JP2008194180A - Switching circuit for high power rf circuit and magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching circuit for an RF circuit of an MRI apparatus capable of blocking large high frequency current passing when conducting a diode and improving isolation when not conducting it. <P>SOLUTION: A transmission circuit 5 is connected through a diode D2 and a capacitor C4 to a transmission/reception coil 7, and a reception circuit 9 and a protective circuit 8 are connected to the transmission/reception coil 7. D0 and D1 are serially connected at polarities opposite to each other between a transmission line and the ground and C0 is inserted between the diodes D0 and D1. A bias power source 6 for controlling the switching of the D2 is connected holding the D2 there between and a bias power source 1 is connected holding the D0 and C0 there between. A low-pass filter circuit 2 is connected holding the C0 and D1 there between. Control is performed so as to conduct the bias current of the D2 and not to conduct the bias current of the D0 and D1 for the time of transmission, and so as not to conduct the bias current of the D2 and to conduct the bias current of the D0 and D1 for the time of reception. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a magnetic resonance imaging (MRI) apparatus, and more particularly to a switching circuit for a high-power high-frequency transmission line.

  The MRI apparatus irradiates high-frequency signals to the nuclei constituting the biological tissue of the subject to cause nuclear magnetic resonance, detects the nuclear magnetic resonance signals (high-frequency signals) generated thereby, and detects the detected nuclear magnetic resonance signals It is possible to reconstruct an image by performing Fourier transform, and display a tomographic image at an arbitrary portion of the subject.

  For irradiation and detection of the high-frequency signal, an RF coil that generates a high-frequency signal in a direction orthogonal to the static magnetic field direction is used. The RF coil includes a transmission / reception coil, and a transmission coil and a reception coil dedicated to transmission and reception.

  In the case of a coil for both transmission and reception, a transmission / reception switching circuit that switches a transmission transmission path and a reception transmission path to the coil in accordance with transmission / reception timing is provided.

  The transmission / reception switching circuit includes a transmission transmission line valid / invalid switching circuit that cuts high-frequency noise from the transmission transmission line during reception or prevents reception signals from being absorbed by the transmission transmission line. This is because high-frequency noise from the transmission transmission line affects the captured image and degrades the S / N of the image.

  However, the valid / invalid switching circuit has a problem that heat is generated by a large amount of power from the transmission transmission line due to a resistance loss component when the switching element is conductive.

  Further, the valid / invalid switching circuit cuts the coil loop using a blocking impedance caused by parallel resonance of capacitance and inductance. For this reason, particularly in the case of an MRI apparatus having a small static magnetic field strength, the Larmor frequency is also lowered, and the capacitance of the capacitance must be increased in order to tune the resonance frequency of the receiving coil. For this reason, the blocking impedance in a parallel resonance state with the inductance is lowered.

  In order to solve the problem of the valid / invalid switching circuit, Patent Literature 1 describes a valid / invalid switching circuit for a coil such as a series-parallel switching type resonance circuit. In the valid / invalid switching circuit described in Patent Document 1, a switching element is arranged between two middle points of a bridge circuit in which inductances and capacitances are arranged to face each other. And when this switching element is non-conductive, it becomes a state where two sets of series resonant circuits are connected in parallel, and when it is conductive, it is a series-parallel switching type resonant circuit where two sets of parallel resonant circuits are connected in series.

  By the way, since the Pin diode used as a switching element of the transmission / reception switching circuit has a capacitance component of about 0.5 to 3.0 pF at the time of non-conduction, invalidation of the transmission transmission line becomes insufficient at the time of reception, and the transmission transmission line May be mixed into the received signal.

  Here, as a well-known method for improving the isolation when the transmission line is not conducting, as shown in FIG. 11, a switching element Da is inserted in the middle of the transmission line, and a switching element Db is inserted between the transmission line and the ground. There is a way to do it. When the transmission transmission line is valid, only the element Da is turned on by the bias current (the element Db is non-conductive), and when it is invalid, only the element Db is turned on (the element Da is non-conductive). Transmission line isolation can be improved.

JP-A-8-322816

  However, since the Pin diode, which is a switching element, becomes conductive at a voltage of about 0.6 to 0.8 V, if it is used for the element D0 that must be non-conductive at the time of transmission, it is caused by the transmission signal of a high-power high-frequency pulse. There is a problem of conduction.

  This also causes the same problem in the case of the series-parallel switching type resonance circuit described in Patent Document 1. In the case of the series-parallel switching type resonance circuit described in Patent Document 1, a high frequency is generated between two midpoints of the bridge circuit due to the influence of a capacitance component of about 0.5 to 3.0 pF that the Pin diode has when it is not conducting. There is a problem that leakage current flows and the series resonance frequency deviates from the Larmor frequency.

  An object of the present invention is to provide a switching circuit for a high-power RF circuit capable of preventing the passage of a large high-frequency current when the diode is on and improving the isolation when the diode is off, and a magnetic resonance imaging using the same. Is to realize the device.

  The switching circuit for the high power RF circuit of the present invention is connected in series so that a pair of switching elements have opposite polarities, and at least one capacitance element is connected between the pair of switching elements. The conduction / non-conduction of the switching element is controlled by a bias power source.

  The high-frequency signal transmission / reception switching circuit for a magnetic resonance imaging apparatus of the present invention includes the above-described switching circuit in a signal transmission path between a high-frequency transmission unit and a high-frequency transmission / reception coil. The pair of switching elements are controlled to be conductive during high frequency reception.

  The high-frequency signal transmission / reception coil of the present invention includes a high-frequency transmission / reception coil and a series-parallel switching type resonance circuit incorporated in the transmission / reception coil, and the series-parallel switching type resonance circuits are connected in series to each other. A first series resonant circuit comprising a first inductance element and a first capacitance element; a second series resonant circuit comprising a second inductance element and a second capacitance element connected in series; and the first A first parallel resonant circuit comprising an inductance element and a second capacitance element, a second parallel resonant circuit comprising the first capacitance element and a second inductance element, and the first and second series resonances. A switching circuit that switches whether the circuit is connected in parallel or the first and second parallel resonant circuits are connected in series. The ching circuit controls a pair of switching elements connected in series so as to have opposite polarities, at least one capacitance element connected between the pair of switching elements, and conduction / non-conduction of the pair of switching elements. The switching power supply is controlled so that the switching element of the switching circuit becomes non-conductive during high-frequency transmission and the switching element of the switching circuit becomes conductive during high-frequency reception.

  The magnetic resonance imaging apparatus of the present invention includes the high-frequency signal transmission / reception switching circuit or the high-frequency signal transmission / reception coil.

  Switching circuit for high-power RF circuit capable of preventing the passage of high-frequency current when the diode is conducting and improving isolation when non-conducting, high-frequency signal transmission / reception switching circuit for MRI apparatus, high-frequency signal transmission / reception A coil and an MRI apparatus using the coil can be realized.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a schematic configuration diagram of a magnetic resonance imaging apparatus (MRI apparatus) to which the present invention is applied. In FIG. 1, an MRI apparatus includes a magnet 302 that generates a static magnetic field around a subject 301, a gradient magnetic field coil 303 that generates a gradient magnetic field in a static magnetic field space, and an RF coil 304 that generates a high-frequency magnetic field in this region. And an RF probe 305 for detecting an MR signal generated by the subject 301.

  The gradient magnetic field coil 303 is composed of gradient magnetic field coils in three directions of X, Y, and Z, and each generates a gradient magnetic field in response to a signal from the gradient magnetic field power supply 309. The RF coil 304 generates a high frequency magnetic field in accordance with the signal from the RF transmission unit 310. The signal of the RF probe 305 is detected by a signal detection unit 306, signal processed by a signal processing unit 307, and converted into an image signal by calculation. The image processing unit 313 performs arithmetic processing on the captured image, and the image is displayed on the display unit 308.

  The gradient magnetic field power supply 309, the RF transmission unit 310, and the signal detection unit 306 are controlled by an arithmetic control unit 311. The control time chart is generally called a pulse sequence. The bed 312 is for the subject to lie down.

  The switching circuit for the high power RF circuit according to the present invention is disposed on the transmission path between the RF transmitter 310 and the RF coil 304 or in the RF coil 304 or the RF probe 305.

  FIG. 2 is a diagram showing an embodiment of a high-frequency circuit (RF circuit) according to the present invention. In FIG. 2, diodes D0 and D1 have opposite polarities and are connected in series via a capacitance C0. It is desirable to select a capacitor C0 having a large capacity so that it does not become a load with respect to the frequency of the flowing high-frequency current.

  A bias power source 1 that controls switching of the diode D0 is connected with the diode D0 and the capacitance C0 interposed therebetween. The low-pass filter circuit 2 is connected with the capacitance C0 and the diode D1 interposed therebetween. However, the bias power source is controlled by the arithmetic control unit 311 of the MRI apparatus.

  In FIG. 2, when the bias current is conductive, the series connection circuit of the diode D0, the capacitance C0, and the diode D1 is conductive with respect to the high-frequency current, and when the bias current is non-conductive, it is also non-conductive with respect to the high-frequency current. . In particular, when the bias current is non-conducting, the diodes D0 and D1 are connected in series with opposite polarity, so that a large high-frequency current that conducts the diodes D0 and D1 can be prevented only in the forward bias direction.

  In the example shown in FIG. 2, the bias power supply shows one unit, but as shown in FIG. 3, the diodes D0 and D1 are controlled using two bias power supplies 3 and 4 respectively. Also, the same effect as the example shown in FIG. 2 can be obtained.

  Furthermore, the same effect can be obtained when the polarities of the diodes D0 and D1 are opposite to those shown in FIG. In this case, it is obvious that the polarity of the bias power supply may be reversed.

  Next, another embodiment of the present invention will be described. FIG. 4 is a diagram showing an example of a transmission / reception switching circuit 100 for an MRI apparatus according to the present invention, which is arranged between a transmission line and a ground. In FIG. 4, the transmission circuit 5 is connected to the transmission / reception coil 7 via a diode D2 and a capacitance C4. A receiving circuit 9 and a protection circuit 8 are connected to the transmitting / receiving coil 7.

  Here, the diode D2 is inserted in the middle of the transmission line, the diodes D0 and D1 are connected in series with opposite polarities between the transmission line and the ground, and a capacitance C0 is inserted between the diodes. It is desirable to select a capacitor C0 having a large capacity so that it does not become a load with respect to the frequency of the flowing high-frequency current.

  Further, a bias power source 6 that controls switching of the diode D2 is connected with the D2 interposed therebetween, and the bias power source 1 is connected with the diode D0 and the capacitance C0 interposed therebetween. A floating circuit 2 having a high impedance with respect to the high-frequency current is connected with the capacitance C0 and the diode D1 interposed therebetween. The diode D0, the capacitance C0, and the diode D1 form a series circuit connected in series. The series circuit has one end connected to the capacitance C4 and the transmission / reception coil, and the other end connected to the transmission circuit and the transmission / reception coil. Is done. The bias power supplies 1 and 6 are controlled by the arithmetic control unit 311 of the MRI apparatus. That is, the bias current of the diode D2 is turned on during high-frequency transmission, and the bias currents of the diode D0 and the diode D1 are turned off. Then, during high frequency reception, the bias current of the diode D2 is controlled to be non-conductive, and the bias currents of the diodes D0 and D1 are controlled to be conductive.

  In FIG. 4, when the bias current of the diode D2 is conductive, the transmission line is conductive with respect to the high-frequency current. At this time, the bias current of the diodes D0 and D1 is nonconductive, and the diode D0, the capacitance C0, and the diode D1 are connected in series. The connection circuit becomes non-conductive even for high-frequency current.

  On the other hand, when the bias current of the diode D2 is non-conductive, the transmission line is also non-conductive to the high-frequency current. At this time, the bias current of the diodes D0 and D1 is conductive, and the diode D0, the capacitance C0, and the diode D1 are connected in series. The connection circuit is in a conductive state with respect to the high-frequency current.

  In particular, when the bias currents of the diodes D0 and D1 are non-conducting, the diodes D0 and D1 are connected in series with opposite polarities, so that a large high-frequency current that conducts the diode only in the forward bias direction is prevented. can do.

  That is, the isolation between the transmission line and the ground at the time of transmission is not deteriorated. Further, the isolation between the transmission transmission line and the reception transmission line at the time of reception can be improved as compared with the case where the switching element of the transmission transmission line is only the diode D2. Furthermore, since the high-power high-frequency pulse during transmission does not flow through the diodes D0 and D1, the problem of heat generation can be avoided.

  The example shown in FIG. 4 shows a case where there are two bias power supplies. However, as shown in FIG. 5, a bias power supply 4 connected between both ends of the diode D1 is added to provide a diode D0. In the case of the transmission / reception switching circuit 101 for controlling D1, D2 as well, the same effect as the example shown in FIG. 4 can be obtained.

  Also, when the polarities of the diodes D0 and D1 are opposite to those in the example shown in FIG. 4, the same effect as in the example shown in FIG. 4 can be obtained by reversing the polarity of the bias power supply.

  Next, another embodiment of the present invention will be described. FIG. 6 is a diagram illustrating an RF coil valid / invalid switching circuit 102 for an MRI apparatus according to another embodiment of the present invention. The example shown in FIG. 6 is arranged as a series-parallel switching type resonance circuit connected to a coil element of a high-frequency signal transmitting / receiving coil.

  Here, the inductance L0 and the capacitance C3 by the coil element form an RF coil for an MRI apparatus that is in a resonance state at the Larmor frequency. The series-parallel switching type resonance circuit 102 is connected in series to the capacitance C3 and the inductance L0, and the transmission / reception circuit 10 is connected to both ends of the capacitance C3.

  The inductance L1 is connected in series with the capacitance C1, and the capacitance C2 is connected in series with the inductance L2. A series circuit composed of the inductance L1 and the capacitance C1 and a series circuit composed of the capacitance C2 and the inductance L2 are connected in parallel to each other, and a connection point between the inductance L1 and the capacitance C2 is connected to one end of the transmission / reception circuit 10. Yes. The connection point between the inductance L2 and the capacitance C1 is connected to the other end of the transmission / reception circuit 10 via the inductance L0.

  A connection point between the inductance L1 and the capacitance C1 is connected to a connection point between the capacitance C2 and the inductance L2 via the diode D0, the capacitance C0, and the diode D1.

  That is, among the coil elements, the inductances L1 and L2 and the capacitances C1 and C2 are opposed to each other, and the diodes D0 and D1 are connected in series with opposite polarities between the two midpoints of the bridge circuit. A capacitance C0 is inserted between them. It is desirable to select a capacitor C0 having a large capacity so that it does not become a load with respect to the frequency of the flowing high-frequency current.

  A bias power source 1 that controls switching of the diode D0 is connected with the diode D0 and the capacitance C0 interposed therebetween, and a low-pass filter circuit 2 is connected with the capacitance C0 and the diode D1 interposed therebetween. However, the bias power source 1 is controlled by the arithmetic control unit 311 of the MRI apparatus. In the case of a transmission coil, control is performed such that bias current is non-conductive during transmission and bias current is conductive during reception. In the case of a receiving coil, bias current conduction is controlled during transmission, and bias current non-conduction is controlled during reception. Furthermore, in the case of the transmission / reception coil, the bias current is controlled to be non-conductive both during transmission and reception.

  In FIG. 6, when the bias current is conductive, the bridge circuit enters a parallel resonance state and becomes high impedance, so that the RF coil loop is disconnected.

  On the other hand, when the bias current is non-conducting, the bridge circuit is in a series resonance state and has a low impedance, and thus operates normally as an RF coil.

  In particular, when the bias current is non-conducting, the diodes D0 and D1 are connected in series with opposite polarities. Therefore, a large high-frequency current that conducts the diode can be prevented only in the forward bias direction.

  In the case of a transmission coil using this bridge circuit, the switching element must be in a non-conductive state at the time of transmission, but the diodes D0 and D1 are not conductive by a high-power high-frequency pulse from the transmission transmission line. Furthermore, since no high-power high-frequency pulse flows through the diodes D0 and D1, the problem of heat generation can be avoided.

  In the case of a receiving coil using this bridge circuit, the switching element must be non-conductive at the time of reception, but the diodes D0 and D1 are connected in series with opposite polarities, so that the capacitance component of the Pin diode is 1 / 2. For this reason, the high frequency leakage current which flows into the two middle points of a bridge circuit can be reduced, and the magnitude | size from which a series resonance frequency deviates from a Larmor frequency can be reduced.

  The example shown in FIG. 6 shows the case where there is one bias power supply, but as shown in FIG. 7, the series-parallel switching that controls the diodes D0 and D1 using the two bias power supplies 3 and 4 is used. In the case of the pattern circuit 103, the same effect as the example shown in FIG. 6 can be obtained.

  Also, when the polarities of the diodes D0 and D1 are opposite to those shown in FIG. 6, the same effect can be obtained if the polarity of the bias power supply is reversed.

  As described above in detail, according to the embodiment of the present invention, a pair of switching elements are connected in series so as to have opposite polarities, and at least one capacitance element is connected between the pair of switching elements. Since the switching circuit is configured, a large high-frequency current that conducts the diode can be prevented only in the forward bias direction.

  Further, in the transmission / reception switching circuit for an MRI apparatus provided with such a switching circuit between the transmission line and the ground, the transmission transmission line and the reception transmission line are isolated at the time of reception without deteriorating the isolation between the transmission line and the ground. Can be improved.

  Alternatively, in an RF coil for an MRI apparatus having a series-parallel switching type resonance circuit having such a switching circuit between two midpoints of a bridge circuit in an element as an effective / ineffective switching circuit at the time of transmission and reception, It is possible to reduce the magnitude of the series resonance frequency of the parallel switching type resonance circuit deviating from the Larmor frequency, and to realize a parallel resonance state with respect to a large high frequency current. Thereby, it is possible to provide a valid / invalid switching circuit with high accuracy and high breakdown voltage.

1 is a schematic configuration diagram of an MRI apparatus to which the present invention is applied. It is a figure which shows the switching circuit for the high power RF circuit which is one Embodiment of this invention. FIG. 3 is a diagram showing a modification of the switching circuit shown in FIG. 2. It is a figure which shows one Embodiment of the transmission / reception switching circuit for MRI apparatuses of this invention. It is a figure which shows the modification of the transmission / reception switching circuit shown in FIG. It is a figure which shows one Embodiment of the valid / invalid switching circuit of RF coil for MRI apparatuses of this invention. It is a figure which shows the modification of the valid / invalid switching circuit of RF coil shown in FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1, 3, 4, 6 ... Bias power supply, 2 ... Low pass filter, 5 ... Transmission circuit, 7 ... Transmission / reception coil, 8 ... Protection circuit, 9 ... Reception circuit, 10 * ..Transmission / reception circuit, 100, 101 ... Transmission / reception switching circuit, 102,103 ... Series-parallel switching type resonance circuit, 301 ... Subject, 302 ... Static magnetic field magnet, 303 ... Gradient magnetic field coil , 304 ... RF coil, 305 ... RF probe, 306 ... signal detection unit, 307 ... signal processing unit, 308 ... display unit, 309 ... gradient magnetic field power supply, 310 ... RF transmission unit, 311 ... operation control unit, 312 ... bed, 313 ... image processing unit, C0 to C4 ... capacitance, D0 to D4 ... diode, L0 to L3 ... inductance

Claims (9)

In a switching circuit for a high power RF circuit,
A pair of switching elements connected in series so as to have opposite polarities;
At least one capacitance element connected between the pair of switching elements;
A bias power source for controlling conduction / non-conduction of the pair of switching elements;
A switching circuit for a high power RF circuit.   2. The switching circuit for a high power RF circuit according to claim 1, wherein the switching element is a diode.   3. The switching circuit for a high power RF circuit according to claim 2, wherein the diode is a Pin diode. In a high-frequency signal transmission / reception switching circuit for a magnetic resonance imaging apparatus that transmits a high-frequency signal to a subject and receives a nuclear magnetic resonance signal from the subject,
The signal transmission path between the high-frequency transmission unit and the high-frequency reception unit and the high-frequency transmission / reception coil includes the switching circuit according to claim 1. A high-frequency signal transmission / reception switching circuit for a magnetic resonance imaging apparatus, wherein the pair of switching elements are controlled to be conductive.   5. The high frequency signal transmission / reception switching circuit for a magnetic resonance imaging apparatus according to claim 4, wherein the pair of switching elements are pin diodes.   A magnetic resonance imaging apparatus comprising the high-frequency signal transmission / reception switching circuit according to claim 4. In a high frequency signal transmitting / receiving coil for a magnetic resonance imaging apparatus that transmits a high frequency signal to a subject and receives a nuclear magnetic resonance signal from the subject,
A coil for high frequency transmission and reception;
A series-parallel switching type resonance circuit incorporated in the transmission / reception coil,
The series-parallel switching type resonance circuit is
A first series resonant circuit comprising a first inductance element and a first capacitance element connected in series with each other;
A second series resonant circuit comprising a second inductance element and a second capacitance element connected in series with each other;
A first parallel resonant circuit comprising the first inductance element and the second capacitance element;
A second parallel resonant circuit comprising the first capacitance element and the second inductance element;
A switching circuit for switching whether the first and second series resonant circuits are connected in parallel or the first and second parallel resonant circuits are connected in series,
The switching circuit includes a pair of switching elements connected in series so as to have opposite polarities, at least one capacitance element connected between the pair of switching elements, and conduction / non-conduction of the pair of switching elements. Magnetic resonance imaging, comprising: a bias power source to be controlled, wherein the switching element of the switching circuit is rendered non-conductive during high-frequency transmission, and the switching element of the switching circuit is rendered conductive during high-frequency reception. High-frequency signal transmission / reception coil for equipment.   8. The high frequency signal transmitting / receiving coil for a magnetic resonance imaging apparatus according to claim 7, wherein the switching element of the switching circuit is a Pin diode. A magnetic resonance imaging apparatus comprising the high-frequency signal transmitting and receiving coil according to claim 7.

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