JP5555081B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention measures nuclear magnetic resonance (hereinafter referred to as `` NMR '') signals from hydrogen, phosphorus, etc. in a subject and visualizes nuclear density distribution, relaxation time distribution, etc. In particular, the present invention relates to a magnetic resonance imaging apparatus with improved openness of a space in which a subject is placed.

  The MRI device measures NMR signals generated by the spins of the subject, especially the tissues of the human body, and visualizes the form and function of the head, abdomen, limbs, etc. in two or three dimensions Device. In imaging, NMR signals are given different phase encodings depending on the gradient magnetic field, frequency encoded, and measured as time-series data, and the measured NMR signals are subjected to two-dimensional or three-dimensional Fourier transform to produce an image. Reconfigure.

  Some of these MRI apparatuses use a static magnetic field generation apparatus for generating a static magnetic field, and a superconducting coil is used as the static magnetic field generation source. Hereinafter, such a static magnetic field generator is simply referred to as a superconducting magnet. This superconducting magnet mainly includes a tunnel type structure and an opposed type structure. The tunnel-type superconducting magnet has a cylindrical gap space inside, and generates a uniform static magnetic field in the cylinder axis direction in the gap space. Photographs are arranged so that the direction of the magnetic field coincides. On the other hand, a counter-type superconducting magnet has a pair of superconducting coils arranged coaxially with a gap space in between, and generates a uniform static magnetic field in the opposite direction in the gap space. Photographs are arranged so that the direction of the static magnetic field is orthogonal.

  However, in the case of a tunnel type superconducting magnet, the subject is placed in a narrow cylindrical space at the time of imaging, which gives the subject a feeling of blockage. The feeling of obstruction here is characterized by the following three. The first is the distance in the line of sight when the subject goes to sleep. Even if the lateral direction is wide open, if there is something like a wall blocking the front, you will feel a sense of blockage. The second is that the degree of freedom on both sides of the subject is hindered. A configuration in which both sides press down gives a feeling of blockage. Third is the accessibility of support personnel. In addition to the above two, the fact that the subject is located at a position where it is difficult to access from the surroundings is a psychologically obstructive feeling.

  These occlusion problems are particularly acute for subjects with claustrophobia and may not be able to be examined. In addition, if an infant is not able to confirm the mother nearby, he / she becomes anxious and may start moving during shooting, and may not be able to take a picture. In addition, although it is necessary to support an unconscious subject or a non-cooperating subject from the surroundings, the tunnel type described above makes it difficult to access the subject from the outside. Furthermore, even in interventional MRI in which a procedure is performed while imaging is performed or a drug is administered via a catheter, it is difficult to access the subject from the outside.

  On the other hand, there is a conventional technique described in Patent Document 1 as a related art related to a magnetic resonance imaging apparatus in which the cross-sectional shape of the static magnetic field generating means is substantially polygonal in order to reduce the feeling of claustrophobia in the subject.

International Publication WO2007 / 007630 Specification

  However, the prior art described in Patent Document 1 only discloses a technique regarding only the cross-sectional shape of the static magnetic field generating means, and discloses what kind of magnetic resonance imaging apparatus can be provided by combining a gradient magnetic field coil and a high frequency coil. There wasn't. An object of the present invention is to provide a magnetic resonance imaging apparatus with little feeling of blockage by combining a gradient magnetic field coil and a high frequency coil. In particular, the object is to solve the above two of the three elements that give a feeling of blockage. In other words, one of the objectives is to increase the distance in the line of sight when the subject is sleeping, and the other is to provide a wide patient space that cannot be pressed from both sides. A method to solve these two purposes simultaneously is provided.

In order to solve the above problems, the magnetic resonance imaging apparatus of the present invention is configured as follows. That is, according to the present invention, a static magnetic field generating unit that is arranged around an imaging space in which a subject is arranged and generates a horizontal static magnetic field in the imaging space, and the static magnetic field generating unit on the imaging space side A magnetic field provided with a gradient magnetic field generating means for generating a gradient magnetic field in the imaging space, and a high frequency magnetic field generating means for generating a high frequency magnetic field in the imaging space, disposed on the imaging space side of the gradient magnetic field generating means In a resonance imaging apparatus,
A magnetic resonance imaging apparatus is provided in which a cross-sectional shape of the high-frequency magnetic field generating means on the imaging space side is an upwardly convex substantially triangular shape.

  Further, the static magnetic field generating means has a circular cross-sectional shape on the imaging space side, and the gradient magnetic field generating means has a cross-sectional shape on the imaging space side formed in a substantially triangular shape protruding upward. A featured magnetic resonance imaging apparatus is provided.

  Further, the static magnetic field generating means has a circular cross-sectional shape on the imaging space side, and the gradient magnetic field generating means has a circular cross-sectional shape on both the static magnetic field generating means side and the imaging space side. A magnetic resonance imaging apparatus is provided.

  In addition, a magnetic resonance imaging apparatus is provided in which the gradient magnetic field generating means is an active shield type gradient magnetic field coil composed of a shield coil having a circular cross section and a substantially triangular main coil having a convex cross section.

  In addition, there is provided a magnetic resonance imaging apparatus characterized in that an inner surface of the gradient magnetic field generating means on the imaging space side and an RF shield on the surface and an RF coil are not constant.

Furthermore, RF coils, magnetic distance between the RF shield the density of the coil conductor in the vicinity of the short point, than the density of the RF shield and the distance the coil conductors kick in the vicinity of the long point of the to be being greater A resonance imaging apparatus is provided.

Further, the RF coil is in the vicinity of the location distance is short and the RF shield, the magnetic resonance imaging apparatus characterized by the length of the coil conductor is longer is provided.

  According to the present invention, a magnetic resonance imaging apparatus with little occlusion can be provided by combining a gradient magnetic field coil and a high-frequency coil.

  In particular, the bore into which the subject enters has a substantially triangular shape that is convex upward, and the distance in the line-of-sight direction can be made longer than before. In addition, for the subject lying on the base of a substantially triangular shape, the distance in the horizontal direction in the bore is wider than before, and a sense of openness can be felt more than before.

FIG. 3 shows Example 1. FIG. 5 shows a second embodiment. The figure which shows the example which forms RF irradiation coil by the rung of substantially equal intervals. The figure which shows the example which arrange | positions a rung conductor with an uneven arrangement | positioning feeling. The figure which shows the example which varies the length of a rung conductor. FIG. 5 shows Example 3. Overview of an example of an MRI system The figure which shows the prior art example of a static magnetic field magnet.

  Hereinafter, preferred embodiments of the superconducting magnet for an MRI apparatus of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments of the invention, and the repetitive description thereof is omitted.

  First, an overall outline of an example of an MRI apparatus will be described with reference to FIG. The MRI apparatus shown in FIG. 7 includes a static magnetic field magnet 12 that generates a static magnetic field to be applied to the subject 10 placed on the table 19, and three different directions that are applied to the subject 10, for example, X, Y, and Z A gradient coil 14 for generating an axial gradient magnetic field is provided. A gradient magnetic field is applied with one of these three directions as a slice direction, a phase encoding direction, and a frequency encoding direction. Further, an RF coil 16 that generates a high-frequency magnetic field pulse to be applied to the subject 10, that is, an RF pulse, and an RF probe 18 that detects an NMR signal or echo signal generated from the subject 10 are provided. Further, a signal processing unit 20 that reconstructs an image based on an echo signal obtained by the RF probe 18 and a display unit 22 that displays an image reconstructed by the signal processing unit 20 are provided.

  The operation of the MRI apparatus configured as described above will be described. A static magnetic field is applied to the subject 10 by the static magnetic field magnet 12. A slice selection gradient magnetic field is applied from the gradient magnetic field coil 14 to the observation site (for example, the heart) of the subject 10 to which the static magnetic field is applied in accordance with the current from the gradient magnetic field power supply 26 based on the command of the control unit 24. Is done. Further, along with the application of the slice selective gradient magnetic field, an RF pulse is applied from the RF coil 16 to the observation site in accordance with the high-frequency current from the high-frequency transmission unit 28 based on the command of the control unit 24. As a result, an NMR phenomenon is induced in a nucleus (for example, a hydrogen nucleus) in the constituent material of the observation site, and an echo signal is generated. In order to acquire position information of the observation site where the NMR phenomenon has been induced, a phase encoding gradient magnetic field and a frequency encoding gradient magnetic field are applied from the gradient coil 14 to the echo signal. This echo signal is detected by the signal detection unit 30 from the RF probe 18 based on a command from the control unit 24. Based on the detected echo signal, the tomographic image of the observation site is reconstructed two-dimensionally or three-dimensionally by the signal processing unit 20, and the reconstructed two-dimensional or three-dimensional image is displayed on the display unit 22.

  A conventional example of the static magnetic field magnet 12 is shown in FIG. FIG. 8 shows a tunnel-type superconducting magnet 12a having an axial length L, and in a cylindrical void space 50 (hereinafter, a void space to which a static magnetic field in which a subject is placed is applied is referred to as “subject space”). The subject 10 is placed and photographed.

  Next, Example 1 of the present invention will be described. Example 1 is a form in which at least the cross-sectional shape perpendicular to the direction of the static magnetic field in the subject space is a substantially triangular shape.

  Example 1 is shown in FIG. In FIG. 1, the tunnel superconducting magnet has a cylindrical inner cylinder, and a gradient magnetic field coil 14 is disposed inside thereof. The gradient magnetic field coil 14 has a cylindrical shape on the outer side together with the inner cylinder of the magnet, while the inner side has a substantially triangular shape. The term “substantially triangular” as used herein refers to a shape having three vertices, and the side is not necessarily a straight line. Further, the vertex may be an inflection point and may not be a sharp vertex as shown in FIG.

  A detailed description of the configuration of the gradient coil is omitted because there are many portions known to those skilled in the art. However, most of today's cylindrical gradient coils are active shield type gradient magnetic fields composed of a main coil and a shield coil. In order to mount the gradient magnetic field coil as shown in FIG. 1, it may be formed of a cylindrical outer shield coil and a substantially triangular inner coil.

  The gradient magnetic field pattern at this time can be optimized using many known techniques.

  A metal foil or metal net (not shown) as an RF shield (not shown) is attached to the inner surface of the substantially triangular magnetic field coil, and the RF irradiation coils are arranged at equal intervals therefrom. A subject's space is formed by a cover disposed inside the RF irradiation coil.

  As shown in the figure, the upwardly convex substantially triangular bore formed in this way makes it possible to take a longer gaze direction distance of the subject. Similarly, the distance in the lateral direction when the subject goes to bed also becomes longer.

In this way, it is possible to provide a subject space that does not cause a sense of blockage.
The superconducting magnet in this embodiment has the same performance as the existing magnetic resonance imaging apparatus, and can enjoy the characteristics of the superconducting magnet such as high magnetic field strength and magnetic field stability as it is.

  As the means for adjusting the magnetic field homogeneity, although not shown, an iron piece arranged in the gradient magnetic field coil is often used. In the case of the embodiment as shown in FIG. 1, it may be arranged on the surface of the substantially triangular prism along the inner diameter of the gradient coil, or may be arranged along the cylindrical shape on the shield coil side. In any case, there is a sufficient space for arranging the iron pieces, and sufficient uniformity can be ensured even by applying the present invention.

Next, Example 2 of the present invention will be described with reference to FIG.
In this embodiment, the superconducting magnet and the gradient magnetic field coil are the same as those in the conventional structure, and only the RF irradiation coil is different. As shown in FIG. 2, a gradient magnetic field coil having a cylindrical outer shape and an inner shape is arranged with respect to a cylindrical superconducting magnet bore. Then, by changing the distance between the RF irradiation coil and the gradient magnetic field coil arranged inside the gradient magnetic field coil, the shape of the RF irradiation coil is made substantially triangular. Specifically, by making the distance from the gradient magnetic field coil narrow at the top and left and right as shown in the figure, a substantially triangular RF irradiation coil shape can be formed. As is apparent from the figure, the effect of the substantially triangular bore obtained by such a method is the same as that of the first embodiment.

  At this time, as shown in FIGS. 3 to 5, the following measures may be taken for the RF irradiation coil. That is, as in Example 1, when the distance between the RF irradiation coil and the RF shield arranged inside the gradient magnetic field coil is constant, the RF irradiation coil is formed by a substantially equidistant rung as shown in FIG. It ’s fine. However, for simplicity, an example of a birdcage type RF irradiation coil is shown.

  On the other hand, when the distance between the RF shield and the RF irradiation coil conductor arranged inside the gradient coil is changed as shown in FIG. 2, the position where the distance between the RF shield and the RF irradiation coil is close. Therefore, the irradiation efficiency of the RF irradiation coil falls due to mutual interference. In order to avoid this, by increasing the density of the RF irradiation coil conductor or increasing the length of the RF irradiation coil conductor, a uniform high-frequency irradiation distribution on the subject can be obtained.

  4 and 5 are examples. In FIG. 4, the rung conductors are arranged at unequal arrangement intervals in order to increase the irradiation efficiency near the apex of the approximately triangle. More specifically, the arrangement interval of the rung conductors is reduced in the vicinity of the apex of the substantially triangle. In FIG. 5, the lengths of the rung conductors are varied in order to increase the irradiation efficiency near the apex of the substantially triangle. In FIG. 5, the length of the rung conductor is increased near the apex of the triangle, that is, in the region where the distance from the RF shield is close.

Next, Example 3 of the present invention will be described with reference to FIG.
In this embodiment, in addition to using the gradient magnetic field coil having a substantially triangular cross section, the distance between the gradient magnetic field coil and the RF irradiation coil is further changed. More specifically, the cross-sectional shape of the inner surface of the gradient magnetic field coil should be a substantially triangular shape with a sharp apex angle, as compared to the triangular shape having a small apex angle. Thus, the distance between the gradient magnetic field coil and the RF irradiation coil is changed. By adopting such a configuration, when the gradient magnetic field coil is a substantially triangular shape with a small acute angle at the apex, the shape of the RF irradiation coil can be formed so as to further widen the space on the imaging space side.

  The above is description of each embodiment of the gradient magnetic field coil or RF irradiation coil of this invention. However, the gradient magnetic field coil or the RF irradiation coil of the present invention is not limited to the contents disclosed in the description of the above embodiment, but can take other forms in consideration of the gist of the present invention.

  For example, although an example in which a superconducting coil is used as a static magnetic field generation source has been described, the same effect as described above can be obtained even if a permanent magnet is used as a static magnetic field generation source in order to generate magnetomotive force.

  The present invention can be used in an MRI apparatus.

  10 subject, 12 superconducting magnet (inner cylinder), 14 gradient coil, 16 RF irradiation coil

Claims (7)

A static magnetic field generating means that is arranged around an imaging space in which the subject is arranged and generates a static magnetic field in the horizontal direction in the imaging space;
A gradient magnetic field generating means that is arranged on the imaging space side of the static magnetic field generating means and generates a gradient magnetic field in the imaging space;
A high-frequency magnetic field generating means disposed on the imaging space side of the gradient magnetic field generating means for generating a high-frequency magnetic field in the imaging space;
The static magnetic field generating means has a circular cross-sectional shape on the imaging space side,
2. A magnetic resonance imaging apparatus according to claim 1, wherein a cross-sectional shape of the high-frequency magnetic field generating means on the imaging space side is a substantially triangular shape protruding upward. The gradient magnetic field generating means is an active shield type comprising a shield coil having a circular cross section disposed on the static magnetic field generation source side and a substantially triangular main coil disposed on the imaging space side and having a convex cross section. The magnetic resonance imaging apparatus according to claim 1, wherein: The gradient magnetic field generating means has a circular cross-sectional shape on the side opposite to the imaging space side and the imaging space side, and the high-frequency magnetic field generating means is disposed on the inner surface of the gradient magnetic field generating means on the imaging space side. The magnetic resonance imaging apparatus according to claim 1, further comprising: an RF shield having a cylindrical shape, and an RF coil whose cross-sectional shape is an upwardly convex triangular shape. 4. The RF coil according to claim 3, wherein the density of the coil conductor in the vicinity of the portion where the distance from the RF shield is short is larger than the density of the coil conductor in the vicinity of the portion where the distance from the RF shield is long. The magnetic resonance imaging apparatus described in 1. 5. The magnetic resonance imaging apparatus according to claim 3, wherein the RF coil has a coil conductor having a long length in the vicinity of a portion where the distance from the RF shield is short. A static magnetic field generating means that is arranged around an imaging space in which the subject is arranged and generates a static magnetic field in the horizontal direction in the imaging space;
A gradient magnetic field generating means that is arranged on the imaging space side of the static magnetic field generating means and generates a gradient magnetic field in the imaging space;
A high-frequency magnetic field generating means disposed on the imaging space side of the gradient magnetic field generating means for generating a high-frequency magnetic field in the imaging space;
The cross-sectional shape on the imaging space side of the high-frequency magnetic field generating means is a substantially triangular convex upward,
The high-frequency magnetic field generating means includes an RF coil,
The interval between the RF shield disposed on the imaging space side inner surface of the gradient magnetic field generating means and the RF coil is not constant,
The magnetic resonance imaging characterized in that the RF coil has a density of a coil conductor in the vicinity of a portion where the distance to the RF shield is short than a density of the coil conductor in the vicinity of a portion where the distance to the RF shield is long. apparatus. The magnetic resonance imaging apparatus according to claim 6, wherein the RF coil has a coil conductor having a long length in the vicinity of a portion where the distance from the RF shield is short.

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