JP2006000295A - Volume coil for MR device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a feeling of blockage of a subject, facilitate an experiment such as fMRI, and enable decoupling of a coil.
A volume coil for an MR device used for transmission / reception of RF radio waves, the volume coil comprising a plurality of conductive rods and having a radio wave shield wound on an outer surface, wherein the radio wave shield includes a plurality of conductive rods. A plurality of conductive segments electrically connected to each other and a capacitor connecting the conductive segments, and the conductive segments have a mesh pattern. One of the conductive rods corresponds to one of the conductive segments, one closed current path including each conductive rod is provided, a blocking circuit is inserted in the closed current path, and corresponds to one end of the closed current path The conductive segments are electrically connected.
[Selection] Figure 3

Description

  The present invention relates to an RF coil for an MR apparatus that transmits or receives an RF (radio frequency) radio wave for measuring an echo signal of MRI (magnetic resonance imaging) or MRS (magnetic resonance spectroscopy). The present invention relates to a volume coil for an MR apparatus used for photographing an image.

  A magnetic resonance diagnostic apparatus (MR apparatus) that performs MRI and MRS measurements applies a static magnetic field having a uniform intensity in the examination region and also applies RF waves polarized in a plane perpendicular to the direction of the static magnetic field to the examination target. It is an apparatus that irradiates a certain nucleus, receives RF radio waves emitted by the excited nucleus, and processes the signal. The position where the signal is emitted is estimated and displayed as an image by MRI (Magnetic Resonance Imaging), and the frequency of the signal is analyzed and the contained component is analyzed by MRS (Magnetic Resonance Spectroscopy). In either method, a gradient magnetic field generator is used to limit the examination area or to identify where the signal is being emitted. The MRI apparatus and the MRS apparatus are exactly the same in terms of hardware configuration, and only the signal processing format including the imaging process is different. The RF coil is used for transmitting and receiving RF radio waves in these devices. A shield is provided between the RF coil and the gradient magnetic field generator to prevent leakage of electromagnetic waves to the outside of the RF coil and reception of external electromagnetic waves. (For example, see Patent Documents 1 and 2)

  The frequency of the RF radio wave transmitted and received by the RF coil increases in proportion to the static magnetic field strength. For example, when the static magnetic field strength exceeds 3T (Tesla), the magnetic resonance frequency of the hydrogen atom becomes 120 MHz or more, and the RF coil used at that time irradiates a large amount of electromagnetic waves to the outside, causing loss of transmission power and reception signal. The SN ratio is degraded. Therefore, it is more important to shield the outer surface of the RF coil to prevent the dissipation of radio waves to the outside during transmission and to suppress the mixing of noise radio waves from the outside during reception. Therefore, according to the conventional technique, a conductor such as a copper foil or a thin copper plate is wound around the outer surface of the RF coil as a shield, thereby preventing leakage of electromagnetic waves to the outside and reception of external electromagnetic waves. (For example, see Patent Documents 2, 3 and 4)

  In an MR apparatus, coil decoupling or detuning (necessary for imaging in an environment where a plurality of coils coexist) is temporarily disabled by electrically disconnecting a coil circuit or breaking resonance conditions. Means) is provided. There is a blocking circuit as this means. This blocking circuit is a circuit for temporarily stopping the function of the RF coil. The RF coil is adjusted to resonate at a specific frequency determined by the measurement nucleus and the magnetic field strength. When transmission is performed with a volume coil and reception is performed with another surface coil, two coils are placed close to each other. However, if these two resonance circuits are magnetically coupled to each other, the resonance characteristics are deteriorated. . For this reason, it is necessary to disconnect the circuit of the reception coil at the time of transmission and to disconnect the circuit of the transmission coil at the time of reception so that a plurality of coils do not work simultaneously. This switching is the role of the blocking circuit.

  According to the prior art, the RF shield attached to the inside of the gradient coil is divided into a plurality of copper foils, a diode is interposed between the copper foils, and a direct current is applied to the copper foil by a blocking circuit. A switch control circuit connected and separated in RF is provided, and the diode is actively controlled by changing the polarity of the DC bias voltage from the switch control circuit. By connecting the foil and blocking each part when the gradient magnetic field changes, the performance of the RF shield can be improved and overcurrent due to the gradient magnetic field can be reduced. (For example, see Patent Document 2)

  Among conventional RF coils, in particular, a volume coil for an MR apparatus used for imaging the head of a subject is formed by a cylindrical cavity having an opening for inserting the head inside. ing. The cylindrical cavity is composed of an outer surface, an inner surface and both end surfaces. For example, 16 elements extending in the axial direction are arranged between the inner surface and the outer surface of the cavity. The outer surface of the coil is covered with a cylindrical shield having a diameter of 34 cm and a height of 22 cm (see Non-Patent Document 1, page 541).

JP 7-39539 A JP-A-8-252234 Japanese Patent Laid-Open No. 62-111542 Japanese Patent Laid-Open No. 5-113473 Brian A. Baertlein et al. "Theoretical Model for an MRI Radio Frequency Resonator" IEEE TRANSACTION ON BIOMEDICAL ENGINEERING. VOL.47. NO.4. APRIL 2000

  According to the prior art, since the outside of the RF coil is blocked with a conductor, the subject is forced to feel occluded. This was particularly noticeable in the case of a volume coil used for inspection of the head or the like. In addition, when performing an experiment using fMRI (magnetic resonance functional imaging), it was difficult to attach a mirror for applying optical stimulation. In addition, coil decoupling and detuning can also be performed in an RF coil having a resonance frequency exceeding 120 MHz because the current flowing in the coil is distributed to the copper foil or copper plate of the shield, so that the coil circuit is electrically disconnected. I couldn't.

  An object of the present invention is to reduce the feeling of obstruction of a subject and facilitate experiments such as fMRI, and in addition to this, to enable coil decoupling.

  According to the present invention, there is provided a volume coil for an MR device used for transmission / reception of RF radio waves, wherein the radio wave shield includes the plurality of conductive rods and has a radio wave shield wound on an outer surface thereof. A plurality of conductive segments electrically connected to the conductive rods and a capacitor connecting the conductive segments, wherein the conductive segments have a see-through structure. An apparatus volume coil is provided.

  One of the conductive rods corresponds to one of the conductive segments, one closed current path including each conductive rod is provided, a blocking circuit is inserted in the closed current path, and the closed current path includes A conductive segment corresponding to one end may be electrically connected.

  The capacitor may be configured by overlapping a part of adjacent conductive segments with a dielectric interposed therebetween.

  The radio wave shield is preferably covered with a transparent insulating structure.

  The insulating structure may be configured to hold the radio wave shield.

  The conductive segment having the transparent structure may be a conductive segment made of a transparent conductive material, a conductive segment having a mesh pattern, or a conductive segment having a perforated pattern.

  According to the present invention, since the conductive segment of the radio wave shield has a structure that can be seen through, the outside can be seen, and the effect of relieving the feeling of blockage of the subject can be obtained. In addition, an effect of facilitating light stimulation in experiments such as fMRI can be obtained.

  According to the present invention, since the conductive segments of the radio wave shield are connected by the capacitor, the eddy current due to the gradient magnetic field can be reduced without reducing the shielding performance. That is, the capacitor cuts the conductive segment for each conductive rod of the volume coil in a direct current manner to suppress the generation of eddy currents caused by switching of the gradient magnetic field, but is connected to a high frequency. Therefore, electromagnetic waves can be effectively blocked.

  According to the present invention, one of the conductive rods corresponds to one of the conductive segments, one closed current path including each conductive rod is provided, and a blocking circuit is inserted in the closed current path, Since the conductive segment corresponding to one end of the closed current path is electrically connected, an effect of easily realizing the decoupling and detuning functions can be obtained.

  According to the present invention, since the radio wave shield is covered with the transparent insulating structure, not only the effect of relieving the blockage of the subject but also the effect of preventing electric shock can be obtained.

  FIG. 1 shows a simple configuration of an MR apparatus to which the volume coil of the present invention can be applied. This MR apparatus includes a static magnetic field coil 11 for applying a static magnetic field having a uniform intensity to an inspection region, a gradient magnetic field coil 12 for limiting the inspection region or specifying a place where a signal is output, and an RF radio wave. A volume coil 13 that receives an RF radio wave emitted from a nucleus excited by irradiating a nucleus to be inspected, a gradient magnetic field generator 14 that drives a gradient magnetic field coil to generate a gradient magnetic field, and a volume coil 13 The RF transmitter / receiver 15 that drives to transmit and receive the RF radio wave and amplifies the radio wave signal received by the volume coil 13, and the gradient magnetic field coil 12 and the volume coil 13 operate in a predetermined operation sequence. A control device 16 that controls the gradient magnetic field generator 14 and the RF transmitter / receiver 15; And a signal processing unit 17 for processing an RF radio signal.

  FIG. 2 is a schematic perspective view showing a part of the volume coil 21 of the present invention. The volume coil 21 has a cylindrical shape so that the head of the subject can be placed therein, and extends in the axial direction between the two conductive end rings 22a and 22b and the conductive end ring. 8 conductive rods 23 are provided. In this embodiment, an example is shown, and the number of conductive rods is eight, but other numbers may be used. For example, the number may be 16.

  FIG. 3 is a schematic perspective view of the volume coil 31 of the present invention, illustrating the relationship between the volume coil 21 of FIG. 2 and the radio wave shield 32 wound around the outer surface thereof. However, in FIG. 3, only the both ends of the conductive rod 23 of FIG. 2 are shown and most of them are omitted. The radio wave shield 32 is composed of eight sets of conductive segments 33 (only three sets of conductive segments 33a, 33b, and 33c are shown) arranged so as to cover the cylindrical outer surface of the volume coil 21. Each conductive segment has a mesh pattern composed of a wire extending in the axial direction of the volume coil and a wire orthogonal to the wire, and corresponds to each conductive rod 23 of the volume coil 21. The length of the conductive segment 33 in the axial direction is longer than the length of the conductive rod 23 in the axial direction, and both ends of each conductive segment extend beyond both ends of the volume coil toward the centers of the end rings 22a and 22b. It is bent inward and electrically connected to both ends of the corresponding conductive rod 23.

  A capacitor 34 (only three capacitors 34a, 34b, and 34c are shown) is connected between the conductive segment and the other conductive segments, so that each conductive segment is disconnected in a direct current manner. The eddy current due to the gradient magnetic field coil is prevented from flowing to the radio wave shield. The mesh pattern of the conductive segments is made of a conductive wire or a narrow tape. Moreover, all the intersections of each wire are electrically connected. It is preferable that the opening ratio of the mesh pattern is as large as possible because the transparency is improved and the feeling of occlusion is reduced. However, considering the shielding performance, the opening width of the mesh putter is about 1 mm to 1 cm, and is exaggerated in the drawing of the present application rather than the actual opening size. In the figure, three capacitors are shown. However, any number of capacitors can be used as long as an eddy current suppressing effect can be obtained.

  When the volume coil of the present invention is used for a human being or an animal, a transparent insulating structure such as an acrylic pipe material is used for a portion where there is a risk of electric shock, such as the outside of the radio wave shield 32 and the inside of the volume coil where the subject enters. It is necessary to cover it with electrical safety. When the radio wave shield 32 is formed by punching an aluminum thin plate and the strength as a structure is not sufficient alone, this transparent insulating structure is attached and held outside or inside the volume coil, and the strength We can make up for the shortage. The attachment method is not limited to screwing or pasting.

  FIG. 4 is a developed view of a part of the radio wave shield as shown in FIG. 3, and only two conductive segments 43a and 43b are shown. The two conductive segments are connected to each other by three capacitors 44a, 44b, and 44c. In the figure, broken lines 45a and 45b indicate two end rings 22a and 22b as shown in FIG. 3, and the portions of the conductive segments extending above and below the broken lines are folded in FIG. This is an expanded version. In FIG. 4, 23a and 23b have shown the both ends of the two conductive rods 23 corresponding to the conductive segments 43a and 43b, respectively.

  5 and 6 are development views showing two conductive segments similar to those in FIG. 4, and show another example of a mesh pattern. The conductive segment in FIG. 5 has a pattern formed of a plurality of parallel wires that obliquely intersect from the left and right with respect to the wires extending in the axial direction of the volume coil. The conductive segment in FIG. 6 does not have a wire extending in the axial direction of the volume coil, but has a rhombus-like mesh pattern formed of a plurality of parallel wires extending obliquely from the left and right with respect to the axial direction and intersecting each other. Have. Each of the two conductive segments shown in FIGS. 5 and 6 is connected by three capacitors and both ends thereof are electrically connected to the corresponding conductive rods, as in the conductive segment of FIG. Connected.

  FIG. 7 shows a radio wave shield according to still another embodiment of the present invention. The mesh pattern shown in FIG. 7 is similar to the mesh pattern shown in FIG. 4, but differs in that each conductive segment constitutes one current circuit with each corresponding conductive rod. That is, in the mesh pattern of FIG. 7, the wire rods 76 a and 76 b extending in the axial direction at the left and right ends of the two conductive segments 73 a and 73 b are respectively the center wire rod 77 a among the plurality of wire rods orthogonal to the wire rod. And 77b only. Further, the connection between each of the conductive segments 73a and 73b and the corresponding conductive rod 23 is achieved by connecting the end portions 23a and 23b of the respective conductive rods only to both ends of the wire rods 76a and 76b. . Thus, one closed current path including each of the wire rods 76a and 76b and each conductive rod is provided, and a conductive segment corresponding to one end of the closed current path is electrically connected. Then, blocking circuits 78a and 78b are inserted into the wire rods 76a and 76b forming the closed current path, respectively. With such a configuration, a current flowing between each conductive segment and each conductive rod always passes through the wires 76a and 76b, and each of the conductive segments and each of the conductive segments is blocked by each of the blocking circuits 78a and 78b. It is possible to control the opening and closing of a current circuit composed of a conductive rod.

  The blocking circuits 78a and 78b have a circuit configuration as shown in FIG. That is, the PIN diode 81 is combined with a resonance circuit using lumped constant components including an inductance L and a capacitance C. The resonance frequency of this resonance circuit is the same as the resonance frequency of the volume coil. As an operation of this blocking circuit, the opening and closing of the coil resonance circuit is controlled by applying a forward or reverse bias voltage to both ends of the PIN diode 81.

  The radio wave shield including such a blocking circuit is useful when dedicated volume coils are used for transmission and reception of RF radio waves, respectively. That is, it is useful when the volume coil is used as a transmission RF coil and the surface coil is used as a reception RF coil.

  FIG. 9 shows a volume coil 91 and a radio wave shield 92 wound around the outer surface of the volume coil 91. The radio wave shield 92 includes a plurality of conductive segments 93 and a plurality of capacitors 94 connected therebetween. Both ends 93a and 93b of one axial wire rod of each conductive segment 93 are electrically connected to both ends 95a and 95b of the plurality of conductive rods 95 of the volume coil 91, similarly to the radio wave shield 32 of FIG. Is done. Further, two surface coils 96 are arranged inside the volume coil 91. The radio wave shield 92 is actually provided with a blocking circuit as shown in FIG. 7, but is not shown in FIG. The surface coil 96 is also provided with another blocking circuit, which is not shown in FIG.

  FIG. 10 simply shows an imaging sequence when imaging is performed by the gradient echo method in an MR apparatus using a volume coil as a transmission RF coil and a surface coil as a reception RF coil. FIG. 10 simply shows the relationship between the blocking timing of the surface coil and the volume coil, the generation timing of the RF transmission pulse, and the generation timing of each pulse of the gradient magnetic field X, the gradient magnetic field Y, and the gradient magnetic field Z by the gradient magnetic field coil. It shows. As shown in FIG. 10, when an RF transmit pulse is generated, the volume coil is not blocked and is in operation, while the surface coil is blocked and the coil circuit is disconnected, Inactive. When the subject is irradiated with RF radio waves from the volume coil, echo signals generated from the measurement site of the subject specified by the gradient magnetic field X, the gradient magnetic field Y, and the gradient magnetic field Z are received by the surface coil. The surface coil is adjusted so that the surface coil is not blocked at the center of the echo so that the echo signal is effectively received by the surface coil.

  As described above, the radio wave shield including the blocking circuit as shown in FIG. 7 is used when the volume coil is used exclusively for transmission or reception without being used for transmission / reception, as shown in FIGS. The radio wave shield not including the blocking circuit is useful when the volume coil is used for both transmission and reception.

  FIG. 11 shows a mesh pattern of a radio wave shield including blocking circuits 78a and 78b similar to FIG. 7, and illustrates an example of a method of forming a capacitor using a conductive segment. The difference from the mesh pattern in FIG. 7 is that the capacitor 4 is formed by overlapping the ends of adjacent conductive segments 73a and 73b with a dielectric interposed therebetween. In general, a commercially available small-capacitance capacitor that allows only high frequencies to pass can be used as the capacitor, but any other means that has an equivalent function can be realized. Such a capacitor can be used not only for a radio wave shield including a blocking circuit as in the mesh pattern of FIG. 7, but also for a radio wave shield not including a blocking circuit as shown in FIGS. In addition, the mesh pattern is not limited to the shape shown in the drawings of the present application, as long as it can achieve a predetermined shielding effect and has an effect of reducing the feeling of obstruction of the subject by providing transparency, Other grid patterns can be used.

  Further, the conductive segment may have a perforated pattern instead of the mesh pattern. This perforation pattern is made by punching a sheet material or the like. When the opening is formed by punching as in this perforation pattern, the shape is not particularly limited to a circle or a square. Further, the formation of the mesh pattern or the perforated pattern does not need to be uniform as a whole. Furthermore, instead of these mesh patterns or perforation patterns, the conductive segments can be made of a transparent conductive material.

  The volume coil of the present invention can be used for an MR apparatus that uses one RF coil for both transmission and reception and an MR apparatus that uses two RF coils for transmission and reception. Further, it can be used for an MR apparatus having a magnetic field strength of 3T or more.

It is a figure which shows the simple structure of MR apparatus which can apply the volume coil by this invention. It is a schematic perspective view of a part of the volume coil according to the present invention. It is a schematic perspective view of the volume coil by this invention. It is an expanded view which shows a part of electromagnetic wave shield of the volume coil by this invention. FIG. 5 is a developed view similar to FIG. 4 showing an example of a mesh pattern. FIG. 5 is a developed view similar to FIG. 4 showing an example of a mesh pattern. It is an expanded view which shows a part of radio wave shield of the volume coil by this invention containing a blocking circuit. It is a simple block diagram of a blocking circuit. It is a disassembled perspective view which shows the volume coil of this invention, and the surface coil used with it. It is a figure which shows the image | video sequence in the case of using the volume coil of this invention with a surface coil. It is a figure which shows another Example of the capacitor | condenser used for the volume coil of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Static magnetic field coil 12 Gradient magnetic field coil 13 Volume coil 14 Gradient magnetic field generator 15 RF transmission / reception apparatus 16 Control apparatus 17 Signal processing apparatus 21, 91 Volume coil 22a, 22b, 45a, 45b Conductive end ring 23, 95 Conductive rod 23a , 23b, 95a, 95b Conductive rod end 31 Volume coil 32, 92 Radio wave shield 33, 43a, 43b, 73a, 73b, 93 Conductive segment 4, 34, 34a, 34b, 34c Capacitors 76a, 76b, 77a, 77b Wire material 78a, 78b Blocking circuit L Inductance C Capacitance 81 PIN diode 93a, 93b Both ends of wire material 96 Surface coil

Claims (6)

  An MR device volume coil used for transmission / reception of RF radio waves, wherein the radio wave shield is electrically connected to the plurality of conductive rods. A volume coil for an MR apparatus, comprising: a plurality of electrically connected conductive segments; and a capacitor for connecting the conductive segments, wherein the conductive segments have a transparent structure.   One of the conductive rods corresponds to one of the conductive segments, one closed current path including each conductive rod is provided, a blocking circuit is inserted in the closed current path, and the closed current path includes The volume coil for an MR device according to claim 1, wherein a conductive segment corresponding to one end is electrically connected.   3. The volume coil for an MR device according to claim 1, wherein the capacitor is configured by overlapping a part of adjacent conductive segments with a dielectric interposed therebetween.   The volume coil for MR apparatus according to claim 1, wherein the radio wave shield is covered with a transparent insulating structure.   The volume coil for MR apparatus according to claim 4, wherein the insulating structure holds the radio wave shield.   The conductive segment having the transparent structure is a conductive segment made of a transparent conductive material, a conductive segment having a mesh pattern, or a conductive segment having a perforated pattern. The volume coil for MR apparatus as described in any one of -5.

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