WO2008059474A1

WO2008059474A1 - Magnet assembly being made from a ferromagnetic or rare-earth material

Info

Publication number: WO2008059474A1
Application number: PCT/IL2007/000583
Authority: WO
Inventors: Doron Kwiat
Original assignee: Doron Kwiat
Priority date: 2006-11-15
Filing date: 2007-05-13
Publication date: 2008-05-22

Abstract

The invention provides a magnet which may be formed using a rare earth material sich as neodymium-iron-borone. The cross section of the magnet is designed by first selecting an initial cross sectional shape having an area A and then removing from the initial cross-sectional shape a region having an area a to produce an intermediate cross- sectional shape. The interior of an angle is then removed from the intermediate cross- sectional shape having a vertex located in the region to produce the final cross-sectional shape. The invention also provides a device containing the magnet and a method for producing the magnet.

Description

MAGNET ASSEMBLY BEING MADE FROM A FERROMAGNETIC OR RARE-EARTH MATERIAL.

FIELD OF THE INVENTION

This invention relates to magnets and more particularly to magnets for use in such applications as nuclear magnetic resonance (NMR), magnetic resonance imaging (MRI), printer heads, magnetic tape readers, hard disks, and microphones.

BACKGROUND OF THE INVENTION

In conventional NMR and MRI, a strong and homogeneous static magnetic field is required for the magnetization of the sampled object. The production of a highly homogeneous, sufficiently intense magnetic field requires costly and large magnets with heavy demands on electricity, cryogenics, and shimming procedures. Since the resulting NMR signal is proportional to the square of the magnitude of the magnetization of the nuclei under investigation, which in turn is proportional to the intensity of the applied static magnetic field, it has always been the goal of NMR and MRI engineers to design magnets that provide the strongest magnetic field possible within the constraints of size, cost, and other factors.

Kwiat D. et al, JMRI 2006 discloses an MRI method in which a locally homogeneous static magnetic field is generated using rare-earth magnets (e.g. Neodymium-Iron-Boron, NIB). Although relatively small in size, these magnets can create magnetic field fluxes over 0.15 Tesla in proximity to the surface of an array of RF detectors which in turn can be used to create magnetic resonance imaging without the use of magnetic field gradients. SUMMARY OF THE INVENTION

In its first aspect, the present invention provides a magnet. In accordance with the invention, the magnet of the invention has a cross-sectional shape perpendicular to a main axis that is designed as follows. Starting from any initial planar shape having area A, a hole of area a is removed from the initial shape,. The interior of an angle having a vertex in the hole is also removed from the initial shape to generate the final cross- sectional shape. In one preferred embodiment, A/a > 100. In another preferred embodiment, the angle is 10° to 240°.

The magnet of the invention may be used for example in MRI and NMR systems as well as in printer heads, magnetic tape readers, hard disks, and microphones.

The inventor of the present invention has found that when A/a > 100, with proper selection of the size of the angle, the field flux can be maximized and field strengths between 0.5 Tesla and up to 30 Tesla may be obtained near the surface of the magnet.

In one embodiment, initial shape is a disk of radius R and the hole is a disk of radius r. In this embodiment, the final cross-sectional shape of the magnet is thus an annular sector shape. As used herein, the term "annular sector" refers to a geometric shape bounded by two concentric circular or polygonal arcs and a central angle. An annular sector is thus defined by an inner radius, an outer radius, and a sector angle. In a preferred embodiment, the outer radius R/r ≥ 10. In another preferred embodiment, the central angle is from 120° to 350°. When the magnet has an annular sector cross- sectional shape, the magnetic field of the magnet has the symmetrical characteristics suitable for MRI purposes, such as annular slices bearing homogeneous static magnetic fields in parallel with the main symmetry axis of the magnet.

In a preferred embodiment, the magnet is made from a ferromagnetic or rare- earth material, such as NIB. The magnetic material preferably has a permeability between 1 and 100,000 and an inherent magnetization grade of at least N40 (N52 is the largest commercially available grade today). The length of the main symmetry axis of the magnet may be determined as required in any application. The magnet may be constructed from a plurality of subunits that are assembled tqgether to achieve a cross-sectional shape in accordance with the invention. The subunits may be unmagnetized prior to incorporation into the magnet in order to facilitate assembly. After each subunit is added to the growing magnet, the subunit may be magnetized. Alternatively, all of the subunits may be magnetized together when assembly of the magnet is completed. When the magnet has an annular sector shape, the magnet may be formed from subunits having an annular sector shape or a trapezoidal shape of a smaller central angle than that of the final magnet. The subunits are assembled radially around the center of the final annular sector cross-section of the magnet. Alternatively, a magnet having an annular sector cross-sectional shape may be constructed from a series of annular sector subunits that are arranged concentrically about the center of the final annular sector, where the inner radius of each subunit matches the outer radius of the subunit on its inner aspect.

In another of its aspects, the invention provides a method for producing highly magnetized (over 0.5 Tesla) slices suitable for obtaining NMR signals from a corresponding portion of a sampled object by appropriate selection of RF excitation at the Larmor frequency.

In yet another of its aspects, the invention provides a device or system comprising a magnet in accordance with the invention. The device may be, for example, an MRI system, an NMR system, a printer head, magnetic tape reader, hard disk, microphone or motor.

Thus, in one of its aspects, the invention provides a magnet having a main axis and a final cross-sectional shape, the cross- sectional shape being designed in a process comprising:

(a) selecting an initial cross sectional shape having an area A;

(b) removing from the initial cross-sectional shape a region having an area a to produce an intermediate cross-sectional shape; and

(c) removing from the intermediate cross-sectional shape an interior of an angle having a vertex located in the region to produce the final cross-sectional shape. In another of its aspects, the invention provides a device comprising a magnet according to any one of the previous claims.

In yet another of its aspects, the invention provides a method for producing a magnet, the magnet having a main axis and a final cross-sectional shape, the cross- sectional shape being designed in a process comprising:

(a) selecting an initial cross sectional shape having an area A;

(c) removing from the intermediate cross-sectional shape an interior of an angle having a vertex located in the region to produce the final cross-sectional shape; the magnet comprising a plurality of subunits formed from a magnetizable material, wherein the method comprises, for each of one or more subunits, assembling the subunit into the magnet in an unmagnetized state and magnetizing the subunit after assembly of the subunit in the magnet.

BMEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Fig. 1 shows a magnet in accordance with one embodiment of the invention;

Fig. 2 shows designing of a cross-sectional shape for a magnet in accordance with the invention;

Fig. 3 shows a magnet in accordance with one embodiment of the invention having an annular sector cross-section;

Fig. 4 shows a magnet having an annular sector cross-section in accordance with another embodiment constructed from a plurality of annular sector subunits arranged radially around the center of the cross-section; Fig. 5 shows a magnet having an annular sector cross-section in accordance with another embodiment constructed from a plurality of annular sector subunits arranged concentrically around the center of the cross-section;

Fig. 6 shows a magnet in accordance with the invention consisting of alternating layers along its main axis of a magnetized material and a solid metal;

Fig. 7a shows a magnet of the invention and flux lines of the magnetic field generated by the magnet; and

Fig. 7b shows the variation of magnetic flux (Tesla) in the tangential direction of the field lines of the magnetic filed generated by a magent of the invention as a function of the central angle of the magnet at a point located 1 cm (•) and 4 cm (X) from center of symmetry of the magnet. DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 shows a magnet 50 in accordance with one embodiment of the invention. The magnet 50 has a main axis 52 and a cross sectional shape observed at an end surface 54 in the perspective view of Fig. 1. Fig. 2 shows a process by which the cross- sectional shape of the magnet 50 is designed. An initial shape 56, shown in Fig. 2a, is selected having an area A. The initial shape 56 may be any planar shape including an irregular shape, as shown in Fig. 2. Then, as shown in Fig. 2b, a hole 58 is introduced into the initial shape 56 to yield an intermediate shape 60, where the hole 58 has an area a. Finally, as shown in Fig. 2c, the final cross sectional shape 64 is obtained by removing an angle 62 from the intermediate shape 60, where the angle 62 has a vertex located in the interior of the hole 58. In one preferred embodiment, A/a > 100. In another preferred embodiment, the size of the angle 62 is from 10° to 240°.

Fig. 3 shows a magnet 1 having an annular sector cross-sectional shape in accordance with one embodiment of the invention. The magnet 1 has an annular sector shape bounded by an inner circular arc 2, an outer circular arc 4 concentric with the inner circular arc 2 and a central angle 6. The radius 8 of the outer circular arc is preferably at least 10 times the radius of the inner circular arc 9. The central angle 6 is preferably between 120° and 350°. The magnet 2 has a first polar edge 10 and a second polar edge 12. The polar edges 10 and 12 are inclined to each other by an angle 14 that is equal to 360° minus the central angle 6 of the magnet. The main axis of the magnet 20 is determined as required in any application. The magnet 2 is preferably made from a rare-earth material such as NIB, for example, as disclosed in Kwiat et al (supra).

Fig. 4 shows a magnet 20 in accordance with another embodiment of the invention. The magnet 20 has an annular sector shape and is constructed from a plurality of annular-sector subunits 22 that are arranged radially around the central axis of the magnet 20. An adhesive is used to hold the subunits 22 together as an integral unit in the magnet 2O.The subunits have a common outer radius 24 equal to the outer radius of the magnet 20, and a common inner radius 26 equal to the inner radius of the magnet 20. The size of the central angle 28 of all of the subunits 22 may be the same as shown in Fig. 4, or the size of the central angle may be different for different subunits 22 (not shown). The size of the central angle 30 of the magnet 20 is equal to the sum of the sizes of the central angle of the subunits 22. The subunits 22 may be unmagnetized prior to incorporation into the magnet 20. In this case, each subunit may be magnetized immediately after its incorporation into the magnet, or all of the subunits may be magnetized together after completion of the assembly of the magnet.

Fig. 5 shows an alternative construction of a magnet 70 having an annular sector cross section in accordance with the invention. The magnet 70 is composed of a plurality of "U" shaped or annular sector subunits 72 arranged concentrically in the magnet 70. Eleven subunits 72a to 72k are shown in Fig. 5. This is by way of example only, and the magnet 70 may be constructed from any number of concentric annular sector subunits, as required in any application. The inner and out radii of the subunits 72 increase with increasing distance of the subunit from the center of the magnet 70. The inner radius of each subunit 72 matches the outer radius of the subunit on its inner aspect. Thus for example, the inner radius of the subunit 72c corresponds to the outer radius of the subunit 72b. Each of the subunits 72 may be composed of smaller subunits. The subunits 72 may be unmagnetized prior to incorporation into the magnet 70. In this case, each subunit may be magnetized immediately after its incorporation into the magnet, or all of the subunits may be magnetized together after completion of the assembly of the magnet. Fig. 6 shows a magnet 84 in accordance with another embodiment of the invention. In accordance with the invention, the magnet 84 has a cross-sectional shape designed as explained above. The magnet 84 consists of alternating layers along its main axis of a magnetized material 86, such as NIB and a solid metal 88. The solid metal layer 88 serves to reinforce the NIB layers and to shield each NIB layer from the magnetic field of neighboring NIB layers in order to allow the NIB layers to be maintained in close proximity to each other. The solid metal layers 88 may be made, for example, from low carbon steel and may be about 3mm in thickness. In this embodiment, the number of solid metal layers 88 may be one or more, and the thickness of the layers 88 may be less than, greater than, or equal to the thickness of the layers 86.

Fig. 7a shows the magnetic filed lines 40 generated by a magnet 42 of the invention made from NIB and having an annular sector shape with an outer radius of 45 mm, an inner radius of 1 mm and a central angle of 295°. The strength of the magnet 42 is 800 milliTelsa at a distance of 1 cm from the center of the magnet, and its permeability is 1.1.

Fig. 7b shows the variation of magnetic flux in the tangential direction of magnetic field lines at a point 80 (see Fig. 6a) located 1 cm (•) and at a point 82 (see Fig. 6a) 4 cm (X) from center of symmetry of an annular sector magnet having the dimensions as the magnet 42, as a function of the central angle of the magnet. As seen in Fig. 3b, at a central angle of about 300°, the flux strength has a maximum of about 3.5 telsa at 1 cm from the center of symmetry and about 0.6 telsa at 4 cm from the center of symmetry.

Claims

CLAIMS:

I. A magnet having a main axis and a final cross-sectional shape, the cross- sectional shape being designed in a process comprising:

(d) selecting an initial cross sectional shape having an area A; (e) removing from the initial cross-sectional shape a region having an area a to produce an intermediate cross-sectional shape; and

(J) removing from the intermediate cross-sectional shape an interior of an angle having a vertex located in the region to produce the final cross-sectional shape. 2. The magnet according to Claim 1 wherein A/a≥lOO.

3. The magnet according to Claim 1 or 2 wherein the angle has a size from between 10° to 240°.

4. The magnet according to any one of the previous claims made from a rare earth material. 5. The magnet according to Claim 4 wherein the magnet is made from a Neodymium-Iron-Boron (NIB) alloy.

6. The magnet according to any one of the previous claims wherein the initial cross-sectional shape is a disk.

7. The magnet according to any one of the previous claims wherein the region has a circular shape.

8. The magnet according to any one of the previous claims wherein the final cross- sectional shape is an annular sector shape.

9. The magnet according to Claim 8 wherein the annular sector has an outer radius and an inner radius, the outer radius being at least 10 times greater than an inner radius. 10. The magnet according to Claim 8 or 9 wherein the annular sector cross-section subtends an arc angle of 170° to 350°.

II. A magnet according to any one of Claims 7 to 10 wherein the magnet comprises two or more U shaped subunits.

12. The magnet according to any one of the previous claims further comprising alternating layers along the main axis of a magnetized material and a solid metal layer. .

13. The magnet according to Claim 12 wherein the metal layer is made of low carbon steel.

14. A device comprising a magnet according to any one of the previous claims.

15. The device according to Claim 14 being an MRI device, an NMR device, a printer head, a magnetic tape recorder or tape player, a hard disk, a microphone or a motor. 16. A method for producing a magnet, the magnet having a main axis and a final cross-sectional shape, the cross-sectional shape being designed in a process comprising:

(a) selecting an initial cross sectional shape having an area A;

(b) removing from the initial cross-sectional shape a region having an area a to produce an intermediate cross-sectional shape; and (c) removing from the intermediate cross-sectional shape an interior of an angle having a vertex located in the region to produce the final cross-sectional shape; the magnet comprising a plurality of subunits formed from a magnetizable material, wherein the method comprises, for each of one or more subunits, assembling the subunit into the magnet in an unmagnetized state and magnetizing the subunit after assembly of the subunit in the magnet.