Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
  • G01R33/383—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using permanent magnets
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F7/00—Magnets
  • H01F7/02—Permanent magnets [PM]
  • H01F7/0273—Magnetic circuits with PM for magnetic field generation
  • H01F7/0278—Magnetic circuits with PM for magnetic field generation for generating uniform fields, focusing, deflecting electrically charged particles

Definitions

• the present invention is due to Mr. Guy AUBERT, Director of the National Service of the Intensive Fields, and it has for its object a magnet of general shape substantially spherical with equatorial access to produce a uniform field of induction. It finds its application particularly in the medical field where magnets are used in nuclear magnetic resonance imaging experiments. It can also find its application in all areas where such magnetic induction fields are required.
• Permanent magnet structures producing a uniform transverse magnetic field in a relatively high volume have been described in the prior art.
• D. LEE and AI. have described such a magnet.
• a cylindrical structure in theory of infinite length
• the blocks are distributed around the perimeter of the rings in a polygonal architecture which reproduces as much as possible the circular aspect of a theoretical cylinder.
• the magnetization in each of the blocks is constant in module and is oriented, relative to the direction of the field of induction to be produced, with an angle equal to twice that which measures the location angle of the block in question.
• the blocks described are preferably prismatic volumes with trapezoidal section.
• anisotropic magnetic materials which moreover have the best magnetic properties, have the disadvantage of being difficult to machine in oblique directions relative to the direction of their anisotropy.
• the patent application cited above indicates, in particular in FIG. 5 and in the associated text, that the production of the blocks can be obtained by stacking elementary bricks. But it is obvious that elementary bricks, of parallelepipedal shapes, have a preferred direction of magnetization parallel to one side of the parallelepiped. Also it remains that it is difficult, on the one hand, to cut the bricks obliquely with respect to the sides of this parallelepiped or, on the other hand, to effectively magnetize the blocks formed in oblique directions with respect to the sides of these parallelepipeds.
• a magnet of this type also has other drawbacks.
• the cylindrical magnets have a drawback linked to the cylindrical shape itself.
• the magnetic blocks located at the ends of the cylinder are large since, in short, they represent and replace the infinite extension of the two ends of the cylinder.
• they contribute little effectively to the intensity of the induced field: they mainly have the effect of improving them. homogeneity.
• Their high mass and therefore their weight linked to the problems of the cost of magnetic materials is an obstacle to the use of permanent magnets.
• the object of the invention is to remedy the aforementioned drawbacks and to this end, it relates to a magnet characterized in that it is substantially spherical and that it comprises two hollow opposite structures, of generally hemispherical shape, magnetized so as to produce an induction field going from one to the other in their internal volume, and sufficiently separated from each other to create an equatorial opening and to allow the introduction into this volume of body to submit to this induction.
• Figures la and lb show schematically and in perspective a magnet according to i'in ⁇ ention.
• the magnet shown is spherical with center O. It has two hollow structures facing each other 1 and 2.
• the two structures are generally hemispherical in shape.
• the structure i of FIG. 1a and the structure 2 of FIG. 1b are seen in perspective from the front, slightly above the base of the hemisphere and partially reveal the hollow interior volume V.
• each structure is magnetized radially and / or tangentially, or with a rotating magnetization, so as to produce a induction field B 0 going from one to the other: here from structure 1 to structure 2.
• the corresponding magnetizations appear respectively in FIGS.
• the magnet comprises, to constitute the structures, a certain number of rings, for example numbered 3 to 8.
• Each ring is in the form of a portion of a spherical crown.
• Each ring is preferably made up of a certain number of magnetic blocks placed side by side, for example the twelve blocks numbered 10 to 21 of the ring 5.
• all the magnetized blocks have the particularity that their magnetization M is either radial (M R for the structure of FIG. 2a), or tangential (M T for the structure of FIG. 2b) with the sphere of center O, is finally rotating, ie such that the direction of the magnetization makes, in each block, an angle (with the normal to the equatorial opening) whose value is double that of l 'locating angle of this block (M ⁇ in Figure 2c).
• the structures 1 and 2 are symmetrical to each other. They therefore have the same number of rings and, preferably also, in each ring, the same number of blocks.
• the ring 8 has the same number of blocks as the ring 3. This constraint is not however totally justified, and, for reasons of industrial production, it may be justified to choose a different number of blocks in the rings located near the spherical caps (3 and 8) of the one used for the equatorial rings (5 and 6): the equatorial rings can have more blocks. It will be seen later that the reasons for simplification as well as reasons for optimum performance lead to the preferred solution and, in this preferred solution, to a particular number of blocks in the rings depending on the number of rings in each structure or in the magnet.
• the rings can be separated from each other. This facilitates assembly and also allows for technical access.
• the top rings can be hollow. We can then easily carry out all the connections with the various pieces of equipment of an NMR imaging device which are introduced into volume V.
• the direction of the magnetization is always radial.
• the value of the magnetization is proportional to the cosine of the angle ⁇ of location of the block. This means that in each block of the same ring the magnetization is the same. This also means that the radial magnetization is strongest in the rings near the hemispherical caps (3 and 8) and is weakest in the equatorial rings (5 and 6).
• the magnetization directions of the different blocks have a radial shape converging towards the center O. To this lower part where the magnetization is convergent corresponds an upper part with divergent magnetization. It follows that at the place of the equatorial opening the induction B 0 has the direction indicated. This structure has the advantage of reducing the cost of the magnet. There are few magnetic materials used.
• the magnetization is tangential to the sphere.
• the orientation of this magnetization is substantially from top to bottom so as to produce the same field B 0 as in the radial embodiment.
• the value of this magnetization is proportional to the sine of the block locating angle: ⁇ 1 , ⁇ 2 and ⁇ 3 .
• the field lines of the magnetization follow the trigonometric direction on the left part the figure and the reverse trigonometric direction on the right part.
• the magnetization is nominal in module in the magnetic blocks of the rings 5 or 6 and it is minimal in the blocks closest to the axis z'z. This solution therefore leads to a greater mass of material for the same field B 0 as the previous solution.
• the cosines of the angles identifying the inclination of the radial faces such as 25 and 26 relative to the axis z'z, portions of spherical crowns constituting the rings have (in a three-ring variant) the following values: 0.98; 0.91; 0.81; 0.67; 0.49 for side 26 and 0.30 for side 25. You can choose one of the three solutions. However, all magnets, including permanent magnets, should preferably have a zero dipole moment. If this is not the case it becomes difficult to approach a control desk, provided with a display console with cathode screen, in the immediate environment of the magnet.
• these two structures can be made concentrically with one another, one being with radial magnetization, the other being with tangential magnetization. It is also possible, in a preferred manner, to make rings comprising n 1 blocks with radial magnetization (in cosine) and n 2 blocks with tangential magnetization (in sine). The zero dipole moment is obtained for n 1 equal to 2n 2 .
• the number of blocks in each ring is a multiple by four of the number of rings in each structure, we are then led to produce, for each elementary block, three sub-blocks: two with magnetization radial and a tangential magnetization. There are therefore in this case 24 sub radial blocks and 12 sub tangentieis blocks. For reasons of homogeneity they are symmetrically and regularly distributed in each ring.
• a variegated structure is produced which is described below. It has been discovered that a variegated solution requires that each solution participates in the mixing with magnetizations in each ring, respectively proportional to a 1 cos ⁇ and to a 2 sin ⁇ times a nominal magnetization common to the two solutions. In this case a 1 and a 2 must be such that (a 1 ) 2 + (a 2 ) 2 is less than or equal to 1. Furthermore the zero dipole moment is obtained in this case when a 1 is equal to 2 a 2 .
• the optimization recommended by the invention suggests a minimal structure, that is to say the number of blocks to be produced is as small as possible in order to obtain homogeneity at a given order.
• This minimal structure is such that the number of blocks in a ring of a structure is equal to 4 times the number of rings in this structure.
• the manufacture of blocks and rings is relatively easy, the installation and adjustment of these rings can be delicate. Also and in a preferred manner, it is arranged to respect the homogeneity with a minimum number of rings.
• the most practical industrial solution is such that the number of blocks per ring is greater than or equal to 4 times the number of rings per structure.
• a first solution consists in using for all the blocks the nominal magnetization of the material by reducing the angular opening around z '0 z of each of the n b blocks constituting a given ring, the removed magnetic material being for example replaced by a material non-magnetic structure (epo y resin or other).
• a material non-magnetic structure epo y resin or other
• FIG. 3 to 5 show the different stages of this general method of constructing a magnet according to the invention.
• Figure 3 indicates that one simply obtains the result by attaching plates (one could also put bars) of magnetizable magnetic material 27 to plates 28 of non-magnetic material.
• the proportion of the magnetic material relative to the general volume is varied by varying the thickness p 1 of the magnetic plates relative to the thickness p 2 of the non-magnetic plates.
• the macroscopically equivalent magnetization of a block thus constituted is equal to the product of the intrinsic magnetization of the parts made of magnetic material by the proportion (p 1 / (p 1 + p 2 )) of these magnetic materials in all. Failing to have plates 27 of adequate size, it is also possible to use blocks 29 to 31 of permanently magnetizable material which are butted against each other with very fine contact so that their direction of magnetic anisotropy A is aligned and continuous, on the one hand with each other, and on the other hand with the direction of magnetization A which is subsequently imposed on the blocks.
• the bricks formed by stacking the plates can be of standard size. They are calculated to be magnetized in a standard magnetizer such as the magnetizer 33 of figure 4.
• the current I which crosses this magnetizer must be sufficiently strong, and produce a sufficiently important excitation, so that all the parts in magnetizable materials of the brick are brought to their magnetization saturation. If we call M this saturation magnetization, the brick will be magnetized with a value macroscopically equal to the product of M by the proportion of magnetic materials.
• the blocks comprise, juxtaposed with one another, bricks magnetized orthogonally to one another.
• a block 34 in FIG. 5 comprises bricks with radial magnetization 35 to 37 juxtaposed, or even stacked, with bricks 38 to 40 with tangential magnetization.
• the proportion of cosine bricks and sine bricks is not equal to one, complete with non-magnetic material.
• the assembly of the bricks in the blocks can be carried out in the same manner as the assembly of the plates or pavers in the bricks: that is to say for example with adhesives based on epoxy resin.
• the blocks thus formed can then be fixed to a crown 41 made of epoxy resin to form the rings. If we choose, to simplify the construction, to use parallel piped blocks, wedges such as 42 in the form of a wedge are inserted to form the rings. If necessary, another ring (not shown) opposite the ring 41 can sandwich the blocks to ensure the rigidity of the ring.
• the rings 41 are not planar but are convex so as to adapt to the spherical shape wanted.
• the production of spherical crown portions constituting the rings then requires the production of non-parallelepipedic blocks. Bricks are already cut at an angle with the right proportion of magnetic material. This cut is made before magnetization.
• the crowns ( Figure 6) can be held by pillars such as 43. To this end the crowns are provided with crows 44 which come to rest on the tops of adjusting screws 45 integral with the pillars. In this way, there is a simple means of industrially correcting the homogeneity of the constructed magnet on the site.
• the magnetic materials used are either ferrites, Strontium or Barium, or alloys of Samarium-Cobait, or alloys of Iron-Neodymium-Boron.
• the demagnetizing excitation in the blocks is never collinear with the magnetization and it is therefore necessary to choose an anisotropic magnetic material.
• These different materials have intrinsic remanent magnetizations with different saturation. They are also of specific mass and different prices. We can choose in this way, according to the different specifications of induction field to produce, which is the most suitable solution.
• the calculation of the outside radius of the sphere and therefore of the size of the blocks is of the same kind, in the case of the invention, as that which leads to the determination of the dimensions of the blocks in the state of the art cited.

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Magnetic Treatment Devices (AREA)
• Magnetic Resonance Imaging Apparatus (AREA)

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• 1986
  • 1986-10-17 FR FR8614419A patent/FR2605452B1/fr not_active Expired
• 1987
  • 1987-10-13 WO PCT/FR1987/000394 patent/WO1988002925A1/fr not_active Ceased
  • 1987-10-13 EP EP87906746A patent/EP0329666A1/de not_active Withdrawn
  • 1987-10-13 JP JP62506140A patent/JPH02501002A/ja active Pending
  • 1987-10-13 US US07/353,675 patent/US5028903A/en not_active Expired - Fee Related

Non-Patent Citations (1)

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