Lightweight asymmetric magnet arrays with mixed-phase magnet rings

Abstract

To provide, generally, magnet assemblies, and particularly, lightweight magnet assemblies comprising permanent magnets, and design methods thereof.SOLUTION: A magnet array (400) includes multiple magnet rings (411-420) and a frame. The multiple magnet rings are positioned along a longitudinal axis and coaxially with the longitudinal axis, where at least two of the magnet rings include mixed-phase magnet rings (411, 413) that are phase-dissimilar. The multiple magnet rings are configured to jointly generate a magnetic field along a direction parallel to the longitudinal axis of at least a given level of uniformity inside a predefined inner volume. The frame is configured to fixedly hold the multiple magnet rings in place.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] Cross - Reference to Related Applications This application claims the benefit of U.S. Provisional Patent Application No. 62 / 772,638, filed on November 29, 2018, and U.S. Provisional Patent Application No. 62 / 780,272, filed on December 16, 2018, the disclosures of which are incorporated herein by reference.

[0002] The present invention generally relates to magnet assemblies, particularly lightweight magnet assemblies with permanent magnets and methods of designing the same.

Background Art

[0003] The design of permanent magnet arrays aimed at achieving a strong and uniform magnetic field has been previously reported in patent literature. For example, U.S. Patent No. 7,423,431 describes a permanent magnet assembly for an imaging device comprising a permanent magnet body having a first surface and a stepped second surface configured to correspond to an imaging volume of an imaging device, and the stepped second surface has at least four steps.

[0004] As another example, U.S. Patent No. 6,411,187 describes an adjustable hybrid magnetic device for use in medical and other applications, the hybrid magnetic device comprising a magnetic flux generator for generating a first magnetic field in an imaging volume and a permanent magnet assembly for generating a second magnetic field superposed on the first magnetic field to provide a substantially uniform magnetic field having an enhanced magnitude within the imaging volume. The permanent magnet assembly comprises a plurality of annular or disk - shaped concentric magnets, and the plurality of concentric magnets may be spaced along their axis of symmetry. The hybrid magnetic device may comprise a high - permeability yoke for increasing the strength of the magnetic field in the imaging volume of the hybrid magnetic device.

[0005] U.S. Patent No. 10,018,694 describes a magnet assembly for a magnetic resonance imaging (MRI) device. The magnet assembly includes a plurality of magnet segments disposed in two or more rings, and the magnet segments are equally spaced from adjacent magnet segments in the same ring and are spaced from magnet segments in adjacent rings. According to one embodiment, the plurality of magnet segments are disposed in two or more rings, and the magnetization directions of at least some of the magnet segments are not along the plane defined by each ring, providing greater control over the resulting magnetic field profile.

[0006] U.S. Patent No. 5,900,793 describes an assembly of a plurality of annular concentric magnets spaced along their axes of symmetry and a method of constructing such an assembly using permanently magnetized equiangular segments.

SUMMARY OF THE INVENTION

[0007] One embodiment of the present invention provides a magnet array including a plurality of magnet rings and a frame. The plurality of magnet rings are disposed coaxially with a longitudinal axis along the longitudinal axis, and at least two of the magnet rings are different mixed-phase magnet rings. The plurality of magnet rings are configured to jointly generate a magnetic field along a direction parallel to the longitudinal axis having at least a given level of uniformity within a predetermined internal volume. The frame is configured to fixedly hold the plurality of magnet rings in a predetermined position.

[0008] In some embodiments, two of the at least two mixed-phase magnet rings include only one permanent magnetic phase having magnetization vectors in directions different by more than 45 degrees from each other.

[0009] In some embodiments, each magnet ring has rotational symmetry about the longitudinal axis with respect to in-plane rotation of the magnet ring.

[0010] In one embodiment, at least one of the plurality of magnet rings surrounds a predetermined internal volume, and the minimum inner radius of the magnet ring disposed on one side of the center of the internal volume along the longitudinal axis is different from the minimum radius of the magnet ring disposed on the other side of the center of the internal volume. In another embodiment, the internal volume is an ellipsoid of revolution about the longitudinal axis.

[0011] In some embodiments, the plurality of magnet rings are arranged in an anti-inversion asymmetric manner with respect to the longitudinal axis.

[0012] In some embodiments, a particular magnet ring consists of one of a single integrated element and an assembly of individual magnet segments.

[0013] In some embodiments, the magnet rings are pre-magnetized in their respective magnetization directions to maximize the uniformity of the magnetic field within the internal volume. In another embodiment, the magnet rings are pre-magnetized in their respective magnetization directions to minimize the fringe field outside the magnet array.

[0014] In one embodiment, the individual magnet segments are electrically insulated from each other. In another embodiment, each of the individual magnet segments has a shape that is one of a sphere, a cylinder, an ellipsoid, and a polyhedron. In yet another embodiment, the individual magnet segments are separated from each other by at least one non-magnetic element including a solid, a gas, and a liquid.

[0015] In some embodiments, the plurality of magnet rings have a shape including any of an ellipse, a circle, and a polygon.

[0016] In some embodiments, the mixed-phase magnet rings each have at least an eighth-order discrete rotational symmetry.

[0017] In one embodiment, the magnet array further comprises one or more additional arrays of a plurality of magnet rings, and the plurality of magnet rings in the additional array are coaxial with their respective longitudinal axes set at respective angles from the longitudinal axis.

[0018] According to one embodiment of the present invention, a method of manufacturing a magnet array is further provided. The method includes arranging a plurality of magnet rings coaxially with the longitudinal axis along the longitudinal axis. At least two of the plurality of magnet rings include hybrid-phase magnet rings having different phases. The plurality of magnet rings are configured to jointly generate a magnetic field along a direction parallel to the longitudinal axis having at least a given level of uniformity within a predetermined internal volume. A frame is used to fixedly hold the plurality of magnet rings in place.

[0019] The present invention will be more fully understood from the following detailed description of its embodiments in conjunction with the following drawings.

Brief Description of the Drawings

[0020]

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[0021] Overview In various fields such as medical, aerospace, electronics, and automotive industries, a strong and uniform magnetic field is required. As an example, the magnets used in magnetic resonance imaging (MRI) of the human brain typically provide a magnetic field with an intensity of 0.1 to 3 tesla. This is uniform up to several million parts per million (ppm) within an imaging volume (volume) of about 3000 cubic centimeters (for example, inside a sphere with a radius of 9 cm). However, such magnets have limited applications because of their relatively large size and weight. Furthermore, in general, in magnet design, there is a severely limited trade-off between weight, magnetic field uniformity, and the size of the volume that can achieve a given uniformity.

[0022] Embodiments of the present invention described below provide a lightweight permanent magnet array that generates a strong and uniform magnetic field (for example, within the range of 0.1 to 1 tesla). Some of the disclosed magnet arrays are configured for use in emergency medical brain mobile MRI systems, such as head MRI systems in ambulances. However, in general, the disclosed technology can be applied to any other suitable system.

[0023] In this specification, the internal volume (volume) is defined as the volume of a rotational ellipsoid around the longitudinal axis using a cylindrical coordinate system (reference system) consisting of coordinates in the longitudinal direction (axial direction) (Z), radial direction (r), and azimuth angle (θ). Examples of the internal volume are a prolate spheroid with the major axis along the longitudinal axis and an oblate spheroid with the minor axis along the longitudinal axis. The transverse plane is further defined as any r-θ plane (that is, a plane perpendicular to the longitudinal z-axis). A specific definition of the internal volume is the imaging volume of an MRI system in which the magnetic field has at least a given level of uniformity.

[0024] In some embodiments of the present invention, a magnet array is provided with a frame, which is fixed at a predetermined position and configured to hold a plurality of magnet rings coaxial with the central longitudinal axis at different positions along the central longitudinal axis. The magnet rings are on the side surface, and at least one ring surrounds a region included in an internal volume through which the longitudinal axis passes (i.e., the ring intersects the internal volume). In this specification, the frame is defined by its mechanical function of holding the rings in a predetermined position, which can be made in various ways, for example, by using a yoke or by embedding the rings in a surrounding material (such as epoxy).

[0025] The plurality of magnet rings are arranged in an inversion-asymmetric manner with respect to the longitudinal axis. In the context of the present disclosure and the scope of the claims, the term "inversion-asymmetric with respect to the longitudinal axis" means that a plane perpendicular to the longitudinal axis is not a plane of symmetry of the magnet array. That is, the magnet array is not symmetric under inversion at any point along the axis with respect to the longitudinal axis. Inversion-asymmetry is also referred to as point-asymmetry or mirror-image asymmetry. For the sake of brevity, in the following description, when referring to the "asymmetry" of the magnet array, it means the inversion-asymmetry defined above.

[0026] The plurality of magnet rings are configured to jointly generate a magnetic field along a direction parallel to the longitudinal axis with at least a given level of uniformity within the internal volume. The magnet array causes each magnet ring to generate a magnetic field having rotational symmetry (continuous or discrete) with respect to in-plane rotation of the ring around the longitudinal axis.

[0027] In some embodiments, each of the magnet rings of the disclosed magnet array has a shape including one of an ellipse, most commonly a circle, or a polygon. Each magnet ring consists of a single integrated element or an assembly of individual magnet segments. The magnet rings are pre-magnetized in a magnetization direction designed to maximize the uniformity of the magnetic field within the internal volume and optionally minimize a safety zone defined by a region around a magnet where the magnetic field exceeds 5 gauss.

[0028] In some embodiments typically configured for head MRI applications, the disclosed asymmetric permanent magnet array can be described as comprising a first magnet assembly having two or more magnet rings with a first inner diameter, and a second magnet assembly having two or more magnet rings with a second inner diameter. The first inner diameter is greater than the maximum transverse diameter of the imaging volume, and the second inner diameter is less than or equal to the maximum transverse diameter of the imaging volume.

[0029] Typically, the plurality of magnet rings are at different longitudinal axis positions. The second magnet assembly is arranged asymmetrically with respect to the imaging volume. Thus, the asymmetric structure of the disclosed magnet array is optimized to fit a human head, and physical access to the internal volume (the same as the imaging volume) including the brain is provided by the first assembly rather than the second assembly. The first magnet assembly and the second magnet assembly are configured to jointly generate a magnetic field parallel to the longitudinal axis with at least a given level of uniformity within the internal volume.

[0030] In some embodiments, the asymmetric magnet array comprises at least two mixed-phase permanent magnet rings with different phases. In the context of the present invention, a mixed-phase magnet ring (MPMR) is defined as a magnet ring including a plurality of repeating segments, each segment consisting of two or more phases, and at least one of the segments consisting of a permanent magnet material.

[0031] A phase is defined as an element characterized by a specific combination of (i) material composition, (ii) geometric shape and relative position within the segment, and (iii) magnetization state. The magnetization state is the three components of the magnetic moment, M=(M r ,M θ ,M z) is represented by these components are shared by the corresponding phases of different segments in the cylindrical coordinate system described above. The materials of the various phases can be, but are not limited to, permanent magnets, ferromagnetic materials, ferrimagnetic materials, paramagnetic materials, diamagnetic materials, antiferromagnetic materials, or non-magnetic materials. The total magnetic field of the MPMR at any point is calculated by superimposing the contributions of all the phases in the ring where the value of M is non-zero.

[0032] The phases fill the entire effective volume of the MPMR. The effective volume of the MPMR is defined as the volume of a polygonal annular ring with the minimum cross-sectional area, which just encloses all the magnetic phases in the ring. The volume ratio of the phases is defined as the ratio of the phase volume to the effective volume of the MPMR.

[0033] If there is a one-to-one correspondence between the phases of two rings such that (a) the volume ratios of the corresponding phases are the same, (b) the corresponding non-permanent magnet phases are the same, and (c) the magnetization vectors of the corresponding phases differ at most by a fixed rotation by a certain angle in the r-Z plane common to all the phases and a fixed scaling factor in the magnitude of the magnetization common to all the phases, then the two MPMRs are said to have similar phases.

[0034] Therefore, if the phases of two MPMRs are different, the relative contribution of an individual phase in that ring to the total magnetic field of a particular ring will be different for the two rings. For example, a computerized magnetic field simulation tool can be used to adjust or "tune" the phases of at least two MPMRs with different phases and the directions of the magnetic moments of the permanent magnet phases of those MPMRs to optimize the uniformity of the total magnetic field within the internal volume. Such additional degrees of freedom are most advantageous when the array is subject to various geometric constraints (such as the position of the rings, the radial / axial thickness, etc.), which usually arise from mechanical or manufacturing limitations.

[0035] It is understood that the integrated magnet ring part (piece) can be magnetized in a repetitive manner in azimuth to create repetitive segments where each segment is magnetized with a different magnetization direction and / or intensity. In the context here, such a magnet part is considered an MPMR that shares a common material composition in phase but has different magnetization states even when there is no actual segmentation of the magnet ring mechanically. The same applies, for example, to an integrated magnet ring made of different material compositions where the material composition varies in a repetitive manner in azimuth. In such a case, different magnetic composition regions are considered different phases. The same also applies to an integrated magnet part where the axial thickness and / or the radial thickness and / or the cross-sectional shape of the magnet part vary in a repetitive manner in the azimuth direction. In this case, the ring is considered an MPMR having phases with different shapes but sharing a common composition and magnetic state even when there is no mechanical segmentation.

[0036] For a given weight of an asymmetric magnet ring array, using two or more MPMRs with different phases results in a level of magnetic field uniformity within the internal volume that is significantly higher than the level achieved by an asymmetric array incorporating only one MPMR or multiple MPMRs with similar phases.

[0037] The various types of magnet rings described above are typically made of a ferromagnetic material such as an alloy of neodymium, iron, and boron (NdFeB), and its Curie temperature is well above the maximum ambient operating temperature. Other material options include ferrite, samarium cobalt (SmCo) magnets, or any other permanent magnet material. Depending on the design and type of the ring, the ring segments can have the shape of a sphere, a cylinder, an ellipsoid, or a polyhedron such as a cuboid, a wedge, or a segmented shape with corners.

[0038] Two disclosed techniques for realizing a magnet array (e.g., using an asymmetric shape, using two or more MPMR rings), either separately or in combination, enable the use of a powerful and uniform magnet array, particularly in applications that require a lightweight magnet solution.

[0039] FIG. 1 is a perspective view of an exemplary asymmetric magnet array 100 comprising a first magnet assembly 110 and a second magnet assembly 120 according to an embodiment of the present invention. As shown, the first magnet assembly 110 and the second magnet assembly 120 each comprise at least two magnet rings coaxial with a central longitudinal axis represented by the "Z-axis" passing through the internal volume 130. The plurality of magnet rings have variable transverse dimensions and variable displacements along the Z-axis. In FIG. 1, by way of example, the first assembly 110 is shown as consisting of four magnet rings 111-114, and the second assembly 120 is shown as consisting of four magnet rings 121-124. Each ring in the first magnet assembly 110 and the second magnet assembly 120 is an integrated ring or a segmented ring, i.e., a ring comprising individual segments. The segments can have the shape of a sphere, a cylinder, an ellipsoid, or a prism, preferably a cuboid. It is understood that the ring can have any cross-section including an irregular cross-sectional shape. All segments belonging to a single ring share a common shape and material composition, as well as the same magnetic moment components in the longitudinal (Z), radial, and azimuthal directions. However, one or more of these properties can be different for each ring.

[0040] In the case of a segmented ring, referring to the magnetic moment of a segment means that the segment is uniformly magnetized in a specific direction in space, and its radial, longitudinal, and azimuthal directions are calculated at the centroid of the segment.

[0041] In the case of an integrated ring, M has continuously varying azimuthal, radial, and longitudinal components that do not depend on the azimuthal coordinates. An integrated magnet portion having a complex shape has M r , M θ , or M zis magnetized so as to change gradually or stepwise according to Z or R, and is mechanically composed of one continuous part, but it is understood that several rings can be effectively created from the perspective of magnetization. In the context here, this type of implementation is regarded as having a plurality of rings where the boundaries of the rings are determined not by mechanical segmentation but by the perspective of magnetization.

[0042] The peripheral shape of the ring can be any closed curve such as a circle, an ellipse, a polygon, etc. In some cases, the choice of the peripheral shape depends on the cross-sectional shape of the internal volume 130. It is understood that the rotational symmetry of the ring means, among other things, that its peripheral shape is also rotationally symmetric (for example, the shape of a circle or an equiangular and equilateral polygon). In the special case where all the rings are circular, the minimum inner radius of the rings 111 - 114 of the first magnet assembly 110 is denoted as R1, and the minimum inner radius of the rings 121 - 124 of the second magnet assembly 120 is denoted as R2. For a given target radius Ri, the target radius Ri has, as an example, the lateral radius 140 of the internal volume 130 that defines the maximum radius of the internal volume of the ellipsoid of revolution used for imaging. When the magnetic field has at least a given level of uniformity, the values of R1 and R2 satisfy the relationship Ri < R1 and 0 ≤ R2 ≤ Ri. When R2 = 0, at least one of the rings of the second magnet assembly 120 is an integrated disk shape. It is understood that the second magnet assembly 120 can include rings having an inner radius larger than R2 and further larger than R1. The magnet assemblies are usually arranged separated in the Z direction with a gap (0 to 10 cm, but not limited to this). For the purposes here, when the ring extends in the Z direction with respect to both magnet assemblies, a part of the ring is regarded as being included in the first magnet assembly and the other part is included in the second magnet assembly. In this case, the gap between the arrays becomes zero.

[0043] In one embodiment, in an asymmetric array, the minimum radius of a ring disposed on one side of the center of the internal volume is different from the minimum radius of a ring disposed on the other side of the center of the internal volume. The center of the internal volume is defined in any suitable manner, for example, as the center of a section of the longitudinal axis within the internal volume. Further, if the internal imaging volume is only partially surrounded by the array, the center is considered as the center of a section of the longitudinal axis within the internal volume and within the array. An array according to the former embodiment can be described as being composed of two sub-assemblies having different minimum inner radii as described above.

[0044] The internal volume 130 is a simply connected region that is at least partially surrounded by the assembly 110, which is typically an ellipsoid or a sphere. As shown, the internal volume 130 is surrounded by the magnet array 110, and the rings 112 - 113 surround the internal volume 130. In one embodiment, the internal volume 130 is an oblate ellipsoid with semi-axes equal to approximately 0.5R1, 0.5R1, and 0.3R1. The parameters of such rings are not limited to the inner and outer radii of the rings, their Z displacement, or the thickness in the Z-axis direction. Further, all the magnetic moment angles are optimized using computational methods such as finite element, finite difference, analytical approach, etc., and in combination with a gradient descent optimization algorithm, to achieve the minimum weight and the highest uniformity for a specific magnetic field strength in the imaging volume. This is possible because each assembly contains a number of rings, all of which are optimized.

[0045] One aspect of the asymmetry of the magnet array 100 is that different rings have different transverse dimensions and magnetic moment directions and are arranged in an array where the rings have inversion asymmetry with respect to the longitudinal axis (i.e., are asymmetric with respect to Z-axis inversion). In the context of the present disclosure and the claims, the term "inversion asymmetric with respect to the longitudinal axis" means that a plane perpendicular to the longitudinal axis is not a plane of symmetry of the magnet array. That is, the magnet array is not symmetric under inversion with respect to the longitudinal axis at any point along the axis. Inversion asymmetry is also referred to as point asymmetry or mirror asymmetry. For the sake of brevity, in the following description, when the "asymmetry" of the magnet array is mentioned, it means the inversion asymmetry defined above.

[0046] Asymmetry in the design is particularly advantageous when imaging an essentially asymmetric specimen such as a human head. For example, in one such case, it has been found that the rings belonging to assembly 110 can be magnetized mainly in a first predetermined direction (e.g., the r direction), while the rings belonging to assembly 120 can be magnetized mainly in another direction (e.g., the z direction).

[0047] Finally, the direction of magnetization of the individual rings can be optimized to obtain both uniformity in the internal volume and reduction of the fringe field, creating a magnetic circuit that closes the magnetic field lines close to the magnet rings. In one embodiment, the individual magnet segments are each pre-magnetized in their respective magnetization directions that minimize the fringe field outside the magnet array.

[0048] Figures 2A and 2B - 2D are perspective views of an asymmetric magnet array 200, and plots of magnetic field lines generated separately by an assembly and jointly generated by the assembly, according to another embodiment of the present invention. The uniformity is not apparent from the uniform density of the magnetic field lines (because the magnetic field lines are drawn more densely in the imaging zone in more detail), but rather is apparent from the alignment of the magnetic field lines in the z-axis direction.

[0049] As shown in FIG. 2A, the internal volume 230 is a singly connected region that is at least partially surrounded by the first magnet assembly 210, which is typically an ellipsoid or a sphere. The second magnet assembly 220 of the asymmetric array "caps" (covers) the internal volume 230. As described above, different rings may have different magnetization directions to optimize the uniformity and fringe field of the magnet array. For example, one ring may have a magnetization vector in a direction that is substantially different (e.g., more than 45 degrees) from another ring. For example, the magnetization vector of a permanent magnet segment may point mainly in the r direction in one ring and mainly in the Z direction in another ring. Further, two rings belonging to the same assembly may have substantially different magnetization directions. For example, one ring of the first assembly may have magnetization mainly in the r direction, another ring of the first assembly may have magnetization mainly in the -z direction, and a third ring of the first assembly may have magnetization at -45 degrees in the r-z plane. In one embodiment, two or more rings have magnetization vectors in directions that differ by more than 45 degrees from each other.

[0050] In certain cases (not shown), the plurality of rings of the assembly 210 are distributed in their inner radii between 15 cm and 30 cm and in their Z positions at a length of 25 cm, while the plurality of rings of the assembly 220 are distributed in their inner radii between 0.05 cm and 30 cm and in their Z positions at a length of 12 cm, and the displacement in the Z direction between the two assemblies is between 0 cm and 10 cm.

[0051] FIG. 2B shows the magnetic field lines of the magnetic field generated by the first magnet assembly 210 (rings shown in cross-section by squares each having a direction of magnetization of the ring in the r-z plane) inside and outside the internal volume 230. As shown, the magnetic field lines inside the inner volume 230 are mostly aligned along the z-axis but bend sharply at the top of the internal volume 230 where the magnetic field becomes very non-uniform.

[0052] Figure 2C shows the magnetic field lines of the magnetic field generated by the second magnet assemblies 220 inside and outside the internal volume 230. As can also be seen here, most of the magnetic field lines inside the internal volume 230 are aligned along the z-axis. However, the magnetic field lines inside the internal volume 230 are inclined to the opposite side of the magnetic field lines in Figure 2B with respect to the z-axis and become very non-uniform at the bottom of the internal volume 230.

[0053] As shown in Figure 2D, when the magnet assemblies 210 and 220 are combined to form a complete array 200, they compensate for the non-uniformity of each other's magnetic fields and achieve a magnetic field that is uniform along the z-axis to a better extent than a predetermined threshold.

[0054] Figures 2A - 2D show an exemplary array including 10 rings. It is understood that the array can include more rings (e.g., dozens or hundreds of rings), all of which are optimized as described above. The more rings included in the array, the better the magnetic performance can be achieved (e.g., higher uniformity level, larger magnetic field, larger imaging volume, etc.). However, there is a drawback in that the complexity and manufacturing cost of the array increase due to the large number of elements. Therefore, those skilled in the art need to consider the required number of rings according to specific applications.

[0055] Figure 3 is a perspective view of one segmented magnet ring 300 that can be either of the rings of the magnet arrays 100 and 200 of FIGS. 1 and 2 according to an embodiment of the present invention. In FIG. 3, each magnet segment 310 has a magnetization vector 320 in the r-Z plane having similar longitudinal (Z) and radial (r) components. Further, each segmented ring has rotational symmetry with an azimuthal period equal to 360 / N degrees, where N is the number of segments in the ring. (In the case of an integrated ring, i.e., when N→∞, the rotational symmetry is continuous). In some embodiments, the disclosed rings have rotational symmetry of N≧8th order. The disclosed arrays include rings having rotational symmetry, and thus it is understood that the resulting magnetic field is along the longitudinal axis. However, it is possible to incorporate into an asymmetric array ring that is non-rotationally symmetric in a way that optimizes the uniformity and fringe field in the internal volume. In such a case, the magnetic field may be along any axis. Such an array may be substantially worse than a rotationally symmetric array, but the use of asymmetry with rings as disclosed can substantially improve the uniformity of the array compared to a symmetric array.

[0056] The individual segments 310 are equally spaced and are attached to each other using an adhesive, preferably non-conductive, for example, or are mechanically held together with the gaps 330 between adjacent segments, preferably filled with an insulating material (but not limited to this). Also, it will be understood that a rotationally symmetric segmented ring can include combinations of multiple types of segments. For the overall thermal stability of the ring 300, the adhesive or gap is preferably made of a material that is also thermally conductive, such as silicon oxide, silicon nitride, aluminum oxide, etc. The individual magnet segments 310 are made of the above-described high ferromagnetic material, and its Curie temperature is much higher than the operating temperature of a related system including elements such as array 200, for example, a mobile MRI system.

[0057] The descriptions of FIGS. 1-3 are intended to be useful only as examples, and it is understood that many other embodiments are possible within the scope of the present invention. For example, the rotation of the magnet moment vector in the r-z-θ plane can be achieved in alternative embodiments by rotating the individual magnet segments 310 at different rotation angles, which may be different for different rings. Further, the magnet arrays 100 and 200 can each be combined with either a static or a dynamic shimming system to further improve the uniformity of the magnetic field within the internal volumes 130 and 230. When a dynamic shimming or gradient pulse magnetic field is used, the presence of an electrically insulating adhesive or an empty gap between adjacent magnet segments 310 helps to minimize the adverse effect of eddy currents on the magnetic field uniformity. Additionally, the magnet arrays 100 and 200 can be combined with resistive coils arranged concentrically with the z-axis to enhance the magnetic field strength within the internal volumes 130 and 230.

[0058] Magnet Array with Hybrid-Phase Magnet Rings FIG. 4 is a perspective view of a magnet array 400 of hybrid-phase magnet rings (MPMRs) having different phases, according to an embodiment of the present invention.

[0059] As shown, by way of example, the array 400 includes ten magnet rings 411-420 that are coaxial with a central Z-axis passing through an internal volume 430. The different rings are disposed at different positions along the Z-axis and generally have different transverse dimensions, radial thicknesses, and axial thicknesses. As shown, the magnet array 400 has inversion asymmetry with respect to its longitudinal axis (i.e., is asymmetric with respect to Z-axis inversion). In the context of the present disclosure and the claims, the term "inversion asymmetric with respect to the longitudinal axis" means that a plane perpendicular to the longitudinal axis is not a plane of symmetry of the magnet array. That is, the magnet array is not symmetric under inversion with respect to the longitudinal axis at any point along the axis. Inversion asymmetry is also referred to as point asymmetry or mirror asymmetry. For the sake of brevity, in the following description, when the "asymmetry" of the magnet array is referred to, it means the inversion asymmetry defined above.

[0060] Furthermore, the internal volume 430 may extend in the z-direction (not shown) either inside (as shown) or at least partially outside the magnet array 400. Further, the disclosed magnet array may or may not be combined with a yoke.

[0061] Ring 411 illustrates an MPMR having rectangular parallelepiped-shaped permanent magnet elements (i.e., phase 1) separated by a relatively small non-magnetic gap (i.e., phase 2). Ring 413 illustrates an MPMR having rectangular parallelepiped-shaped permanent magnet elements (i.e., phase 1) separated by a relatively large non-magnetic gap (i.e., phase 2). Clearly, the proportion of the non-magnetic gap in the total ring volume is small in the case of ring 411 and relatively large in the case of ring 413. Thus, rings 411 and 413 are MPMRs with different phases, and the array 400 may include many MPMRs with different phases.

[0062] Furthermore, ring 411 may also have a magnetization vector in a direction substantially different (e.g., more than 45 degrees) from that of ring 413. For example, the magnetization vector of the permanent magnet segment may point in the -Z direction in ring 411 and point at -45 degrees in the r-Z plane in ring 413. In one embodiment, two or more mixed-phase magnet rings include only one magnetic phase having magnetization vectors in directions different by more than 45 degrees from each other. Each MPMR ring has rotational symmetry with an azimuthal period equal to 360 / N degrees. Here, N is the number of segments in the ring. (In the case of a continuous ring, i.e., when N→∞, the rotational symmetry is continuous). In some embodiments, the disclosed MPMR rings have a discrete rotational symmetry of N≧8.

[0063] Figure 5 is a plot of the magnetic field lines generated by the magnet array 400 of FIG. 4 according to an embodiment of the present invention. The uniformity is not apparent from the uniform density of the magnetic field lines (since the magnetic field lines are drawn more densely in the imaging zone for more details), but rather is apparent from the alignment of the magnetic field lines in the z-axis direction. The MPMR array 400 can achieve a more uniform magnetic field along the z-axis compared to what can be achieved with the same array weight, for example, with arrays 100 and 200. Further, the uniform magnetic field extends radially to approximately a ring. Thus, such a lightweight MPMR array can be particularly useful for applications such as mobile MRI, for example, in an MRI ambulance.

[0064] FIGS. 4 and 5 show an exemplary array 400 that includes 10 MPMRs. The array can include more MPMRs (e.g., dozens or hundreds of MPMRs), all of which are optimized as described above, and many of which will have different phases, it is understood. The more rings included in the array, the better the magnetic performance can be achieved (e.g., higher levels of uniformity, larger magnetic fields, larger imaging volumes, etc.). There is a drawback, however, in that the complexity and manufacturing cost of the array increase due to the large number of elements. Thus, one of ordinary skill in the art will need to consider the number of MPMRs required for a particular application.

[0065] FIG. 6 is a perspective view of an exemplary MPMR 600 according to an embodiment of the present invention. The ring consists of six repeating segments 610, and each segment has four elements 620a, 620b, 620c, 620d.

[0066] In one embodiment, element 620a is made of the ferromagnetic material described above. Element 620a is typically pre-magnetized with a specific value for the component of the magnetic moment. The shape of element 620a can be cylindrical as shown in FIG. 3, or can be other shapes such as, for example, a sphere, an ellipsoid, a rectangular prism, a polyhedron, etc.

[0067] Element 620c typically has a different phase from element 620a. For example, element 620c can have the same material composition and geometric shape as element 620a, but one or more components of the magnetic moment M can be different. Alternatively, element 620c can be made of a non-ferromagnetic material such as, for example, ferrimagnetic, paramagnetic, or non-magnetic material. In this case, the phase of element 620c is different from the phase of element 620a due to its different material composition.

[0068] As shown in FIG. 6, element 620b fills the gap of length LI that separates element 620a from element 620c, and similarly, element 620d fills the gap of length L2 that separates element 620c from the adjacent segment of element 620a. In many cases, for the thermal stability of MPMR600, elements 620b and 620d are preferably made of a non-magnetic non-conductive material that has at least moderately high thermal conductivity, such as, for example, silicon oxide, silicon nitride, aluminum oxide.

[0069] To further illustrate the concept of MPMR with similar phases, consider an MPMR600 in which elements 620a and 620c have axial magnetizations M0 and -M0, respectively. Next, consider a different MPMR600* (not shown) that is identical to MPMR600 in all respects except that elements 620a* and 620c* have radial magnetizations 2M0 and -2M0, respectively. Since MPMR600 can be converted to MPMR600* by rotating the magnetic moment by 90° in common in the r-Z plane and then multiplying by a common factor of 2, the two MPMRs are considered to have similar phases. For each ring, the effective strength of the ring can be defined by dividing the magnitude of the volume average r-Z projection of the magnetization vector by the magnitude of the maximum magnetization of all the permanent magnet phases. The parameter is a value between 0 and 1 and has the qualitative meaning of how effectively the ring generates a magnetic field nearby. If the phases of the two rings are not similar, the relative effective strengths of the two rings are different, and the contributions of the two rings to the magnetic field can be different.

[0070] Generally, adjacent elements in the MPMR are held together by mechanical means or adhesives. If the total volume occupied by the adhesive layer is small, for example, less than 1% of the total volume of the ring, the adhesive layer need not be treated as an additional phase for the purpose of magnetic field calculations. Minor adjustments in the position and angle of the segments can be performed to compensate for segment imperfections and remaining non-uniformities.

[0071] The above description is intended to serve only as an example, and it is understood that many other embodiments are possible within the scope of the present invention. For example, the magnet array 400 can be combined with either a static or dynamic shimming system to further improve the uniformity of the magnetic field within the internal volume 430. If a dynamic shimming or gradient pulse magnetic field is used, an electrically insulating material is present in the gap between adjacent magnet elements, which helps to minimize the adverse effect of eddy currents on the magnetic field uniformity. Further, the magnet array 400 can be combined with a resistive coil arranged concentrically with the z-axis to enhance the magnetic field strength within the internal volume 430.

[0072] In the exemplary embodiments described herein, the mixed-phase ring is part of an asymmetric magnet array. However, in alternative embodiments, the mixed-phase ring can be used in a symmetric array or any other type of magnet array, with or without a yoke, to enhance their uniformity. Further, the exemplary magnet arrays described herein include a plurality of rings coaxial with a common axis. However, it is possible to combine the described arrays with one or more additional ring arrays that are coaxial with one or more different axes at an angle from the first common longitudinal axis. The combination of arrays results in a magnetic field being jointly generated in any direction within the space. The additional ring arrays can include mixed-phase rings, but such rings are defined according to a separate cylindrical coordinate system in which the z-axis is defined as a separate common coaxial axis.

[0073] For example, it is possible to provide two arrays of rings where the axes of each coaxial pair are 45 degrees different from each other. Each array can include two or more different MPMRs and can be optimized to obtain a substantially uniform magnetic field in the internal volume along each of the array axes. The combination of the two arrays results in a uniform magnetic field in a direction between the first longitudinal axis and the second longitudinal axis.

[0074] The embodiments described herein mainly address applications for mobile MRI, but the methods and systems described herein can also be used for other applications, such as aerospace applications that require a powerful, uniform, and lightweight magnet, for example, a scanning electron microscope (SEM).

[0075] Accordingly, the above embodiments are cited by way of example, and it is understood that the present invention is not limited to what has been shown and described in detail above. Rather, the scope of the present invention includes both the various combinations and sub - combinations of the features described above, as well as variations and modifications thereof that are not disclosed in the prior art and would occur to a person of ordinary skill in the art upon reading the above description. Documents incorporated by reference in this patent application should be considered as an integral part of this application, except when terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly herein, and only the definitions herein should be considered.

Claims

1. 1. A magnet array for use in a brain magnetic resonance imaging (MRI) system, comprising: A plurality of magnet rings; a frame configured to hold a plurality of magnet rings in place; Equipped with 11. A magnet array comprising: a plurality of magnet rings made of permanent magnetic material, arranged along a longitudinal axis and coaxial with the longitudinal axis, at least one of the magnet rings surrounding a predetermined imaging internal volume of the MRI system, the magnet rings being divided into: (i) a first assembly characterized by a first minimum inner diameter that is the smallest of inner diameters of magnet rings of the first assembly; and (ii) a second assembly arranged along the longitudinal axis next to the first assembly, characterized by a second minimum inner diameter that is the smallest of inner diameters of magnet rings of the second assembly, the first minimum inner diameter of the first assembly being greater than the second minimum inner diameter of the second assembly, a center of the imaging internal volume being located outside of the second assembly, at least one of the magnet rings of the first assembly having a non-zero inner diameter and a longitudinal magnetization component along the longitudinal axis, the magnet rings collectively generating a magnetic field of at least a given level of uniformity within the imaging internal volume.

2. The magnet array of claim 1 , wherein the magnetic field generated by the magnet array is along a direction parallel to a longitudinal axis.

3. The magnet array of claim 1 , wherein each magnet ring has rotational symmetry with respect to an in-plane rotation of the magnet ring about the longitudinal axis.

4. 2. The magnet array of claim 1, wherein the magnet rings are pre-magnetized with respective magnetization directions that maximize magnetic field homogeneity within the imaging interior volume.

5. 2. The magnet array of claim 1, wherein the magnet rings are pre-magnetized with respective magnetization directions that minimize fringe fields outside the magnet array.

6. The magnet array of claim 1 , wherein the individual magnet segments are electrically isolated from one another.

7. 2. The magnet array of claim 1, wherein each of the individual magnet segments has one of the following shapes: a sphere, a cylinder, an ellipsoid, or a polygon.

8. 10. The magnet array of claim 1, wherein the individual magnet segments are separated from one another by at least one non-magnetic element including a solid, gas, or liquid.

9. The magnet array of claim 1 , wherein each of the plurality of magnet rings has a shape including any one of an ellipse, a circle, and a polygon.

10. 1. A method of manufacturing a magnet array for use in a brain magnetic resonance imaging (MRI) system, comprising: Positioning a plurality of magnet rings; Fixing the plurality of magnet rings in place using a frame; Equipped with 11. A method according to claim 10, wherein the plurality of magnet rings are made of permanent magnetic material, are arranged along a longitudinal axis and are coaxial with the longitudinal axis, at least one of the magnet rings surrounds a predetermined imaging internal volume of the MRI system, the magnet rings being divided into: (i) a first assembly characterized by a first minimum inner diameter that is the smallest of the inner diameters of the magnet rings of the first assembly, and (ii) a second assembly arranged along the longitudinal axis next to the first assembly, characterized by a second minimum inner diameter that is the smallest of the inner diameters of the magnet rings of the second assembly, the first minimum inner diameter of the first assembly being greater than the second minimum inner diameter of the second assembly, a center of the imaging internal volume is located outside of the second assembly, at least one of the magnet rings of the first assembly has a non-zero inner diameter and has a longitudinal magnetization component along the longitudinal axis, and the magnet rings collectively generate a magnetic field of at least a given level of uniformity within the imaging internal volume.

11. 11. The method of claim 10, wherein each magnet ring has rotational symmetry with respect to an in-plane rotation of the magnet ring about the longitudinal axis.

12. 11. The method of claim 10, wherein the magnet rings are pre-magnetized with respective magnetization directions that maximize magnetic field homogeneity within the imaging interior volume.

13. 11. The method of claim 10, wherein the magnet rings are pre-magnetized with respective magnetization directions that minimize fringe fields outside the magnet array.

14. The method of claim 10, wherein the individual magnet segments are electrically insulated from one another.

15. 11. The method of claim 10, wherein each of the individual magnet segments has one of the following shapes: a sphere, a cylinder, an ellipsoid, or a polygon.

16. 11. The method of claim 10, wherein the individual magnet segments are separated from one another by at least one non-magnetic element including a solid, gas, or liquid.

17. The method of claim 10, wherein each of the plurality of magnetic rings has a shape including any of the following: an ellipse, a circle, and a polygon.

18. The method of claim 10, wherein the magnetic field generated by the magnet array is along a direction parallel to a longitudinal axis.

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