JP3906418B2 - Magnetic field generator for MRI - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an improvement in a magnetic field generator used in a medical magnetic resonance tomography apparatus (hereinafter referred to as MRI) and the like, and a magnetic field is generated by mounting a gradient magnetic field coil on a pair of opposing magnetic pole pieces forming a gap. After installing the device, when adjusting the magnetic field due to the influence of the installed environment, etc., magnetic field generation for MRI that can easily adjust the magnetic field uniformity in the imaging field of view in the air gap without removing the gradient coil Relates to the device.
[0002]
[Prior art]
A medical magnetic resonance tomography apparatus (hereinafter referred to as MRI) inserts a part or all of a subject into a gap of a magnetic field generator that forms a strong magnetic field, obtains a tomographic image of an object, and It is a device that can depict the nature of the organization.
In the magnetic field generator for MRI described above, the gap needs to be wide enough to allow a part or all of the subject to be inserted. In order to obtain a clear tomographic image, the gap is usually within the imaging field of view within the gap. Therefore, it is required to form a stable and strong uniform magnetic field having an accuracy of 0.02 to 2.0 T and 1 × 10 ⁻⁴ or less.
[0003]
As a magnetic field generator used for MRI, a pole piece is fixed to one end of each of a pair of permanent magnet structures using R-Fe-B magnets as a magnetic field generating source, and the other end is made of a yoke. A configuration is known in which a static magnetic field is generated in the gap between the pole pieces (Japanese Patent Publication No. 223010).
[0004]
In addition, as a magnetic field generation source, a configuration in which an electromagnetic coil (including a normal conductive coil, a superconductive coil, etc.) is wound around an iron core instead of the above-described permanent magnet structure is also known (special feature). (Kaihei 4-288137) and these configurations also employ the same pole pieces as described above.
[0005]
As described above, the uniformity of the magnetic field of the magnetic field generator is required to have an accuracy of 1 × 10 ⁻⁴ or less in a predetermined space, but it is particularly affected by the shape of the magnetic pole piece in the magnetic circuit, and the shape of the yoke Since various adjustments are inevitable in order to make the magnetic field uniformity within the imaging field of view a predetermined value, the magnetic field strength in the required air gap can be localized without reworking the pole pieces. Therefore, a configuration that can increase or decrease by a predetermined amount and obtain a magnetic field with extremely high uniformity is required.
[0006]
Therefore, the applicant assumes that the imaging space is a sphere, and at a predetermined position on the air gap facing surface of the pole piece according to the strength of the magnetic field strength on each circumference measured across the sphere space by a plurality of horizontal planes. By arranging magnetic material pieces and / or permanent magnet pieces, there is no need to process the pole pieces, the magnetic field strength in the air gap can be locally increased or decreased by a required amount, and the magnetic field uniformity can be improved. (Japanese Patent Publication No. 5-87962).
[0007]
[Problems to be solved by the invention]
In MRI, in order to obtain positional information in the air gap, a gradient magnetic field coil (GC) consisting of three groups of coils corresponding to the three directions of X, Y, and Z is generally arranged near each pole piece. By applying a pulse current to the magnetic field coil, a gradient magnetic field in a desired direction can be generated in the gap. In other words, position information is given to the nuclear magnetic resonance signal by applying a gradient magnetic field to the uniform magnetic field formed in the air gap, and in order to obtain one image, a large number of pulsed gradient magnetic fields are added. Is required.
[0008]
On the other hand, there are cases where the MRI is already installed in the field or the magnetic field may be adjusted due to the influence of the installed environment. However, after the gradient magnetic field coil is installed in the magnetic pole piece, as described above, the magnetic material piece and / or It is complicated and difficult to adjust the magnetic field uniformity by arranging and moving the permanent magnet pieces, and it is impossible to complete the work in a short time.
[0009]
In a magnetic field generator for MRI, as a method of arranging magnetic field adjusting permanent magnet pieces for adjusting the magnetic field uniformity, Japanese Patent Application Laid-Open No. 1-164356 discloses a plurality of magnetic field adjusting permanent magnet pieces through screws. A method has been proposed in which the holding plate fixed in (1) is bolted to the annular protrusion of the magnetic pole piece with the small permanent magnet piece side of the magnetic pole piece, or arranged with respect to the gap from the outer peripheral side of the pair of magnetic pole pieces. In this configuration, since the permanent magnet pieces are fixed to the holding plate with screws via the pedestal, there is a problem that it takes a lot of labor to remove and install the shim.
[0010]
That is, the magnetic material piece or permanent magnet piece for adjusting the magnetic field uniformity is not composed of the same shape and dimension, and first, a magnetic material or a permanent magnet is selected according to the spatial position and adjustment amount for adjustment, Furthermore, since the shape and dimensions are also selected as appropriate, it is necessary to create a pedestal according to the shape and dimensions of the magnetic material in order to fix it to the holding plate with screws via the pedestal. In order to direct the direction to a specific direction, it is necessary to create a pedestal having a required shape in accordance with the shape and dimensions.
[0011]
The present invention adjusts the magnetic field uniformity in the imaging field of the air gap safely and easily without attaching or removing the gradient magnetic field coil after installing the magnetic field generator for MRI by mounting the gradient magnetic field coil on a pair of magnetic pole pieces. An object of the present invention is to provide a magnetic field generator for MRI that enables the above.
[0012]
[Means for Solving the Problems]
The inventors have configured a magnetic field generator that can easily and reliably adjust the magnetic field uniformity in the imaging field of the air gap without removing the gradient magnetic field coil after the installation of the magnetic field generator for MRI. As a result of various examinations for the purpose, a small piece of magnetic material having various shapes, dimensions, and magnetization directions by using a shim holder comprising a main body provided with a pit capable of accommodating a small magnetic piece and a lid detachable to the gap side. Alternatively, the inventors have found that the purpose of the permanent magnet pieces can be achieved simply by putting them in the pits of the main body and covering them with a spacer as required, thereby completing the present invention.
[0013]
This invention has a pair of pole pieces which face each other to form a gap, in the MRI magnetic field generating device for generating a magnetic field in the air gap, the magnetic field homogeneity adjusting magnetic at a predetermined position of the gap facing surfaces of each pole piece A small piece is disposed, and a non-magnetic shim holder is detachably disposed on the air gap facing surface of the gradient magnetic field coil opposed to each magnetic pole piece. The shim holder includes a base disk and a magnetic field uniformity. It has a laminated structure of a holder main body composed of a holder disk in which a plurality of holes for accommodating a magnetic piece for adjustment are perforated and a lid disk detachably attached to the gap side, and a plurality of the holder disks The holes are concentrically arranged so as to correspond to the respective positions on the respective circumferences in the horizontal plane when the spherical space assumed in the center of the gap is crossed by a plurality of horizontal planes, and the magnetic field uniform After storing place the adjustment magnetic pieces to the required hole of the holder disc, a magnetic field generator for MRI, characterized in that arranged in the air gap face opposing gradient coils.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a configuration example of the magnetic field generator for MRI according to the present invention will be described in detail with reference to the drawings. FIG. 1 is a longitudinal sectional view showing an embodiment of a magnetic pole piece of a magnetic field generator according to the present invention. FIG. 2 is an exploded perspective view of the shim holder according to the present invention. Note that the size of the pits is exaggerated and enlarged for illustration.
[0015]
The pole piece 2 in FIG. 1 shows the lower pole piece of a pair opposed to each other in the vertical direction, and is an upper part of the magnet structure 1 as a magnetic field generation source such as an R-Fe-B magnet or an electromagnetic coil. Is placed. Pole pieces 2, the outer peripheral portion of the base portion 3 made of an iron or soft iron to place the annular protrusion 4 pure iron, central of iron or soft iron, laminate silicon steel sheets or soft ferrite, also flat convex The protrusion 5 is disposed.
[0016]
Reference numeral 6 in FIG. 1 denotes a gradient magnetic field coil, which is disposed at a required position in the pole piece 2 by a support member (not shown). A shim holder 10 is provided so as to be fixed to the gradient coil 6 or its support member. As shown in FIG. 2, the shim holder 10 is composed of three layers of non-magnetic thin discs, a holder disc 13 in which a hole 12 serving as a pit for housing a base disc 11 and magnetic pieces is perforated, It consists of a laminated structure of each thin plate of the lid disk 14. Hole for accommodating the magnetic pieces 12, the magnetic pieces are arranged corresponding to the respective positions on the circumference in each horizontal plane when traversing a spherical space is assumed in the gap center portion in a plurality of horizontal plane to so that is drilled concentrically arranged on the holder disk 13.
[0017]
Each disk constituting the shim holder 10 is fastened with an appropriately arranged screw . In other words , after the holder disk 13 is laminated and fixed on the base disk 11, a magnetic material piece or a permanent magnet piece for adjusting the magnetic field uniformity is inserted into the required place of the pit arranged in a predetermined pattern. If necessary, place a spacer in the gap between the lid disk 14 in the pit and position it, then screw the lid disk 14 together and stack the integrated shim holder 10 into the gradient magnetic field. The coil 6 or its supporting member is aligned and fixed.
[0018]
In this invention, the shim holder 10 can be made of a non-magnetic material such as vinyl chloride resin or glass fiber reinforced plastic. In addition to the three-layer structure shown in FIG. There may be.
[0019]
【Example】
A shim holder with a three-layer structure shown in Fig. 2 was made using vinyl chloride resin as the material. It was about 900 mm in diameter and about 15 kg in thickness and was about 11 kg lightweight and easy to handle . Only the shim holder can be attached and detached while it is mounted in the required position . By using a shim holder, magnetic field adjustment can be performed using the linear programming method on the side of the indoor unit where the MRI magnetic field generator is installed, and the number of steps for magnetic field adjustment is small. Even the adjustment was completed in a short time.
[0020]
【The invention's effect】
According to the present invention, the shim holder uses a method such as linear programming on the apparatus side without attaching and removing the gradient magnetic field coil after installing the magnetic field generator for MRI by mounting the gradient magnetic field coil on a pair of magnetic pole pieces. The magnetic field can be adjusted and the uniformity of the magnetic field in the imaging field of the air gap can be adjusted safely and easily.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view showing an embodiment of a pole piece of a magnetic field generator according to the present invention.
FIG. 2 is an exploded perspective view of a shim holder according to the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Magnet structure 2 Magnetic pole piece 3 Base 4 Annular protrusion 5 Flat convex protrusion 6 Gradient magnetic field coil 10 Shim holder 11 Base disk 12 Hole 13 Holder disk 14 Cover disk

Claims (1)

In a magnetic field generator for MRI having a pair of magnetic pole pieces opposed to each other by forming a gap and generating a magnetic field in the gap, a magnetic piece for adjusting the magnetic field uniformity is arranged at a predetermined position on the gap facing surface of each pole piece. In addition, a non-magnetic shim holder is detachably disposed on the air gap facing surface of the gradient magnetic field coil disposed to face each of the magnetic pole pieces. The shim holder includes a base disk and a magnetic piece for adjusting the magnetic field uniformity. a holder body comprising a plurality of holders discs the holes drilled arrangement for housing, has a laminated structure of the lid disc removable to void side, and a plurality of holes of the holder disc is When the spherical space assumed in the center of the air gap is crossed by a plurality of horizontal planes, the magnetic field uniformity adjusting magnets are concentrically arranged so as to correspond to the respective positions on the respective circumferences in the horizontal plane. After the pieces were housed and arranged in the desired hole of the holder disc, MRI magnetic field generating apparatus, characterized in that arranged in the air gap face opposing gradient coils.

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