High-frequency coil unit and magnetic resonance imaging apparatus

Abstract

To provide an RF shield capable of avoiding complexity of conductor pattern design for eddy current suppression by a gradient magnetic field coil, and achieving balance of improvement of radiation efficiency and suppression of eddy current heat generation by the gradient magnetic field coil.SOLUTION: A cylindrical RF shield which is one element forming a high frequency coil unit is formed by rolling patterned conductive thin films which are formed on both surfaces of an insulation sheet, front and back patterns of the patterned conductive thin films are in a mirror symmetrical relationship in a center axis direction of the cylinder, the front pattern being divided into two parts in the center axis direction, and one side of the divided patterns is further divided into two parts in a circumferential direction. Islands of the three independent patterns have a slit in a form of forming a strip from a cylinder axis direction, and the respective strips are connected in a vertical direction at one position.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present invention relates to a high-frequency coil unit used in a magnetic resonance imaging apparatus, and particularly to an RF shield provided in the high-frequency coil unit.

Background Art

[0002] In a magnetic resonance imaging (MRI) apparatus (hereinafter referred to as an "MRI apparatus"), a high-frequency signal (hereinafter referred to as an "RF (Radio Frequency) signal"), which is an electromagnetic wave, is irradiated onto a subject placed in a uniform static magnetic field generated by a static magnetic field magnet, and the nuclear spins in the subject are excited. At the same time, an NMR (Nuclear Magnetic Resonance) signal, which is an electromagnetic wave generated by the nuclear spins, is received and signal-processed to obtain a magnetic resonance image of the subject.

[0003] Irradiation of the RF signal and 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 at radio frequencies. As such an antenna device, a high-frequency coil unit using a birdcage-type coil is known. >

[0004] Normally, a frequently used birdcage-type coil has two annular ring conductors serving as end rings, a plurality of linear rung conductors connecting the two ring conductors, a capacitor, a diode, a power supply cable (not shown), etc. The capacitor is inserted into a gap provided at equal intervals in the ring conductor, and the diode is inserted into the gap of the rung conductor.

[0005] These cylindrical birdcage coils are typically surrounded by a cylindrical conductor called an RF shield. The capacitor in the birdcage coil is tuned to resonate at a specific frequency in the MRI device by the RF shield, lung conductor, and ring conductor. A key feature of the birdcage coil is that the uniform spread of the RF magnetic field (also called the "irradiation magnetic field") created by the irradiated RF signal is higher than that of simple loop coils or saddle coils. Due to this feature, the birdcage coil is currently the standard type of transmitting coil in tunnel-type horizontal magnetic field MRI devices.

[0006] In horizontal magnetic field MRI systems, the gradient coils that provide a magnetic field gradient are typically cylindrical gradient coils formed into a cylindrical shape. In the case of birdcage-type coils, the RF shield is often installed on the inner surface of the cylindrical gradient coil.

[0007] One important requirement that such RF shielding must meet is to have the best possible electrical conductivity at RF frequencies, for example, approximately 64 MHz for a 1.5 Tesla MRI device. Another requirement is that it must not overheat or burn even when eddy currents are induced by the magnetic field generated by the gradient coils in the frequency range of a few kHz. However, if a copper thin film with excellent electrical conductivity is used as RF shielding, for example, it may overheat and melt due to eddy currents, so it is necessary to satisfy both of the above requirements.

[0008] To satisfy both requirements, one method involves using copper foil on both sides of the RF shield, creating a tile-like pattern, and connecting the front and back surfaces as a capacitor. This results in almost no resistance around 64MHz, but eddy currents are generated only within a single tile (Patent Documents 1 and 2). This method is called the tiling method.

[0009] For example, the RF shield pattern disclosed in Patent Document 2 describes overlapping tiles so that they are staggered in the circumferential direction on the front and back sides. However, in this pattern, there are no circumferential connections in a location close to the ring portion of the birdcage-type coil, causing the Q value of the birdcage-type coil to drop, making it difficult to use in practice. Even if the RF shield pattern disclosed in Patent Document 2 had circumferential connections, it would not have slits perpendicular to the cylindrical axis, so it would not be able to handle the eddy currents created by the gradient magnetic fields of X, Y, and Z, and would be insufficient to suppress the heat generated by eddy currents.

[0010] On the other hand, in the RF coil disclosed in Patent Document 1, in an RF shield with a double-sided copper foil pattern tiled, as shown in Figure 9, the region 192 and 194 on both sides in the axial direction of the cylinder is divided into a region in between (the central region) 184. In the regions on both sides, the longitudinal direction of the tiles (strips) 185 is aligned in the circumferential direction, while in the central region 184, the longitudinal direction of the tiles is aligned in the axial direction of the cylinder. Note that Figure 9 shows the pattern on the front side (upper figure) and the pattern on the back side (lower figure), but the explanation of the pattern on the back side is omitted. With this configuration, in the RF shield, current flows along the current flowing through the ring conductor of the birdcage type coil in the regions 192 and 194 on both sides, and current flows along the current flowing through the lung conductor of the birdcage type coil in the central region 184, thereby limiting the location of eddy current generation to within the tiles while maintaining good electrical conductivity. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] U.S. Patent No. 5,367,261 [Patent Document 2] U.S. Patent No. 5574372 [Overview of the Initiative] [Problems that the invention aims to solve]

[0012] The magnitude of eddy currents generated on the RF shield by the magnetic field created by the gradient coils is proportional to the magnetic field strength created by the gradient coils. A gradient coil consists of three sets of gradient coils that generate gradient magnetic fields in the three axial directions: X, Y, and Z. In a cylindrical gradient coil, these three sets of gradient coils are stacked in the thickness direction of the cylinder. Therefore, the eddy currents generated on the RF shield are most strongly influenced by the gradient coil located furthest inside the cylinder, i.e., the gradient coil closest to the RF shield. For example, in the gradient coil shown in Figure 8, the elliptical pattern of the X-direction gradient coil (402) is closest to the RF shield and is located on the left and right sides in Figure 8. Typically, the location where the magnetic field created by the X-direction gradient coil is strongest is near the center of the left and right elliptical patterns (indicated by the thick dashed lines). In other words, the magnetic field created by the gradient coil is strongest at a distance of D502 from the magnetic field center on the cylindrical axis.

[0013] The RF shield described in Patent Document 1 has a tiled pattern along the circumferential direction in the regions on both sides of the axial direction of the cylinder. However, in this RF shield pattern, in order to suppress the generation of eddy currents as much as possible at the position where the magnetic field created by the gradient coils is strongest, i.e., at a distance D502 from the center of the magnetic field (Figure 8), it is necessary to design with different tile widths. In other words, if one attempts to reduce eddy currents in the RF shield described in Patent Document 1 by considering the arrangement of the gradient coils, the pattern becomes complex and the design becomes difficult. In particular, determining the tile width requires a great deal of effort, as there are many items that need to be verified, whether by experimental or computational methods.

[0014] This invention has been made in view of the above circumstances, and aims to provide an RF shield pattern for a birdcage-type irradiation coil that maintains irradiation efficiency, prevents heat generation due to eddy currents, and has a relatively simple design. [Means for solving the problem]

[0015] To solve the above problems, the high-frequency coil unit of the present invention has the following features in the RF shield pattern. The RF shield is a cylindrical sheet with conductive thin film patterns on both sides of an insulator, and in order to prevent heat generation due to eddy currents, strip-shaped patterns are formed on the conductive thin films on the front and back, and these strips are arranged in the axial direction of the cylinder, with the strips on the front and back overlapping alternately at both ends of the axial direction of the cylinder. Furthermore, in the part corresponding to the ring conductor of the RF coil, the strips are connected in order to ensure good electrical conductivity at RF frequencies and to ensure capacitance as a capacitor, by avoiding breaks as much as possible in the part through which the mirror current of the current flowing through the ring conductor passes.

[0016] That is, the high-frequency coil unit of the present invention includes a cylindrical RF coil having ring conductors arranged at both ends and a plurality of lung conductors connecting the ring conductors at both ends, and a cylindrical RF shield arranged to cover the outer circumference of the RF coil, the RF shield including an insulating sheet and a conductor pattern formed on the front and back surfaces of the insulating sheet. The conductor pattern is divided into first and second conductor regions of different sizes in the axial direction of the cylinder, and each conductor region includes a plurality of strips having a connection portion along the circumferential direction of the cylinder at at least one location in the axial direction of the cylinder and partially separated by a plurality of slits partially formed along the axial direction of the cylinder, and the connection portion along the circumferential direction of the cylinder is cut at at least one location around the circumference of the cylinder, and the positions of the cut portions are offset in the circumferential direction (not the same position) on the front and back surfaces of the insulating sheet. The conductor pattern on the front surface and the conductor pattern on the back surface are arranged so that one first conductor region and the other second conductor region overlap, and in the portion where the first and second conductor regions overlap, the slits in the conductor pattern on the front surface and the slits in the conductor pattern on the back surface are offset in the circumferential direction. [Effects of the Invention]

[0017] Provided is an RF shield for a birdcage type irradiation coil that can reduce the number of man-hours required for design, with little heat generation due to eddy currents caused by the gradient magnetic field, without reducing the efficiency of the irradiation coil. As a result, a high-frequency coil unit with good irradiation efficiency and excellent durability can be obtained.

Brief Description of the Drawings

[0018] [Figure 1] It is a schematic configuration diagram of an MRI apparatus. [Figure 2] It is a perspective view showing the configurations of an irradiation coil and a gradient magnetic field coil. [Figure 3] It is a perspective view of a birdcage type irradiation coil. [Figure 4] It is a schematic cross-sectional view of a birdcage type irradiation coil and an RF shield. [Figure 5] It is a schematic cross-sectional view of an irradiation coil and a gradient magnetic field coil. [Figure 6] It is a diagram for explaining the RF shield pattern (Example 1) of the RF coil unit of the present invention. [Figure 7] It is a diagram combining the front and back surfaces of the RF shield pattern of FIG. 6. [Figure 8] It is a view of the configuration of the gradient magnetic field coil seen from the right. [Figure 9] It is a diagram showing the RF shield pattern of the prior art. [Figure 10] It is an example of an actual design pattern diagram using the prior art. [Figure 11] It is a diagram for explaining the RF shield pattern (Example 2) of the RF coil unit of the present invention. [Figure 12] It is a diagram combining the front and back surfaces of the RF shield pattern of FIG. 11.

Modes for Carrying Out the Invention

[0019] <0000​​​[Overall configuration of the MRI system] Figure 1 shows a schematic diagram of the MRI apparatus 100. The MRI apparatus 100 comprises a magnet 101 that forms a static magnetic field in the measurement space where the subject 112 is placed, a gradient coil 102 that applies a magnetic field gradient in a predetermined direction to the static magnetic field, an RF antenna 103 that transmits a high-frequency signal (RF signal) to the subject 112 and receives a nuclear magnetic resonance signal (NMR signal) generated from the subject 112, a transceiver 104 that generates a pulse waveform of the RF signal (RF wave) and transmits it to the RF antenna 103 and performs signal processing on the NMR signal received by the RF antenna 103, a gradient power supply 109 that supplies current to the gradient coil 102, a data processing unit 105 that controls the driving of the transceiver 104 and the gradient power supply 109 and accepts various information processing and operator operations, a display device 108 for displaying the processing results of the data processing unit 105, and a bed 111 on which the subject 112 is placed.

[0021] MRI devices 100 are classified into horizontal magnetic field types and vertical magnetic field types depending on the direction of the static magnetic field formed by the magnet 101. In horizontal magnetic field type MRI devices to which the RF coil unit according to this embodiment is applied, the magnet 101 generally has a cylindrical bore (central space) and generates a static magnetic field in the left-right direction (the direction coinciding with the central axis of the RF coil unit) in Figure 1, and is called a tunnel-type MRI device.

[0022] The gradient magnetic field coil 102 consists of three sets of coils that generate gradient magnetic fields in three mutually orthogonal axis directions: X, Y, and Z. As shown in Figure 2, the horizontal magnetic field type MRI device described above has a cylindrical structure in which the three sets of coils, namely the X coil 402, Y coil 403, and Z coil 404, are stacked with insulating sheets in between. In most cases, the X coil 402 is positioned on the innermost side of the cylinder and the Z coil 404 is positioned on the outermost side of the cylinder.

[0023] The Z coil 404 is a solenoid coil wound around the circumference of the cylinder, generating a gradient magnetic field in the axial direction of the cylinder. The Y coil 403 and X coil 402 generate gradient magnetic fields perpendicular to the axis. Each pair of coils is positioned symmetrically with respect to the axial center of the cylinder, and the Y coil 403 and X coil 404 are positioned 90 degrees apart in the circumferential direction of the cylinder. In Figure 2, the illustration is simplified and the Y coil 403 and X coil 404 are shown as simple loops, but in reality, they have complex patterns to generate the desired gradient magnetic fields.

[0024] The RF antenna 103 is a birdcage-type transmitting or transmitting antenna with two channels that resonates at a predetermined frequency, and is positioned inside the gradient magnetic field coil 102 described above. When imaging various parts of the human body in detail, different antennas are almost always used for transmitting and receiving. For transmission, a large irradiation antenna installed inside the gradient magnetic field coil, covering the entire body, is often used, while for reception, a local antenna placed near the surface of the human body is frequently employed. In this case, the local antenna is almost always for reception only.

[0025] The gradient magnetic field power supply 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 transceiver cable 106. The transceiver 104 includes a synthesizer, power amplifier, receiver mixer, analog-to-digital converter, transmit / receive switch, etc. (none of which are shown).

[0026] The data processing unit 105 controls the transceiver 104 and the gradient magnetic field power supply 109 to intermittently irradiate the subject 112, which is placed in a static magnetic field, with RF signals from the RF antenna 103 and the gradient magnetic field coil 102, and also applies a gradient magnetic field. Furthermore, the RF antenna 103 receives the 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.

[0027] [RF antenna configuration] The details of the RF antenna 103 applied to the MRI apparatus of this embodiment will be described below. The RF antenna 103 consists of a birdcage-type coil and an RF shield (hereinafter abbreviated as shield) arranged to surround the birdcage-type coil.

[0028] As shown in Figure 3, the birdcage-type coil 200 includes two annular ring conductors 203, a plurality of linear lung conductors 204, a capacitor, a diode, a power supply cable (not shown), etc. The central axes of the ring conductors 203, which are positioned at both ends of the RF coil to form an end ring, and the central axis of the cylindrical surface on which the lung conductors 204 are arranged are aligned. Each ring conductor 203 has gaps 201 at equal intervals in the circumferential direction, and the lung conductors 204 are arranged at equal intervals along the cylindrical surface on which the lung conductors are arranged. The ends of the lung conductors 204 are connected to portions of the ring conductors 203 separated by gaps 201 (front side) and portions of the ring conductors 203 separated by gaps 201 (back side), respectively.

[0029] In this birdcage-type coil 200, capacitors are inserted into equally spaced gaps 201 in each of the ring conductors 203 and connected between the sections of the ring conductor separated by the gaps 201, and diodes are inserted into gaps 202 in the lung conductor 204 and connected between the sections of the lung conductor separated by the gaps 202. The capacitors are tuned to resonate at the frequency of a high-frequency signal or nuclear magnetic resonance signal with respect to the ring conductors 203, lung conductors 204, and RF shielding.

[0030] The specifications of the Birdcage Coil 200 are not particularly limited, but as an example, it has a diameter of approximately 710 mm and a total length of approximately 550 mm, and is a birdcage coil called a high-pass type in which capacitors are installed only in the ring conductor section. There are 24 lung conductors, and a diode is installed in the notch in the center of each lung conductor. When no RF signal is being irradiated, a reverse bias is applied to the diode to prevent coupling with the receiving coil and to prevent resonance of the coil. This birdcage coil can be used in a 1.5 Tesla tunnel-type MRI machine, and if the value of the capacitors installed at 24 locations on each of the ring conductors located at both ends of the cylinder is approximately 200 pF, it will resonate at 63.8 MHz, which is the RF resonant frequency of a 1.5 Tesla MRI machine.

[0031] Figure 4 shows a cross-section of an RF antenna 103 using a birdcage-type coil 200, cut by a plane perpendicular to the cylindrical axis. As shown in Figure 4, the RF shield 300 is arranged to surround the outer circumference of the birdcage-type coil 200, and the birdcage-type coil 200 and the RF shield 300 constitute a high-frequency coil unit, i.e., the RF antenna 103. The RF shield 300 is made of a cylindrical tube made of a conductive material and may be attached to the inner surface of the gradient magnetic field coil.

[0032] The RF shield 300 is formed by partially removing conductive thin films formed on both the front and back surfaces of an insulating sheet by etching or other means to create a pattern, then rolling the patterned sheet around a cylindrical axis and connecting parts of it to form a cylindrical shape. In the RF shield 300 with this structure, the conductive thin films on the front and back surfaces function as capacitors. The thickness of the insulating sheet and the conductive thin film is determined considering the capacitor function of the RF shield and its mechanical strength. Specifically, the thickness of the insulating sheet is related to the capacitance of the capacitor formed by the conductors on the front and back surfaces. A thinner sheet increases the capacitance and reduces RF resistance, but if it is too thin, the structure becomes weaker, increasing the risk of damage such as holes forming. Similarly, a thicker conductive thin film can reduce RF resistance, but increasing its thickness also increases eddy currents due to gradient magnetic fields, so it is preferable to keep it as thin as possible without increasing RF resistance. As an example, consider a case where the conductive material on the front and back is 18 microns thick copper, and the insulating sheet in between is 100 microns thick FR-4 (glass epoxy). While the copper thickness is set at 18 microns as an example, it is preferable to make it thicker than the skin thickness at the RF frequency (approximately 64 MHz for a 1.5 Tesla MRI device) to avoid increasing RF resistance. Skin thickness is defined by the following formula.

number

[0033] Furthermore, the distance between the ring conductor of the birdcage-type coil 200 and the RF shield 300, i.e., the difference between the radius d300 of the RF shield and the radius d200 of the ring conductor of the birdcage-type coil 200 (d300-d200), is a design-critical parameter that significantly affects the irradiation efficiency of the high-frequency coil unit. Therefore, the size of the RF shield 300 is determined and the RF shield 300 is manufactured to ensure this difference is an appropriate value.

[0034] Furthermore, regarding the positional relationship with the gradient magnetic field coils 102 (402-404), as shown in Figure 2, in the gradient magnetic field coils 102 where the X coil 402 is positioned inside the cylinder, the X coil 402 is located closest to the RF shield 300, as shown in Figure 5. Figure 5 shows the gradient magnetic field coils and RF coils cut in a cross section perpendicular to the cylindrical axis. In the RF shield 300, it is important to suppress eddy currents generated by the current flowing through the gradient magnetic field coils 102, and in particular, countermeasures against eddy currents by the X coil 402, which is closest to the shield, are important.

[0035] In other words, as mentioned above, the main requirements that the RF shield must meet are: (1) having good electrical conductivity at RF frequencies, for example, approximately 64 MHz for a 1.5 Tesla MRI device; and (2) not overheating or burning even when eddy currents are induced by the magnetic field generated by the gradient coils in the frequency range of several kHz. The RF shield of this embodiment has the following configuration.

[0036] To satisfy the requirements of (1) and (2) above, the RF shield of this embodiment is configured as follows. That is, it is formed of double-sided copper foil sheets and has a pattern consisting of strips whose longitudinal direction is the axial direction of the cylinder, with the strips of the front and back patterns overlapping alternately in the regions at both ends of the cylinder, and overlapping without alternation in the central region. In addition, in the parts of both sides that are close to the ring conductor of the birdcage type coil, there is a pattern in which the strips (conductor parts) are connected. Furthermore, at least one cut section (a part where the conductor is interrupted: a discontinuity) is provided in the circumferential direction, and the front and back are connected by capacitors over a wide area. The part that is close to the lung conductor of the birdcage type coil (central region) also has at least one cut section in the direction of the cylindrical axial direction, and the front and back are connected by capacitors over a wide area.

[0037] The width of the cut section is determined based on the eddy currents that may occur in the cut section and the current flowing through the ring conductor of the RF coil. Furthermore, at least one cut section is sufficient in the side and central regions, and fewer cut sections are desirable in order to secure the area of ​​the capacitor.

[0038] In this embodiment, the RF shield has a structure in which strip-shaped patterns with slits are arranged in the areas on both sides where eddy currents are strong, thereby suppressing heat generation caused by eddy currents. Furthermore, both the part of the birdcage-type coil near the ring conductor and the part near the lung conductor are connected by large-area front and back capacitors, resulting in virtually no resistance around the magnetic resonance frequency (e.g., 64 MHz), providing an RF coil unit with excellent irradiation efficiency and durability.

[0039] The RF shield pattern of this embodiment will be described below with specific examples.

[0040] <Example 1> The conductive thin film pattern of the RF shield 300 in Example 1 will be described with reference to Figures 6 and 7. Figure 6 shows the cylindrical RF shield 300 unfolded in the circumferential direction. The upper part of Figure 6 shows the pattern on the front side (outer surface of the cylinder) (hereinafter referred to as the front pattern) 601, and the lower part shows the pattern on the back side (inner surface of the cylinder) (hereinafter referred to as the back pattern) 602. That is, the horizontal direction in Figure 6 is the axial direction of the cylinder, and the illustrated front pattern 601 and back pattern 602 form a cylindrical shape when their upper and lower ends are connected. However, the patterns shown in Figure 6 differ from the scale of an actual RF shield for the sake of simplicity of explanation.

[0041] As shown in the figure, the cylindrical RF shield 300 has a front pattern 601 and a back pattern 602 that are symmetrical when the axis of the cylinder's center is considered as left and right. Below, the details of the front pattern will be explained as a representative example.

[0042] The surface pattern 601 is divided into two regions (conductor regions) 604 and 605 by a circumferential cut portion 614, and the left region 604 is further divided vertically by an axial cut portion 613 at position 606. The size of the left and right regions is such that the width in the cylindrical axis direction from the circumferential joint of the smaller region 604 to the end toward the magnetic field center (i.e., the difference between the width 704 shown in Figure 7 and half the width of the central portion 715) is less than half the length in the cylindrical axis direction of the birdcage coil 200 (the distance between ring conductors), and preferably the width in the cylindrical axis direction of region 604 is three times or more the width in the cylindrical axis direction of the ring conductors of the birdcage coil 200. The front pattern, divided by these two cut sections, is further divided into a total of three regions (hereinafter also referred to as islands). Region 605 is longer in the left-right direction than region 604. Therefore, the cut section 614 is shifted in the axial direction of the cylinder between the front and back sides.

[0043] Each island has slits 611 and 612 forming strips from the left and right, but the left and right slits are not connected, and each strip has a section 603 that connects vertically at one point. The position (axial position) of this section 603 where the strips connect vertically is preferably such that the distance 704 (Figure 7) from the center of the pattern is half the distance between the ring conductors, and its width (axial length) is preferably as wide as is permissible for eddy current heating. The width of the strips in the circumferential direction (vertical direction in the figure) is the same for both regions 604 and 605, but one is formed with a half-width offset from the other. In the illustrated example, the upper and lower strips of region 604 are half the width.

[0044] The reverse pattern 602 is the same as the front pattern 601, but reversed left to right. These front and back patterns are made into a sheet, then rolled into a cylinder, and the ends are electrically connected by soldering or other means. At this time, the top and bottom edges of the right region 604 are connected. As a result, when the right region 604 is rolled into a cylinder, the pattern at position 606 is cut off, and the rest is electrically connected as a single region. On the other hand, in the left region, the top and bottom edges are not connected with a gap of about 1 mm. Similarly, on the back side, region 604 is electrically connected, but region 605 is not.

[0045] Figure 7 shows the state when the front pattern and back pattern are superimposed. In Figure 7, the thick lines represent the front pattern and the thin lines represent the back pattern. As shown in the figure, when the front and back strip shapes are superimposed, in the left portion 714 where the region 604 of the front pattern 601 and the region 605 of the back pattern 602 overlap, and in the right portion 714 where the region 605 of the front pattern 601 and the region 604 of the back pattern 602 overlap, the arrangement of the strips in regions 604 and 605 is shifted by half a width, so the front and back strips are superimposed alternately. On the other hand, in the central portion (center) 715 where the region 605 of the front pattern 601 and the region 605 of the back pattern 602 overlap, the slits coincide, and the strips are superimposed without being alternately oriented.

[0046] As a result, in the left and right sections 714, the alternating arrangement of the strips allows the ring current of the birdcage coil, i.e., the circumferential current, to flow effectively. Furthermore, in section 703, which is connected circumferentially without being cut by the slits, and the sections on either side of it, the circumferential current can be connected via capacitors made of copper foil on both sides. On the other hand, in the central section 715, the lung conductor of the birdcage coil 200 is located, and no circumferential current is generated, so there is no need to alternate the strips in this section. Moreover, not alternating them has the advantage of increasing the overlapping area of ​​one strip on each side.

[0047] The effect of the RF shield in this embodiment will be explained in comparison with the thin film pattern of the prior art RF shield shown in Figure 9.

[0048] The thin film pattern of the prior art RF shield, as shown in Figure 9, is common to the RF shield of this embodiment, as it is made up of strips (tiles) and the patterns are different on the front and back sides, and on the left and right sides in the axial direction and on the central part, corresponding to the ring conductor of the birdcage type coil and the lung conductor connecting it.

[0049] However, in the prior art thin film pattern, the longitudinal direction of the strips is axial in the center and circumferential at both ends, whereas in this embodiment, the longitudinal direction of the strips is entirely axial. Furthermore, in the prior art pattern, the thickness of the strip-shaped slits differs in various places, resulting in non-uniform eddy currents due to the gradient magnetic field, making it difficult to predict the heat distribution. In contrast, in the RF shield pattern of this embodiment, the width of the strips is uniform everywhere, so the heat generated by eddy currents mainly depends on the strength of the magnetic field created by the gradient magnetic field coil, which has the advantage of being easier to predict and countermeasures.

[0050] Furthermore, as shown in Figure 8, in a cylindrical gradient magnetic field coil, the point where the gradient magnetic field is strongest in the X or Y direction is at a distance D502 from the center, and it is necessary to determine the width of the strips so that eddy currents do not become excessive in that area. For this reason, in the prior art RF shield, as shown in Figure 10, it is desirable that the width of the strips at both ends be narrower on the side 802 closer to the end, and wider near the ring conductor 803 in order to maximize the area where the front and back patterns overlap, that is, a design that changes the width in stages is necessary. Also, since strips are formed in the circumferential direction in this area, if the width of the strips is W and the length of one circumference of the cylinder is L, the overlapping area of ​​the front and back is W × L / 2.

[0051] On the other hand, in the RF shield of this embodiment, it is sufficient to determine one width such that eddy currents do not become excessive at a distance D502, and only one width of strip is needed. Furthermore, in this section (at the position of distance D502), the front and back patterns are connected to each other in the circumferential direction, so if the width of region 604, i.e., the length of 714, is V, then the overlapping area is V × L. Since the length V of 714 can be about 7 times the width (702) W of the strip, the overlapping area V × L of the strips becomes about 14 times larger than the overlapping area W × L / 2 in the prior art. In other words, the capacitance of the capacitor formed by the front and back patterns is 14 times larger, the impedance component of the capacitor becomes 1 / 14, and this greatly contributes to the improvement in electrical conductivity at the RF frequency (reduction of complex impedance) mentioned above.

[0052] Furthermore, regarding points where multiple strips are adjacent, in this embodiment, the strips of two regions 604 and 605 are arranged alternately. For example, on one surface, at position 711 shown in Figure 7, three strips meet on only one side of the front and back. However, in the conventional technology, the patterns of regions 192 and 194 on both sides in the direction of the cylindrical axis have many points where four strips meet on only one side of the front and back. Electromagnetic field simulations have shown that if there are points where many corners of strips converge in areas where the mirror image current of the birdcage-type coil is relatively large, a large RF current may flow locally, causing heat generation. The RF local current is expected to be several times larger at points where four strips converge than at points where three strips converge. Therefore, the configuration of this embodiment is more effective in suppressing heat generation due to RF local currents compared to the conventional technology pattern.

[0053] As described above, the RF shield of this embodiment has the effect of suppressing the generation of eddy currents caused by the gradient magnetic field and the heat generated therefrom, while improving the electrical conductivity at RF frequencies and maintaining a high irradiation efficiency of the coil. This is achieved by (1) dividing the thin film patterns on the front and back into two regions with different axial lengths, forming strips in each region whose longitudinal direction is the axial direction of the cylinder, and (2) providing portions on both ends of the cylinder where the strips are connected in the circumferential direction. Furthermore, the RF shield of this embodiment has the effect of improving the electrical conductivity at RF frequencies and maintaining a high irradiation efficiency of the coil by widening the capacitor area formed by the strips on both sides of the cylinder that are joined together on the axial ends where the gradient magnetic field strength is high.

[0054] <Example 2> The thin film pattern of the RF shield in this embodiment is similar to the RF shield pattern of Embodiment 1, in which strips are divided in the direction of the cylindrical axis at one location on both the front and back sides, and also divided in the circumferential direction. However, in Embodiment 1, the strips are offset by half a width in the two regions divided in the direction of the cylindrical axis, whereas in this embodiment, there is no offset in the width direction (circumferential direction) of the strips in the two regions. Instead, the strips are offset by half a width in the circumferential direction between the front pattern and the back pattern.

[0055] The thin film pattern of the RF shield in this embodiment will be described below with reference to Figures 11 and 12. Figure 11 shows the front pattern 1101 and the back pattern 1102, and, similar to Figure 6, shows the cylindrical RF shield unfolded circumferentially around the axis of the cylinder. Figure 12 shows the front pattern 1101 and the back pattern 1102 superimposed, with the back pattern indicated by a dotted line.

[0056] The front pattern 1101 is cut by the cutting section 1114, dividing it into two regions 1104 and 1105 in the direction of the cylindrical axis. Region 1105 includes the center in the axial direction and is wider than region 1104. Furthermore, the front pattern 1101 is cut by the cutting section 1111, dividing it into four regions (islands) when unfolded. Each region has slits 1103 forming strips from the direction of the cylindrical axis, but each strip has two points (positions indicated by 1113) where it connects in the circumferential direction. The upper and lower ends of the strips are half the width (circumferential length) of the other strips.

[0057] When this type of table pattern 1101 is rolled up into a cylinder, the strips that are half the width of the pattern remain electrically connected. In other words, the end 1112 becomes a cut section where the conductor connection is broken.

[0058] The reverse pattern 1102 is also divided along the axial direction of the cylinder at the cutting section 1114. The position of the cutting section 1114 is symmetrical to the position of the cutting section 1114 of the front pattern. The reverse pattern 1102 does not have a cutting section (cutting section 1111 of the front pattern) that cuts the pattern along the axial direction of the cylinder. As a result, the reverse pattern 1102 is divided into two islands. The reverse pattern 1102 also has slits forming strips from the axial direction of the cylinder, and each strip has a part that connects in the circumferential direction at two places (positions indicated by 1113). The width of all the strips is the same as the width of the strips of the front pattern 1101, excluding the top and bottom strips.

[0059] In this type of backing pattern 1102, when rolled up into a cylindrical shape, the upper and lower strips are not electrically connected to each other.

[0060] In the integrated front-to-back state of these patterns, as shown in Figure 12, the strip arrangement is the same as in Example 1 at the end of the X gradient magnetic field coil where the intensity is high, and the generation of eddy currents can be limited, just as in Example 1. Furthermore, since the pattern is continuous along the current flowing through the ring conductor of the birdcage-type coil at position 1113, i.e., the current flowing in the circumferential direction, the electrical conductivity at RF frequencies can be improved, and the irradiation efficiency of the coil can be maintained at a high level. In other words, the same effects as in Example 1 can be obtained with the pattern of this embodiment.

[0061] Regarding the points where multiple strips meet, in Example 1, when viewed as a single-sided pattern, there were points where up to three strips met on one side of the front and back. In contrast, in this embodiment, for example, at point 1211, four strips meet on one side of the front and back. Therefore, if these points are located in areas where the mirror image current of the birdcage-type coil is large, a large RF current may flow locally. However, this can be avoided by adjusting the position of the cut section 1114.

[0062] Furthermore, the thin film pattern of this embodiment only requires electrical connection at the top and bottom of one of the two patterns (front and back) when forming it into a cylinder, making the manufacture of the RF shield easier compared to the thin film pattern of Embodiment 1, which has electrical connection points on both the front and back surfaces. For example, by designating only the inner pattern, which is easier to connect, as the front pattern shown in Figure 11, the front and back patterns can be formed into a sheet, then rolled into a cylinder with the back pattern facing outwards, and electrical connections can be made from the inside in that state, thus improving work efficiency.

[0063] As described above, the RF shield of the present invention is composed of strips whose longitudinal direction is the axis of the cylinder, and is characterized in that at least on both sides of the cylinder, the front and back strips overlap alternately and there are parts where the strips connect. This suppresses the generation of eddy currents due to gradient magnetic fields and the heat generated therefrom, and also improves electrical conductivity at RF frequencies, thereby maintaining a high irradiation efficiency of the RF coil. The RF shield of the present invention has a simple pattern, and unlike the conventional technology, it is not necessary to consider and determine the width of the strips by considering many different lengths. It is mainly necessary to determine the length (703) of the parts where the strips connect (connection parts) and the width W (702) of the strips, so the design is simple. [Explanation of Symbols]

[0064] 100: MRI device, 101: Magnet, 102: Gradient coil, 103: RF antenna, 104: Transmitter / receiver, 105: Data processing unit, 106: Transmitter / receiver cable, 107: Gradient control cable, 108: Display device, 109: Gradient power supply, 111: Bed, 112: Subject, 200: Birdcage coil, 203: Ring conductor, 204: Lung conductor, 300: RF shield, 402, 403, 404: Gradient coil, 601, 602: RF shield pattern (front and back), 1101, 1102: RF shield pattern (front and back)

Claims

1. A high-frequency coil unit comprising a cylindrical RF coil having ring conductors positioned at both ends and a plurality of lung conductors connecting the ring conductors at both ends, and a cylindrical RF shield positioned to cover the outer circumference of the RF coil, The RF shield includes an insulating sheet and a conductor pattern formed on the front and back surfaces of the insulating sheet, wherein the conductor pattern is divided into first and second conductor regions of different sizes along the axial direction of the cylinder, and each conductor region includes a plurality of strips that have a connection portion along the circumferential direction of the cylinder at at least one location along the axial direction of the cylinder and are partially separated by a plurality of slits partially formed along the axial direction of the cylinder, and the connection portion along the circumferential direction of the cylinder is cut at at least one location around the circumference of the cylinder, and the position of the cut portion is offset in the circumferential direction between the conductor pattern on the front surface and the conductor pattern on the back surface. A high-frequency coil unit characterized in that the conductor pattern on the surface and the conductor pattern on the back surface are arranged so that one first conductor region and the other second conductor region overlap, and in the portion where the first conductor region and the second conductor region overlap, the slits in the conductor pattern on the surface surface and the slits in the conductor pattern on the back surface are offset in the circumferential direction.

2. A high-frequency coil unit according to claim 1, The high-frequency coil unit is characterized in that each of the plurality of slits has a discontinuity in a part of its axial direction, and adjacent strips are connected through the discontinuity.

3. A high-frequency coil unit according to claim 2, The high-frequency coil unit is characterized in that the discontinuous portion is formed at a position that overlaps with the ring conductor of the RF coil.

4. A high-frequency coil unit according to claim 2, The high-frequency coil unit is characterized in that the width of the discontinuity is determined based on the eddy currents that may occur in the discontinuity and the current flowing through the ring conductor of the RF coil.

5. A high-frequency coil unit according to claim 1, A high-frequency coil unit characterized in that, of the first and second conductor regions, the width from the magnetic field center-side end of the smaller conductor region to the circumferential connection portion is less than half the length in the cylindrical axis direction of the RF coil, and the width in the cylindrical axis direction of the smaller conductor region is three times or more the width in the cylindrical axis direction of the ring conductor of the RF coil.

6. A high-frequency coil unit according to claim 1, A high-frequency coil unit characterized in that a plurality of slits formed in a first conductive region and a plurality of slits formed in a second conductive region are all spaced at the same interval, and the slits in the first conductive region and the slits in the second conductive region are formed in an alternating pattern.

7. A high-frequency coil unit according to claim 1, The multiple slits formed in the first conductive region and the multiple slits formed in the second conductive region each have the same slit spacing. A high-frequency coil unit characterized in that the conductive patterns on the front surface and the conductive patterns on the back surface are arranged such that the slits formed in the first and second conductive regions of one conductive pattern and the slits formed in the first and second conductive regions of the other conductive pattern are staggered.

8. A magnetic resonance imaging apparatus characterized by comprising a high-frequency coil unit according to any one of claims 1 to 7.

9. The magnetic resonance imaging apparatus according to claim 8, wherein the high-frequency coil unit is an irradiation coil.

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