JPH0821498B2 - Magnetic field generator for MRI - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of a magnetic field generator using a permanent magnet used in a nuclear magnetic resonance tomography apparatus for medical use,
In a permanent magnet structure comprising a pair of one or more magnet segments facing each other with a gap formed, the magnetic poles are arranged by changing the height of the magnet segment at the required position or at the required position in the void direction, that is, the magnetizing direction. The present invention relates to a magnetic field generator that generates a uniform magnetic field without using a piece to reduce eddy current generation and reduce weight.

BACKGROUND ART Medical Magnetic Resonance Tomography (hereinafter referred to as MRI)
Is a device that can insert a part or all of a subject into a void of a magnetic field generator that forms a strong magnetic field, obtain a tomographic image of an object, and depict the properties of the tissue.

In the above-mentioned magnetic field generator for MRI, the space needs to be wide enough to allow insertion of a part or all of the subject,
In order to obtain a clear tomographic image, it is usually required to form a stable and strong uniform magnetic field with an accuracy of 0.05 to 1.5T and 1 × 10 ^-4 or less in the imaging field of view in the void. It

As a magnetic field generator used for MRI, as shown in FIG. 9, magnetic pole pieces (2) (2) (2) (2) are provided on one end of each of a pair of permanent magnet constituents (1) (1) using Fe-BR magnets. ) Are fixed and made to face each other, and the other end is connected by plate yokes (4) and (5), and these are further connected by four columnar yokes (6) to form a gap between the pole pieces (2) and (2). A configuration for generating a static magnetic field in (7) is known.

For example, in the above structure, in order to form a more stable and uniform magnetic field, an annular protrusion (3) is provided on each peripheral edge of the facing surface of the pole piece, or a convex shape is formed on each central portion of the facing surface of the pole piece. A magnetic field generator provided with a protrusion (8) has been proposed (Japanese Utility Model Laid-Open No. 60-166110).

In such a magnetic field generator, a magnetic pole piece is provided on the opposing surface of the permanent magnet to obtain a uniform magnetic field. However, with this configuration, leakage from the side surface of the magnetic pole piece becomes large, and the magnetic field strength in the air gap decreases. is there. Further, since the magnetic pole pieces occupy about 10% of the total weight of the magnetic field generator, it has been an obstacle to weight reduction.

On the other hand, in order to obtain position information in the air gap, a gradient magnetic field coil that generates a magnetic field inclined in a desired direction in a short time by applying a pulse current, that is, usually X, Y, Z
The three sets of coil groups corresponding to the three directions are arranged near each magnetic pole piece, but there is a problem that an eddy current is generated by the gradient magnetic field coil and it takes a lot of time for the gradient magnetic field to reach a predetermined strength. .

SUMMARY OF THE INVENTION The present invention has been proposed in view of the above circumstances, and it is possible to reduce the generation of eddy current and reduce the weight without using a magnetic pole piece, and to obtain a uniform magnetic field equal to or higher than that of a configuration using a magnetic pole piece.
The purpose is to provide a magnetic field generator for RI.

SUMMARY OF THE INVENTION The present invention has variously studied in order to generate a highly accurate uniform magnetic field in a required air gap in a magnetic field generator for MRI without using magnetic pole pieces. The present invention has been completed by finding that a uniform magnetic field can be obtained and the above-mentioned object can be achieved by partially adjusting the length in the magnetization direction.

That is, the present invention relates to a magnetic field generator for MRI that magnetically couples a pair of permanent magnet constructs, which are opposed to each other with a gap and are composed of a single or a plurality of magnet segments, by a yoke to generate a magnetic field in the gap. The magnetic field generator for MRI is characterized in that a required position or a magnet segment at a required position of the permanent magnet structure is arranged by changing its magnetization direction length to generate a uniform magnetic field in the gap.

More specifically, according to the present invention, in the magnetic field generator for MRI having the above-mentioned structure, when the number of the magnet segments forming the permanent magnet structure is single, the magnetizing direction length of the magnet at the required position is changed, or the plurality of magnet segments are provided. In this case, the magnet segments are arranged at different positions with different lengths in the magnetization direction, and a uniform magnetic field is generated in the gap.

Further, according to the present invention, in addition to changing the length of the permanent magnet in the magnetization direction (height in the air gap direction), at least one of the magnet length, magnet width, magnetic characteristics, and magnet material is changed so that a magnetic field is evenly applied to the air gap. It is characterized by generating.

Configuration of the Invention In the present invention, the magnetic circuit is a configuration in which a pair of permanent magnet constituents facing each other with a gap formed therein are magnetically coupled with a yoke, and the yoke, the magnet shape and dimensions of the permanent magnet constituent, The arrangement method or the like may be any configuration, and it is desirable to appropriately select and select the magnetic characteristics, the shape size, the yoke shape size, etc. of the permanent magnet structure according to the size of the required voids and the like.

According to the present invention, in order to generate a uniform magnetic field in a required air gap without providing pole pieces on the air gap facing surface of the permanent magnet structure, the magnetizing direction lengths (height in the air gap direction) of the plurality of permanent magnets are arrayed. The basic technical idea is, for example, that a predetermined permanent magnet structure is formed with permanent magnets of the same size and characteristics, and the generated magnetic field strength in the required air gap. This is to analyze the distribution, change the magnetization direction length of the permanent magnets at the required locations, that is, the magnet height in the air gap direction, and adjust the magnets so that a uniform magnetic field is formed in the air gaps.

Further, in the present invention, the pair of permanent magnet constituents that form a gap and are opposed to each other can have any shape as described above, and the plurality of permanent magnets forming this permanent magnet constituent are as described above. In addition to changing the magnetizing direction length of the permanent magnet, that is, the magnet height in the air gap direction, the magnet length, magnet width, magnetic characteristics, and magnet material may be changed, and these magnets can be arranged in various patterns. .

According to the present invention, a required position or a required position of a pair of permanent magnet constructions composed of a single or a plurality of magnet segments is arranged by changing its magnetizing direction length.
It is characterized in that a uniform magnetic field is generated in the air gap. However, the dimensions for changing the length in the magnetizing direction, the shape of the permanent magnet, the arrangement pattern, etc. are the main objectives such as weight reduction and improvement of magnetic field uniformity. Depending on various uses of the apparatus, the analysis method such as the magnetic field strength in the required air gap and the homogeneity is selected as shown in the examples described later, and the position of changing the analysis result and the magnet height can be selected. Since the specific relationship with the selection and change amount of the magnet segment and the preferable configuration are changed, it is necessary to appropriately select the magnet segment, the length of the magnetizing direction of the magnet, etc. according to each condition.

Ferrite magnets, alnico magnets, and rare earth cobalt magnets can be used as the permanent magnets of the magnet structure composed of one or a plurality of magnet segments used in such a magnetic circuit. By using abundant light rare earths and using Fe-BR permanent magnets containing B and Fe as the main components and having an extremely high energy product of 30 MGOe or more, the size can be significantly reduced.

Disclosure of the Invention Based on the Drawings FIGS. 1A and 1B are explanatory views showing a permanent magnet structure (lower side) of a magnetic field generator according to the present invention.

FIG. 2a is a vertical cross-sectional explanatory view of a magnetic field generator having no pole piece, and FIG. 2b is a cross-sectional explanatory view thereof.

FIG. 3 is a perspective explanatory view of a gap showing a method for measuring a magnetic field in a spherical space of a magnetic field generator.

4A to 4D are explanatory views showing an example in which the magnetization direction height of the required segment of the permanent magnet structure (upper side) of the present invention is changed.

5 and 6 are graphs showing the magnetic field inhomogeneities of P _{1 to} P ₇ in the spherical space of FIG. 3, and FIG. 5 shows the conventional magnetic field generator of FIG. 9 with pole pieces removed. In this case, FIG. 6 shows the case of the permanent magnet construction of FIGS.

FIG. 7 shows P ₁ -in the spherical space in the embodiment of FIG. 1a.
₇ is a graph showing the magnetic field inhomogeneity of P ₇ .

FIG. 8 is an explanatory view showing one embodiment of the optimized permanent magnet structure (upper side) of the present invention.

Device Configuration Here, as shown in FIG. 2, the magnetic pole pieces (2) and (2) are removed from the configuration of the magnetic field generator shown in FIG.
-A pair of permanent magnet components using a B-R magnet (1)
(1) face each other and the other end has a rectangular plate yoke (4)
(5) Further, these are connected by four columnar yokes (6) having four corners, and a static magnetic field is generated in the space (7) between the permanent magnet constructs (1) and (1). An example in which the present invention is applied to a magnetic field generator having a configuration will be described.

First, assuming that the permanent magnet structure (1) is formed into a disk shape in a predetermined pattern with a large number of permanent magnets having the same size and characteristics, or is formed in this structure, the magnetic field state generated in the air gap (7). , Analyze intensity distribution, etc.

Analysis Method Various analysis methods such as a finite element method and a boundary element method can be adopted as the analysis method. Further, as the modernization method, a mathematical programming method such as a linear programming method is used. Here, a case where the finite element method and the linear programming method are used together will be described.

The state of the magnetic field in the magnetic field generator gap (7) is examined by the finite element method.

Further, in the air gap (7) of the magnetic field generator having the above structure, a spherical space having a predetermined radius r is set from the center (O), and as shown in FIG. 3, a required number of horizontal planes of the spherical body,
Here, the cross section is taken along seven horizontal planes (P _{1 to} P ₇ ), and the crossing circle between each horizontal plane (P _{1 to} P ₇ ) and the spherical space is equally divided at the required angle around the Z axis. The magnetic field strength at the point is calculated, and the variation of the magnetic field strength on the circumference of each horizontal plane (P _{1 to} P ₇ ) is examined.

Next, the magnetization direction length of each of the multiple magnet segments forming the permanent magnet structure (1) (1) is determined by the horizontal plane (P ₁ ~
The effect of P ₇ ) on the magnetic field strength on the circumference is investigated.

As shown in FIG. 2, except for the pole pieces, when the surfaces of the permanent magnet constituents (1) and (1) that face the air gap are flat, that is, when the magnetizing direction heights of all the magnet segments are all equal (P ₁ ~ When the magnetic field strength on the circumference of P ₇ ) was measured,
As shown in FIG. 5, as indicating the percentage of non-uniformity of the magnetic field intensity in the deviation from magnetic field strength of the gap center (%), horizontal (P ₁
The magnetic field strength of ~ P ₇ ) is extremely uneven.

Therefore, as shown in FIGS. 4a to 4d, the positions of the magnet segments whose magnetizing heights are lowered are moved horizontally from the center of the permanent magnet structure (1) to the outside (P _{1 to} P ₇ ).
When the effect of the above on the magnetic field strength on the circumference is examined, the results shown in FIG. 6 are obtained.

In this way, the effect of the position of the magnet segment and the amount of change in the magnetization direction height on the magnetic field strength on the circumference of the horizontal plane (P _{1 to} P ₇ ) is examined, and the optimum magnet segment is selected and Horizontal planes (P _{1 to} P ₇ ) by overlapping
The magnetic field strength of becomes constant.

Similarly, the optimization of the magnet segments can be expanded to the whole area, and a uniform magnetic field can be obtained by overlapping the optimal magnet segments, but trial and error requires a large number of steps, so optimization here is performed. The height of the magnetization direction of each magnet segment was determined by linear programming, which is one of the methods.

Based on this analysis, the magnetization direction length of the required permanent magnet is selected from among the many permanent magnets of the permanent magnet structure (1) (1) in accordance with the strength of the magnetic field in the air gap (7),
For example, a uniform magnetic field can be obtained as shown in FIG. 7 by changing and arranging as shown in FIG.

In the permanent magnet structure (1) shown in FIG. 1a, various permanent magnets having different lengths in the magnetization direction are arranged on the plate yoke (5), but as shown in FIG. The same effect can be obtained by providing irregularities on the surface of the magnetic yoke (5), arranging permanent magnets in the same manner, and making the surface of the permanent magnet constituting body (1) facing the void (7) a flat surface.

Furthermore, in order to reduce the weight of the magnets, the height of the magnetization direction of each magnet segment was optimized by the above-mentioned analysis method. As shown in FIG.
In particular, by increasing the magnetizing direction at the peripheral edge to form an annular protrusion and changing the magnetizing direction at a predetermined position in the center to arrange the magnets, a more accurate uniform magnetic field can be obtained, and the weight is further reduced. Was achieved.

Practical example Using a Fe-BR permanent magnet with a (BH) max of 35 MGOe at room temperature, a convex protrusion with a diameter of 300 mm and a height of 3.0 mm and an outer diameter
9th with the pole piece having an annular projection of 1100mm, inner diameter 900mm, and height 80mm arranged and the facing distance of the pole piece set to 500mm
In the conventional magnetic field generator shown in the figure, the magnetic field uniformity within a radius of 200 mm from the center of the air gap was measured and found to be 50 ppm.
Met.

In the above conventional magnetic field generator, the pole piece is removed,
The above-mentioned analysis method is applied to optimize the magnetization direction length of each permanent magnet of the permanent magnet structure, the facing distance of the permanent magnet structure is set to 500 mm, and the permanent magnet structure shown in FIG. 8 is provided. A magnetic field generator according to the present invention was produced.

The radius from the center in the air gap of the magnetic field generator according to the present invention
When the magnetic field homogeneity within 200 mm was measured, it was the same as that of the conventional device.
%, A weight reduction of more than 10% has been achieved, and it has become possible to reduce the weight of the entire device.

Effects of the Invention In the magnetic field generator according to the present invention, by arranging permanent magnets having different lengths in the magnetization direction without using the magnetic pole pieces, a uniform magnetic field equal to or more than that of the configuration using the magnetic pole pieces can be obtained. Since no piece is used, generation of eddy current is suppressed, and further weight reduction can be achieved.

[Brief description of drawings]

1A and 1B are explanatory views showing a permanent magnet structure (lower side) of a magnetic field generator according to the present invention. FIG. 2a is a vertical cross-sectional explanatory view of a magnetic field generator having no pole piece, and FIG. 2b is a cross-sectional explanatory view thereof. FIG. 3 is a perspective explanatory view of a gap showing a method for measuring a magnetic field in a spherical space of a magnetic field generator. 4A to 4D are explanatory views showing an example in which the magnetization direction height of the required segment of the permanent magnet structure (upper side) of the present invention is changed. 5 and 6 are graphs showing the magnetic field inhomogeneities of P _{1 to} P ₇ in the spherical space of FIG. 3, and FIG. 5 shows the conventional magnetic field generator of FIG. 9 with pole pieces removed. In this case, FIG. 6 is the fourth
The case of the permanent magnet construction of FIGS. FIG. 7 shows P _{1 to} P ₇ in the spherical space in the embodiment of FIG. 1a.
3 is a graph showing the magnetic field non-uniformity of FIG. FIG. 8 shows an optimized permanent magnet structure of the present invention (upper side).
It is explanatory drawing which shows one Example. FIG. 9A is a vertical cross-sectional explanatory view of a conventional magnetic field generator, and FIG. 9B is a cross-sectional explanatory view thereof. 1 ... Permanent magnet structure, 2 ... Pole piece, 3 ... Annular protrusion, 4,5 ... Plate yoke, 6 ... Column yoke, 7 ... Void, 8 ... Convex projection.

Claims (1)

[Claims] 1. A magnetic field generator for MRI for magnetically coupling a pair of permanent magnet constituents, which are opposed to each other with a gap and are composed of a single or a plurality of magnet segments, by a yoke to generate a magnetic field in the gap. A magnetic field generator for MRI, characterized in that required positions of a magnet structure or magnet segments at required positions are arranged with varying lengths in the direction of magnetization thereof to generate a uniform magnetic field in the gap.