this 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 the high frequency receiving coil tuned to the frequency. At this time, a gradient magnetic field in the three axial directions of X, Y, and Z is applied to the static magnetic field space in order to add position information to the space. As a result, it is possible to capture position information in space as frequency information.

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

The static magnetic field generating magnet 10 is for generating 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 space having a certain space around the subject 6. The magnetic field generating means is arranged. The sequencer 12 operates under the control of the CPU 11 and sends various commands necessary for collecting tomographic image data of the subject 6 to the transmission system 13, the gradient magnetic field generation system 14, and the reception system 1.
To send to 5.

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.
The high frequency pulse output from the high frequency generator 17 is modulated by the modulator 18 in accordance with the instruction from the sequencer 12, and the modulated irradiation pulse is supplied to the power amplifier 19
After being amplified in (1), the electromagnetic wave is irradiated to the subject 6 by supplying it to the irradiation coil 20 arranged close to the subject 6.

The gradient magnetic field generating system 14 is composed of a gradient magnetic field coil 21 wound in three directions of X, Y and Z and a gradient magnetic field power source 22 for driving each coil, and an instruction from the sequencer 12 is given. By driving the gradient magnetic field power source 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 comprises a receiving coil 2, a preamplifier 23, a quadrature phase detector 24 and an A / D converter 25, and the response of the subject 6 by the electromagnetic wave emitted from the transmitting side irradiation coil 20. Electromagnetic wave (NMR signal) of 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 amount, and further converted into two systems of collected data sampled by the quadrature phase detector 24 at the timing according to the instruction from the sequencer 12, and the signals are sent to the signal processing system 16. It is like this. The signal processing system 16 includes a CPU 11, a recording device such as a magnetic disk 26 and an optical disk 27, and C
It comprises a display 28 such as an RT, and the CPU 1
In step 1, Fourier transform, correction coefficient calculation, image reconstruction, and other processing are performed, and the distribution (signal distribution) obtained by performing appropriate calculation on the signal intensity distribution of an arbitrary cross section or a plurality of signals is displayed on the display 28. It is supposed to be 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 the imaging of the anatomical structure of the subject, different pulse sequences are used for pulse irradiation, so that an image in which the emphasis state is changed according to the nature of the tissue is obtained. You can also get it. 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 showing a simple manner of irradiation of high frequency pulses and reception of NMR signals in the MRI apparatus. Different irradiation outputs are 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 in 3. Each coil 20, 2 is composed of 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, since the irradiation and reception coils 20 and 2 are resonant circuits tuned to the same frequency and are arranged relatively close to each other, 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, in the tuning circuit of the receiving coil 2,
There has been provided a coupling blocking circuit which is connected in series with the conductor loop 4 and comprises an LC parallel resonance circuit tuned to the irradiation frequency of the irradiation coil 20 and a cross diode connected between L and C thereof ( JP-A-62-298344
(See the official gazette).

According to this, the cross diode is turned on by the voltage due to the irradiation output, the LC parallel resonance circuit resonates, the conductor loop 4 and the resonance capacitor 3 are cut off, and the receiving coil 2 is separated from the irradiation frequency. It is possible to prevent damage to the circuit components of the reception system.

[0015]

The coupling inhibition according to this prior art is premised on that a sufficient input can be obtained because it is supplied with an irradiation output and operates. That is, it is necessary to generate a voltage in the receiving coil 2 to make the cross diode conductive.

In general, a diode that performs high-speed switching has different characteristics depending on the current capacity, and a diode having a relatively large capacity has a large turn-on voltage (forward voltage drop VF) near 1 V as shown in FIG. 4 (a). The on-resistance in the conductive state has a small characteristic. On the other hand, a diode having a small current capacity is 0.
Although it turns on at a low VF of about 3 V, the ON resistance in the conductive state is large and the inclination 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 characteristic of FIG. 4A) in order to withstand a large current during irradiation and to improve the blocking efficiency. ) Was used.

As described above, such a diode has a large VF and is satisfactory in terms of protection of the receiving coil 2 against a high level irradiation output. However, subject 6
Depending on the pulse sequence of the irradiation pulse to, the irradiation output of a comparatively low level is used. In this case,
The voltage generated in the receiving coil 2 becomes low, a sufficient coupling blocking effect cannot be expected, and the excitation power changes due to the coupling radiation from the receiving coil 2, resulting in a problem that it cannot be controlled with high accuracy.

In the MRI apparatus, the precision of the excitation power level is important, and if it is insufficient, problems such as a decrease in SN ratio, deterioration of slice characteristics, and changes in image enhancement state occur. Was requested.

The object of the present invention is to improve the imperfections of the operation of the prior art of coupling inhibition during low-level irradiation, and to improve the imperfections during low-level irradiation, that is, from low-level irradiation to high-level irradiation. It is to provide a receiving coil for an MRI apparatus that can obtain an image.

[0021]

The above-mentioned problems at the time of low-level irradiation are due to the large VF of the cross diode for blocking coupling. Therefore, in the present invention, this V
The above problem was dealt with by connecting another set of cross diodes each having a small VF in parallel to the cross diode having a large F to improve the characteristics.

That is, the above-mentioned object is to detect a magnetic resonance signal emitted from the subject by applying an irradiation pulse to the subject from the irradiation coil, and it 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 receiver coil for a magnetic resonance imaging apparatus having a coupling blocking circuit comprising a parallel resonant circuit and a cross diode connected between L and C thereof, two sets of the coupling blocking circuit cross diodes are connected in parallel. The pair of cross diodes each have a large turn-on voltage and a small on-resistance in the conductive state, and the other pair of cross diodes have both a small turn-on voltage and a conductive on-resistance. This is achieved by consisting of a pair of diodes with great characteristics.

[0023]

When the two kinds of diodes shown in FIGS. 4 (a) and 4 (b) are connected in parallel, the combined characteristic is that VF is small, the current capacity is large, and the on-state is on as shown by the solid line in FIG. 4 (c). It has an ideal characteristic of low resistance.

Of the two sets of cross diodes connected in parallel in the coupling blocking circuit, one set is given a characteristic that the turn-on voltage (VF) is large and the on-state resistance in the conducting 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 shown by the solid line in FIG. 4C are obtained as a whole. This makes it possible to protect the circuit components of the receiving system and 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 comprises a resonance loop 3 (3a, 3b, 3) that forms a tuning circuit with a conductor loop 4 that forms the coil.
It consists of 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. The resonance capacitance 3 is composed of three resonance capacitances 3a, 3b and 3c, one of which is connected to a variable capacitance diode 7, and a tuning frequency is generated by a DC voltage applied to the receiving coil 2. Can be controlled. The variable-capacitance diode 7 has a property that the capacitance between junctions largely changes due to a reverse bias voltage, and the capacitance decreases as the voltage increases. Therefore, by adjusting this, the resonance frequency at which the maximum sensitivity is obtained can be matched with the signal frequency f0.

Further, another resonance capacitance 3a is connected in series to the conductor loop 4, and a coil 5 is connected in parallel to this resonance capacitance 3a so that an LC parallel resonance tuned to the irradiation frequency of the MRI apparatus. A circuit is formed. In this case, on one side of the connecting portion of the resonance capacitor 3a and the coil 5,
Two sets of cross diodes 1 (1a, 1a) connected in parallel
b) is inserted, and by these, the coupling blocking 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
It is tuned to the irradiation frequency of the RI apparatus, and when the cross diode 1 is turned on by the irradiation output mixed in the receiving coil 2, the LC is in a parallel resonance state, and acts to cut off the receiving coil 2 with high impedance.

As a result, a circuit component of the receiving coil 2 and the preamplifier 2 connected to the receiving coil 2 are generated from a strong irradiation output.
It is possible to prevent circuit components of the receiving system such as 3 from being damaged. Further, although the received NMR signal is very weak, the coupling blocking circuit 100 is cut off by the cross diode 1 at the time of receiving the NMR signal, so that the reception of the NMR signal is not adversely affected.

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

Therefore, the characteristics of the cross diode 1 are ideal characteristics in which the forward voltage drop VF is small, the current capacity is large, and the on-resistance is small (FIG. 4 (c)).
The characteristics shown by the solid line in the figure). As a result, the coupling blocking circuit 100 will perform a reliable coupling blocking operation at any illumination 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 solve the problem of coupled radiation at a low irradiation level. It is possible to accurately apply the excitation power to the subject. As a result, the SN ratio in various pulse sequences is improved, the slice characteristic is improved, the desired enhancement state can be accurately realized, and a good MRI image can be obtained. There is an effect that you only need to add.

[Brief description of 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 showing the overall configuration of an MRI apparatus.

FIG. 3 Irradiation of high frequency pulse in MRI device, NM
It is a figure which shows the mode of reception of an R signal.

FIG. 4 is a characteristic explanatory view of a diode.

[Explanation of symbols]

 1 2 sets of cross diodes 1a, 1b cross diode 2 receiving coil 3, 3a, 3b, 3c resonance capacitance 4 conductor loop 5 coil 6 subject 7 variable capacitance diode 20 irradiation coil 100 coupling block circuit

Claims (1)

[Claims] 1. A magnetic resonance signal emitted from an object is detected by applying an irradiation pulse from the irradiation coil to the object. The object comprises a conductor loop and a tuning circuit. A receiving coil for a magnetic resonance imaging apparatus, comprising a LC parallel resonance circuit connected in series and tuned to an irradiation frequency of the irradiation coil, and a coupling prevention circuit comprising a cross diode connected between L and C thereof, wherein the coupling Two sets of cross diodes of the blocking circuit are connected in parallel, one set of which includes a pair of diodes having a large turn-on voltage and a small turn-on resistance in the conductive state, and the other set of cross diodes. Is a pair of diodes that both have a low turn-on voltage and a large on-resistance in the conductive state. A receiving coil for a magnetic resonance imaging apparatus, comprising: