JP2006218141A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a horizontal magnetic field type MRI apparatus improving the openness by shortening the axial length and suitably improving a static magnetic field uniformity. <P>SOLUTION: This magnetic resonance imaging apparatus is provided with cylindrical magnets 33-36 generating the static magnetic field along the Z axis 3 in a uniform space 32 inside an approximately cylindrical gantry, gradient magnetic field coils 37 disposed along the cylinder in the side of the uniform space 32 of a static magnetic field generating means and generating a gradient magnetic field in the uniform space 32, and irradiation coils disposed along the cylinder in the side of the uniform space 32 of the gradient magnetic field coils 37 and generating a high frequency magnetic field in the uniform space. This magnetic resonance imaging apparatus is disposed with internal circumference compensating members 38 comprising ferromagnetic bodies and/or permanent magnets in the Z-directional outside of the gradient magnetic field coils and/or the irradiation coils 37. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), and more particularly to a technique for improving the uniformity of a static magnetic field of a horizontal magnetic field type MRI apparatus having a short axial length and high openness.

  The MRI system utilizes the nuclear magnetic resonance phenomenon that occurs in the nuclei of the atoms that make up the subject when the subject is placed in a uniform static magnetic field and radiates electromagnetic waves. Hereinafter, an NMR signal is detected, and an image is reconstructed using the NMR signal, thereby obtaining a magnetic resonance image (hereinafter referred to as an MR image) representing the physical properties of the subject.

  The MRI apparatus is generally arranged on the imaging space side of the static magnetic field generating magnet for generating a static magnetic field in a predetermined direction in the imaging space, and generates a gradient magnetic field in the imaging space. A gradient magnetic field coil and an irradiation coil that is disposed on the imaging space side of the gradient magnetic field coil and generates a high-frequency magnetic field in the imaging space are configured. However, the MRI device is a tunnel type MRI device of the horizontal magnetic field type, depending on which configuration the magnet is arranged around and the static magnetic field is generated from which direction, etc. It can be broadly divided into vertical magnetic field type open MRI systems.

  Here, the horizontal magnetic field type tunnel MRI apparatus is an MR image of a subject placed in an imaging space in a gantry by generating a static magnetic field along a central axis of the cylinder in a substantially cylindrical gantry. It is an MRI device that obtains. Also, the vertical magnetic field type open MRI apparatus refers to the imaging space by generating a static magnetic field in the opposing direction of a static magnetic field generating magnet arranged facing the top and bottom or the left and right sides of the imaging space. It is an MRI apparatus which acquires the MR image of the patient arrange | positioned in.

  In an MRI apparatus, it is important to improve openness in order to reduce the feeling of claustrophobia of a patient lying in an imaging space. In order to improve openness, it is effective to increase the cylindrical bore diameter and shorten the axial length of the cylindrical gantry in the horizontal magnetic field type tunnel MRI apparatus. On the other hand, in order to obtain a high-quality image in the MRI apparatus, it is necessary to improve the static magnetic field uniformity. However, if the axial length of the gantry is shortened in the horizontal magnetic field type tunneling MRI apparatus, the super-electric coil becomes the axis. Since it cannot be arranged outside the direction, there is a problem that the magnetic field in the Z-axis direction becomes weak, particularly in a region away from the center of the axial static magnetic field in the imaging space.

A conventional technique described in Patent Document 1 is a technique for improving the uniformity of a static magnetic field in a horizontal magnetic field type MRI apparatus. In the prior art of Patent Document 1, FIG. 6 discloses a magnet having a relatively large passive correction member in the peripheral region in the internal space.
JP-A-8-38457

Patent Document 2 also discloses a technique for improving the static magnetic field uniformity using a ferromagnetic ring.
US 5594401

The present inventor has found the following problems as a result of studying the above prior art.
That is, in the prior art described in Patent Document 1, the primary and secondary gradient coils are arranged on the test volume side of the relatively large passive auxiliary member, but in this prior art, only the space for arranging the gradient coils is provided. There was a problem that the bore diameter was shortened. Further, in the prior art described in Patent Document 2, a technique for correcting the distortion of the static magnetic field using a ferromagnetic ring is disclosed, but the positional relationship between the gradient coil and the ferromagnetic ring has not been disclosed. .

  An object of the present invention is to provide a horizontal magnetic field type MRI apparatus that can improve the static magnetic field uniformity suitably while shortening the axial length to increase the openness.

  According to the first feature of the present invention, a static magnetic field generating means for generating a static magnetic field along a central axis of the cylinder in an imaging space inside a substantially cylindrical gantry, and the imaging of the static magnetic field generating means A gradient magnetic field generating means for generating a gradient magnetic field in the imaging space on the space side; a gradient magnetic field generating means for generating a gradient magnetic field in the imaging space; In a magnetic resonance imaging apparatus comprising a high-frequency magnetic field generating means for generating a magnetic field, a static magnetic field correction comprising a ferromagnetic material and / or a permanent magnet outside the gradient magnetic field generating means and / or the high-frequency magnetic field generating means in the central axis direction A magnetic resonance imaging apparatus is provided in which a member is disposed.

  According to the second feature of the present invention, the static magnetic field correction member is also arranged on the outer surface in the central axis direction of the static magnetic field generating means. A magnetic resonance imaging apparatus is also provided.

  According to the third aspect of the present invention, the gradient magnetic field generated by the gradient magnetic field generating means is between the gradient magnetic field generating means and / or the high frequency magnetic field generating means and the static magnetic field correcting member. Provided is a magnetic resonance imaging apparatus having the first and second features of the present invention, characterized in that shielding means for shielding entry into the correction means is provided.

  According to a fourth feature of the present invention, the static magnetic field correction member is divided and arranged in a radial direction and / or a circumferential direction with respect to the central axis. A magnetic resonance imaging apparatus having the three features is provided.

  According to a fifth aspect of the present invention, there is provided the magnetic resonance combined with the first to fourth aspects of the present invention, characterized in that a temperature control means is provided to keep the temperature of the static magnetic field correction member constant. An imaging device is provided.

  According to a sixth feature of the present invention, the static magnetic field correction member includes a detachable movable portion that is detachably and / or movably attached. A magnetic resonance imaging apparatus is also provided.

  According to a seventh aspect of the present invention, the static magnetic field correcting member is fixed to the gradient magnetic field generating means and / or the fixed support member of the high frequency magnetic field generating means and / or the static magnetic field generating means. A magnetic resonance imaging apparatus having the features of the first to sixth features of the present invention is provided.

  ADVANTAGE OF THE INVENTION According to this invention, while shortening axial length and making openness high, the horizontal magnetic field type | system | group MRI apparatus which can improve a static magnetic field uniformity suitably is provided.

Hereinafter, a system configuration of a general MRI apparatus will be described in detail with reference to FIG.
The MRI apparatus is roughly classified into a central processing unit (hereinafter abbreviated as CPU) 1, a sequencer 2, a transmission system 3, a static magnetic field generating magnet 4, a receiving system 5, a gradient magnetic field generating system 21, and a signal. It consists of a processing system 6.

  The CPU 1 controls the sequencer 2, the transmission system 3, the reception system 5, and the signal processing system 6 according to a predetermined program. The sequencer 2 operates based on a control command from the CPU 1, and sends various commands necessary for collecting image data of the tomographic plane of the subject 7 to the transmission system 3, the gradient magnetic field generation system 21, and the reception system 5. ing.

  The transmission system 3 includes a high-frequency oscillator 8, a modulator 9, and an irradiation coil 11. The reference high-frequency pulse from the high-frequency oscillator 8 is amplitude-modulated by the modulator 9 according to a command from the sequencer 2, and the amplitude-modulated high-frequency signal is transmitted. The subject 7 is irradiated with a predetermined pulsed electromagnetic wave by amplifying the pulse via the high-frequency amplifier 10 and supplying the amplified pulse to the irradiation coil 11.

  The static magnetic field generating magnet 4 is for generating a uniform static magnetic field in a predetermined direction around the subject 7. In the static magnetic field generating magnet 4, an irradiation coil 11, a gradient magnetic field coil 13, and a receiving coil 14 are arranged. The gradient magnetic field coil 13 is included in the gradient magnetic field generation system 21 and receives a current supplied from the gradient magnetic field power supply 12 and generates a gradient magnetic field under the control of the sequencer 2.

  The receiving system 5 detects a high-frequency signal (NMR signal) emitted by nuclear magnetic resonance of the nucleus of the biological tissue of the subject, and includes a receiving coil 14, an amplifier 15, a quadrature detector 16, and an A / D converter. The high frequency signal (NMR signal) of the response of the subject due to the electromagnetic wave irradiated from the irradiation coil 14 is detected by the receiving coil 14 disposed close to the subject, and the amplifier 15 and the orthogonal The signal is input to the A / D converter 17 via the phase detector 16, converted into a digital quantity, and the signal is sent to the CPU 1.

  The signal processing system 6 includes an external storage device such as a magnetic disk 20 and an optical disk 19 and a display 18 such as a CRT. When data from the reception system 5 is input to the CPU 1, the CPU 1 performs signal processing and image re-processing. Processing such as configuration is executed, and the resulting image of a desired tomographic plane of the subject 7 is displayed on the display 18 and stored in the magnetic disk 20 of the external storage device or the like.

  FIG. 2 is a cross-sectional view of the horizontal magnetic field type MRI apparatus according to Embodiment 1 of the present invention viewed from the horizontal direction. However, the static magnetic field generation source in the present embodiment is a cylindrical magnet using a superconducting coil, and is composed of 33 to 36 described below. In FIG. 2, 31 is a Z-axis which is a direction for generating a static magnetic field, 32 is an imaging space for generating a static magnetic field, 33 is a vacuum container for heat insulation by evacuating the inside, and 34 is a cryogenic temperature such as liquid helium. A helium tank that houses the superconducting coil together with the cryogen, 35 is the superconducting coil of the main coil group that generates a uniform static magnetic field in the imaging space inside the bore, and 36 is the superconducting shield group to suppress the leakage magnetic field to the outside Coil, 37 is a gradient magnetic field coil or RF coil for generating a gradient magnetic field or a high frequency magnetic field in the imaging space, 38 is an inner circumference compensation member for compensating for the distortion of the static magnetic field, and 39 is the openness / access in the cylindrical magnet It is the elevation angle of the inner surface edge of the opening (magnet bore inner surface edge or gradient magnetic field coil, RF coil inner surface edge) viewed from the magnetic field center, which is a parameter that determines the property.

  In general, the superconducting coil 35 of the main coil group requires a higher magnetomotive force as the coils arranged at both ends in the Z-axis direction. That is, it is necessary to arrange a coil with a large number of turns or a coil with a large current value. Further, a gradient coil for generating a gradient magnetic field in the imaging space and an RF coil for generating a high-frequency magnetic field are arranged between the bore inner surface side of the cylindrical magnet and the imaging space. At this time, the openness / accessibility of the cylindrical magnet is determined by the elevation angle of the inner surface edge of the opening (magnet bore inner surface edge or gradient magnetic field coil, RF coil inner surface edge) viewed from the center of the magnetic field, and the elevation angle is large. The more open and accessible the better. In order to increase the elevation angle, it is effective to shorten the axial length of the cylindrical magnet or widen the bore diameter. At this time, if the short axis of the magnet or the bore diameter is enlarged, the magnetic field strength of both extreme portions (opening side end portions) 40a and 40b in the imaging space is lowered, so that the magnetic field uniformity in the imaging space is deteriorated. was there. An example of this will be described with reference to FIG. 11 showing the flow of magnetic flux lines. First, FIG. 11 (a) is an example of a horizontal magnetic field type tunneling MRI apparatus when the length in the Z-axis 31 direction is long. In this example, the superconducting coil 35a on the outermost side in the axis 31 direction is in the Z-axis 31 direction. Since there is a position away from the center of the static magnetic field, the magnetic flux lines in the imaging space 32 are aligned in the Z-axis 31 direction. However, in the horizontal bore type tunnel MRI apparatus short bore with a short length in the Z-axis 31 direction (axial length), as shown in FIG. There is a problem that the magnetic flux lines in the imaging space 32 are difficult to align in the Z-axis 31 direction because there is no position away from the center of the magnetic field.

  In this embodiment, in order to compensate the magnetic field at the extreme portions of the imaging space in the outer space (outer edge of the gradient magnetic field coil) in the Z-axis direction of the gradient magnetic field coil (or RF coil) disposed on the bore inner surface side of the cylindrical magnet. The inner circumference compensating member 38 is arranged. With this configuration, the magnetic field strength at both extremes (opening side end) in the imaging space is compensated, and the magnetic field uniformity in the imaging space can be improved. Specifically, the flow of magnetic flux lines is as shown in FIG. 11C, and the component in the Z-axis direction in the imaging space 32 becomes uniform due to the influence of the inner circumference compensation member 38 being magnetized. It is considered that the inner circumference compensating member is desirably in an arrangement position and shape that does not reduce the elevation angle of the inner surface edge of the opening from the magnetic field center. However, in this embodiment, in order to secure a space for arranging the inner periphery compensation member, the length of the gradient magnetic field coil and the RF coil in the Z-axis direction is longer than the length (axial length) of the cylindrical magnet in the Z-axis direction. Is shorter. Further, the inner periphery compensation member 38 may be fixed to a member for fixing the gradient magnetic field coil (or RF coil), or may be fixed to the vacuum vessel 33 constituting the cylindrical magnet with a screw or the like.

  FIG. 3 is a cross-sectional view of a horizontal magnetic field MRI apparatus according to Embodiment 2 of the present invention viewed from the horizontal direction. In this embodiment, the annular compensation member 41 is fixed to the outer surface (outer end surface) in the Z-axis direction of the cylindrical magnet with a screw or the like, so that the magnetic field strength can be compensated more effectively than in the first embodiment. Is going. The change in the flow of magnetic flux lines in this case is shown in FIG. 12 (a) and FIG. 12 (b). However, FIG. 12A shows a case where the annular compensation member 41 is not placed, and FIG. 12B shows a case where the annular compensation member 41 is placed. According to FIGS. 12 (a) and 12 (b), by placing the annular compensation member 41, the direction of the magnetic flux lines flows closer to the outer side in the Z-axis 31 direction near the superconducting coil 35a. The magnetic field component in the Z-axis direction becomes more uniform. Here, the material of the inner periphery compensation member 38 and the annular compensation member 41 disposed on the outer edge portion of the gradient magnetic field coil and the outer end face of the cylindrical magnet may be any material as long as it is a ferromagnetic material. In addition, since a silicon steel plate has a high magnetic permeability and a high saturation magnetization, the magnetic field strength can be compensated more efficiently.

  FIG. 4 is a cross-sectional view of a horizontal magnetic field MRI apparatus according to Example 3 of the present invention as seen from the horizontal direction. In this embodiment, a permanent magnet is used as the material of the inner circumference compensation member 38 and the annular compensation member 41. If the direction of the magnetic flux created by the permanent magnet is arranged as shown by the arrow as shown in FIG. 4, there is a compensation effect of the magnetic field strength in the same manner as in the first and second embodiments. Since the magnetic field is generated actively, it is considered that the effect of compensating the magnetic field strength is higher than in the case of the first and second embodiments.

  FIG. 5 is a cross-sectional view of a horizontal magnetic field MRI apparatus according to Example 4 of the present invention when viewed from the horizontal direction. In the present embodiment, a combination of a ferromagnetic body 38a and a permanent magnet 38b is used as the inner circumference compensating member 38. In this embodiment, it is considered that the compensation effect of the magnetic field strength is reduced as compared with the case where only the permanent magnet is used as in the case of Embodiment 3, but the cost is considered to be lower than the case where only the permanent magnet is used.

  FIG. 6 (a) is a cross-sectional view of the horizontal magnetic field type MRI apparatus according to the fifth embodiment of the present invention when viewed from the horizontal direction, and FIG. 6 (b) is a fifth embodiment of the present invention when viewed from the Z-axis direction. FIG. However, this embodiment is an example in which a shim tray 42 for arranging shimming members for adjusting the magnetic field uniformity of the imaging space is provided. In this case, the shim tray 42 is inserted from the magnet opening direction (Z-axis direction). In the present embodiment, the inner periphery compensating member 38 disposed at the outer edge of the gradient magnetic field coil has a divided structure. By making such a divided structure, the manufacture becomes easy and the installation property becomes easy. In this embodiment, the shim tray may be pulled out from the valley of the divided inner circumference compensation member 38, or a shimming member (shim tray) may be arranged in the valley of the divided inner circumference compensation member 38. .

  FIG. 7 is a cross-sectional view of a horizontal magnetic field MRI apparatus according to Embodiment 6 of the present invention when viewed from the horizontal direction. In the present embodiment, the pulsed gradient magnetic field generated by the gradient magnetic field coil penetrates into the inner periphery compensation member 38, which is a ferromagnetic or permanent magnet, and is magnetized hysterically, and is nonlinear to the gradient magnetic field generated in the imaging space. A pulse magnetic field shielding material 43 is provided in order to prevent such adverse effects. In the present embodiment, the material of the pulse magnetic field shielding material 43 is preferably a nonmagnetic good electrical material such as aluminum or a directional ferromagnetic material such as a directional silicon steel plate.

  FIG. 8 is a cross-sectional view of a horizontal magnetic field type MRI apparatus according to Embodiment 7 of the present invention. In this embodiment, the inner periphery compensation member 38 made of a ferromagnetic material has a strip shape. By doing so, the inner periphery compensating member 38 having a strip shape is likely to be magnetically saturated and can be prevented from being magnetized by hysteresis and having a non-linear adverse effect.

  FIG. 9 is a cross-sectional view of a horizontal magnetic field type MRI apparatus according to Example 8 of the present invention. In this embodiment, an attaching / detaching / moving mechanism 41c is provided on an annular compensation member 41 made of a ferromagnetic material. In this way, the magnetic field adjustment (shimming) of the imaging space can be performed using the detachable / movable mechanism 41c. As a specific installation method, a method is conceivable in which a screw hole is provided in the annular compensation member 41, and a screw made of a ferromagnetic material is fitted therein to adjust the position. However, the attaching / detaching / moving mechanism may not be a ferromagnetic material, and may be a permanent magnet.

  FIG. 10 is a cross-sectional view of a horizontal magnetic field MRI apparatus according to Embodiment 9 of the present invention. However, in this embodiment, the annular compensation member 41 disposed on the outer surface (outer end surface) in the Z-axis direction of the cylindrical magnet is configured as a part of the vacuum vessel that is the outermost shell of the cylindrical magnet. By doing in this way, the external dimension of a cylindrical magnet can be made small and a more compact horizontal magnetic field type | system | group MRI apparatus can be provided.

  The present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, in the above embodiment, the magnetization characteristics of the ferromagnetic material and the permanent magnet depend on the change in temperature. Therefore, in the above embodiment, a heat retaining member, a heater, and the like are arranged around the inner circumference compensation member 38 and the annular compensation member 41. It is desirable to provide a mechanism for keeping the temperature constant. In addition, as a method for fixing the inner circumference compensation member 38 and the annular compensation member 41, a method of firmly fixing the inner circumference compensation member 38 and the annular compensation member 41 by a method such as direct welding to the vacuum vessel or fastening with a bolt can be considered. Further, in the above embodiment, the case where a ferromagnetic material or a permanent magnet is used as the inner circumference compensation member 38 and the annular compensation member 41 for correcting the static magnetic field is exemplified, but the same effect is obtained by the inner circumference compensation member 38. It can also be obtained by arranging a normal conducting coil at the position where the annular compensation member 41 is arranged.

The figure which shows the system configuration | structure of a general MRI apparatus. 1 is a cross-sectional view of a horizontal magnetic field type MRI apparatus according to Embodiment 1 of the present invention. Sectional drawing of the horizontal magnetic field type | system | group MRI apparatus which concerns on Example 2 of this invention. Sectional drawing of the horizontal magnetic field type | mold MRI apparatus which concerns on Example 3 of this invention. Sectional drawing of the horizontal magnetic field type MRI apparatus which concerns on Example 4 of this invention. The figure which looked at the horizontal magnetic field type MRI apparatus which concerns on Example 5 of this invention from the direction of the Z-axis. Sectional drawing of the horizontal magnetic field type | system | group MRI apparatus which concerns on Example 6 of this invention. Sectional drawing of the horizontal magnetic field type | system | group MRI apparatus which concerns on Example 7 of this invention. Sectional drawing of the horizontal magnetic field type | system | group MRI apparatus which concerns on Example 8 of this invention. Sectional drawing of the horizontal magnetic field type | system | group MRI apparatus which concerns on Example 9 of this invention.

Explanation of symbols

  31 Z-axis, 32 uniform space, 33 vacuum vessel, 34 helium tank, 35 superconducting coil of main coil group, 36 superconducting coil of shield coil group, 37 gradient magnetic field coil and RF coil, 38 internal compensation member, 39 inner end of opening Elevation angle

Claims (7)

  A static magnetic field generating means for generating a static magnetic field along a central axis of the cylinder in an imaging space inside a substantially cylindrical gantry, and being arranged along the cylinder on the imaging space side of the static magnetic field generating means, Gradient magnetic field generating means for generating a gradient magnetic field in the imaging space, and high frequency magnetic field generating means for generating a high frequency magnetic field in the imaging space, arranged along the cylinder on the imaging space side of the gradient magnetic field generating means. In the magnetic resonance imaging apparatus, a static magnetic field correction member made of a ferromagnetic material and / or a permanent magnet is disposed outside the gradient magnetic field generating means and / or the high-frequency magnetic field generating means in the central axis direction. Magnetic resonance imaging device.   The magnetic resonance imaging apparatus according to claim 1, wherein the static magnetic field correction member is also disposed on an outer surface in the central axis direction of the static magnetic field generation unit.   Shielding means for shielding the gradient magnetic field generated by the gradient magnetic field generating means from entering the static magnetic field correction means between the gradient magnetic field generating means and / or the high frequency magnetic field generating means and the static magnetic field correcting member. The magnetic resonance imaging apparatus according to claim 1, further comprising:   The magnetic resonance imaging apparatus according to claim 1, wherein the static magnetic field correction member is divided and arranged in a radial direction and / or a circumferential direction with respect to the central axis.   5. The magnetic resonance imaging apparatus according to claim 1, further comprising a temperature control unit for keeping the temperature of the static magnetic field correction member constant.   The magnetic resonance imaging apparatus according to claim 1, wherein the static magnetic field correction member includes a detachable movable part that is detachably and / or movably attached.   7. The magnetism according to claim 1, wherein the static magnetic field correcting member is fixed to a fixed support member and / or the static magnetic field generating means of the gradient magnetic field generating means and / or the high frequency magnetic field generating means. Resonance imaging device.

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