JP5886053B2 - RF coil unit and magnetic resonance imaging apparatus - Google Patents

Description

  Embodiments described herein relate generally to an RF coil unit and a magnetic resonance imaging apparatus including the RF coil unit.

  In a magnetic resonance imaging apparatus using a magnetic resonance phenomenon, a gradient magnetic field is applied to a subject placed in a static magnetic field and an RF pulse is applied from a transmission RF coil. A magnetic resonance signal (MR signal) generated from the subject in response to the RF pulse is received by the receiving RF coil, and various processing such as inverse Fourier transform processing is performed on the received MR signal, and the MR inside the subject. I have an image.

  A signal cable such as a coaxial cable is connected to the reception RF coil, and an MR signal received via the signal cable is transmitted to a reception processing unit in the subsequent stage.

  Since the reception RF coil and the signal cable are usually arranged inside the transmission RF coil, an unbalanced current is induced in the reception RF coil and the signal cable with the application of the RF pulse, which adversely affects the received MR signal. .

  In order to suppress this unbalanced current, a device called a balun is often provided in the signal cable (see, for example, Patent Document 1).

JP 2011-56247 A

  The balun suppresses the unbalanced current by finally converting the unbalanced current into heat. Therefore, although the unbalanced current generated by the application of the RF signal is suppressed by the balun, the balun itself generates heat and the balun temperature rises. Since the signal cable provided with the balun runs close to the patient lying on the inside of the bore of the magnetic resonance imaging apparatus, if the temperature rise of the balun increases, the patient may be burned. Further, if it is attempted to keep the temperature increase of the balun within the reference value, the function and performance of the apparatus must be suppressed to some extent, such as by reducing the duty ratio of the RF pulse.

  Therefore, there is a demand for a unit or apparatus having a balun that can reliably reduce the unbalanced current while ensuring safety to the patient and without suppressing the function and performance of the apparatus.

The RF coil unit of the embodiment is connected to the receiving coil that receives the magnetic resonance signal in the RF coil unit that receives the magnetic resonance signal in the magnetic resonance imaging apparatus that reconstructs the image of the subject, and the receiving coil. A cable for transmitting the received magnetic resonance signal; and a balun provided at a predetermined position in the longitudinal direction of the cable so as to surround the outer periphery of the cable and having a resonance circuit using a capacitor and an inductor. Is between the outer periphery of the cable and the outer periphery of the balun , and is provided at a position closer to the outer periphery of the cable than the outer periphery of the balun.

1 is a diagram illustrating a configuration example of a magnetic resonance imaging apparatus according to an embodiment. The figure which shows the structural example of the RF coil unit of this embodiment. The figure which shows an example of the condition which mounted | worn with the RF coil unit around the subject lying on a bed. Sectional drawing which shows an example of the internal structure of a cable. The figure which shows the structural example of the balun of this embodiment. The figure which shows the structure of the conventional balun as a comparative example of the balun of this embodiment. The figure which shows an example of RF coil unit which has a 1st insulation means. The figure which shows an example of RF coil unit which has a 2nd insulation means.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

(1) Magnetic Resonance Imaging Device FIG. 1 is a block diagram showing the overall configuration of a magnetic resonance imaging device 1 in the present embodiment. As shown in FIG. 1, the magnetic resonance imaging apparatus 1 includes a cylindrical static magnetic field magnet 22 that forms a static magnetic field, a cylindrical shim coil 24 provided with the same axis inside the static magnetic field magnet 22, A gradient magnetic field coil 26, a transmission or reception RF coil 28, a control system 30, a bed 32 on which a subject (patient) P is placed, and the like are provided. Furthermore, the control system 30 includes a static magnetic field power source 40, a shim coil power source 42, a gradient magnetic field power source 44, an RF transmitter 46, an RF receiver 48, a bed driving device 50, a sequence controller 56, a computer 58, and the like. The computer 58 includes an arithmetic device 60, an input device 62, a display device 64, a storage device 66, and the like as its internal configuration.

  The static magnetic field magnet 22 is connected to a static magnetic field power supply 40 and forms a static magnetic field in the imaging space by a current supplied from the static magnetic field power supply 40. The shim coil 24 is connected to a shim coil power source 42 and equalizes the static magnetic field by the current supplied from the shim coil power source 42. The static magnetic field magnet 22 is often composed of a superconducting coil, and is connected to the static magnetic field power source 40 and supplied with current when excited, but after being excited, it is disconnected. Is common. The static magnetic field magnet 22 may be formed of a permanent magnet without providing the static magnetic field power supply 40.

  The gradient magnetic field power supply 44 includes an X-axis gradient magnetic field power supply 44x, a Y-axis gradient magnetic field power supply 44y, and a Z-axis gradient magnetic field power supply 44z. In FIG. 1, the axial direction of the static magnetic field magnet 22 and the shim coil 24 is the Z-axis direction, the vertical direction is the Y-axis direction, and the direction orthogonal thereto is the X-axis direction.

  The gradient magnetic field coil 26 has an X-axis gradient magnetic field coil 26 x, a Y-axis gradient magnetic field coil 26 y, and a Z-axis gradient magnetic field coil 26 z and is formed in a cylindrical shape inside the static magnetic field magnet 22. The X-axis gradient magnetic field coil 26x, the Y-axis gradient magnetic field coil 26y, and the Z-axis gradient magnetic field coil 26z are connected to an X-axis gradient magnetic field power source 44x, a Y-axis gradient magnetic field power source 44y, and a Z-axis gradient magnetic field power source 44z, respectively.

  Gradient magnetic fields Gx, Gy, and Gz in the X-axis, Y-axis, and Z-axis directions are formed in the imaging space by currents respectively supplied to the gradient magnetic field coils 26x, 26y, and 26z from the gradient magnetic field power supplies 44x, 44y, and 44z. Is done.

  By synthesizing the gradient magnetic fields Gx, Gy, and Gz in the three axis directions of the apparatus coordinate system, the slice direction gradient magnetic field Gss, the phase encode direction gradient magnetic field Gpe, and the readout direction (frequency encode direction) gradient magnetic field Gro as the logical axes Each direction can be set arbitrarily. Each gradient magnetic field in the slice direction, the phase encoding direction, and the readout direction is superimposed on the static magnetic field.

  The RF transmitter 46 generates an RF pulse having a Larmor frequency for causing nuclear magnetic resonance based on the control information input from the sequence controller 56, and transmits this to the RF coil 28 for transmission. The RF coil 28 is provided in the vicinity of a whole body coil (WBC) for transmitting and receiving an RF pulse and receiving a magnetic resonance signal (MR signal) from the subject, the bed 32 or the subject P. There is a coil dedicated to reception (also called a local coil).

  The MR signal received by the RF coil 28 is supplied to the RF receiver 48 via a signal cable.

  The RF receiver 48 performs various signal processing such as pre-amplification, intermediate frequency conversion, phase detection, low-frequency amplification, and filtering on the received MR signal, and then performs A / D (analog to digital) conversion. To generate raw data (raw data) that is digitized complex data. The RF receiver 48 inputs the generated raw data of the MR signal to the sequence controller 56.

  The arithmetic device 60 includes a processor and the like, and performs system control of the entire magnetic resonance imaging apparatus 1 as well as image reconstruction processing and various image processing.

  The sequence controller 56 generates gradient magnetic fields Gx, Gy, Gz, and RF pulses in accordance with predetermined imaging conditions and pulse sequences stored in the storage device 66 of the computer 58 and instructions from the arithmetic device 60 based on these. Further, the sequence controller 56 inputs MR signals received in response to these R gradient magnetic fields Gx, Gy, Gz and RF pulses from the RF receiver 48 as raw data, and inputs them to the arithmetic device 60. Output.

  The arithmetic device 60 performs reconstruction processing including inverse Fourier transform and various image processing on the input raw data to generate image data of the subject. The generated image data is displayed on the display device 64. The input device 62 is used for inputting imaging conditions and various information by a user operation.

(2) RF Coil Unit FIG. 2 is a diagram illustrating an example of the configuration of the RF coil unit 100 according to the present embodiment. FIG. 3 is a diagram illustrating an example of a situation where the RF coil unit 100 is mounted around the subject P lying on the bed 32.

  As shown in FIG. 2, the RF coil unit 100 includes at least a receiving coil 28R that receives MR signals, a cable 200 that transmits MR magnetic resonance signals received by the receiving coil 28R, and a balun 300.

  The reception coil 28R is a reception-dedicated coil (also referred to as a local coil) described above, and is configured by, for example, a loop coil. The RF coil unit 100 illustrated in FIG. 2 has two receiving coils 28R, but the number of receiving coils 28R is not limited to two, and may be one or three or more. Also good. The receiving coil 28R is housed in an appropriate cover 281.

  The MR signal received by the receiving coil 28R is usually transmitted to the subsequent RF receiver 48 via a coaxial cable. As illustrated in FIG. 2, when the number of receiving coils 28R is two, MR signals are transmitted by two coaxial cables. In this case, the cable 200 may be a bundle of two coaxial cables. In addition to a plurality of coaxial cables, single wires and pair wires are combined, and the whole is covered with a conductive shield (external conductor). One cable (composite cable) 200 may be used.

  FIG. 4 is a cable cross-sectional view showing an example of the internal configuration of the cable 200 as a composite cable. A cable 200 illustrated in FIG. 4 includes two coaxial cables 201 and four single wires 205 concentrated in one place, bundled into one, and the outer periphery thereof covered with a conductive shield 206. It is said. Each coaxial cable 201 has a center conductor 203 and an outer conductor 202.

  One end of the coaxial cable 201 is connected to the receiving coil 28 </ b> R, and the other end is connected to the connector 210 of the cable 200.

  As shown in FIG. 3, the connector 210 is fitted with a connector 210 a provided on the bed 32. That is, the RF coil unit 100 is configured to be detachable from the bed 32 by the connectors 210 and 210a.

  Furthermore, the RF coil unit 100 according to the present embodiment is configured to provide the balun 300 at a predetermined position in the longitudinal direction of the cable 200. 2 and FIG. 3, the configuration has three baluns 300. However, the number of baluns 300 is not limited to this, and may be one, two, or four or more. .

(3) Balun The balun 300 is a device for suppressing the unbalanced current of the cable 200, and is constituted by a resonance circuit having a capacitor and an inductor. The balun 300 of the present embodiment is provided so as to surround the outer periphery of the cable 200 at a predetermined position of the cable 200.

  The balun 300 serves to suppress the unbalanced current of the coaxial cable 202 in the cable 200 or the unbalanced current flowing through the shield 206 of the cable 200 and reduce the adverse effect of the unbalanced current on the received MR signal. Specifically, the unbalanced current flowing through the cable 200 is absorbed by the resonance circuit of the balun 300, and transmission of the unbalanced current is blocked.

  On the other hand, when an RF pulse is radiated from the transmitting RF coil 28, the RF pulse is also applied to the receiving coil 28R and the cable 200, and an unbalanced current is also generated in the cable 200 in this case. The unbalanced current accompanying the application of the RF pulse shows a larger value than the unbalanced current when receiving the MR signal, but this large unbalanced current is also absorbed by the resonant circuit of the balun 300. At this time, the unbalanced current is absorbed by the unbalanced current being converted into heat mainly by the capacitor in the resonance circuit of the balun 300.

  As described above, since the unbalanced current accompanying the application of the RF pulse is converted into heat by the capacitor, the temperature of the capacitor of the balun 300 rises when the RF pulse is applied. As shown in FIG. 3, since the cable 200 passes near the subject P lying on the bed 32, the temperature of the capacitor of the balun 300 becomes high when the capacitor is located outside or near the outside of the balun 300. The capacitor is easy to touch the subject P, and the subject P is uncomfortable, and in some cases, the subject P may be burned.

  Therefore, in the balun 300 of the RF coil unit 100 according to the present embodiment, the capacitor is provided between the cable 200 and the balun 300 and at a position away from the outer periphery of the balun. Hereinafter, a specific structural example of the balun 300 will be described.

  FIG. 5A is a schematic perspective view of the balun 300 according to the present embodiment, and FIG. 5B is a longitudinal sectional view of the balun 300. 5A and 5B show the cable 200 in addition to the balun 300. FIG.

  The balun 300 includes a capacitor 301 and an inductor 302, and the capacitor 301 and the inductor 302 constitute a resonance circuit that resonates at a high frequency of an RF pulse.

  As shown in FIGS. 5A and 5B, the inductor 302 has a double structure having a first cylindrical conductor 310 on the outer peripheral side and a second cylindrical conductor 312 on the inner peripheral side. It is a cylindrical shaped conductive member. A predetermined gap is formed between the first cylindrical conductor 310 and the second cylindrical conductor 312, while the first and second cylinders are provided at both ends in the axial direction. End conductors 313 that electrically connect the respective conductors 310 and 312 are provided. As described above, the inductor 302 has a cylindrical box shape having an internal space surrounded by the first and second cylindrical conductors 310 and 312 and the end conductor 313. The internal space of the inductor 302 may be hollow or may be filled with an appropriate dielectric. The material forming the first and second cylindrical conductors 310 and 312 and the end conductor 313 is preferably a highly conductive metal, such as copper. The inductance of the inductor 302 is determined by the physical dimensions of the first and second cylindrical conductors 310 and 312 and the end conductor 313, the dielectric constant of the dielectric (or air) filled in the internal space, and the like.

  On the other hand, the second cylindrical conductor 312 on the inner peripheral side of the inductor 302 is formed with a slit (gap) 314 on the entire circumference in a part of the axial direction (in the example of FIG. 5, approximately the center in the axial direction). Has been. Then, both electrodes of the capacitor 301 are connected so as to span the slit 314. The capacitor 301 is, for example, a normal discrete capacitor element. The number of capacitors 301 and the capacity of each capacitor 301 are not particularly limited, and may be appropriately determined from the resonance frequency and the inductance of the inductor 302.

  Conversely, the total capacity of the capacitor 301 may be set in advance, the inductance of the inductor 302 may be obtained from the resonance frequency and the total capacity, and the physical dimensions of the inductor 302 may be determined from the obtained inductance.

  The cable 200 is inserted into the axial center portion of the cylindrical box-shaped inductor 302, that is, inside the second cylindrical conductor 312 on the inner peripheral side. The shield (outer conductor) 206 of the cable 200 and the second cylindrical conductor 312 on the inner peripheral side may be electrically connected or insulated.

  When an unbalanced current flows through the shield 206 of the cable 200 or the coaxial cable 201 inside the cable 200, an induced current is excited in the balun 300 so as to cancel the unbalanced current. It flows through a resonance circuit composed of the inductor 302. Due to this induced current, the capacitor 301 generates heat, and the temperature of the capacitor 301 rises.

  However, in the balun 300 of this embodiment, as described above, the capacitor 301 is provided only in the slit 314 of the second cylindrical conductor 312 on the inner peripheral side, and is located away from the outer periphery of the balun 300. . The first cylindrical conductor 310 on the outer peripheral side of the balun 300 does not have the capacitor 301 that is a heating element.

  Therefore, the heat of the capacitor 301 is sufficiently blocked by the internal space between the first and second cylindrical conductors 310 and 312 and the first cylindrical conductor 310 on the outer peripheral side. Further, by setting the position of the slit 314 to be approximately the center of the second cylindrical conductor 312, the propagation of heat to both ends of the balun 300 is also reduced. As a result, even if the subject P lying on the bed 32 touches the balun 300, the subject P is not discomforted by heat, and there is no risk of causing the subject P to be burned.

  6A and 6B are diagrams showing a configuration example of a conventional balun 500 for comparison with the balun 300 of the present embodiment. The conventional balun 500 is the same in basic configuration that the cable 200 is inserted into the center of the cylindrical box-type inductor 502, but the mounting position of the capacitor 501 is greatly different from that of the balun 300 of this embodiment.

  In the conventional balun 500, slits 514 are formed in the cylindrical conductor 510 on the outer peripheral side instead of the cylindrical conductor 512 on the inner peripheral side, and the capacitor 501 is spanned on the slit 514 in the cylindrical conductor 510 on the outer peripheral side. Connected to be passed. That is, the conventional balun 500 is configured such that the capacitor 501 is mounted on the outer periphery thereof. For this reason, when the capacitor 501 generates heat due to the application of the RF pulse, the outer periphery of the balun 500 becomes high temperature, giving the subject P an uncomfortable feeling due to heat, and possibly causing a burn or the like in some cases. Even if a countermeasure such as covering the outer periphery of the balun 500 with a case of resin or the like is performed in order to prevent the high-temperature capacitor 501 from coming into direct contact with the subject P, the effect is not always sufficient.

  Further, as a technique for keeping the temperature increase of the balun 500 within the reference value, a countermeasure such as reducing the duty ratio of the RF pulse is conceivable. However, this technique sacrifices the function and performance of the apparatus to some extent.

  Furthermore, as schematically shown in FIG. 6B, not only the heat T but also an unnecessary electromagnetic wave R is radiated from the capacitor 501, and this unnecessary radiation is radiated to the receiving coil 28R. This will also adversely affect the MR signal.

  In contrast, in the balun 300 according to the present embodiment, as shown in FIGS. 5A and 5B, the capacitor 301 is disposed on the innermost side of the balun 300, and the outer side of the capacitor 301 is the first The dielectric (or air) in the internal space between the second cylindrical conductors 310 and 312 is completely covered. In addition, the first cylindrical conductor 310 on the outer peripheral side covers the outer side of the inner space. For this reason, the heat of the capacitor 301 is sufficiently blocked by the dielectric or air in the internal space and the first cylindrical conductor 310 on the outer peripheral side, giving the subject P a risk of discomfort or burns due to heat. There is no fear.

  Further, since the heat of the capacitor 301 is sufficiently cut off from the outside, even if the temperature of the capacitor 301 rises, the duty ratio and power of the RF pulse can be increased without being so much restricted by the raised temperature. It becomes possible. As a result, the SNR of the MR signal can be increased, and a more flexible pulse sequence can be set.

  Moreover, since the 1st cylindrical conductor 310 has covered the outer side of the capacitor | condenser 301, the balun 300 of embodiment prevents the unnecessary electromagnetic wave radiated | emitted from the capacitor | condenser 301 from leaking into the receiving coil 28R. You can also.

(3) Modified Example of RF Coil Unit The RF coil unit 100 of the embodiment shown in FIG. 5 is premised on that the capacitor 301 of the balun 300 is mounted at a position where it is not short-circuited by the shield 206 of the cable 200. However, since the capacitor 301 is mounted inside the balun 300, it may be considered that a short circuit is caused by the shield 206 depending on the mounting position. Since the resonance circuit is not established when the capacitor 301 is short-circuited, it is preferable to provide an insulating means for insulating the capacitor 301 and the shield 206.

  FIGS. 7A and 7B are diagrams showing an embodiment of the RF coil unit 100 having the first insulating means. The first insulating means is a notch 250 formed in the shield 206 of the cable 200 at a position facing the capacitor 301. The notch 250 is notched in a slit shape, for example, along the outer periphery of the shield 206 at a position where the capacitor 301 faces. The width of the notch 250 of the shield 206 is made slightly larger than the width of the slit 314 formed in the second cylindrical conductor 312 of the inductor 302. By doing so, it is possible to reliably avoid contact between the shield 206 and the capacitor 301 that spans the slit 314.

  FIG. 8 is a diagram showing an embodiment of the RF coil unit 100 having the second insulating means. The second insulating means is an insulating sheet 260 provided between the shield 206 of the cable 200 and the capacitor 314. The insulating sheet 260 is wound, for example, along the outer periphery of the shield 206 at a position where the capacitor 301 faces. The insulating sheet 260 may be a thin and flexible member having electrical insulation.

  In the RF coil unit 100 shown in FIGS. 7B and 8, the balun 300 itself is the same as the balun 300 shown in FIG.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

DESCRIPTION OF SYMBOLS 1 Magnetic resonance imaging device 28R Reception coil 100 RF coil unit 200 Cable 250 1st insulation means 260 2nd insulation means 300 Balun 301 Capacitor 302 Inductor 310 1st cylindrical conductor 312 2nd cylindrical conductor

Claims (9)

In an RF coil unit that receives a magnetic resonance signal in a magnetic resonance imaging apparatus that reconstructs an image of a subject,
A receiving coil for receiving the magnetic resonance signal;
A cable connected to the receiving coil and transmitting the received magnetic resonance signal;
A balun having a resonance circuit using a capacitor and an inductor, provided to surround an outer periphery of the cable at a predetermined position in a longitudinal direction of the cable;
With
The capacitor is provided between the outer periphery of the cable and the outer periphery of the balun , and at a position closer to the outer periphery of the cable than the outer periphery of the balun.
An RF coil unit characterized by that. The inductor is a double-structured cylindrical inductor formed of an inner circumferential cylindrical conductor and an outer circumferential cylindrical conductor,
The capacitor is provided in a part of the inner cylindrical conductor.
The RF coil unit according to claim 1. The inductor is
A cylindrical first conductor into which the cable is inserted;
A cylindrical second conductor provided outside the first conductor with a predetermined gap from the outer periphery of the first conductor;
Comprising
The first conductor has a slit formed in a part of the entire circumference in the axial direction,
The capacitor is connected so as to span the slit,
The RF coil unit according to claim 1. The cable has a shield on its outer periphery,
An insulating means for insulating the capacitor and the shield;
The RF coil unit according to claim 1. The insulating means is a notch formed at a position of the shield facing the capacitor;
The RF coil unit according to claim 4. The insulating means is an insulating sheet provided between the shield and the capacitor.
The RF coil unit according to claim 4. The cable has a shield on its outer periphery,
The inductor is
A cylindrical first conductor into which the cable is inserted;
A cylindrical second conductor provided outside the first conductor with a predetermined gap from the outer periphery of the first conductor;
Comprising
The first conductor has a slit formed in a part of the entire circumference in the axial direction,
The capacitor is connected to span the slit,
An insulating notch is formed at a position of the shield facing the capacitor.
The RF coil unit according to claim 1. The cable has a shield on its outer periphery,
The inductor is
A cylindrical first conductor into which the cable is inserted;
A cylindrical second conductor provided outside the first conductor with a predetermined gap from the outer periphery of the first conductor;
Comprising
The first conductor has a slit formed in a part of the entire circumference in the axial direction,
The capacitor is connected to span the slit,
An insulating sheet is provided between the shield member and the capacitor.
The RF coil unit according to claim 1. A magnetic resonance imaging apparatus comprising the RF coil unit according to any one of claims 1 to 8.