JPH0528138B2 - - Google Patents

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention excites a subject with an excitation high-frequency pulse transmitted from a transmitting coil of an MRI apparatus (magnetic resonance diagnostic apparatus). The present invention relates to a receiving coil device for receiving magnetic resonance signals induced after excitation, and particularly to a receiving coil device for an MRI apparatus that eliminates coupling with a transmitting coil.

(Prior Art) In a general medical MRI apparatus, a gradient magnetic field and an excitation high-frequency pulse are applied to a subject placed in a static magnetic field according to a predetermined excitation/detection procedure. By creating a magnetic resonance phenomenon in a specific part of the subject, detecting the induced magnetic resonance signal, and performing signal processing, anatomical information and qualitative information of the specific part of the subject can be imaged. ing.

In the above, a transmitting coil is used to apply (transmit) the excitation high-frequency pulse, and a receiving coil is used to detect (receive) the induced magnetic resonance signal. Here, the transmitting coil and receiving coil are:
The same coil may be used for both purposes.

In addition, there are various types of coils, such as whole-body coils fixed to a gantry or bed that can excite and detect a wide area, and small surface coils that can be hand-held and can excite and detect localized areas. There are forms.

Generally, when trying to image a local area with high S/N, a whole-body coil is used as the transmitting coil to excite a wide area, and then a surface coil is used as the receiving coil. Signals are collected from a local region within the wide excited region to perform high S/N imaging.

The above excitation/detection procedure has the following problems. In other words, when a high-frequency excitation pulse is transmitted from a transmitter coil, a voltage is induced in the surface coil, which is a receiver coil that is close to the transmitter coil, due to the magnetic coupling between the transmitter coil and the surface coil. An induced current flows. This induced voltage and induced current disturb the magnetic field generated by the excitation high-frequency pulse in the vicinity of the surface coil, resulting in a deterioration in image quality.

Generally, in order to prevent the above-mentioned problems from occurring, the transmitting coil and the surface coil are arranged in a positional relationship that reduces magnetic coupling between them (both coils are arranged coaxially). (The coupling is maximum, and the orthogonal arrangement of both coils minimizes the coupling.) For this reason, the surface coil can only be placed at a specific position, and therefore the areas that can be imaged with high S/N are limited.

(Problem to be Solved by the Invention) In this way, in the conventional technology, magnetic coupling between the transmitting coil and the receiving coil is reduced by arranging the receiving coil only at a specific position. Therefore, the areas that can be imaged with high S/N are limited, which is a problem.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a receiving coil device for an MRI apparatus that allows imaging with a high S/N ratio without specifying a region.

[Structure of the Invention] (Means for Solving the Problems) The present invention has a structure in which the following means are taken to solve the above problems and achieve the objects.

That is, the present invention provides a receiving coil device that excites a subject with an excitation high-frequency pulse transmitted from a transmitting coil of an MRI apparatus, and receives the induced magnetic resonance signal after the excitation. a decoupling coil disposed close to the receiving coil and having substantially the same characteristics as the receiving coil; When transmitting a high frequency pulse, the receiving coil and the decoupling coil are connected to cancel the induced voltage generated in the receiving coil and the induced voltage occurring in the decoupling coil, and when receiving the magnetic resonance signal, the receiving coil and the decoupling coil are connected, The apparatus further includes a switch that disconnects the decoupling coil and causes the receiving coil to receive the magnetic resonance signal.

(Function) According to such a configuration, when transmitting a high frequency pulse for excitation, terminals of the same polarity of the receiving coil and the decoupling coil are connected via the switch, and the magnetic connection between the transmitting coil and the receiving coil is Even if a voltage is induced in the receiving coil due to the large coupling, this induced voltage is canceled by the induced voltage similarly induced in the decoupling coil, which is placed close to the receiving coil and has approximately the same characteristics as the receiving coil. , does not disturb the magnetic field caused by the excitation high-frequency pulse. Therefore, imaging can be performed with high S/N no matter where the receiving coil device is placed within the magnetic field generated by the excitation high-frequency pulse.

(Example) An example of a receiving coil device for an MRI apparatus according to the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of the device of this embodiment, and FIG. 2 is an electrical circuit diagram of the device.

As shown in FIGS. 1 and 2, the receiving coil device TC of this embodiment has a receiving coil 1, and a decoupling coil 2 having substantially the same characteristics as the receiving coil 1 is attached to the receiving coil 1. They are provided in close proximity and via automatic switch elements 3A and 3B.

These automatic switch elements 3A and 3B are elements of a means for controlling the electromotive force of the receiving coil 1 and the electromotive force of the decoupling coil 2 to cancel each other out when transmitting high-frequency pulses for excitation, and this means is an element of the means described above. Automatic switch elements 3A, 3B, and a trap circuit 4 with both ends connected to the receiving coil 1
It is composed of A and 4B. The other ends of the trap circuits 4A and 4B are used as external connection terminals.

The automatic switch elements 3A and 3B are composed of cross diodes and the like that are turned on under high voltage bias and turned off under low voltage bias. When the high-frequency pulse for excitation is transmitted, a high-level induced voltage is generated in the receiving coil 1 and the decoupling coil 2 due to the magnetic field caused by the high-frequency pulse for excitation, and this causes the automatic switch elements 3A, 3
B becomes a high voltage bias and turns on, connecting the receiving coil 1 and the decoupling coil 2.

On the other hand, even when receiving a magnetic resonance signal induced after the excitation high-frequency pulse is not transmitted (when the excitation high-frequency pulse is not transmitted), an induced voltage is generated due to the received signal. The level is lower than that at the time of transmission, and the automatic switching elements 3A and 3B become a low voltage bias and turn off, cutting off the connection between the receiving coil 1 and the decoupling coil 2.

The trap circuits 4A and 4B include coils 4Aa and 4Ba with an inductance L, capacitors 4Ab and 4Bb with a capacitance C, and an automatic switch element 4Ac, which is composed of a cross diode, etc., which is turned on at high voltage bias and turned off at low voltage bias. , 4Bc.

Here, the trap circuits 4A, 4B operate the coils 4Aa, 4B when the automatic switch elements 4Ac, 4Bc are on.
Ba and capacitors 4Ab and 4Bb form a parallel resonant circuit under the following resonance conditions.

Resonance condition 2πf=1/L·C Here, the frequency of the excitation high-frequency pulse and the resonance frequency of the trap circuits 4A and 4B are made to match the above-mentioned frequency f. That is, the trap circuits 4A and 4B
It is assumed that the inductance L of Aa and 4Ba and the capacitance C of the capacitors 4Ab and 4Bb are selected so as to resonate at the same value as the frequency of the excitation high-frequency pulse.

In addition, the automatic switch elements 4Ac and 4Bc are on when transmitting high-frequency pulses for excitation,
At this time, a high-level induced voltage is generated in the receiving coil 1 and the decoupling coil 2 by the magnetic field generated by the excitation high-frequency pulse, so that the automatic switch elements 4Ac and 4Bc are turned on with a high voltage bias.

Furthermore, if the Q value of coils 4Aa and 4Ba is Q, a parallel resonant circuit is formed by coils 4Aa and 4Ba and capacitors 4Ab and 4Bb when they are on, so RA, RB=Qω, ω=2πf The resistances of RA, RB is formed. Here, the Q values of the coils 4Aa and 4Ba are selected in accordance with the frequency f of the excitation high-frequency pulse so that the values of the resistors RA and RB are large.

Therefore, when transmitting high frequency pulses for excitation, the second
The circuit shown in the figure becomes the equivalent circuit shown in FIG. That is, as shown in FIG. 3, the receiving coil 1 and the decoupling coil 2 have equivalent voltage sources e1 and e2 generated by transmitting excitation high-frequency pulses, and the trap circuits 4A and 4B have only resistors RA and RB. It becomes a circuit. In other words, the receiving coil device TC at the time of transmitting the excitation high-frequency pulse consists of the receiving coil 1, the voltage source e
1. A closed circuit is formed by the decoupling coil 2 and the voltage source e2.

Here, the voltage sources e1 and e2 are the receiving coil 1
Since the decoupling coil 2 and the decoupling coil 2 have substantially the same characteristics and are close to each other, they have the same induced voltage. Therefore, i1 flowing through the receiving coil 1 due to the voltage source e1 and i2 flowing through the decoupling coil 2 have the same magnitude. When transmitting high-frequency pulses for excitation, the above-mentioned closed circuit is formed, and i1 flows to the receiving coil 1 by the voltage source e1, and i2 flows to the decoupling coil 2 by the voltage source e2 of the same magnitude as this i1. They cancel each other out.

As a result, no induced current flows through the receiving coil 1 and the decoupling coil 2.
Therefore, when an excitation high-frequency pulse is transmitted from a transmitting coil (not shown), no induced current flows through the receiving coil 1, so that there is nothing magnetically seen from the transmitting coil. In other words, the transmitting coil and the receiving coil are not coupled.

On the other hand, when receiving the induced magnetic resonance signal after the excitation high-frequency pulse is not transmitted (when the excitation high-frequency pulse is not transmitted), the automatic switch elements 3A and 3B are set to the low voltage bias as shown in FIG. Since the receiving coil 1 and the decoupling coil 2 are disconnected from each other, the trap circuits 4A and 4B are switched off by the automatic switching elements 4Ac and 4Bc.
is turned off, so the circuit consists of only capacitors 4Ab and 4Bb. That is, the receiving coil device TC has a circuit configuration in which the capacitors 4Ab and 4Bb are connected to the receiving coil 1, and normal receiving operation is performed. Here, the capacitors 4Ab and 4Bb can be used as elements of a tuning circuit (not shown).

As described above, according to this embodiment, the following effects can be obtained.

According to this embodiment, even if a voltage is induced in the receiving coil 1 due to magnetic coupling between the transmitting coil and the receiving coil 1 during transmission of high-frequency pulses, this induced voltage will be transferred to the decoupling coil 2. It is canceled out by the induced voltage and does not disturb the magnetic field caused by the excitation high-frequency pulse. Therefore, imaging can be performed with high S/N no matter where the receiving coil device is placed within the magnetic field generated by the excitation high-frequency pulse.

According to this embodiment, the connection between the receiving coil 1 and the decoupling coil 2 is disconnected when receiving the induced magnetic resonance signal after the excitation high-frequency pulse is not transmitted (when the excitation high-frequency pulse is not transmitted). The receiving coil device TC can perform normal receiving operations.

According to this embodiment, in the above, the switching of the circuit and operation between the transmission of the excitation high-frequency pulse and the reception of the induced magnetic resonance signal is performed by an automatic switch that is turned on and off at the time of transmission and reception. Advantageously, this is done automatically by elements 3A, 3B, 4Ac, 4Bc.

Next, another embodiment of the present invention will be described with reference to FIG. That is, a decoupling coil 2 is provided to the receiving coil 1 via PIN diodes 5A, 5B, and both ends of the receiving coil 1 are used as external connection terminals via PIN diodes 6A, 6B.

Here, when transmitting the excitation high frequency pulse,
On/off control is performed by a control signal so that the PIN diodes 5A, 5B are turned on, and the PIN diodes 6A, 6B are turned off. In addition, when receiving and transmitting magnetic resonance signals, PIN diode 5A,
5B is turned off, and PIN diodes 6A and 6B are turned on, so that they are controlled to be turned on and off by a control signal.

With the above configuration, the effects listed above can be obtained in the same way as the configurations shown in FIGS. 1 and 2. In this case, control signals for on/off control of the PIN diodes 5A, 5B, 6A, and 6B can be obtained from the controller of the MRI apparatus, so this can be handled with a simple configuration.

In addition, various modifications can be made without departing from the gist of the present invention.

[Effects of the Invention] As described above, the present invention includes a receiving coil that receives magnetic resonance signals, a decoupling coil that is disposed close to the receiving coil and has substantially the same characteristics as the receiving coil, and the receiving coil. is inserted between terminals of the same polarity as the decoupling coil and the decoupling coil, and connects the receiving coil and the decoupling coil when excitation high-frequency pulses are transmitted, and the induced voltage generated in the receiving coil and the decoupling coil are The structure includes a switch that cancels the induced voltage and disconnects the receiving coil and the decoupling coil when receiving the magnetic resonance signal and causes the receiving coil to receive the magnetic resonance signal. When transmitting high-frequency pulses, the terminals of the receiving coil and the decoupling coil of the same polarity are connected via a switch, and a voltage is induced in the receiving coil due to the magnetic coupling between the transmitting coil and the receiving coil. Also, this induced voltage is
This is canceled by the induced voltage similarly induced in the decoupling coil, which is placed close to the receiving coil and has substantially the same characteristics as the receiving coil, and does not disturb the magnetic field caused by the excitation high-frequency pulse.

Therefore, high S/N can be achieved no matter where the receiving coil device is placed within the magnetic field generated by the excitation high-frequency pulse.
This has the effect of allowing you to take pictures with

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an embodiment of a receiving coil device of an MRI apparatus according to the present invention, FIG. 2 is an electric circuit diagram of the same embodiment, and FIGS. 3 and 4 respectively show the operation of the same embodiment. FIG. 5 is a circuit diagram showing the configuration of another embodiment of the present invention. 1... Receiving coil, 2... Decoupling coil, 3A, 3B, 4Ac, 4Bc... Automatic switch element, 4A, 4B... Trap circuit, 4Aa,
4Ba...Coil, 4Ab, 4Bb...Capacitor,
5A, 5B, 6A, 6B...PIN diode.

Claims (1)

[Claims] 1. A receiving coil device that excites a subject with an excitation high-frequency pulse transmitted from a transmitting coil of an MRI apparatus, and receives the induced magnetic resonance signal after the excitation, comprising: a receiving coil for receiving data; a decoupling coil disposed close to the receiving coil and having substantially the same characteristics as the receiving coil; inserted between terminals of the receiving coil and the decoupling coil having the same polarity; When transmitting a high frequency pulse for excitation, the receiving coil and the decoupling coil are connected to cancel the induced voltage generated in the receiving coil and the induced voltage occurring in the decoupling coil, and when receiving the magnetic resonance signal, the receiving coil and the decoupling coil are connected. A receiving coil device for an MRI apparatus, comprising: a switch for disconnecting the decoupling coil and causing the receiving coil to receive the magnetic resonance signal.