JP2019041795A - High-frequency coil unit and magnetic resonance imaging device - Google Patents

Abstract

To expand an imaging space by expanding a region where an RF magnetic field is uniformly irradiated, and acquire an MRI image of a wide range and high image quality without increasing the number of components or device cost.SOLUTION: A high-frequency coil unit includes a cylindrical RF coil 210 applied to an MRI device for executing at least one of transmission of a high-frequency signal to a static magnetic field space, and reception of a nuclear magnetic-resonance signal generated from a subject placed in a static magnetic field, and a shield composed of a sheet-like conductor, which is cylindrically shaped so as to surround the RF coil. The RF coil includes two ring conductors 203 formed annularly and disposed at both ends of the cylindrical shape, a plurality of rung conductors 214 disposed in parallel with a central axis of the cylindrical shape along the cylindrical outer peripheral surface, which have N (N is an integer of 2 or more) linear parts arranged roughly in parallel with each other in a circumferential direction, and connect the two ring conductors, and a plurality of capacitors inserted between the adjacent rung conductors in the ring conductors.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a high-frequency coil unit and a magnetic resonance imaging apparatus, and more particularly to a technique for performing electromagnetic wave irradiation and magnetic resonance signal detection in a high-frequency coil unit.

  In a magnetic resonance imaging apparatus (hereinafter referred to as “MRI apparatus”), a high-frequency signal (hereinafter referred to as “RF signal”) is applied to a subject placed in a uniform static magnetic field generated by a static magnetic field magnet. ) To excite nuclear spins in the subject, and receive and process an NMR signal that is an electromagnetic wave generated by the nuclear spins, thereby obtaining a magnetic resonance image of the subject.

  Thus, the MRI apparatus irradiates the subject with the RF signal. Here, the irradiation of the RF signal and the reception of the NMR signal are performed by an antenna device such as an RF antenna or an RF coil that transmits or receives electromagnetic waves of a radio frequency. As such an antenna device, a high-frequency coil unit called a birdcage type (birdcage type) coil is known (for example, Patent Document 1 and Patent Document 2).

  Normally, a birdcage type coil has two cylindrical ring conductors 203, a plurality of linear rung conductors 204, a capacitor, a diode, a feeding cable (not shown) and the like as shown in FIG. The ends of the rung conductors arranged at equal intervals on the cylindrical surface and the ring conductors are connected, the capacitors are inserted into the gaps 201 provided at equal intervals in the ring conductor 203, and the diodes of the rung conductors 204 are connected. It is configured to be inserted into the gap.

  Note that the cylindrical birdcage coil 200 of FIG. 13 is surrounded by a cylindrical conductor called an RF shield (not shown). The capacitor provided in the birdcage coil 200 is adjusted so as to resonate at a specific frequency in the MRI apparatus by the RF shield, the rung conductor, and the ring conductor. Normally, the RF shield is further installed on the inner cylindrical surface of a cylindrical gradient magnetic field coil existing outside.

  The birdcage type coil is characterized in that the spread of the uniform space of the RF magnetic field created by the RF signal to be irradiated is higher than that of a simple loop coil or saddle (saddle type) coil. Due to this feature, the birdcage type coil is now a standard type of transmission coil of the tunnel type horizontal magnetic field MRI apparatus.

  In the birdcage type coil, the extent of the uniform space in the cross-sectional direction of the cylinder depends on the number of rungs arranged at equal intervals on the cylindrical surface if the cylinder has the same diameter. In other words, in the case of a birdcage type coil used for a 1.5 Tesla transmission coil, the number of rung conductors is usually 16 or 24, and the larger the number, the more the uniform space spreads in the cross-sectional direction of the cylinder. large.

  Usually, the uniformity of the RF magnetic field is high at the center of the cylinder and decreases as it approaches the periphery. In a birdcage coil, when the number of rung conductors is small, there is a large difference in magnetic field between the rung conductor and the space near the rung conductor and between the rung conductor and the rung conductor. Thus, the uniformity of the magnetic field is reduced.

US Pat. No. 7,688,070 International Publication 2012/059574

  By the way, in general, when a person enters the cylindrical irradiation coil while lying down, the shoulders, waist, abdomen, arms, etc. of the person spread sideways, so the shoulders, waist, arms, In order to clearly image the abdomen using an MRI apparatus, it is desirable that a static magnetic field, a gradient magnetic field, and an RF magnetic field can be generated uniformly in a spherical space having a diameter of about 50 cm from the center of the magnet.

  On the other hand, for example, a superconducting magnet that generates a strong static magnetic field of 1.5 Tesla has a cylindrical shape called a tunnel type, and it is preferable to enlarge the imaging space that is the tunnel type internal space. Increasing the imaging space by increasing the size increases the cost. Therefore, when it is intended to create a wide RF magnetic field space without changing the size of the magnet, it is desirable to generate an RF magnetic field that is as wide and uniform as possible without changing the size of the birdcage coil. For this purpose, it is effective to increase the number of rung conductors as described above. However, increasing the number of rung conductors complicates the shape of the coil and increases the number of circuit elements such as capacitors. Therefore, the cost increases.

  The present invention has been made in view of the above circumstances, and without increasing the number of parts and the cost of the apparatus, the imaging space can be expanded by expanding the region where the RF magnetic field is uniformly irradiated, and a wide range and high image quality. It aims at acquiring the MRI image of.

  One aspect of the present invention is a cylindrical shape that is applied to an MRI apparatus and performs at least one of transmission of a high-frequency signal to a static magnetic field space and reception of a nuclear magnetic resonance signal generated from a subject placed in the static magnetic field. An RF coil and a shield made of a sheet-like conductor and formed in a cylindrical shape so as to surround the RF coil, and the RF coil is formed in an annular shape, and the cylindrical shape is formed at both ends of the cylindrical shape. Two ring conductors arranged so that the center axis of the ring and the center of the ring coincide with each other and along the outer peripheral surface of the cylindrical shape are arranged at a predetermined interval in parallel to the central axis of the cylindrical shape. A plurality of rung conductors that electrically connect the two ring conductors, and a plurality of capacitors inserted between the adjacent rung conductors in the ring conductor, the rung conductors in the circumferential direction N (N is an integer of 2 or more) to provide a high frequency coil unit characterized by having a linear portion disposed almost in parallel with each other.

  According to the present invention, without increasing the number of components and the apparatus cost, the imaging space can be expanded by expanding the region where the RF magnetic field is uniformly irradiated, and a wide range and high quality MRI image can be acquired. .

1 is a schematic configuration diagram of an MRI apparatus to which an RF coil according to an embodiment of the present invention is applied. It is a perspective view of RF coil concerning an embodiment of the present invention. The map of the RF magnetic field produced | generated by the conventional RF coil is shown, (A) is a map which expressed the magnetic field produced when a total of about 24 watts are input into two input terminals about 12 watts in gray scale. Yes, (B) is a diagram in which the map shown in (A) is shown by 30 contour lines at equal intervals with an intensity of 0 to 2 [A / m]. FIG. 5A shows a map of an RF magnetic field generated by an RF coil according to an embodiment of the present invention, and FIG. 5A shows a gray scale of a magnetic field generated when approximately 24 watts are input to each of two input terminals in total. (B) is a diagram in which the map shown in (A) shows the intensity of 0 to 2 [A / m] with 30 contour lines. It is the figure which plotted the magnetic field intensity on the vertical axis | shaft, (A) plotted the calculation point of the Z = 0 cross section of the conventional type RF coil, (B) is Z = of the RF coil which concerns on this embodiment. This is a plot of the calculation points of the 0 cross section. It is a figure which shows the diode circuit provided in a rung conductor, (A) is a figure which shows the diode circuit applied to the conventional type RF coil, respectively, (B) is the diode circuit applied to the RF coil which concerns on this embodiment. FIG. It is a perspective view of RF coil concerning modification 1 of this embodiment. It is a perspective view of RF coil concerning modification 2 of this embodiment. It is a perspective view of the RF coil which concerns on the modification 3 of this embodiment. It is sectional drawing explaining the space | interval of RF coil and a shield. It is sectional drawing explaining the space | interval of RF coil and a shield. It is a perspective view of the RF coil which concerns on the modification 4 of this embodiment. It is a perspective view of the conventional RF coil.

  Hereinafter, an MRI apparatus to which an RF coil unit according to an embodiment of the present invention is applied will be described with reference to the drawings.

[Overall configuration of MRI system]
As shown in FIG. 1, it is a schematic block diagram of the MRI apparatus 100 which concerns on this embodiment. The MRI apparatus 100 includes a magnet 101 that forms a static magnetic field in a measurement space in which the subject 112 is disposed, a gradient magnetic field coil 102 that applies a magnetic field gradient in a predetermined direction to the static magnetic field, and a high-frequency signal (RF signal) as the subject. An RF antenna 103 that transmits a nuclear magnetic resonance signal (NMR signal) generated from the subject 112 and transmits a pulse waveform of the RF signal (RF wave) to the RF antenna 103; The transmitter / receiver 104 that performs signal processing on the NMR signal received by the 103, the gradient magnetic field power supply 109 that supplies a current to the gradient magnetic field coil 102, the drive of the transceiver 104 and the gradient magnetic field power supply 109, and various information The data processing unit 105 that receives processing and operations by the operator, and the processing result of the data processing unit 105 are displayed. And a display device 108 for a bed 111 for supporting the patient 112.

  The gradient magnetic field power source 109 and the gradient magnetic field coil 102 are connected by a gradient magnetic field control cable 107. The RF antenna 103 and the transceiver 104 are connected by a transmission / reception cable 106 that transmits and receives signals between the RF antenna 103 and the transceiver 104. The transceiver 104 includes a synthesizer, a power amplifier, a reception mixer, an analog / digital converter, a transmission / reception changeover switch, and the like (all not shown).

The RF antenna 103 includes a multi-channel transmission or transmission / reception antenna that resonates at a predetermined frequency and has two or more channels.
In the example shown in FIG. 1, a single RF antenna is shown as the RF antenna 103 that transmits an RF signal and receives an NMR signal, but is not limited thereto. For example, an RF antenna composed of a plurality of antennas may be used as the RF antenna 103, for example, a combination of an RF antenna for wide range imaging and an RF antenna for local use.

  In particular, when each part of the human body is imaged in detail, in most cases, different antennas are used for the transmission antenna and the reception antenna. For transmission, a large irradiation antenna installed inside the gradient magnetic field coil covering the entire body is used, and for reception, a local antenna installed near the human body surface is often used. In this case, the local antenna is mostly dedicated to reception. In some cases, a local transmission / reception antenna that is installed locally near the human body and performs both transmission and reception may be used. In this case, the local transmission / reception antenna is also often composed of a plurality of channels.

  The MRI apparatus 100 is classified into a horizontal magnetic field method and a vertical magnetic field method depending on the direction of the static magnetic field formed by the magnet 101. In the case of the horizontal magnetic field method to which the RF coil unit according to the present embodiment is applied, generally, the magnet 101 has a cylindrical bore (central space), and generates a static magnetic field in the horizontal direction in FIG. It is called a tunnel type MRI apparatus. On the other hand, in the case of the vertical magnetic field method, a pair of magnets are arranged above and below the subject 112, and a vertical static magnetic field is generated in FIG.

  The data processing unit 105 controls the transmitter / receiver 104 and the gradient magnetic field power source 109 to intermittently irradiate the subject 112 disposed in the static magnetic field from the RF antenna 103 and the gradient magnetic field coil 102 with an RF signal. Apply a gradient magnetic field. Further, the RF antenna 103 receives an NMR signal emitted from the subject 112 in resonance with the RF signal, performs signal processing, and reconstructs an image. The subject 112 is, for example, a predetermined part of the human body.

[Configuration of RF transceiver system]
In the present embodiment, a birdcage type coil (hereinafter simply referred to as “RF coil”) 210 shown in FIG. 2 is applied as the RF antenna 103. Although not shown here, the RF coil 210 is surrounded by a shield in which a sheet-like conductor is formed in a cylindrical shape. As shown in FIG. 2, the RF coil 210 includes two annular ring conductors 203, a plurality of rung conductors 214, a capacitor, a diode, a feeding cable (not shown), and the like.

  More specifically, the ring conductor 203 is formed in an annular shape, and is disposed at both ends of the cylindrical RF coil so as to be an end ring, and so that the central axis of the cylinder coincides with the annular center. Moreover, the rung conductor 214 is arrange | positioned at equal intervals along a cylindrical outer peripheral surface, and the edge part of the rung conductor 214 and the ring conductor 203 are connected.

  Further, in the RF coil 210, the capacitor is inserted into the gap 201 provided at equal intervals on the ring conductor 203, and the diode circuit is inserted into the gap 212 of the rung conductor 214. The capacitor is adjusted to resonate at the frequency of the high frequency signal or the nuclear magnetic resonance signal by the ring conductor 203, the rung conductor 214, and a shield (not shown).

  The RF coil 210 shown in FIG. 2 is different from the conventional birdcage type coil 200 shown in FIG. That is, the rung conductor 214 has two straight portions 218 that are bifurcated at both ends and are positioned substantially parallel to each other, so that the two conductors are parallel to each other at the axial center portion of the cylindrical shaft. It is a connected ring.

  By forming the rung conductor 214 in such a shape, the number of rung conductors can be substantially increased without increasing the number of capacitors inserted into the ring conductor. The interval between the parts can be made close. Thereby, the uniform space of the irradiation RF magnetic field generated inside the cylinder of the RF coil 210 can be expanded to the periphery of the cylinder.

  One rung conductor may have a plurality of straight portions (in this embodiment, two (FIG. 2)), thereby forming a current loop between the plurality of straight portions. However, by adjusting the capacitor inserted into the ring conductor, a current flows in parallel in the plurality of straight portions. For gradient magnetic fields in the X and Y directions, where the occurrence of overcurrent can be a problem, the loops made by the rung conductors are arranged symmetrically so as to cancel the magnetic field from the gradient magnetic field coils, which adversely affects image quality. It can be said that an overcurrent caused by a gradient magnetic field does not occur.

  Here, the magnetic field distribution generated by the RF coil will be described. With the current calculation technique, the magnetic field distribution of the RF coil used for MRI can be calculated by simulation in substantially the same manner as in actual practice. The magnetic field distribution calculated by simulation is shown below.

FIG. 3 shows a map of the RF magnetic field (B1 + circularly polarized wave) generated by the conventional RF coil 200 shown in FIG. 13 in the Z = 0 cross section when the cylindrical axis is the Z axis and the magnetic field center is Z = 0. Indicates.
FIG. 3A is a map representing the magnetic field generated when approximately 12 watts are input to each of the two input terminals in a total of approximately 24 watts in gray scale, and FIG. ) Is shown by 30 contour lines at equal intervals with an intensity of 0 to 2 [A / m]. In the center of the magnetic field, a cylindrical container having a diameter of 30 cm and a length of 50 cm containing saline having an electric conductivity of 0.613 [S / m] is placed instead of a human load. The map shows the magnetic field generated in units of A / m when approximately 12 watts are input to the two input terminals of the antenna and a total of approximately 24 watts are input.

  Here, the conventional RF coil 200 of FIG. 13 is designed on the assumption that it is used in a 1.5 Tesla tunnel type MRI apparatus. This RF coil 200 is a birdcage type coil called a high-pass type having a diameter of about 610 mm and a length of a rung conductor of about 530 mm, and installing a capacitor only in the ring conductor portion.

The number of rung conductors is 16, and a diode is installed in the notch at the center of each rung conductor. During the time when RF is not irradiated, a reverse bias is applied to the diode to prevent coupling with the receiving coil. Avoid resonance of the coil.
The value of the capacitors installed at 16 locations on each of the ring conductors arranged at both ends of the cylinder is about 100 pF, and resonates at 63.8 MHz, which is the RF resonance frequency of a 1.5 Tesla MRI apparatus.

  On the other hand, as described above, in the RF coil 210 of FIG. 2, each rung conductor 214 has an annular shape in which both ends are branched and both ends are connected. Therefore, in the RF coil 210, when the cylindrical axis is the Z axis and the magnetic field center is Z = 0, the 16 rung conductors substantially correspond to the 32 rung conductors in the cross section of Z = 0. It has become. The width of the branched rung conductor 214 is half the width of the rung conductor 202 of the RF coil 200, and the distance between the branched conductors is also arranged at equal intervals in the circumferential direction as exactly 32 rungs in the cross section of Z = 0. It has become so.

  FIG. 4 shows a map of the RF magnetic field (B1 + circularly polarized wave) generated by the RF coil 210 according to the present embodiment shown in FIG. FIG. 4A shows, in gray scale, the magnetic field generated when approximately 24 watts are input to the two input terminals of the RF coil 210 in a total of approximately 24 watts, and FIG. The map shown in (A) shows the intensity of 0 to 2 [A / m] with 30 contour lines. A similar saline phantom is installed at the center of the magnetic field.

  As can be seen from FIGS. 3 (A) and 4 (A), in the slightly dark portion of the donut shape near the center of the map, the magnetic field in the inner portion of about 4 cm from the surface of the phantom cross section with a diameter of 30 cm is the electric conductivity of the saline solution. It is a weakened part due to the relative dielectric constant. In addition, in the part with the rung conductor with a diameter of 61 cm, the black part with weak magnetic field and the white part with strong magnetic field appear alternately, the part with the rung conductor has strong magnetic field, and the space between the rung conductor and the rung conductor is It can be seen that the magnetic field is weak. When FIG. 3A is compared with FIG. 4A, there is a significant difference in the magnetic field in the space between the rung conductor and the rung conductor.

  In FIGS. 3B and 4B, a portion where the interval between contour lines is wide (sparse portion) is a space where the magnetic field is uniform. Comparing FIG. 3 (B) and FIG. 4 (B), in the map of FIG. 4 (B), compared with the map of FIG. That is, it can be seen that the uniform space region of the magnetic field is wide. It should be noted that there is no contour line in the portion where the magnetic field intensity on the outermost side exceeds 2 [A / m].

FIG. 5 shows the results of more quantitative confirmation of the above comparison results.
In the diagrams shown in FIGS. 5A and 5B, the electromagnetic field is calculated with a Z = 0 cross section at intervals of about 5 mm, and the respective calculation points are the distance from the origin of X = Y = 0 on the horizontal axis. The magnetic field strength is plotted with the vertical axis. FIG. 5A is a plot of calculation points for the Z = 0 cross section of the conventional RF coil 200, and FIG. 5B is a calculation point for the Z = 0 cross section of the RF coil 210 according to the present embodiment. It is a plot. The point where the distance from the origin on the horizontal axis is 0 is the magnetic field intensity at the origin.

  The magnetic field intensity at the origin in FIG. 5 (A) is 1.16 [A / m], and the magnetic field intensity at the origin in FIG. 5 (B) is 1.13 [A / m]. The magnetic field strength of is obtained. From this, it can be seen that the irradiation efficiency of the central portion of the cylinder of the RF coil 210 according to the present embodiment is substantially the same as that of the conventional RF coil 200.

  In the plots of FIGS. 5A and 5B, the magnetic field B1 + greatly deviates up and down as the distance from the origin increases. FIG. 5A shows the minimum value 502 and the maximum value 503 of the magnetic field at the distance 0.25 [m] of the conventional RF coil 200. FIG. 5B shows the minimum value 512 and the maximum value 513 of the magnetic field at the distance 0.25 [m] of the RF coil 210 according to this embodiment.

  In the conventional RF coil 200 shown in FIG. 5 (A), the magnetic field strength varies in the range of 0.8 to 1.4 [A / m] in the 50 cm diameter portion, but this embodiment shown in FIG. 5 (B). It can be seen that only a variation of about 0.9 to 1.2 [A / m] occurs in the RF coil 210 of FIG. This is because, even when a human shoulder or the like is placed in a 50 cm diameter portion, the RF magnetic field variation in the 50 cm diameter portion, that is, the peripheral portion of the cylinder, is different from that in the conventional RF coil 200 in the RF coil 210 according to this embodiment. It means less improvement. If there is little variation in the RF magnetic field, a uniform image can be acquired, so that the image quality can be improved.

FIG. 6 shows details of the diode circuit provided in the rung conductor.
FIG. 6A shows a diode circuit 601 in the conventional RF coil 200, and FIG. 6B shows a diode circuit 602 of the RF coil 210 according to this embodiment.
A diode circuit 601 shown in FIG. 6A is inserted in the gap 202 of the rung conductor 204 shown in FIG. 13 and includes a diode 613 and an inductor 612. In order to allow a DC current to flow through the diode 613, a branch of a conducting wire is provided at both ends of the diode 613, and an inductor 612 is provided so that an RF current does not flow in a path through which the DC current flows.

The diode circuit 601 in FIG. 6A merely shows an example of a diode circuit provided on one rung conductor 204, but the diode circuit 601 provided on each rung conductor 204 has a cylindrical circumference. They are arranged continuously in the direction so that a DC current can flow through the diode 613 of each rung conductor 204.
At the time of RF irradiation, a DC current is passed to bring the RF coil 200 into a resonance state. At the time of reception, no DC current is passed to bring the RF coil 200 into a non-resonant state. By doing so, coupling with the receiving antenna is prevented, and a decrease in receiving sensitivity is prevented.

  A diode circuit 602 shown in FIG. 6B is inserted in the gap 212 of the rung conductor 214 and includes a diode 613 and an inductor 612. The diodes 613 are respectively provided in the gaps 212 of the respective straight portions 218 branched at the end portions of the rung conductors 214. The inductor 612 is also provided between the diodes 613 inserted in the adjacent linear portions 218.

  Therefore, the number of diodes 13 and inductors 612 provided on the rung conductor 214 is increased by one as compared to the diodes 613 and inductors 612 provided on the rung conductor 204 of the conventional RF coil 200. The diode circuits 602 provided on the respective rung conductors 214 are connected in series with each other through the inductor 612. With the configuration like the diode circuit 602, the diode 613 can be inserted into the rung conductor even if the rung conductor 214 branches into two straight portions 218.

  As described above, according to the present embodiment, the rung conductor is branched at both ends, and a plurality of straight portions are provided, thereby increasing the number of capacitors inserted into the gap of the ring conductor without increasing the number of capacitors. Since the number can be increased substantially, the interval between the rung conductors can be reduced without increasing the number of parts.

  Thereby, the uniform space of the irradiation RF magnetic field produced | generated inside the cylinder of RF coil can be expanded to the peripheral part of a cylinder. That is, according to the RF coil according to the present embodiment, the imaging space can be expanded by enlarging the region where the RF magnetic field is uniformly irradiated without increasing the number of components and the apparatus cost. When applied to the above, a wide range and high quality MRI image can be acquired.

(Modification 1)
In the above-described embodiment, the RF coil 210 provided with the rung conductor 214 branched at both ends and having two straight portions has been described. The RF coil is not limited to such a shape. For example, the rung conductor may have a shape having three straight portions.

  FIG. 7 shows an RF coil 310 according to this modification. The RF coil 310 includes a rung conductor 314 that branches at both ends and has three straight portions. In the rung conductor 314 of this modification, since the number of rung conductors 314 itself is not changed, it is not necessary to increase the number of capacitors inserted into the ring conductor 303. In this example, 16 capacitors are inserted in one ring conductor 303, the number of rung conductors 314 is 16, and each rung conductor 314 has three straight portions 318. In this case, substantially the same effect as that of 48 linear rung conductors can be obtained, and the uniform space of the magnetic field generated by the RF coil is expanded.

(Modification 2)
In the above-described embodiment and Modification 1 thereof, one rung conductor is branched into a plurality of straight portions at both ends, and the rung conductor and the capacitor are alternately provided on the ring conductor. In the present modification, an example will be described in which two independent straight line portions that are not branched at both ends and are not connected at both ends constitute a rung conductor, and the rung conductor and one capacitor are alternately provided.

  FIG. 8 shows an example of an RF coil in which the number of capacitors is reduced and a rung conductor 814 having two straight portions 818 and one capacitor are alternately provided. In the example of FIG. 8, the RF coil 810 includes an annular ring conductor 803 and a rung conductor 814, and the rung conductor 814 has two straight portions 818. In FIG. 8, the rung conductor 814 is connected to the ring conductor 803 so that the straight portions 818 are arranged at equal intervals.

In the ring conductor 803, the rung conductor 814 and the capacitor are alternately inserted. As a result, one capacitor is inserted each time two straight portions 818 are provided. That is, the ring conductor 803 is provided with two straight portions 818 and capacitors alternately, and at the connection point between the ring conductors 818 and the straight portions 818, the two straight portions 818 are paired. In order to install a capacitor between them, no gap is provided.

  This is because the number of capacitors in the ring conductor 803 is reduced as compared with the case where the linear rung conductors and the capacitors are alternately arranged one by one. Usually, in a high-pass birdcage type coil having 32 straight rung conductors, 32 capacitors are installed in the ring conductor, but in the RF coil 801 in FIG. 8, the number is reduced to 16 which is half the number. It can be said. When the number of capacitors is reduced, the number of parts is reduced, so that the cost and manufacturing cost required for the parts are reduced, and the apparatus cost is reduced. Further, since the number of parts is small, there is an advantage that the possibility of failure is reduced.

(Modification 3)
In the above-described embodiment and each modified example, the RF coil in which a plurality of rung conductors having a plurality of straight portions branched at both ends between two annular ring conductors has been described. The RF coil is not limited to such a shape, and the ring conductor may have an elliptical annular shape.

  FIG. 9 shows an example in which a rung conductor 214 branched into two straight portions 218 at both ends is applied to an elliptical birdcage type coil 910. The elliptical birdcage type coil 910 in FIG. 9 has rung conductors 214 arranged at equal intervals in the elliptical circumferential direction. Thus, the present embodiment can be applied even to an elliptic birdcage type coil, and in this case, the uniformity of the irradiation magnetic field around the rung can also be improved. In the elliptic birdcage type coil, the rung conductors 214 do not necessarily have to be installed at regular intervals as in the example shown in FIG. 9, and the rung conductor installation intervals can be changed as appropriate.

  As shown in FIG. 10, it is conceivable that the interval (gap) between the rung conductor of the elliptical birdcage type coil 910 and the shield 950 is equal. However, as shown in FIG. 11, a wider patient space can be secured in the left-right direction by narrowing the left-right distance and widening the vertical distance. At this time, the uniformity of the irradiation magnetic field is maintained or improved by narrowing the gap between the rung conductors in the vertical direction and widening in the horizontal direction. In addition, the space | interval (gap) of predetermined (usually about 20-30 mm) is required for the space | interval of a shield and RF coil, in order to improve the power efficiency of irradiation.

(Modification 4)
Also, as shown in FIG. 12, even in the case of an elliptic birdcage type coil, a rung conductor composed of two separate linear rung portions and one capacitor are alternately provided without branching at both ends. be able to. The elliptic birdcage type coil 1101 of FIG. 12 is also provided with a rung conductor 814 having two straight portions 818 and a capacitor alternately. Therefore, at the connection point of the ring conductor with the straight line portion 818, a gap is not provided between the two straight line portions 818 that form a pair.

  In the ring conductor 1103, the number of capacitors is reduced as compared with the conventional elliptic birdcage type coil in which linear rung conductors and capacitors are alternately arranged one by one. There is a merit that the risk of the decrease. Thus, the elliptical birdcage coil 1101 can also be an RF coil in which the uniformity of the irradiation magnetic field around the rung conductor is improved by applying the rung conductor according to this embodiment.

  Note that the embodiment of the present invention and its modifications can be variously added and changed without departing from the spirit of the invention.

DESCRIPTION OF SYMBOLS 100 ... MRI apparatus, 101 ... Magnet, 102 ... Gradient magnetic field coil, 103 ... RF antenna, 104 ... Transceiver, 105 ... Data processing part, 106 ... Transmission / reception cable, DESCRIPTION OF SYMBOLS 107 ... Gradient magnetic field control cable, 108 ... Display apparatus, 109 ... Gradient magnetic field power supply, 111 ... Bed, 112 ... Subject, 200 ... Conventional RF coil, 201 ... Gap, 202 ... Gap, 203 ... Ring conductor, 204 ... Lang conductor, 210 ... RF coil, 212 ... Gap, 214 ... Lang conductor, 218 ... Linear part, 310 ... Birdcage RF coil, 303 ... Ring conductor, 314 ... Lang conductor, 318 ... Straight line part, 601 ... Diode circuit, 602 ... Diode Circuit, 612 ... Inductor, 613 ... Diode, 803 ... Ring conductor, 810 ... Bird cage type RF coil, 814 ... Lang conductor, 818 ... Straight line part, 910 ... Ellipse Birdcage type RF coil, 950 ... Shield, 1101 ... Elliptical birdcage type RF coil, 1103 ... Ring conductor

Claims (8)

A cylindrical RF coil that is applied to an MRI apparatus and performs at least one of transmission of a high-frequency signal to a static magnetic field space and reception of a nuclear magnetic resonance signal generated from a subject placed in the static magnetic field,
The RF coil is
Two ring conductors that are formed in an annular shape and are arranged so that the cylindrical center axis and the annular center coincide with each other at both ends of the cylindrical shape, and the cylindrical shape along the outer peripheral surface of the cylindrical shape. A plurality of rung conductors arranged parallel to the central axis and spaced apart from each other and connecting the two ring conductors;
In the ring conductor, having a plurality of capacitors inserted between the adjacent rung conductors,
The rung conductor includes N (N is an integer of 2 or more) linear portions arranged in parallel with each other in the circumferential direction.   The high-frequency coil unit according to claim 1, wherein the rung conductor has straight portions arranged substantially parallel to each other by branching into N pieces in the circumferential direction at both ends.   2. The high-frequency coil unit according to claim 1, wherein both end portions of each of the straight portions constituting the rung conductor are directly connected to the ring conductor and arranged substantially parallel to each other.   The high frequency coil unit according to any one of claims 1 to 3, wherein the capacitors are alternately arranged with the rung conductors in the ring conductor. A sheet-shaped conductor, further comprising a shield formed in a cylindrical shape so as to surround the RF coil;
The RF coil and the shield are formed in a cylindrical shape having a substantially circular cross section perpendicular to the central axis,
5. The high-frequency coil unit according to claim 1, wherein the RF coil unit is disposed such that a center axis of the RF coil and a center axis of the shield coincide with each other. A sheet-shaped conductor, further comprising a shield formed in a cylindrical shape so as to surround the RF coil;
The RF coil and the shield are formed in a cylindrical shape having a substantially elliptical cross section perpendicular to the central axis,
5. The high-frequency coil unit according to claim 1, wherein the RF coil unit is disposed such that a center axis of the RF coil and a center axis of the shield coincide with each other.   7. The high-frequency coil according to claim 1, wherein the capacitor is adjusted to resonate at the frequency of the high-frequency signal or the nuclear magnetic resonance signal by the ring conductor, the rung conductor, and the shield. unit.   A magnetic resonance imaging apparatus comprising the high-frequency coil unit according to any one of claims 1 to 5.

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