CN1248895A - MRI magnetic field generator - Google Patents

Abstract

An MRI magnetic field generator which does not give a subject an imaged space stably, and produces a clear image with a high sensitivity while its size is small and its cost is low. A magnetic field reinforcing coil is attached to the peripheral parts of a pair of pole pieces which face each other with a gap therebetween. Hence the adjustment of the magnetic field intensity is possible with a simple construction without changing the form of a magnetic circuit and increasing the weight and area of a magnet. Further, the direction of a current applied to the coil is adjusted to enable the adjustment of the magnetic field distribution and the improvement of the magnetic field uniformity.

Description

magnetic resonance imaging magnetic field generator

The present invention relates to an improvement of a magnetic field generator used in, for example, a medical magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), and more particularly relates to an MRI magnetic field generator as a magnetic field adjusting coil wound around a pair of As a result of the perimeter winding of the magnetic pole pieces facing so as to form an air gap, the MRI magnetic field generator generates a powerful, precise, and uniform magnetic field within the air gap.

An MRI apparatus is a device by which all or part of a patient's body is inserted into an air gap of a magnetic field generator forming a strong magnetic field, and a tomographic image is obtained so that its tissue can be estimated. The MRI magnetic field generator must be wide enough to be inserted into all or part of the patient's body, and clear tomographic images must be obtained. Usually, it is necessary to form a field of view with an area equal to or less than 1×10 ^{- 4} accuracy and a stable, powerful, and uniform magnetic field from 0.02 to 2.0T (Tesla).

As shown in FIGS. 7A and 7B, a known magnetic field generator used in an MRI apparatus has a structure in which magnetic pole pieces 2 are fixed at one end of each of a pair of permanent magnet structures 1 facing each other, at An R-Fe-B base magnet in the permanent magnet structure 1 is used as a magnetic field generating source, and the other ends of the permanent magnet structure 1 are connected by a yoke 3, and a static magnetic field is generated in an air gap 4 between the magnetic pole pieces 2 ( Japanese Patent Laid-Open No. 2-23010). An annular protruding portion 5 is provided around the circumference of the pole piece 2 so as to increase the uniformity of the magnetic field distribution in the air gap 4, and a raised protruding portion (not shown) is provided at the center (Japanese Utility Model Publication 5-37446), and the above structure has been adopted and other structures, and [these elements] are generally made of sheet-shaped blocks produced by flattening electromagnetic soft iron, pure iron, or other magnetic material. In this figure, 6 is a gradient coil.

However, with the magnetic field generators described above, magnetic flux tends to leak from the poles into the air gap, so that the magnetic field strength increases closer to the pole surface along the vertical centerline of the air gap. Therefore, in order to obtain a desired high-strength and uniform magnetic field within the used magnetic field space, the air gap must be several times [big] the used magnetic field space, for example by using a large number of expensive permanent magnets or increasing the distance between magnetic poles or This is achieved by increasing the pole surface area, but this results in a larger magnetic circuit and also increases cost.

In view of this situation, the applicant has found that the magnetic field generator can be made more compact by using a magnetic field generating source comprising a permanent magnet arrangement whose basic phase includes tetragonal crystals, and using these permanent magnets for the magnetic field generator And its essential components are 8 to 30 at% (atomic percent) of R (wherein R is one or more rare earth elements including Y), 2-28 at% of B, and 42-90 at% of Fe; and also found that, Because of the temperature characteristics of these permanent magnets, by keeping these permanent magnets cool, a much higher maximum energy product can be obtained and the magnet weight can be further reduced (Japanese Patent Laid-Open No. 4-22009).

It has also been found that by arranging separate permanent magnet structures around a flat permanent magnet structure so that the pole surfaces of a pair of permanent magnet structures form curved surfaces that are concave relative to the air gap, the magnetic field strength and magnetic field uniformity within the air gap of the magnetic circuit can be increased. properties, and can expand the region of a uniform magnetic field with high precision (Japanese Patent Laid-Open No. 3-20053).

However, the former invention requires a cooling device and the latter requires various permanent magnet structures, both of which in particular make the construction more complicated, resulting in longer assembly time and higher price.

Moreover, in an effort to improve the uniformity of the magnetic field within the air gap, the applicant has proposed a structure in which at least one auxiliary permanent magnet having the same magnetization direction as the permanent magnet is installed inside and/or outside the permanent magnet, thereby It is movable in the magnetization direction of the permanent magnet, which makes it possible to adjust the magnetic flux generated in the air gap (Japanese Patent Laid-Open No. 3-7124); and a structure in which a magnetic force adjustment rod composed of a magnetic material is provided to the yoke , so that at least the amount they protrude into the air gap can be changed, which allows the amount of magnetic flux short circuit to be increased or decreased (Japanese Utility Model Laid-Open No. 2-41841). However, if the short-circuiting of the magnetic flux is increased too much by means of these auxiliary permanent magnets or magnetic adjustment rods, this has the risk of causing a reduction in the magnetic field strength.

In addition, in order to form a static magnetic field with high uniformity and stability, there has been proposed a nuclear magnetic resonance apparatus having a structure in which a static magnetic field generating device includes a pair of permanent magnets, and a space forming a nearly closed space with these magnets. magnetically coupled yokes, providing a plurality of auxiliary coils arranged adjacent to each of the pair of permanent magnets for correcting the uniformity of said static magnetic field, thereby determining the current applied to these auxiliary coils so as to cancel said pair of yokes Influence of magnetic field formed by auxiliary coil, and influence on static magnetic field distribution of permanent magnet (Japanese laid-open patent application 61-17053).

However, with the above structure, the auxiliary coils are arranged on the side of each pole piece facing the air gap, and the air gap is narrowed by the arrangement of these auxiliary coils. If the air gap is widened so that the patient does not feel constrained, the strength of the static magnetic field will be weakened and good quality images will not be obtained.

In addition, there is such a problem that if, during the process of generating images using the above-mentioned MRI magnetic field generator, due to temperature changes of the magnet or movement of objects, such as movement of an elevator in a building where the device is installed, or movement of a car or a train in a Movement in the vicinity of buildings, causing sudden dynamic changes in the magnetic field strength, makes conventional mechanical methods for regulating the magnetic field inappropriate.

In view of the above circumstances, the present invention has been proposed according to the desired magnetic field strength of the MRI magnetic field generator, and an object of the present invention is to provide the MRI magnetic field generator with such a structure: although the structure is compact and the cost is low, it is possible A uniform static magnetic field is stably obtained in the space without making the patient feel constrained, and produces clear images with high resolution.

As a result of various studies aimed at achieving the object, the inventors who have accomplished the present invention have found that in a pair of magnetic poles facing each other to form an air gap and magnetically coupled by a yoke, and in the In an MRI magnetic field generator that generates a magnetic field in an air gap, the magnetic field intensity can be increased or decreased by installing magnetic field adjustment coils around the periphery of the pole shoes installed at these magnetic poles and sending current to these coils; and by adjusting the current The direction can adjust the magnetic field distribution and improve the uniformity of the magnetic field.

The present inventors have also proposed the MRI magnetic field generator constructed as above, having a structure in which the magnetic pole piece includes ring-shaped protrusions, and magnetic field adjustment coils are wound on the peripheries of these ring-shaped protrusions, and used as a means for distributing the magnetic field. A structure in which high-precision adjustment is possible; a structure in which the ring-shaped protrusion includes a plurality of arc-shaped ring-shaped protrusions, and a magnetic field adjustment coil is wound around each of these ring-shaped protrusions.

Fig. 1 shows an embodiment belonging to the magnetic field generator of the present invention, 1A is a vertical sectional view, and 1B is an oblique view;

Fig. 2 is a vertical sectional view showing another embodiment of the magnetic field generator belonging to the present invention;

Figure 3 is an oblique view showing an embodiment of a separate annular projection belonging to the magnetic field generator of the present invention;

Fig. 4 is the magnetic field distribution curve in each plane when spherical space is divided into 15 horizontal planes, and 4A is a kind of conventional magnetic field generator, and 4B is the magnetic field generator belonging to the present invention;

Figure 5 is a top view of one embodiment of a split annular projection belonging to the magnetic field generator of the present invention;

Fig. 6 is the magnetic field distribution curve of X direction, and 6A is a kind of conventional magnetic field generator, and 6B belongs to the magnetic field generator of the present invention;

Fig. 7 shows the structure of a conventional MRI magnetic field generator, 7A is a front view, and 7B is a side sectional view thereof;

Figure 8 shows the structure of a magnetic pole piece used in a conventional MRI magnetic field generator, 8A is a front view, and 8B is a side sectional view thereof; and

Fig. 9 shows the structure of a conventional MRI magnetic field generator, 9A is a front view, and 9B is a side sectional view thereof.

1A and 1B are vertical sectional views and oblique views showing an embodiment of a magnetic field generator pertaining to the present invention. For example, an annular protruding portion 12 composed of laminated silicon steel sheets having a pentagonal cross section is arranged on the air gap facing side of a magnetic material base 11 composed of pure iron or the like. An inner magnetic pole piece 13 is disposed therein, and a magnetic field adjustment coil 14 is wound on the periphery of the annular protrusion 12 . The object of the present invention can also be achieved by a structure in which the inner magnetic pole piece 13 is not provided, the annular protrusion 12 is directly arranged on the magnetic material base 11, and the magnetic field adjustment coil 14 is wound on its periphery. As shown in FIG. 2, the same effect as the above-mentioned structure can also be obtained with a structure in which the magnetic field adjustment coil 14 is wound on the periphery of the annular protrusion 12 constituting the magnetic material base 11 and the magnetic pole piece, or wound on the magnetic material On the periphery of the base 11, the annular protrusion 12, and a permanent magnet structure.

A material known in the past for use as a pole piece, such as electromagnetic soft iron or pure iron, can be used in the present invention to form the magnetic material base of the pole piece. The magnetic material base makes the magnetic field intensity more uniform, ensures that the magnetic pole piece has good mechanical strength as a whole, and facilitates the work of assembling the magnetic field generator.

The material constituting the magnetic pole piece in the present invention is not limited to the material in the embodiment, and a material or the like produced by molding pure iron or soft magnetic powder together with an electric insulating material can be used, and by using a laminate of a silicon steel sheet Or soft ferrite, such as one based on Mn-Zn or Ni-Zn, because of its low coercive force and high electrical resistance, it can reduce the generation of residual magnetism or eddy current at the base of the magnetic material during the application of a pulsed magnetic field . Also, by using these materials together, the advantages of laminated silicon steel sheet and soft ferrite can be more fully utilized. Laminated silicon steel sheets are economically superior to soft ferrites due to their low cost.

As far as the magnetic pole piece 20 shown in FIG. 8 is concerned, if the above-mentioned material is made into a plurality of block-shaped magnetic pole pieces 23 and 24 to configure the material on the magnetic material base 21, the reduction of eddy current and residual magnetism will be even better. Good, and the pieces can be connected with less work. Optimizing the thickness of the pole pieces 23 and 24 or the thickness ratio of the magnetic material base can ensure that the pole piece 20 has good mechanical strength, balance the required magnetic field strength of the pole piece 20, and prevent eddy current and residual magnetism.

In addition, in the present invention, it is preferable to form a protruding part composed of a magnetic material ring 22, such as electromagnetic soft iron or pure iron, around the periphery of the disk-shaped magnetic material base 21, so as to increase the uniformity of the magnetic field in the air gap. Preferably, these rings of magnetic material are electrically insulated from the base of magnetic material in order to reduce the effects of eddy currents. In particular, if one or more slots are provided in the circumferential direction, the influence of eddy currents can be reduced even further.

The cross-section of the annular protrusion is appropriately selected from approximate triangles, quadrilaterals, and the like. A similar effect would be obtained if the entire surface on the opposite side of the pole piece was a concavely curved surface with a smooth arc of single or compound radius of curvature.

In the present invention, winding the magnetic field adjustment coil around the magnetic pole piece including the above-mentioned ring-shaped protrusion and then passing power therethrough makes it possible to increase the magnetic field of the magnetic field generation source and raise or lower the magnetic field intensity, and especially for The magnetic pole piece has a structure with a ring-shaped protrusion, and the winding of the magnetic field adjustment coil around the periphery of the pole piece combines with the magnetic field equalization effect of the ring-shaped protrusion to generate a more uniform magnetic field.

As for a structure in which the annular protrusion is divided into segments by providing a plurality of slits in the circumferential direction, the magnetic field adjustment coil 14 is wound around each of the segmented annular protrusions 121, 122, and 123, as shown in FIG. 3, and adjusting the amount of current through each segment, has the advantage of being able to fine-tune the magnetic field distribution.

When the magnetic field distribution in the imaging space is symmetrical in the positive and negative directions of the Y axis via the X axis, as shown in FIG. Other magnetic field adjustment coils 141 and 142 on the demagnetization side can adjust the magnetic field distribution and improve the uniformity of the magnetic field.

When using the MRI magnetic field generator of the present invention constructed above, by winding the magnetic field adjustment coil around the periphery of the magnetic pole piece that has been used in the past, the magnetic field strength can be raised or lowered without greatly changing the shape of the magnetic circuit or increasing the The weight or surface area of a magnet. Furthermore, adjustment of the magnetic field distribution in the air gap can be achieved relatively simply by adjusting the direction of current flowing to each of the magnetic field adjustment coils wound around the pair of magnetic pole pieces, which improves the uniformity of the magnetic field.

In addition, as for the structure in which a plurality of slits are provided to the annular protrusions in the circumferential direction for the purpose of suppressing eddy currents, and arc-shaped annular protrusion structures are joined together, by Winding magnetic field adjustment coils around them and adjusting the current flow through each enables fine tuning of the magnetic field distribution.

When an R-Fe-B based permanent magnet is used as the source of the magnetic field, the rare earth magnet has a temperature coefficient of -0.1%/C, but when the current passes continuously, the magnetic pole piece or around the periphery of the permanent magnet or the magnetic pole piece The magnetic field adjustment coil generates heat, and this heats up the permanent magnet, thereby maintaining the temperature of the magnet and stabilizing the magnetic field strength.

Thus, if a sensor is placed in a magnetic field, the magnetic field can be kept stable by passing a current through the field-regulating coil as the temperature changes cause the field strength to change. Even if the motion of an elevator or car disturbs the magnetic field during imaging, the dynamic change of the magnetic field can be corrected by the provision of the magnetic field adjustment coil of the present invention.

The MRI magnetic field generator which is the object of the present invention is not limited to the embodiments given above, and can be applied to any structure as long as a pair of magnetic poles facing each other to form an air gap is magnetically coupled through a yoke. In addition, the size and shape of the yoke used to form the magnetic path can be suitably selected as determined by the required size of the air gap, magnetic field strength, magnetic field uniformity, and various other variables.

In addition to the above structure shown in FIGS. 7A and 7B, in which a pair of yokes 3 facing each other via an air gap are connected by four cylindrical yokes 7, it is also possible to adopt the structure shown in FIGS. 9A and 9B. structure in which pole pieces 2 are fixed at one end of each of a pair of permanent magnet structures 1 facing each other, and yokes 3 fixed at the other ends are coupled by a flat connection yoke 8 .

As far as the structure in FIGS. 9A and 9B is concerned, the yoke 3 is coupled at one end thereof by a single connection yoke 8, and since the air gap 4 between the pole pieces 2 is opened on three sides (closed only on the connection yoke 8 side ), so the device has a more open feel without restricting the patient.

Ordinary conducting magnets, superconducting magnets, permanent magnets, etc. can be used as the magnet structure functioning as a magnetic field generating source, and when permanent magnets are used, ferrite magnets, rare earth cobalt-based magnets, or other such known magnetic Material. In particular, the device can be made much more compact by using light rare earths which are abundant in resources, mainly R of Nd or Pr, and using R-Fe-B-based permanent magnets exhibiting a very high energy product above 30MG0e as the main component . Furthermore, by arranging a combination of these known permanent magnets, a less expensive magnetic field generator can be provided without greatly hindering efforts to make the device more compact.

Example 1

A Nd-Fe-B based permanent magnet having a BHmax of 40 MGOe, an outer diameter of 1300 mm (millimeters), an inner diameter of 190 mm, and a height of 150 mm was used in a magnetic field generator constructed as shown in Figures 1A and 1B, one consisting of mild steel A ring-shaped protruding structure with an outer diameter of 1200mm, an inner diameter of 1000mm and a height of 70mm is arranged on a magnetic material base composed of pure iron with an outer diameter of 1200mm and a height of 40mm, and a A laminate of silicon steel sheets produced with integrated bulk silicon steel sheets and having an outer diameter of 1000 mm and a height (in the center) of 25 mm is provided in the annular protrusion.

Provide a pair of upper and lower annular protrusions so that the distance between the opposite sides is 500mm, a magnetic field adjustment coil composed of electrical copper wire is wound 1000 turns around the circumference of the annular protrusion, and a current of 10A is passed through a coil on the magnetized side (in the same direction as the static magnetic field), and the current also passes through the other coil on the magnetized side (in the same direction as the static magnetic field). This configuration allows an increase in strength of about 120 Gauss at the center of the magnetic field.

The goal in this case is to enhance the magnetic field at the center of the air gap, but when the goal is to correct field changes caused by car or elevator motion, the field can be corrected with fewer wire windings or smaller currents than those given above. In this case, the uniformity of the magnetic field within the air gap can be maintained by detecting the magnetic field from the outside by means of a magnetic sensor placed outside the magnetic field generator, and increasing or decreasing the current flowing to the magnetic field adjustment coil according to the sensor information . According to the desired magnetic field strength and the part to be corrected, the number and current value of the wire winding can be properly selected.

Assuming that the imaging space in the structure of Embodiment 1 is spherical, Fig. 4B shows the magnetic field distribution in each plane when the spherical space is divided into 15 horizontal planes. FIG. 4A shows the results of producing a conventional magnetic circuit having the same structure as in Embodiment 1 and examining the magnetic field distribution in the same manner as in Embodiment 1 except that no magnetic field adjustment coil was provided. As is apparent from FIG. 4, the structure of the present invention greatly increases the uniformity of the magnetic field at very low current values.

Example 2

As shown in FIGS. 3 and 5, a plurality of slits are provided thereto in the circumferential direction of the annular protrusion, the arc-shaped annular protrusion members are joined together, and a magnetic field is wound around each of the annular protrusion members. Adjust the coil. As shown in FIG. 6B, the magnetic field uniformity is higher than when no magnetic field adjustment coil is provided (as shown in FIG. 6A).

As far as the present invention is concerned, a magnetic field adjustment coil is wound around the periphery of a pair of magnetic pole pieces facing each other to form an air gap, and as a result, it is possible to raise or lower the magnetic field strength with a simple structure without changing The shape of the magnetic circuit either increases the weight or surface area of the magnet. Also, when the magnetic field distribution in the imaging space is vertically symmetrical about the X axis, the magnetic field distribution can be adjusted by passing current through one coil in the opposite direction to the other coils, which increases the magnetic field uniformity.

In addition, when a plurality of slits are provided to the annular protrusions of the pole piece in the circumferential direction thereof for the purpose of suppressing eddy currents, and a plurality of segmental annular protrusions are joined together, by winding each of these annular protrusions Magnetic field adjustment coils and adjustment of the current flow through each, can fine tune the magnetic field distribution.

Finally, when a rare-earth magnet sensitive to the influence of ambient temperature is used as the main magnetic field generation source, the temperature of the permanent magnet can be maintained by the heat generated when the current flows through the magnetic field adjustment coil, which allows a stable and uniform magnetic field to be obtained. Furthermore, if there is a dynamic change in magnetic field strength due to any reason, such as movement of a car or elevator during imaging, the magnetic field distribution can be adjusted on the fly.

Claims (12)

1. An MRI magnetic field generator having a pair of magnetic poles facing each other to form an air gap and magnetically coupled by a yoke and generating a magnetic field in said air gap, said MRI magnetic field generator comprising A magnetic field adjustment coil wound around the periphery of the pole piece arranged on the above-mentioned pole. 2. The MRI magnetic field generator according to claim 1, wherein the magnetic pole piece includes some annular protrusions, and the magnetic field adjustment coil is wound on the periphery of the annular protrusions. 3. The MRI magnetic field generator according to claim 1, wherein the magnetic pole piece includes an annular protrusion and a magnetic material base, and a magnetic field adjustment coil is wound on a periphery of the magnetic material base. 4. The MRI magnetic field generator according to claim 1, wherein the magnetic pole piece comprises an annular protrusion and a magnetic material base, and a magnetic field adjustment coil is wound on the periphery of the annular protrusion and the periphery of the magnetic material base . 5. The MRI magnetic field generator according to claim 1, wherein the magnetic pole piece comprises an annular protrusion, a magnetic material base, and an inner magnetic pole piece, and a magnetic field adjustment coil is wound around the periphery of the annular protrusion and the magnetic material on the periphery of the base. 6. The MRI magnetic field generator according to claim 5, wherein the inner magnetic pole piece is composed of a silicon steel sheet. 7. The MRI magnetic field generator according to claim 2, wherein the ring-shaped protrusion is composed of a plurality of arc-shaped ring-shaped protrusions, and a magnetic field adjustment coil is wound around each of the ring-shaped protrusions. 8. The MRI magnetic field generator according to claim 7, wherein the annular protrusion consists of four arc-shaped annular protrusions. 9. The MRI magnetic field generator according to any one of claims 2 to 8, wherein at least a part of the annular protrusion is formed of silicon steel. 10. The MRI magnetic field generator according to any one of claims 1 to 9, wherein the pole piece pairs are fixed to an upper yoke and a lower yoke, and the upper and lower yokes are formed by a single-joint yoke to magnetic coupling. 11. The MRI magnetic field generator according to any one of claims 1 to 9, wherein pairs of magnetic pole pieces are fixed to upper and lower yokes, and the upper and lower yokes are magnetically coupled by a plurality of connecting yokes. 12. The MRI magnetic field generator according to any one of claims 1 to 9, wherein pairs of pole pieces are arranged facing each other in a tubular yoke and are magnetically coupled by said yoke.

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