WO2018225484A1

WO2018225484A1 - Array coil and magnetic resonance imaging device

Info

Publication number: WO2018225484A1
Application number: PCT/JP2018/019388
Authority: WO
Inventors: 陽介大竹; 浩二郎岩澤; 悦久五月女
Original assignee: 株式会社日立製作所
Priority date: 2017-06-09
Filing date: 2018-05-18
Publication date: 2018-12-13
Also published as: JP6745244B2; JP2018202105A

Abstract

The objective of the present invention is to acquire an image having an improved SNR by reducing noise. Provided is an array coil which is provided with first and second RF coils capable of receiving a magnetic resonance signal from a subject and respectively including a first loop coil portion and a second loop coil portion from which magnetic coupling has been eliminated and each of which comprises a conductor, and an electric field generating circuit which includes a loop portion in which the end portions of at least one conductor are connected to one another, and which is magnetically coupled to the first RF coil and the second RF coil. The placement of the electric field generating circuit is adjusted in such a way that an inner product volume integral value of electric fields generated respectively in the subject by a first sub-current induced in the electric field generating circuit by the first RF coil and a second sub-current induced in the electric field generating circuit by the second RF coil has a sign and a value that cancel out an inner product volume integral value of electric fields generated in the subject by a first current flowing through the first loop coil portion and a second current flowing through the RF coil of the second loop coil portion.

Description

Array coil and magnetic resonance imaging apparatus

The present invention relates to a magnetic resonance imaging (MRI) apparatus, and more particularly to an RF coil that detects a nuclear magnetic resonance signal by irradiating a high frequency magnetic field (RF magnetic field).

The MRI apparatus is an apparatus that images an arbitrary cross section across a subject using a nuclear magnetic resonance phenomenon. Specifically, the MRI apparatus irradiates a subject placed in a spatially uniform magnetic field (static magnetic field) with an RF magnetic field to cause nuclear magnetic resonance, and detects and detects the generated nuclear magnetic resonance signal. A cross-sectional image is acquired by performing image processing on the processed signal.

A device that irradiates a subject with an RF magnetic field and detects a nuclear magnetic resonance signal generated from the subject is called an RF coil (hereinafter referred to as an RF (Radio Frequency) coil). The RF coil has a loop unit (coil loop) that performs irradiation and detection of an RF magnetic field. The smaller the coil loop, the narrower the sensitivity region, but the higher the sensitivity. On the other hand, if the coil loop is enlarged, the sensitivity region can be expanded. As described above, in the RF coil, there is a trade-off relationship between the high sensitivity and the wide sensitivity area.
Further, since the nuclear magnetic resonance signal is a signal of a rotating magnetic field generated in a direction perpendicular to the static magnetic field generated by the magnet, the RF coil is preferably arranged in a direction in which a magnetic field perpendicular to the static magnetic field can be irradiated and detected. .

As described above, the smaller the RF coil, the higher the sensitivity but the narrower the sensitivity region. As a solution to this problem, there is a multi-channel array coil in which a plurality of highly sensitive small-diameter RF coils are arranged in an array (see, for example, Non-Patent Document 1). Since the multi-channel array coil has high sensitivity and a wide sensitivity region, it can acquire a high SNR (Signal to Noise Ratio) image, which is the mainstream of the current receiving RF coil.

通常 When RF coils with the same resonance characteristics are usually arranged close to each other, they interfere with each other by magnetic coupling. Since interference due to magnetic coupling degrades the performance of the RF coil, it is essential to remove the magnetic coupling between the RF coils in a multi-channel array coil. Therefore, in Non-Patent Document 1, the magnetic coupling is reduced to the maximum by arranging the coil loops of adjacent RF coils so that a part of the coil loops overlap each other. Further, by using a low-input preamplifier, an inductor, and a capacitor, a part of the coil loop is made to have a high impedance, thereby reducing interference from other than the RF coil.

Patent Documents

1 and 2 disclose techniques for reducing magnetic coupling between RF coils constituting a multichannel coil by providing decoupling means for the multichannel coil. Patent Document 3 discloses a technology that can realize high sensitivity by effectively using magnetic coupling.
Thus, in recent years, RF coils have been increased in sensitivity by increasing the number of channels and reducing interference due to magnetic coupling.

Japanese Patent No. 3197262 JP 2002-119495 A JP 2016-054785 A

However, in the RF coil, thermal noise generated from the subject is also detected as noise through the electric field created by the coil. For this reason, in order to improve the sensitivity of the RF coil, it is not sufficient to reduce interference due to magnetic coupling, and it is necessary to consider noise due to thermal noise generated from the subject.

Here, generally, noise due to thermal noise is Gaussian noise. Therefore, when the noise is ideally independent, the noise of the combined signal increases by the 1/2 power of the number of channels. However, in the array coil, in the channels adjacent to each other, the electric field generated by one RF coil induces a current to the other RF coil through the subject, so the detected noise is shared through the subject and has a correlation. Noise is detected on both channels. For this reason, when noise is shared, the noise of the composite signal increases at a faster rate than the 1/2 power of the number of channels.

Therefore, in a multi-channel array coil (particularly an array coil of 32ch or more) in which the noise is strongly correlated, even if the number of channels is further increased, only the noise increases and the expected SNR does not increase. That is, even if the number of channels is further increased, an effect commensurate with the increase in the number of channels cannot be obtained.

By the way, Non-Patent Document 1 proposes an image reconstruction method that uses a noise correlation matrix to minimize noise and maximize SNR. However, when the noise correlation matrix cannot be obtained correctly or when the SNR is low, Conversely, this approach may reduce the SNR.
Patent Document 3 proposes a method of obtaining high sensitivity by magnetically coupling coils, but the coils that are magnetically coupled are RF coils that detect signals from each other, so that adjustment of the coils may be difficult. . Therefore, in any of the methods, noise reduction may not be sufficient, and it is difficult to improve the SNR.

The present invention has been made in view of the above circumstances, and an object thereof is to obtain an image in which noise in an array coil is reduced and SNR is improved.

In order to solve the above problems, the present invention provides the following means.
One embodiment of the present invention includes a first loop coil portion made of a conductor, a first RF coil capable of receiving a magnetic resonance signal from a subject, and a second loop coil portion made of a conductor. The magnetic resonance signal can be received from the subject, the second RF coil from which the magnetic coupling is removed from the first RF coil, and a loop portion in which at least one conductor end is connected to each other. And an electric field generation circuit magnetically coupled to the first RF coil and the second RF coil, wherein the electric field generation circuit is induced in the electric field generation circuit by the first RF coil. The volume integral value of the inner product of the electric fields generated in the subject by the one sub current and the second sub current induced in the electric field generation circuit by the second RF coil is the first loop coil. The first flowing in the department The arrangement is adjusted such that the volume integral value of the inner product of the electric field generated in the subject is canceled by the current and the second current flowing in the RF coil of the second loop coil section. An array coil is provided.

In another aspect of the present invention, a static magnetic field forming unit that forms a static magnetic field, a gradient magnetic field forming unit that forms a gradient magnetic field, and a transmission RF coil that irradiates an inspection target placed in the static magnetic field with an RF magnetic field A reception RF coil for detecting a nuclear magnetic resonance signal from the inspection object, and a signal processing unit for processing the nuclear magnetic resonance signal detected by the reception RF coil, wherein the reception RF coil is the array described above. A magnetic resonance imaging apparatus that is a coil is provided.

According to the present invention, it is possible to obtain an image with reduced noise in the array coil and improved SNR.

BRIEF DESCRIPTION OF THE DRAWINGS It is an external view of the MRI apparatus which concerns on embodiment of this invention, (a) is an MRI apparatus of a horizontal magnetic field system, respectively, (b) is an external view of an open type perpendicular magnetic field type MRI apparatus. It is a block diagram which shows schematic structure of an MRI apparatus. It is explanatory drawing for demonstrating the connection of the transmission RF coil and the reception RF coil in the MRI apparatus which concerns on embodiment of this invention. (A) is a figure which shows the structure of the birdcage type | mold RF coil used as a transmission RF coil, (b) is a figure which shows an example of the magnetic coupling prevention circuit between transmission / reception of a transmission RF coil. (A) is a figure which shows one Embodiment of the array coil used as a receiving RF coil, (b) And (c) is a figure which shows the example of the magnetic coupling prevention circuit between transmission / reception of a receiving RF coil. (A)-(b) is explanatory drawing for demonstrating operation | movement and an effect | action of a conventional array coil. (A)-(d) is explanatory drawing for demonstrating operation | movement and an effect | action of the array coil of Example 1. FIG. (A)-(c) is a figure which shows the modification of this embodiment, respectively. (A) is a figure explaining the adjustment example of Example 1, (b) is a figure explaining the arrangement | positioning. (A)-(c) is a figure explaining the principle which an RF coil and a conductor loop magnetically couple and induce an electric current. (A) is a graph of the sensitivity profile of an Example and a comparative example, (b) is a graph which shows the relationship between the electric current ratio of a subcurrent, and a noise correlation. FIG. 10 is a diagram illustrating a circuit for explaining an operation of the array coil according to the second embodiment. (A) And (b) is explanatory drawing explaining the inner product distribution of the electric field which two conventional RF coils which do not use a conductor loop make. It is a figure explaining the inner product distribution of the array coil which concerns on Example 2, (a) shows the example of arrangement | positioning of a to-be-photographed object and an array coil, (b) is 1st RF coil at the time of signal reception to 1st RF coil (C) shows the operation when the second current flows through the second RF coil during signal reception, and (d) shows the inner product of the electric field created by the two RF coils. The concept of distribution is shown. It is explanatory drawing which shows the structure of the array coil 400 which concerns on a modification. It is a reference figure concerning the example which removed the magnetic coupling of two RF coils via common capacitor 451C. It is a figure explaining the inner product distribution of the electric field which the conventional two RF coils which do not use a conductor loop, (a) shows an example of arrangement of a subject and an array coil, and (b) shows two RF coils. The concept of the inner product distribution of the electric field is shown. It is a figure explaining the inner product distribution of a present Example, (a) shows the example of arrangement | positioning of a to-be-photographed object and an array coil, (b) is the 1st RF coil 1st electric current (solid line in a figure) flows at the time of signal reception. (C) shows the operation when a second current flows through the second RF coil during signal reception, and (d) shows the concept of the distribution of the inner product of the electric fields produced by the two RF coils. Indicates. It is a figure which shows the structure of the array coil at the time of providing three RF coils. It is a figure which shows the structure at the time of providing the inductor for performing the magnetic coupling of RF coil and a conductor loop in the coil element and conductor loop of RF coil.

Hereinafter, an MRI apparatus according to an embodiment of the present invention will be described with reference to the drawings. Hereinafter, in the drawings according to each embodiment or example, the same components are denoted by the same reference numerals, and the repeated description thereof is omitted.

<Overall configuration of MRI apparatus>
First, an MRI apparatus to which this embodiment can be applied will be described.
FIG. 1 shows the appearance of an example of an MRI apparatus to which the present embodiment can be applied. In particular, FIG. 1A shows a horizontal magnetic field type MRI apparatus 100 using a tunnel magnet 110 that generates a static magnetic field by a solenoid coil. FIG. 1B shows an open-type vertical magnetic field type MRI apparatus 101 in which a magnet 111 is separated into upper and lower parts to enhance the feeling of opening. These

MRI apparatuses

100 and 101 include a table 102 on which an inspection target (subject) 103 is placed. The subject 103 is placed on a table and placed in an examination space where a uniform magnetic field (static magnetic field) is generated by the

magnets

110 and 111.

Magnets

110 and 111 constitute a static magnetic field forming unit that forms a static magnetic field.

The MRI apparatus according to the present embodiment applies a so-called multi-channel RF coil having a plurality of RF coils and a conductor pool as a reception RF coil. The above-described horizontal magnetic field type MRI apparatus 100 and vertical magnetic field type MRI apparatus are used. Any of the devices 101 can be applied.
The form of the MRI apparatus shown in FIG. 1 is an example, and various known MRI apparatuses can be used in the present invention regardless of the form and type of the apparatus. In the following description, as a coordinate system common to the horizontal magnetic field method and the vertical magnetic field method, a coordinate system 090 in which the static magnetic field direction is the z direction and the two directions perpendicular thereto are the x direction and the y direction, respectively.

Hereinafter, in the present embodiment, a schematic configuration of the MRI apparatus 100 will be described by taking as an example a case where a horizontal magnetic field type MRI apparatus is applied.
As shown in FIG. 2, the MRI apparatus 100 includes a horizontal magnetic field type magnet (static magnetic field magnet) 110, a gradient magnetic field coil 131, a transmission RF coil 151, a reception RF coil 161, a gradient magnetic field power supply 132, a shim coil 121, and a shim power supply 122. , An RF magnetic field generator 152, a receiver 162, a magnetic coupling prevention circuit driving device 180, a computer (PC) 170, a sequencer 140, and a display device 171. Reference numeral 102 denotes a table on which the inspection object (subject) 103 is placed.

The gradient magnetic field coil 131 is connected to the gradient magnetic field power supply 132 and generates a gradient magnetic field. The gradient magnetic field coil 131 and the gradient magnetic field power source 132 constitute a gradient magnetic field forming unit that forms a gradient magnetic field. The shim coil 121 is connected to the shim power source 122 and adjusts the uniformity of the magnetic field. The transmission RF coil 151 is connected to the RF magnetic field generator 152 and irradiates (transmits) the subject 103 with the RF magnetic field.

The reception RF coil 161 is connected to the receiver 162 and receives a nuclear magnetic resonance signal from the subject 103. Here, as the reception RF coil 161 according to the present embodiment, a multi-channel RF coil (hereinafter referred to as an array coil) including a plurality of RF coils and a conductor loop is applied. In the following description, the number of RF coils constituting the array coil and the number of channels are assumed to be the same. Details of the array coil as the reception RF coil 161 will be described later.

The magnetic coupling prevention circuit driving device 180 is connected to a magnetic coupling prevention circuit (described later). The magnetic coupling prevention circuit is a circuit that prevents magnetic coupling between the transmission RF coil 151 and the reception RF coil 161 connected to the transmission RF coil 151 and the reception RF coil 161, respectively.

The sequencer 140 sends commands to the gradient magnetic field power supply 132, the RF magnetic field generator 152, and the magnetic coupling prevention circuit driving device 180 to operate them. The command is sent in accordance with an instruction from the computer (PC) 170. Further, in accordance with an instruction from the computer (PC) 170, the receiver 162 sets a magnetic resonance frequency as a reference for detection. For example, the subject 103 is irradiated with the RF magnetic field through the transmission RF coil 151 in accordance with a command from the sequencer 140. The nuclear magnetic resonance signal generated from the subject 103 by irradiating the RF magnetic field is detected by the reception RF coil 161 and detected by the receiver 162.

The computer (PC) 170 controls the operation of the entire MRI apparatus 100 and performs various signal processing. For example, a signal detected by the receiver 162 is received via an A / D conversion circuit, and signal processing such as image reconstruction (function of the image reconstruction unit) is performed. The result is displayed on the display device 171. The detected signal and measurement conditions are stored in a storage medium as necessary. In addition, the sequencer 140 is made to send an instruction so that each device operates at a preprogrammed timing and intensity. Further, when it is necessary to adjust the static magnetic field uniformity, the sequencer 140 sends a command to the shim power supply 122 to cause the shim coil 121 to adjust the magnetic field uniformity.

<Outline of transmit RF coil and receive RF coil>
As described above, the MRI apparatus according to the present embodiment uses two types of RF coils, that is, the transmission RF coil 151 and the reception RF coil 161. As the transmission RF coil 151 and the reception RF coil 161, one RF coil can serve both, or separate RF coils can be used.

Hereinafter, the transmission RF coil 151 and the reception RF coil 161 are separate RF coils, the transmission RF coil 151 is an RF coil having a birdcage shape (birdcage type RF coil), and the reception RF coil 161 is composed of a plurality of RF coils. The details of the RF coil will be described using a multi-channel array coil as an example.

First, the arrangement of the birdcage type RF coil 300 used as the transmission RF coil 151 and the array coil 400 used as the reception RF coil 161, the birdcage type RF coil 300, the array coil 400, the RF magnetic field generator 152, the receiver 162, and the magnetism. A connection mode of the coupling prevention circuit driving device 180 will be described with reference to FIG.

As shown in FIG. 3, the birdcage type RF coil 300 has a substantially cylindrical shape (including an elliptical column and a polygonal column) in appearance, and the axis of the substantially cylindrical axis is the central axis of the magnet 110 (in the Z direction). Arranged so as to be coaxial with the shaft. The subject 103 is disposed inside the birdcage type RF coil 300. The array coil 400 is disposed in the birdcage type RF coil 300 in the vicinity of the subject 103. Further, as described above, the birdcage type RF coil 300 is connected to the RF magnetic field generator 152. Array coil 400 is connected to receiver 162.

Further, the birdcage type RF coil 300 is provided with a magnetic coupling prevention circuit 210 that prevents magnetic coupling with the array coil 400, and the array coil 400 includes magnetic coupling prevention that prevents magnetic coupling with the birdcage type RF coil 300. A circuit 220 is provided. These are called a magnetic coupling prevention circuit between transmission and reception. The inter-transmission / reception magnetic coupling prevention circuit enables transmission of an RF magnetic field and reception of a nuclear magnetic resonance signal without magnetic coupling to each other in the arrangement as described above.

[Transmission RF coil]
Next, a birdcage type RF coil 300 used as the transmission RF coil 151 of this embodiment will be described with reference to FIG.
The birdcage type RF coil 300 of this embodiment is adjusted so that the resonance frequency (magnetic resonance frequency) of the element to be excited becomes the resonance frequency, and irradiates the RF magnetic field of the magnetic resonance frequency. In this embodiment, it is adjusted to the magnetic resonance frequency f0 of the hydrogen nucleus that can excite the hydrogen nucleus. Hereinafter, the magnetic resonance frequency of the irradiated RF magnetic field is set to f0.

FIG. 4A is a block diagram for explaining the configuration of the birdcage type RF coil 300 of the present embodiment. As shown in the figure, the birdcage type RF coil 300 of this embodiment includes a plurality of straight conductors 301, end conductors 302 connecting the ends of the respective straight conductors 301, and a capacitor inserted into the end conductors 302. 303.

The birdcage type RF coil 300 includes two

input ports

311 and 312. The first input port 311 and the second input port 312 are configured so that transmission signals having phases different from each other by 90 degrees are input and an RF magnetic field is efficiently applied to the subject 103.
Further, in the birdcage type RF coil 300 of the present embodiment, a transmission / reception magnetic coupling prevention circuit 210 for preventing magnetic coupling with the reception RF coil 161 (array coil 400) is provided on the linear conductor 301 of the birdcage type RF coil 300. Inserted in series.

For example, as shown in FIG. 4B, the inter-transmission / reception magnetic coupling prevention circuit 210 can be constituted by a PIN diode 211 inserted in series with a straight conductor 301, and control signal lines 212 are provided at both ends thereof. Connected. The control signal line 212 is connected to the magnetic coupling prevention circuit driving device 180. It is desirable to insert a choke coil (not shown) into the control signal line 212 in order to avoid high frequency mixing.

The PIN diode 211 normally has a high resistance (off), and has a characteristic of being generally in a conductive state (on) when the value of the direct current flowing in the forward direction of the PIN diode 211 exceeds a certain value. In this embodiment, this characteristic is used, and on / off of the PIN diode 211 is controlled by a direct current output from the magnetic coupling prevention circuit driving device 180. That is, at the time of high-frequency signal transmission, a control current for turning on the PIN diode 211 is passed through the control signal line 212 to cause the birdcage RF coil 300 to function as the transmission RF coil 151. Further, when receiving a nuclear magnetic resonance signal, the control current is stopped, and the birdcage type RF coil 300 is made to have a high impedance and opened.

As described above, in the present embodiment, by controlling the direct current (control current) from the magnetic coupling prevention circuit driving device 180, the birdcage type RF coil 300 is made to function as the transmission RF coil 151 at the time of high-frequency signal transmission, and the nuclear magnetism. When receiving the resonance signal, the magnetic coupling with the array coil 400 which is the reception RF coil 161 is removed as an open state.

[Receiving RF coil]
Next, the array coil 400 used as the reception RF coil 161 of this embodiment will be described with reference to FIG. Here, for simplicity of explanation, an electric field generation circuit having two RF coils (surface coils) having a loop-shaped coil loop portion and a loop portion in which ends of one conductor are connected to each other as an example. A description will be given using arrayed coils. Hereinafter, for convenience of explanation, the loop portion of the electric field generation circuit is simply referred to as “conductor loop”. Note that the receiving RF coil of the present embodiment is not limited to this, as long as it is an array coil in which at least two or more RF coils and one or more conductor loops (electric field generating circuits) are arranged.

The array coil 400 of the present embodiment includes two RF coils 410 and a conductor loop 415 as shown in FIG. The two RF coils 410 constituting the array coil 400 are referred to as a first RF coil 410A and a second RF coil 410B, respectively. However, the components of each RF coil 410 constituting the array coil 400 are not particularly distinguished from each RF coil 410, and the last letter of the symbol is omitted (hereinafter the same).

The first RF coil 410 </ b> A, the second RF coil 410 </ b> B, and the conductor loop 415 are surface coils each having a loop arranged and configured to cover a subject on a substantially plane. This magnetic coupling is prevented by an overlap 451A that overlaps a part of the conductors of the coil loop, which is one of magnetic coupling preventing means. In addition, the conductor loop is arranged so as to have a figure of 8 when the direction in which the two RF coils are arranged, that is, the X direction is the vertical direction. Further, each RF coil 410 and the conductor loop 415 are arranged and configured so as to be intentionally magnetically coupled 450. The function and details of each magnetic coupling will be described later.

Each of the two RF coils 410 is adjusted so that the birdcage RF coil 300 can receive a nuclear magnetic resonance signal of an excitable element in a state where each of the two RF coils 410 is magnetically coupled to the conductor loop. Function. The signals received by the first RF coil 410A and the second RF coil 410B are sent to the receiver 162, respectively.

Since the configuration of the two RF coils 410 is the same, the configuration of the first RF coil 410A will be described below as a representative. The first RF coil 410A includes a loop coil unit 420 (first loop coil unit 420A) that detects a nuclear magnetic resonance signal (RF magnetic field), a low input impedance preamplifier 430 (first low input impedance preamplifier 430A), and the like. The inductor 441 (first inductor 441A) that connects the loop coil unit 420 and the low input impedance preamplifier 430 is connected to the receiver 162 via the low input impedance preamplifier 430.

Note that the inductor 441 may be substituted for a capacitor. Looking at the low input impedance preamplifier side from the coil loop, the impedance of the parallel resonant circuit composed of the capacitor 421, the low input impedance preamplifier, the inductor 441 or the capacitor to be substituted, and the conductor connecting them is f0 and becomes higher than the impedance of other frequencies. Configured as follows.

The loop portion of the first loop coil portion 420A is formed by the conductor 21A. The first loop coil section 420A includes a capacitor 424 that is inserted in series with the inductor component of the first loop coil section 420A. The inductor component and the capacitor 424 constitute a parallel resonance circuit. This capacitor 424 is referred to as a parallel capacitor 424 in order to distinguish it from other capacitors.

Also, a capacitor 422A for adjusting the resonance frequency and a magnetic coupling prevention circuit 220 between transmission and reception are inserted in series in the first loop coil section 420A. The capacitor 422A is called a series capacitor in order to distinguish it from other capacitors. Here, a case where two first series capacitors (422A) are provided is illustrated, but the number of first series capacitors may be one or more.

Thus, the first RF coil 410A is a circuit element for adjustment, the first inductor 441A and the first series capacitor 422A inserted in series with the inductor component of the first loop coil section 421A. And a first parallel capacitor 424 that is inserted in series with the inductor component and uses the first loop coil portion 420A as a parallel resonant circuit. Similarly, the second RF coil 410B includes, as adjustment circuit elements, a second inductor 441B and a second series capacitor 422B inserted in series with respect to the inductor component of the second loop coil portion 421B. And a second parallel capacitor 424B inserted in series with the inductor component and having the second loop coil portion 420B as a parallel resonant circuit.

One terminal on the loop coil unit 420 side of the low input impedance preamplifier 430 is connected to one end of the parallel capacitor 424 of the loop coil unit 420 via the inductor 441. The other terminal on the loop coil unit 420 side of the low input impedance preamplifier 430 is directly connected to the other end of the parallel capacitor 424 of the loop coil unit 420.

The transmission / reception magnetic coupling prevention circuit 220 removes magnetic coupling with the birdcage type RF coil 300 which is the transmission RF coil 151. For example, as shown in FIG. 5B, the inter-transmission / reception magnetic coupling prevention circuit 220 includes a capacitor 423 inserted in series with the conductor 21 constituting the loop 421, and a PIN diode 221 connected in parallel with the capacitor 423. , And inductor 222.

A control signal line 223 is connected to both ends of the PIN diode 221. The control signal line 223 is connected to the magnetic coupling prevention circuit driving device 180. It is preferable that a choke coil (not shown) is inserted in the control signal line 223 in order to avoid high frequency mixing. Inductor 222 and capacitor 423 are adjusted to resonate in parallel at the frequency of the received nuclear magnetic resonance signal.

Generally, a parallel resonant circuit has a characteristic that a resonance frequency has a higher impedance (high resistance) than other frequencies. Therefore, when a current flows through the PIN diode 221, the PIN diode 221 is turned on, and the capacitor 423 of the loop 421 enters into a high impedance state in parallel with the inductor 222 at the frequency of the received nuclear magnetic resonance signal. Therefore, a part of the loop coil unit 420 becomes high impedance and opens at the frequency of the received nuclear magnetic resonance signal, and the RF coil 410 having the loop coil unit 420 is also opened.

Thus, when the current flows through the PIN diode 221 and is turned on, the magnetic coupling between the RF coils 410A and 410B and the birdcage type RF coil 300 is removed. Therefore, the magnetic coupling between the array coil 400 having each RF coil 410 as a coil element and the birdcage type RF coil 300 is also removed.

5A shows an example in which one transmission / reception magnetic coupling prevention circuit 220 is inserted into the RF coil 410, but the number of transmission / reception magnetic coupling prevention circuits 220 inserted into the RF coil 410 is as follows. It is not limited to one. Two or more may be inserted into each loop 421. By inserting a plurality, the magnetic coupling between the transmission RF coil 151 and the reception RF coil 161 can be sufficiently lowered.

Further, the configuration of the transmission / reception magnetic coupling prevention circuit 220 is not limited to the above configuration. For example, a cross diode 221m may be used instead of the PIN diode 221 as in a modification of the transmission / reception magnetic coupling prevention circuit 220m shown in FIG. As a result, when a large signal flows through the conductors constituting the loop 421, the cross diode 221m is turned on, and the capacitor 423 of the loop 421 resonates in parallel with the inductor 222 at the frequency of the received nuclear magnetic resonance signal to generate high impedance. It becomes a state. In this case, the magnetic coupling prevention circuit driving device 180 may not be provided.

In the array coil 400 of the present embodiment, each RF coil 410A is adjusted by adjusting the inductance and capacitance of the circuit elements for adjustment included in the RF coils 410A and 410B and the value of the inductance and capacitance provided by the electromagnetic coupling unit 450. , 410B can receive nuclear magnetic resonance signals, and each RF coil and conductor loop are magnetically coupled so that part of the current flowing in the RF coil flows as a sub-current in the conductor loop. Adjusted.

Specifically, in the first RF coil 410A, the magnetic field generated by the first RF coil 410A during signal reception is applied to the loop 416 on the first RF coil side, which is the left loop in the figure of the conductor loop. The conductor loop is arranged so as to induce a stronger current than the loop 417 on the second RF coil side which is the loop on the right side of the conductor loop, and the current induced in the loop 416 on the first RF coil side is intentionally controlled. The target current is made to flow through the conductor loop.

At the same time, the magnetic field generated by the second RF coil 410B is transferred to the second RF coil side loop 417, which is the right-hand side loop of the conductor loop, and the first RF coil, which is the left-hand side loop of the conductor loop. It arrange | positions so that the electric current stronger than the loop 416 of a side may be induced | guided | derived, and the electric current which the electric current induced in the loop 417 of the 2nd RF coil side intentionally flows flows through a conductor loop intentionally.

Specific examples of the adjustment method and the electromagnetic coupling means will be described in an embodiment of the RF coil described later. Here, first, as described above, it will be described that noise is reduced by magnetically coupling each RF coil so that a current flows through the conductor loop.

The noise of each channel of the reception RF coil of the MRI apparatus is mainly thermal noise generated from the subject, and is Gaussian noise detected by electric field coupling between the coil and the subject. Such noise is random noise that follows a Gaussian distribution with the same intensity as long as the input impedance of each coil is adjusted to 50Ω.

Therefore, in the image reconstruction, when the square sum square root synthesis of each channel is performed, the noise generally increases by a power of 1/2 according to the number of synthesis channels. However, in the array coil, a plurality of RF coils are arranged adjacent to each other so as to cover the subject, so that the RF coil is electrically coupled via the subject, so that some noise signals are shared. Therefore, the detected noise does not have an independent Gaussian distribution, but a correlation occurs in the noise signal. This correlation (noise correlation) takes a value from −1 to +1. If it is 0, it is an independent noise, if it is +1, it is the same noise, and if it is −1, it is the same noise with a different sign.

That is, when the noise correlation is 1, the noise increases in proportion to the number of combined channels, and therefore increases at a faster pace than when the noise correlation is 0. Therefore, generally, it is preferable that the noise correlation is low (the absolute value is close to 0).
Generally, the noise correlation Ψ between the RF coils is obtained by the following equations (1) and (2).

Where σ is the conductivity of the subject, V is the volume of the subject, Ei is the electric field (complex number) created by the i-th RF coil, and Ej is the electric field (complex number) created by the j-th RF coil. E ^* represents the complex conjugate of E. That is, the magnitude of the noise correlation is determined by the volume integral Rij of the inner product of the electric fields created by each coil in the subject. By reducing Rij, the noise correlation can be kept low, and the SNR can be prevented from decreasing due to an increase in noise correlation.

In the array coil 400 of this embodiment, the inner product of the electric field generated by the electric field generated when the first RF coil 410A and the second RF coil 410B are magnetically coupled to the conductor loop 415 to induce a current is shown in FIG. The reduction of noise will be described with reference to FIG.

FIG. 6 is a diagram for explaining the inner product distribution of the electric field created by the two conventional RF coils 410A and 410B, in which the conductor loop 415 is not used. As shown in FIG. 6A, the first RF coil 410A and the second RF coil forming the array coil are arranged so as to cover the object 103 and not be magnetically coupled.

At the time of signal reception, a clockwise first current (solid line in the figure) flows through the first RF coil 410A as shown in the figure, and a clockwise second current flows through the second RF coil 410B as shown in the figure. (Broken line in the figure) flows. In the present embodiment, the two RF coils 410 are arranged adjacent to each other, have substantially the same sensitivity region when viewed from a distance, and detect similar signals, so that the currents induced in the RF coils are handled in the same direction.

FIG. 6B is a view of FIG. 6A viewed from the Z-axis direction, and shows the concept of the inner product distribution of the electric field created by the two RF coils. The area where the inner product of the electric field is positive is indicated by fine dots, and the negative area is indicated by coarse dots. As shown in FIG. 6, the first RF coil 410A and the second RF coil 410B exhibit a positive inner product distribution in the subject. The first RF coil 410A and the second RF coil 410B are present in substantially the same direction when viewed from the subject, and currents flowing in the same direction flow in the first RF coil. This is because the electric fields generated by 410A and the second RF coil 410B in the subject are similar.

As described above, since the positive inner product component is dominant in the conventional coil configuration, the integral value is positive, and the noise correlation obtained by Expression (2) is a large positive value. Therefore, the noise of the synthesized value of the signals acquired with these configurations is larger than that without correlation. That is, the SNR is lowered.

FIG. 7 is a diagram for explaining the inner product distribution of the present embodiment. As shown in FIG. 7A, the first RF coil 410A and the second RF coil 410B forming the array coil are arranged so as to cover the object 103 and not to be magnetically coupled, and further, the conductor loop 415 is provided. The first RF coil 410A and the second RF coil 410B are magnetically coupled to each other and are arranged to cover the subject.

FIG. 7B shows the operation when a first clockwise current (solid line in the figure) flows through the first RF coil 410A as shown in the figure at the time of signal reception, as in the conventional configuration of FIG. Show. In the present embodiment, since the first RF coil 410A is magnetically coupled to the conductor loop, the magnetic field generated by the first RF coil 410A is on the first RF coil side, which is the left loop in the figure of the conductor loop. A left-handed current is induced in the loop 416, and a first sub-current 418A as shown in FIG.

FIG. 7C shows the operation when a second clockwise current (broken line in the figure) flows through the second RF coil 410B as shown in the figure at the time of signal reception, as in the conventional configuration of FIG. Show. In this embodiment, since the second RF coil 410B is magnetically coupled to the conductor loop, the magnetic field generated by the second RF coil 410B is on the second RF coil side, which is the loop on the right side of the conductor loop in the figure. A left-handed current is induced in the loop 417, and a second sub-current 418B as shown in FIG.

That is, in the present embodiment, the induced first sub-current 418A and the second sub-current 418A and the second RF coil, even if the directions of the currents flowing through the conductor loop are both clockwise and the same direction. The sub-current 418B has a reverse current direction on the conductor loop 415.

FIG. 7D is a view of FIG. 7A viewed from the Z-axis direction, and shows the concept of the inner product distribution of the electric field created by the two RF coils. Positive electric fields are indicated by fine dots, and negative electric fields are indicated by coarse dots. As shown in this figure, the first RF coil 410A and the second RF coil 410B form a current similar to that in the conventional configuration shown in FIG. 6, and thus show the same positive inner product distribution in the subject. On the other hand, a negative inner product distribution is shown near the conductor loop 415. As described above, the negative polarity is because the direction of the current generated by the magnetic coupling is reversed. Since the sign of the current is different, the sign of the electric field is also different, and the inner product is negative.

As described above, in the present embodiment, not only the positive area but also the negative area is generated as in the conventional configuration. Therefore, as shown in the above formulas (1) and (2), since the noise correlation is obtained by volume integration of the electric field, the integral value can be brought close to zero by configuring positive and negative in this embodiment. . Therefore, if it is close to zero, the noise can be reduced and the SNR can be improved compared to the conventional configuration shown in FIG.

In the present embodiment, the direction of the current flowing through each RF coil and the direction of the current flowing through the loop (416 or 417) in the vicinity of the conductor loop are reversed (the phase difference of the current is 180 degrees). However, it is not limited to this. The integral value of the inner product of the electric field generated in the subject by the first sub-current 418A and the second sub-current 418B is determined by the loop coil portion 420A of the first RF coil 410A and the loop coil portion 420B of the second RF coil 410B. What is necessary is just to be comprised so that the sign of the integral value of the inner product of the electric field produced in a to-be-photographed object may differ. As a result, the degree of freedom in design is improved, and the SNR in the region of interest can be improved.

Also, the symmetry of the two loops constituting the figure 8 is not limited to this. It may be asymmetric as shown in FIG. It is sufficient that the current can be configured so that the integral value of the inner product of the sub-currents to be configured is negative even if it is asymmetric. As a result, the degree of freedom in design is improved, and the SNR in the region of interest can be improved.

In the present embodiment, it has been described that a clockwise current flows through both the first RF coil 410A and the second RF coil 410B, but the effect is the same even if the current is defined in the reverse direction. The sign of the integral value of the inner product of the electric field created in the subject by the first RF coil 410A and the second RF coil 410B is magnetically coupled to the conductor loop by the first RF coil 410A and the second RF coil 410B. If the sign of the integral value of the inner product produced by the sub-current produced in (5) becomes an opposite sign, the integral value approaches zero, the noise is reduced, and the SNR is improved.

The imaging method using the MRI apparatus configured as described above is the same as the operation of the conventional MRI apparatus, and the subject 103 arranged in the static magnetic field space generated by the static magnetic field magnet 110 is subjected to, for example, the imaging method. In accordance with the selected pulse sequence, an RF magnetic field pulse is applied from the transmission RF coil 151 (for example, the birdcage type RF coil 300) and a gradient magnetic field pulse is applied by the gradient magnetic field coil 131.

When the transmission RF coil 151 is in operation, the transmission / reception magnetic coupling prevention circuit 220 of the reception RF coil 161 is opened, and the magnetic coupling with the reception RF coil 161 is removed. A reception RF coil 161 (multi-channel coil: array coil 400) arranged close to the subject 103 after a predetermined time has elapsed from the application of the RF magnetic field pulse, and a nuclear magnetic resonance signal generated from an atomic nucleus of an element constituting the living tissue of the subject 103 Receive at. During the reception operation, the transmission / reception magnetic coupling prevention circuit 210 is opened, and the magnetic coupling between the transmission RF coil 151 and the reception RF coil 161 is removed.

The computer (signal processing unit) 170 processes each MR signal received by the RF coil of the reception RF coil 161. For example, if the imaging method is a high-speed imaging method employing parallel imaging, it follows a parallel imaging algorithm. An image of the subject is created by the image reconstruction method. Alternatively, the image obtained by the signal of each channel is MAC synthesized to create an image. At this time, sensitivity distribution information of each RF coil is used as appropriate.

According to the MRI apparatus of the present embodiment, by using a multi-channel coil with specific adjustment as a reception RF coil, noise correlation between each RF coil is reduced, and a high-quality image can be obtained.

That is, the MRI apparatus according to the present embodiment includes a static magnetic field forming unit that forms a static magnetic field, a gradient magnetic field forming unit that forms a gradient magnetic field, and a transmission RF coil that irradiates an RF magnetic field to an inspection object arranged in the static magnetic field, A reception RF coil for detecting a nuclear magnetic resonance signal from the inspection object, and a signal processing unit for processing the nuclear magnetic resonance signal detected by the reception RF coil.

The reception RF coil includes at least two RF coils and a conductor loop as an electric field generation circuit, and the integral value of the inner product of the electric fields generated in the subject by the current flowing through the first RF coil and the second RF coil is The integral value and sign of the inner product of the electric field generated in the subject by the current induced in the conductor loop by the magnetic coupling of one RF coil and the current induced in the conductor loop by the magnetic coupling of the second RF coil These are RF coils (array coils) for the MRI apparatus that are arranged and adjusted so as to be different from each other.

Specifically, an array coil including a first RF coil, a second RF coil, and a conductor loop (electric field generation circuit), and the coil loops of the first RF coil and the second RF coil are mutually connected. Arranged and configured so as not to be magnetically coupled.

When the cosine value of the angle formed by the first axis substantially perpendicular to the coil loop surface of the first RF coil and the second axis substantially perpendicular to the coil loop surface of the second RF coil is smaller than 0 Are further arranged so that the substantially circular conductor loop covers the subject. On the other hand, when the cosine value of the angle formed by the first axis perpendicular to the coil loop surface of the first RF coil and the second axis perpendicular to the coil loop surface of the second RF coil is greater than zero , 8 is further arranged so as to cover the subject.
That is, the first RF coil and the conductor loop, and the second RF coil and the conductor loop are adjusted so as to be magnetically coupled.

The array coil 400 of this embodiment arranged and adjusted as described above is tuned to the magnetic resonance frequency f0. In addition, when the signal is received, the integral value of the inner product of the electric field generated in the subject by the first RF coil 410A and the second RF coil 410B is obtained by magnetically coupling the first RF coil 410A and the second RF coil 410B. Each of the currents flowing through the conductor loops is configured to have a sign different from the sign of the integral value of the inner product of the electric field, and the sum of the integral values of these inner products approaches zero.

As a result, the noise correlation between the first RF coil 410A and the second RF coil 410B decreases, and the SNR of the composite image increases.

Thus, according to the array coil 400 of this embodiment, both multi-channel and low noise can be achieved. In addition, this multi-channel and low noise is realized by arrangement and adjustment of circuit element values. Therefore, the configuration is not complicated. By using this array coil 400 as the reception RF coil 161, the MRI apparatus of this embodiment can obtain a high-quality image in a wide area.

Note that the MRI apparatus according to the present embodiment is characterized by using a reception RF coil that has been subjected to specific adjustment, and various changes can be made to the other configurations. For example, omitting some of the elements shown in FIG. 2 and adding elements not shown in FIG. 2 are also included in the present embodiment. In the above-described embodiment, the horizontal magnetic field type MRI apparatus has been described. However, the present invention can be similarly applied to a vertical magnetic field type MRI apparatus.

<Example of RF coil>
Next, the array coil as the reception RF coil will be described in more detail. The array coil is a multi-channel RF coil used as a reception RF coil of the MRI apparatus, and includes a plurality of RF coils. Each of the plurality of RF coils can receive a nuclear magnetic resonance signal generated by an MRI apparatus, and is adjusted so that an electric field generated in a subject is strengthened between adjacent RF coils by a current flowing through each RF coil. Has been. Details of a specific coil configuration and adjustment method for realizing such adjustment will be described below.

Example 1
Hereinafter, an array coil as a reception RF coil according to the present embodiment will be described.
As shown in FIG. 9, the reception RF coil according to the present embodiment includes two RF coils, a first RF coil 410A, a second RF coil 410B, and a conductor loop 415. The first coil surface created by the coil element of the first RF coil 410A and the second coil surface created by the coil element of the second RF coil 410B are arranged so as to cover the subject, and The cosine of the angle formed by the perpendicular line of the coil surface and the perpendicular line of the second coil surface is arranged at a positive position.

In the coordinate system 090 shown in FIG. 9, the vertical direction on the paper is the X-axis direction, the horizontal direction is the Y-axis direction, and the direction perpendicular to the paper surface is the Z-axis direction. In the example of FIG. 9, the RF coils are arranged substantially in the XZ plane, so the perpendiculars of these coil elements intersect at infinity, and the angle formed by the coils is almost 0 degrees. That is, the cosine is +1.

In addition, the coil surface in each Example including the above-mentioned embodiment handles not only the inside of a coil loop but the surface extended outside as a coil surface. Moreover, the perpendicular of the first coil surface and the perpendicular of the second coil surface are not limited to the inside of the coil loop. Think of an infinite plane so that these perpendiculars intersect, draw a perpendicular.

In general, when two identical RF coils are arranged in such an arrangement, the integral value of the inner product of the electric field produced by these coils in the subject becomes positive, and as a result, a positive noise correlation occurs.
Each

RF coil

410A, 410B is arranged with a magnetic coupling removing portion 451 partially overlapping each other so that the magnetic coupling is removed, thereby removing the magnetic coupling between the RF coils. A magnetic coupling portion 450 is provided between each of the RF coils 410A and 410B and the conductor loop.

In this embodiment, magnetic coupling is achieved by the arrangement of each RF coil and the conductor loop. The first RF coil 410A and the second RF coil 410B are each adjusted to the same frequency as the magnetic resonance frequency fo so that a magnetic resonance signal can be received in a state of being coupled to the conductor loop. The conductor loop 415 includes a capacitor 422C and a magnetic coupling adjustment circuit.

A part of the current (sub-current: first sub-current) detected by the first RF coil 410A flows through the conductor loop 415 when the signal of the first RF coil 410A is received, and the second RF coil When the 410B signal is received, a part of the current detected by the second RF coil 410B (sub-current: second sub-current) is adjusted to flow. Specifically, the resonance frequency of the conductor loop is adjusted to a lower frequency f _L from the gas resonance frequency f _0.

The intensity of the sub-current can be adjusted by the arrangement or the magnetic coupling adjustment circuit 452 inserted in the conductor loop and / or the RF coil, and the first RF coil or the current flowing through the second RF coil can be adjusted. 5% to 30% is preferable. As a result, the integral value of the inner product of the electric fields generated by the coils approaches zero, and the noise correlation can be effectively reduced.

The first RF coil 410A and the second RF coil 410B are arranged on substantially the same plane so as to cover the subject. Here, “substantially the same plane” means that the surfaces of adjacent coil elements are arranged substantially along the subject surface, and other regions (other end portions) may be bent. Note that the coil elements of the first RF coil 410A and the second RF coil 410B do not have to be overlapped as long as the magnetic coupling is sufficiently removed.

Hereinafter, the reception RF coil will be described in more detail.
As shown in FIG. 9B, in the present embodiment, the first RF coil 410A and the second RF coil 410B have surfaces formed by the loops 421 of the respective loop coil portions 420 in the direction of the static magnetic field of the magnet. They are arranged so as to be substantially parallel to the (Z-axis direction). Hereinafter, the case where the surface of the conductor loop is a surface substantially perpendicular to the static magnetic field direction of the magnet will be described as an example. In addition, the shape of the loop 421 of the loop coil unit 420 may be a polygon or a circle (including an ellipse) in addition to a rectangle as illustrated, and is arbitrary.

In the present embodiment, the two

RF coils

410A and 410B have magnetic couplings removed by an overlapping method in which magnetic couplings can be removed by overlapping part of the coil elements. Two techniques are used to adjust the magnetic coupling between the two

RF coils

410A, 410B and the conductor loop. That is, in order to adjust the magnetic coupling, one adjusts the distance between the loop coil section 420 and the conductor loop 415, and the other is provided with a magnetic coupling adjusting circuit 452 on the circuit.

Further, each RF coil of the array coil of this embodiment includes a low input impedance preamplifier 431. By using the low input impedance preamplifier 431, the signal detected by the loop coil unit 420 can be immediately amplified, so that data with less noise due to disturbance can be obtained. In addition, since both ends of the capacitor 421 have high impedance due to the parallel resonance circuit formed by the low input impedance preamplifier 431, the inductor 441, and the capacitor 421, the RF coil is difficult to be magnetically coupled when viewed from other RF coils.

The magnitude of the input impedance of the low input impedance preamplifier 431 is not limited, but is, for example, about 2Ω or less. The low input impedance preamplifier 430 is not limited to the signal amplifier 431 having a low input impedance.
The series capacitor 422, the transmission / reception magnetic coupling prevention circuit 220 (including the capacitor 423), and the parallel capacitor 424 inserted in each loop coil unit 420 have the same configuration as described in the array coil of FIG. Also in this embodiment, three capacitors (422, 424) are inserted into the loop 421 of the loop coil unit 420, but the present invention is not limited to this. It is not necessary.

In the example of FIG. 9, the coil element 420 and the input impedance preamplifier 431 are connected using an inductor, but the inductor 441 is not limited to using an inductor. Usually, the parallel capacitor 424 and the inductor 441 are connected by a conductor. Since the conductor also has an inductor component, a parallel resonant circuit is formed by the parallel capacitor 423, the inductor 441, and the inductor component of the conductor connecting them without adding an additional inductor. The inductor 441 may be a capacitor as long as the resonance frequency of the parallel resonance circuit can be adjusted by some method.

A parallel circuit of a capacitor and an inductor may be used. In the following description, for simplicity of explanation, it is assumed that there is no inductor component of the conductor connecting the parallel capacitor 424 and the inductor 441.
The conductor loop includes a magnetic coupling adjustment circuit 451. By providing the magnetic coupling adjustment circuit 451, the amount of sub-current and the phase of the sub-current can be adjusted. The magnetic coupling adjustment circuit 451 is configured by a parallel resonance circuit of a capacitor 451A and an inductor 451B. In the present embodiment, the parallel resonant circuit resonant frequency comprised capacitor 451A and inductor 451B is adjusted to a lower frequency f _L from the gas resonance frequency f _0.

Next, the operation and adjustment of each circuit element of the array coil 400 of this embodiment will be described. Here, it is assumed that the transmission RF coil 151 is always in an open state, and a description of removing the magnetic coupling between the transmission RF coil 151 and the reception RF coil 161 is omitted.
The first RF coil 410A and the second RF coil 410B of the array coil 400 of the present embodiment adjust the value of the capacitor 451A or the inductor 451B of the conductor coupling adjustment circuit 452 and the arrangement and configuration of the conductor loop. Realize the above functions.

Prior to the description of the operation of the array coil of the present embodiment, the basic principle that one RF coil 410 shown in FIG. 10A is magnetically coupled to the conductor loop 415 to induce a current in the conductor loop is described with an equivalent circuit. It explains using.

FIG. 10B is an equivalent circuit of the RF coil 410 and the conductor loop 415 shown in FIG. L ₁₁ and L ₂₁ indicate the combined reactance of the RF coil and the conductor loop. L ₁₂ represents the inductance of the inductor 441. L ₂₂ represents the inductance of the inductor 452A of the magnetic coupling adjustment circuit. C ₂₂ indicates the capacitance of the capacitor 452B of the magnetic-coupling adjusting circuit. Z ₁₁ represents an input resistance of a low input impedance preamplifier, in subsequent equivalent circuit for at 2Ω or less treated as a short circuit. Here, from the reciprocity theorem, the sensitivity of the RF coil is equivalent to the magnetic field strength generated when an RF signal of unit power is applied to the coil, and in FIG. 10B and FIG. The operation principle of this coil is shown in the operation when is applied.

FIG. 10B is an equivalent circuit when RF power having the same frequency as the magnetic resonance frequency f ₀ is applied to the RF coil 410. When RF power is applied to the RF coil 410, I ₁ shown in the following formula (3) flows through the coil element.

Here, i is the magnitude of the current, ω (= 2πf ₀ ) is the angular frequency of the applied high frequency signal, and θ is a phase difference constant. When I ₁ flows through the coil element of the RF coil 410, a magnetic field that fluctuates in the conductor loop 415 is generated. Therefore, as shown in the following formula (4), an electromotive force Ve ₂ is generated in the element (L ₂₁ ) of the conductor loop.

As described above, the parallel resonance circuit composed of L ₂₂ and C ₂₂ constituting the magnetic coupling adjustment circuit 452 is set to a frequency f _L lower than the magnetic resonance frequency f ₀ . In general, a parallel resonant circuit operates as a capacitor when a frequency higher than the resonant frequency is applied. That is, when a magnetic resonance frequency signal is applied to the parallel resonance circuit composed of L ₂₂ and Cm ₂ , the parallel resonance circuit operates as a capacitor (C). An equivalent circuit at that time is shown in FIG. Therefore, if Ve ₂ is applied, the charge q of the capacitor can be obtained from the following equation (5).

Further, the current flowing through the capacitor and the current I ₂ flowing through the conductor loop 415 are obtained from the following equation (6).

As can be seen from equation (6), in the circuit shown in FIG. 10A, I ₂ becomes a current opposite to I ₁ by the adjustment.

As described above, one RF coil 410 shown in FIG. 10A is magnetically coupled to the conductor loop 415 and forms a current in the conductor loop. In the above, the current distributions that are opposite to each other can be realized and the operation principle has been described. However, with the same configuration as described above, the resonance frequency of the series resonance circuit composed of L ₂₂ and C _{22 is set} to a frequency f _H higher than the magnetic resonance frequency. Then, it is possible to form a current distribution that flows in the same direction in the first RF coil 410 and the conductor loop 415.

The operation of the array coil of this embodiment can be explained by the above principle.
First, the operation when only the first RF coil 410A and the conductor loop 415 are present will be described.
In this embodiment, the shape of the conductor loop 415 is different from the circuit of the basic principle described in FIG. The conductor loop 415 of this embodiment is composed of a loop 416 on the first RF coil side and a loop 417 on the second RF coil side so that currents in opposite directions flow, and an 8-shaped current flows. It is configured. The distance between the first RF 410A coil and the first RF coil side loop 416 is different from the distance between the first RF 410A coil and the second RF coil side loop 417. Therefore, the strength of the magnetic field generated by the first RF 410A generated in the loop 416 on the first RF coil side and the loop 417 on the second RF coil side is different, and the current induced in each loop is different.

Generally, the amount of current induced is attenuated in proportion to the distance. In addition, since these currents are connected in a figure eight shape, the result of combining them with different signs is a sub-current that is formed in the conductor loop by magnetically coupling the first RF 410A coil and the conductor loop 415. That is, in this embodiment, because of the distance relationship, a stronger current is induced in the loop 416 on the first RF coil side closer to the first RF coil 410A than in the loop 417 on the second RF coil side. , Become the dominant current. Therefore, a current as shown in FIG.

Similarly, the case where only the second RF coil 410B and the conductor loop 415 are present can be explained by the same principle, and a current as shown in FIG.

As described above, in the array coil of this embodiment, the first RF coil 410A and the second RF coil 410B are magnetically coupled to the conductor loop 415, respectively, and currents flowing in opposite directions flow through the conductor loop.

As a result, the first RF coil 410A is magnetically coupled to the conductor loop 415 to create the conductor loop, and the second RF coil 410B is magnetically coupled to the conductor loop 415 to create the conductor loop. The integrated value of the inner product of the electric field generated in the subject by the sub-current of the current is the current flowing through the first loop coil portion 420A of the first RF coil 410A and the first loop coil portion 420A of the second RF coil 410B. Is different from the integral value of the inner product of the electric field generated in the subject by the current flowing in the object. For this reason, the absolute value of the composite value is reduced, and as a result, the noise correlation is lowered and the SNR of the composite image is improved.

Further, in the present embodiment, since the figure 8 element that is a conductor loop is arranged on the XY plane of the phantom, a magnetic field in the Z direction that does not contribute to the sensitivity produced by the first RF coil or the second RF coil is obtained. Since the 8-shaped conductor loop detects and forms an induced current, a magnetic field in the X direction is generated in the deep part of the phantom, so that the sensitivity is also improved.
By adjusting as described above, each RF coil 410 can receive a nuclear magnetic resonance signal to be detected. Moreover, since the electric field as shown in FIG. 7D is generated by forming the current path as described above, the noise correlation is lowered.

[Adjustment Example 1]
Hereinafter, the adjustment procedure of each circuit element of the present embodiment will be specifically described.
Here, an example in which the array coil 400 is adjusted to resonate at a magnetic resonance frequency of 128 MHz (f0 = 128 MHz) of a hydrogen nucleus at a static magnetic field strength of 3 T (Tesla) will be described as an example.

The shape and size of the RF coil are, for example, a rectangular loop, and the vertical and horizontal sizes of the loops 421A and 421B of the first loop coil portion 420A and the second loop coil portion 420B are 10 cm, 20 Centimeters. Further, the distance between the coil centers of the two RF coils 410 is 10 cm. The size of the conductor loop 415 is 16 cm and 8 cm in the vertical and horizontal sizes. The subject was a cylinder having a diameter of 17 cm and a height of 28 cm as shown in FIG. As for the arrangement relationship between each RF coil 410 and the conductor loop 415, the distance in the Z direction of the center point of each coil was 11 cm.

Here, when the first RF coil 410A and the second RF coil 410B are electromagnetically coupled to the conductor loop 415 as shown in FIG. 7, respectively, as shown in FIGS. 7B and 7C. Adjust the current to flow. Specifically, adjustment is performed so that 15% of the current flowing through the coil element of each RF coil flows through the conductor loop by the magnetic coupling.

First, the conductor loop adjustment 415 is performed. The conductor loop adjusts the capacitance of the series capacitor 422C. Here, only the capacitor 452A of the magnetic coupling adjustment circuit is parallel to the series capacitor 422C inserted in series with the figure 8 element so that the conductor loop is efficiently magnetically coupled to the first RF coil or the second RF coil. 422C and 452A are adjusted so that the parallel resonant circuit connected to the terminal resonates at the magnetic resonance frequency. Thereafter, in order to facilitate the adjustment of the entire circuit at the same time, an inductor 452B is connected in parallel to the capacitor 452A which is the magnetic coupling adjustment circuit 451, and the resonance frequency of the magnetic coupling adjustment circuit 451 is adjusted to be f0. As a result, the magnetic coupling adjusting circuit has a high impedance, and therefore hardly magnetically couples with other coils.

The

capacitors

422A and 422C are adjusted so that each RF coil resonates at the same frequency as the magnetic resonance frequency. At the same time, the capacitor 421 is adjusted so that the input impedance of the circuit on the RF coil side viewed from the low input impedance preamplifier becomes 50Ω. Further, the capacitor 421 is adjusted so that the parallel resonance circuit of the capacitor 421, the low input impedance preamplifier, and the inductor 441 has a high impedance.

At this time, about 10% of the coil elements of the RF coil 410 are overlapped so that the magnetic coupling between the first RF coil 410A and the second RF coil 410B is removed.
Next, the magnetic coupling of the first and second RF coils and the conductor loop is adjusted. Here, after the magnetic coupling is adjusted so that the first RF coil and the second RF coil resonate at the same frequency as the magnetic resonance frequency f0 even after the magnetic coupling, the RF coil is also adjusted again.

When adjusting the amount of magnetic coupling, first determine the direction of the current. In the array coil of the present embodiment, the first RF coil 410A and the loop 416 on the first RF coil side of the conductor loop 415 are not affected by the magnetic field created in the space between them. The relationship between the directions of the currents of the two coils was determined to be 8 characters.

That is, it was determined in the reverse direction as shown in FIG. Therefore, as described above, a reverse current is generated by lowering the resonance frequency fL of the magnetic coupling adjustment circuit 451. In this embodiment, the value of the inductor 415B is adjusted, and 102 MHz, which is 20% lower than the value of the resonance frequency of the magnetic coupling adjustment circuit due to f0, is used.

In this embodiment, the current is generated in a symmetrical manner in the X direction, so that by performing this adjustment, the second RF coil 410B and the reverse current in the loop 416 on the first RF coil side of the conductor loop 415 are also reversed. The same adjustment is performed.

Next, since the frequency of the first RF coil 410A and the second RF coil 410B is slightly deviated from f0 by the above adjustment, the

capacitors

422A and 422B are adjusted again to match the frequency f0, and the

capacitors

422A and 422B are simultaneously adjusted. The input impedance of each coil was readjusted to 50Ω. As a result, the value of the sub current was 15% of the current flowing through each RF coil.
The adjustment of the first RF coil 410A, the second RF coil 410B, and the conductor loop may be repeated several times as necessary.

By adjusting in this way, the array coil 400 of this embodiment resonates at the nuclear magnetic resonance frequency and receives the nuclear magnetic resonance signal. In addition, the first RF coil 410A is magnetically coupled to the conductor loop 415 to create a conductor loop, and the second RF coil 410B is magnetically coupled to the conductor loop 415 to create a conductor loop. The inner product of the electric field generated in the subject by the sub current has a different sign from the current flowing in the first RF coil 410A and the inner product of the electric field generated in the subject by the current flowing in the second RF coil 410B. The absolute value of the composite value becomes small, resulting in a decrease in noise correlation and an improvement in the SNR of the composite image.

Further, in this embodiment, since the figure 8 element which is a conductor loop is arranged on the XY plane of the phantom, a magnetic field in the Z direction which does not contribute to the sensitivity produced by the first RF coil or the second RF coil is represented by 8 Since the U-shaped conductor loop detects and forms an induced current, a magnetic field in the X direction is generated in the deep part of the phantom, so that the sensitivity is also improved.

[Results of Adjustment Example 1]
FIG. 11 shows a simulation result of imaging a water phantom with an MRI apparatus (3 Tesla, horizontal magnetic field method) using the array coil adjusted as described above. FIG. 11A shows an SNR profile in the Z direction at the center of the phantom after image synthesis. In FIG. 11A, the solid line is the profile of this embodiment, and the broken line is the sensitivity distribution profile when the magnetic coupling of the conventional (comparative example) RF coil is removed.

As can be seen from these results, the array coil 400 of this embodiment improves the SNR of the composite image by magnetic coupling.
In this embodiment, the resonance frequency of the magnetic coupling adjustment circuit is 20% smaller than f0. However, the present invention is not limited to this. The first RF coil 410A and the loop 416 on the first RF coil side of the conductor loop 415 do not cancel each other out of the magnetic field formed in the space of interest, and at the same time, the second RF coil 410B and the conductor loop 415 Other values may be used as long as the second RF coil side loop 417 does not cancel each other out of the magnetic field created in the space of interest.

For example, when each RF coil and the conductor loop are arranged so as to overlap each other, the direction of the voltage induced in the conductor loop and the direction of the current are reversed, so that the loop 416 on the first RF coil side of the conductor loop 415 is reversed. In order not to cancel each other out in the magnetic field generated in the space which is the region of interest, it is necessary to select a value higher than f0 for the resonance frequency of the magnetic coupling adjustment circuit. By setting the adjustment value according to the configuration and arrangement state of the coil model, the degree of freedom of adjustment is improved, and as a result, the final SNR can be improved.

In addition, by using the magnetic coupling adjustment circuit, the phase difference between the currents of each RF coil and the conductor loop can be adjusted, but basically the phase difference between the magnetic fields finally generated in the region of interest is adjusted to be equal. It is preferable to do.

Furthermore, in this embodiment, the amount of sub-current (current ratio) is adjusted to 15%, but the amount of sub-current can change the noise correlation to an arbitrary value and improve the SNR of the target image area. What is necessary is just to adjust and according to the distance between coils etc. suitably. FIG. 11B shows a graph showing the relationship between noise correlation and current. Even if the magnetic coupling increases and the current ratio increases, the noise decreases if the noise correlation decreases accordingly. Since the signal distribution also changes due to the magnetic coupling, it is preferable to set conditions that increase the SNR in the region of interest by comprehensively considering these.

In this embodiment, the figure 8 loop is arranged on the XY plane, but the present invention is not limited to this. Other arrangements may be used from the viewpoint of noise reduction. However, from the viewpoint of increasing the signal, it is preferable that the direction of the magnetic field formed by the 8-shaped loop is perpendicular to the static magnetic field in the region of interest. As a result, a signal can also be acquired efficiently, so that the SNR can be improved.

(Example 2)
In the first embodiment described above, the first coil surface created by the first loop coil portion 420A of the first RF coil 410A and the second loop coil portion 420B created by the second loop coil portion 420B of the second RF coil 410B. The case where the coil surface is arranged to cover the subject and the cosine of the angle formed by the intersection of the perpendicular of the first coil surface and the perpendicular of the second coil surface is arranged at a positive position. did.

In the present embodiment, a case will be described in which the cosine of the angle formed by the perpendicular line of the first coil surface and the perpendicular line of the second coil surface is arranged at a negative position.
In general, when two identical RF coils are arranged at such an angle, the integral value of the inner product of the electric fields produced by these coils in the subject becomes negative, resulting in a negative noise correlation. In this case, since the noise correlation can be made zero by adding the conductor loop 415 and adding a positive correlation in the subject, the circuit configuration is different. Specifically, as shown in FIG. 12, the conductor loop has a circular or rectangular loop shape instead of an 8-shaped shape.

Hereinafter, the operation of this embodiment will be described using the array coil 400 shown in FIG.
The present embodiment is basically the same as the first embodiment described above, except for the arrangement of the first RF coil 410A and the second RF coil 410B and the shape of the conductor loop, and the other configurations are the same. Yes, the basic principle and adjustment method are the same as in the first embodiment.

In the array coil 400 of the present embodiment, the inner product of the electric field generated by the electric field generated when the first RF coil 410A and the second RF coil 410B induce current in the conductor loop, respectively, will be described with reference to FIGS. explain.

FIG. 13 is a diagram for explaining the inner product distribution of the electric field created by two conventional RF coils not using a conductor loop. As shown in FIG. 13A, the first RF coil 410A and the second RF coil forming the array coil are arranged so as to cover the subject 103 and not be magnetically coupled.

In this embodiment, as the magnetic coupling removing means 451, an inductor method is used, in which an inductor is inserted into the coil loop of each RF coil shown in FIG. When a signal is received, a first current (solid line in the figure) flowing in the positive direction of the X direction flows through the first RF coil 410A through the element in the negative direction of the Z axis as shown in FIG. In 410B, as shown in the figure, a second current (broken line in the figure) flows in the X direction plus direction through the element in the Z axis minus direction.

FIG. 13B shows the concept of the distribution of the inner product of the electric field created by the two RF coils. Positive electric fields are indicated by fine dots, and negative electric fields are indicated by coarse dots. As shown in the figure, the first RF coil 410A and the second RF coil 410B show a negative inner product distribution in the subject. The reason for this negative is that the first RF coil 410A and the second RF coil 410B exist in substantially opposite directions when viewed from the subject, and currents in the same direction flow in both directions. This is because the electric field generated in the subject has the opposite sign, and the inner product of each region is almost negative.

Thus, since the negative inner product component is dominant in the conventional coil configuration, the noise correlation obtained by Equation (2) is a large negative value. Therefore, the noise of the composite value of the signals acquired with these configurations is larger than that with zero correlation, and the SNR is low.

FIG. 14 is a diagram for explaining the inner product distribution of this embodiment. As shown in FIG. 14A, the first RF coil 410A and the second RF coil forming the array coil are arranged so as to cover the object 103 and not to be magnetically coupled, and further have two conductor loops. It is magnetically coupled to the RF coil and arranged to cover the subject.

FIG. 14B shows the operation when a first current (solid line in the figure) flows through the first RF coil 410A as shown in the figure at the time of signal reception, as in the conventional configuration of FIG.
In this embodiment, since the first RF coil 410A is magnetically coupled to the conductor loop, the magnetic field generated by the first RF coil 410A is the same as the basic principle described with reference to FIG. Current flowing in the right direction is induced in the other element, and the first sub-current 418A flows.

FIG. 14C shows the operation when a second current (broken line in the figure) flows through the second RF coil 410B as shown in the figure at the time of signal reception as in the conventional configuration of FIG.
In this embodiment, since the second RF coil 410B is magnetically coupled to the conductor loop, the magnetic field generated by the second RF coil 410B is the same as the basic principle described in FIG. Current flowing in the right direction is induced in the other element, and the second sub-current 418A flows.

That is, in this embodiment, the direction of the current flowing through the conductor loop through the first RF coil and the second RF coil is the X direction plus direction, and the induced first sub current 418A and second sub current 418B. Both are positive in the X direction.

FIG. 14 (d) shows the concept of the inner product distribution of the electric field created by the two RF coils. Positive electric fields are indicated by fine dots, and negative electric fields are indicated by coarse dots. As shown in this figure, the first RF coil 410A and the second RF coil 410B form a current similar to that in the conventional configuration shown in FIG. 13, and thus show the same negative inner product distribution in the subject. On the other hand, a positive inner product distribution is shown near the conductor loop 415. The reason for being positive in this way is that the directions of currents generated by magnetic coupling are both the same as described above. Since the current direction is the same, the signs of the electric fields are also equal, and the inner product is negative.

Thus, in this embodiment, not only the negative area but also the positive area is generated as in the conventional configuration. Therefore, as shown in Expression (1) and Expression (2), since the noise correlation is obtained by volume integration of the electric field, in this embodiment, the integration value can be brought close to zero by configuring negative and positive.

Therefore, if the noise correlation is brought close to zero, the noise can be reduced and the SNR can be improved compared to the conventional configuration shown in FIG. Also in the present embodiment, the first RF coil 410A and the second RF coil 410B are respectively adjusted to be coupled to the conductor loop and resonate at the same frequency as the magnetic resonance frequency f0. Can be obtained.

In this embodiment, the conductor loop is installed at a negative position in the Y-axis direction as shown in FIG. 14, but the present invention is not limited to this. It may be a positive position in the Y-axis direction. Further, the position in the X and Z directions is not limited, but it is preferable to arrange the first RF coil 410A and the second RF coil 410B and the conductor loops at the same magnetic coupling strength.

The examples of the MRI apparatus and the receiving RF coil according to the present embodiment and each example have been described above, but the RF coil of each embodiment can be appropriately changed without being limited to the drawings and the above description. Hereinafter, typical modifications will be described.

(Modification of electromagnetic coupling means)
In the first embodiment described above, the overlap method is used for the magnetic coupling removal means of the first RF coil 410A and the second RF coil 410B. However, in this modification, a magnetic circuit using the inductor 451B is used as in the second embodiment. Use coupling means.

FIG. 15 shows a configuration of an array coil 400 according to this modification. As shown in FIG. 15, in this array coil 400, an inductor is inserted in series in the first loop coil portion 420A and the second loop coil portion 420B, and these are magnetically coupled, whereby the first RF coil 410A. And the magnetic coupling of the second RF coil 410B is removed.
By removing the magnetic coupling in this manner, the arrangement of the two array coils is less likely to be restricted, and the degree of freedom in design is increased.

Further, for example, as shown in FIG. 16, the magnetic coupling of two RF coils may be removed via a common capacitor 451C.
By removing in this way, adjustment can be easily performed by using a variable capacitor or the like. In addition, even if it is difficult to adjust with an inductor, it is possible to adjust with this embodiment.

In the second embodiment, the inductor system is used as the magnetic coupling removing means for the first RF coil 410A and the second RF coil 410B. However, the two RF coils are magnetically removed through a common capacitor 451C. You may do it.
By removing in this way, adjustment can be easily performed by using a variable capacitor or the like. In addition, even if it is difficult to adjust with an inductor, it is possible to adjust with this embodiment.

Also in the modification, each circuit element is adjusted so that each RF coil can receive a nuclear magnetic resonance signal, and combined with the conductor loop 415 to form a sub-current, and the first sub-current is generated. The integral value of the inner product of the electric field and the electric field generated by the second sub-current is different from the integral value of the inner product of the electric field generated in the subject by the first RF coil 410A and the second RF coil 410B. adjust.

It is the same as in the above-described embodiments that adjustment is performed so that a part of the current detected by one RF coil flows as a sub-current to the other RF coil and the direction of the sub-current is opposite to that of the one RF coil. is there.

(Application example of vertical magnetic field)
In the first embodiment and the second embodiment described above, the example applied to the horizontal magnetic field is shown, but the present invention may be applied to the vertical magnetic field. By applying to the vertical magnetic field, the noise correlation can be reduced even in the array coil of the vertical magnetic field, and the SNR is improved.

Hereinafter, an example in which the subject penetrates the surfaces of the two loop coils shown in FIG. 17 will be described.
In this example as well, the operation is basically based on the same principle as in the first and second embodiments, and the adjustment is performed in the same manner. In this example, the cosine of the angle formed by the perpendicular line of the first coil surface and the perpendicular line of the second coil surface is arranged at a negative position as in the second embodiment. Therefore, generally, when two identical RF coils are arranged at such an angle, the integral value of the inner product of the electric field generated by these coils in the subject becomes negative, resulting in a negative noise correlation. In this case, the noise correlation can be reduced to 0 by adding a conductor loop 415 and adding a positive correlation in the subject, so the circuit configuration is the same as that of the second embodiment.

Hereinafter, in the array coil 400 of this modification, the inner product of the electric field generated by the electric field generated when the first RF coil 410A and the second RF coil 410B induce currents in the conductor loops will be described with reference to FIGS. Will be used to explain that noise is reduced.

FIG. 17 is a diagram for explaining the inner product distribution of the electric field formed by two RF coils without using a conventional conductor loop.
As shown in FIG. 17A, the first RF coil 410A and the second RF coil forming the array coil are arranged so as to cover the object 103 and not be magnetically coupled. In this example, as the magnetic coupling removing means 451, an inductor method shown in FIG. 12 in which an inductor is inserted into the coil loop of each RF coil to remove the magnetic coupling is used.

When a signal is received, a first current (solid line in the figure) flows in the first RF coil 410A through the element in the Z-axis minus direction as shown in FIG. 17A, and the second RF coil. In 410B, as shown in the figure, a second current (broken line in the figure) flows in the X direction plus direction through the element in the Z axis minus direction. FIG. 17B shows the concept of the inner product distribution of the electric field created by the two RF coils.

As shown in the figure, the first RF coil 410A and the second RF coil 410B show a negative inner product distribution in the subject. The reason for this negative is that the first RF coil 410A and the second RF coil 410B exist in substantially opposite directions when viewed from the subject, and currents in the same direction flow in both directions. This is because the electric field generated in the subject has the opposite sign, and the inner product of each region is almost negative. As described above, in the conventional coil configuration, the negative inner product component is dominant, so that the noise correlation obtained by Expression 2 is a large negative value. Therefore, the noise of the synthesized value of the signals acquired with these configurations is larger than that with zero correlation, and the SNR is low.

FIG. 18 is a diagram for explaining the inner product distribution of this embodiment. Positive electric fields are indicated by fine dots, and negative electric fields are indicated by coarse dots. As shown in FIG. 18A, the first RF coil 410A and the second RF coil that form the array coil are arranged so as to cover the object 103 and not to be magnetically coupled, and further have two conductor loops. It is magnetically coupled to the RF coil and arranged in the X-axis direction so as to cover the subject.

FIG. 18B shows the operation when a first current (solid line in the figure) flows through the first RF coil 410A as shown in the figure at the time of signal reception, as in the conventional configuration of FIG. In the present embodiment, since the first RF coil 410A is magnetically coupled to the conductor loop, the magnetic field generated by the first RF coil 410A is the same as the basic principle described in FIG. Current flowing in the left direction is induced in the other element, and the first sub-current 418A flows.

FIG. 18 (c) shows the operation when a second current (broken line in the figure) flows through the second RF coil 410B as shown in the figure at the time of signal reception, as in the conventional configuration of FIG. In this example, since the second RF coil 410B is magnetically coupled to the conductor loop, the magnetic field generated by the second RF coil 410B is the same as the basic principle described in FIG. A second sub-current 418A flows by inducing a current flowing in the left direction in the element.

That is, in this example, the direction of the current flowing through the conductor from the Z direction minus of the conductor loop of the first RF coil and the second RF coil is the minus direction of the X direction, and the induced first sub-current 418A and the first The second sub-current 418B is also negative in the Y direction.

FIG. 18 (d) shows the concept of the inner product distribution of the electric field created by the two RF coils. Positive electric fields are indicated by fine dots, and negative electric fields are indicated by coarse dots. As shown in this figure, the first RF coil 410A and the second RF coil 410B form a current similar to that in the conventional configuration shown in FIG. 13, and thus show the same negative inner product distribution in the subject. On the other hand, a positive inner product distribution is shown near the conductor loop 415. The reason for being positive in this way is that the directions of currents generated by magnetic coupling are both the same as described above. Since the current direction is the same, the signs of the electric fields are also equal, and the inner product is negative.

Thus, in this example, not only the negative area but the positive area is generated as in the conventional configuration. Therefore, as shown in the equations (1) and (2), the noise correlation can be obtained by volume integration of the electric field. Therefore, in this embodiment, it is possible to cancel each other by constructing negative and positive, and to bring the integral value close to zero. it can. Therefore, if the noise correlation is brought close to zero, the noise can be reduced and the SNR can be improved compared to the conventional configuration shown in FIG.
Also in this example, the first RF coil 410A and the second RF coil 410B are adjusted so as to be coupled to the conductor loop and resonate at the same frequency as the magnetic resonance frequency f0, and thus acquire a magnetic resonance signal. be able to.

As described above, even a vertical magnetic field operates in the same manner as a horizontal magnetic field. Also, the conductor loop 415 is configured in the same manner as the horizontal magnetic field. When the cosine of the angle formed by the perpendicular of the first coil surface and the perpendicular of the second coil surface is positive, the 8-shaped coil is used. If a negative or negative coil is used, noise is reduced. Further, the signal acquisition efficiency is improved by arranging the magnetic field to be perpendicular to the direction of the static magnetic field of the magnet so that the magnetic field generated by the current flowing through the conductor loop contributes to the MRI sensitivity.

In this example, the loop coils 420 of the first RF coil 410A and the second RF coil 410B are arranged so as to wrap around the subject, but the present invention is not limited to this. The loop coil unit 420 may be wound twice. Thereby, a sensitivity area can be expanded.

Further, in the first and second embodiments and their modifications, the case where there is one conductor loop 415 is shown, but two or more conductor loops may be provided. By using a plurality, it is possible to increase the adjustment range of the noise correlation and improve the SNR.

Further, in the first and second embodiments and the modified examples, two RF coils are shown, but three or more RF coils may be provided. Thereby, an array coil of three or more RF coils can be produced. The example shown in FIG. 19 is a case where three RF coils are provided. Only the combination of the coils (first RF coil 410A and second RF coil 410B) whose noise correlation is to be reduced is magnetically coupled to the conductor loop, and the other coils are arranged so that the magnetic coupling to the conductor loop is broken. Thus, the noise correlation can be lowered and the SNR can be improved only where necessary.

In the first and second embodiments and these modifications, the magnetic coupling between the RF coil and the conductor loop is magnetically coupled depending on the arrangement, but the magnetic coupling method is not limited to this. Magnetic coupling using an inductor may be performed. The example shown in FIG. 20 includes an inductor for magnetically coupling the RF coil and the conductor loop to the coil element and the conductor loop of each RF coil. By adjusting these magnetic couplings, the magnetic coupling between the RF coil and the conductor loop can be adjusted. As a result, the adjustment range of the magnetic coupling between the RF coil and the conductor loop is widened, so that the amount of magnetic coupling can be adjusted and noise can be reduced without depending on the arrangement.

090 ... coordinate system, 100 ... MRI apparatus, 101 ... MRI apparatus, 102 ... table, 103 ... inspection object, 110 ... magnet, 111 ... magnet, 121 ... Shim coil 122 ... Shim power source 131 ... Gradient magnetic field coil 132 ... Gradient magnetic field power source 140 ... Sequencer 151 ... Transmission RF coil 152 ... RF magnetic field generator 161 ..Receiving RF coil, 162... Receiver, 170... Computer, 171... Display device, 180... Magnetic coupling prevention circuit driving device, 210. ..PIN diode, 212 ... control signal line, 220 ... transmission / reception magnetic coupling prevention circuit, 221 ... PIN diode, 221 ... cross diode, 222 ... inductor 223: control signal line, 300: birdcage type RF coil, 301 ... straight conductor, 302 ... end conductor, 303 ... capacitor, 311 ... input port, 312 ... Input port 400 ... array coil 410 ... RF coil 415 ... conductor loop 420 ... loop coil section 421 ... loop 422 ... series capacitor 424 ... Parallel capacitor 431 ... Low input impedance preamplifier, 450 ... Magnetic coupling unit, 452 ... Magnetic coupling adjustment circuit

Claims

A first RF coil having a first loop coil portion made of a conductor and capable of receiving a magnetic resonance signal from a subject;
A second loop coil portion made of a conductor, capable of receiving a magnetic resonance signal from a subject, and a second RF coil from which magnetic coupling is removed from the first RF coil;
An electric field generating circuit having a loop portion in which ends of at least one conductor are connected to each other and magnetically coupled to the first RF coil and the second RF coil;
The electric field generation circuit includes:
Generated in the subject by a first sub-current induced in the electric field generation circuit by the first RF coil and a second sub-current induced in the electric field generation circuit by the second RF coil, respectively. The volume integral value of the inner product of the electric field to be generated is the electric field generated in the subject by the first current flowing through the first loop coil section and the second current flowing through the RF coil of the second loop coil section. An array coil that is arranged and adjusted so as to have a sign and value that cancel the volume integral value of the inner product.
The first loop coil unit and the second loop coil unit are arranged and configured so as not to be magnetically coupled to each other by magnetic coupling preventing means, and the loop unit of the electric field generating circuit is arranged to cover the subject. And
The electric field generating circuit when a cosine value of an angle formed between a first axis substantially perpendicular to the first loop coil surface and a second axis substantially perpendicular to the second loop coil surface is negative. The array coil according to claim 1, having a substantially circular loop portion.
The first loop coil unit and the second loop coil unit are arranged and configured so as not to be magnetically coupled to each other by magnetic coupling preventing means, and the loop unit of the electric field generating circuit is arranged to cover the subject. And
The electric field generating circuit when a cosine value of an angle formed by a first axis substantially perpendicular to the first loop coil surface and a second axis substantially perpendicular to the second loop coil surface is positive. The array coil according to claim 1, wherein the array coil has an eight-shaped loop portion.
The array coil according to claim 1, wherein the electric field generating circuit is a series resonant circuit having a capacitor inserted in series in a loop portion.
The array coil according to claim 1, wherein the loop section includes at least one magnetic coupling adjustment circuit.
The array coil according to claim 5, wherein the magnetic coupling adjustment circuit is a parallel resonance circuit in which a capacitor and an inductor are connected in parallel, and is inserted in series in the loop portion.
Having a first inductor inserted into the first loop coil section;
The magnetic coupling adjustment circuit is a second inductor inserted in series in the loop portion;
The array coil according to claim 5, wherein the magnetic coupling is adjusted by magnetically coupling the first inductor and the second inductor.
The array coil according to claim 1, wherein the surface of the first loop coil portion and the surface of the second loop coil portion are arranged to cover the subject.
The array coil according to claim 1, wherein the surface of the first loop coil portion and the surface of the second loop coil portion are arranged so as to be penetrated by a subject.
By arranging a part of the first coil loop part and a part of the second coil loop part so as to overlap each other, magnetic coupling between the first RF coil and the second RF coil is achieved. The array coil according to claim 1, wherein an overlapping method for removing is applied.
The first RF coil has a third inductor;
The second RF coil has a fourth inductor;
2. The array coil according to claim 1, wherein an inductor system is applied in which magnetic coupling between the first RF coil and the second RF coil is eliminated by magnetically coupling the third inductor and the fourth inductor. .
Capacitor system that removes the magnetic coupling between the first RF coil and the second RF coil by connecting the first RF coil and the second RF coil in series to the same capacitor. The array coil according to claim 1.
When the first RF coil or the second RF coil is coupled to the electric field generating circuit, the phase of the RF magnetic field generated in the region of interest of the subject by the current flowing through the loop portion, and the first RF coil or the The array coil according to claim 1, wherein the difference between the phase of the RF magnetic field generated by the second RF coil in the region of interest is from -30 degrees to +30 degrees.
The array coil according to claim 1, further comprising a third RF coil that is not magnetically coupled to the electric field generating circuit.
A static magnetic field forming unit for forming a static magnetic field;
A gradient magnetic field forming section for forming a gradient magnetic field;
A transmission RF coil for irradiating an RF magnetic field to the inspection object arranged in the static magnetic field;
A receiving RF coil for detecting a nuclear magnetic resonance signal from the inspection object;
A signal processing unit for processing a nuclear magnetic resonance signal detected by the reception RF coil,
The magnetic resonance imaging apparatus according to claim 1, wherein the reception RF coil is an array coil according to claim 1.