WO2002029431A2 - Ensemble aimants permanents - Google Patents

Ensemble aimants permanents Download PDF

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Publication number
WO2002029431A2
WO2002029431A2 PCT/GB2001/003297 GB0103297W WO0229431A2 WO 2002029431 A2 WO2002029431 A2 WO 2002029431A2 GB 0103297 W GB0103297 W GB 0103297W WO 0229431 A2 WO0229431 A2 WO 0229431A2
Authority
WO
WIPO (PCT)
Prior art keywords
air gap
permanent magnets
permanent magnet
permanent
magnet assembly
Prior art date
Application number
PCT/GB2001/003297
Other languages
English (en)
Other versions
WO2002029431A3 (fr
Inventor
Neil Marks
Original Assignee
Council For The Central Laboratory Of The Research Councils
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Council For The Central Laboratory Of The Research Councils filed Critical Council For The Central Laboratory Of The Research Councils
Priority to AU2001270911A priority Critical patent/AU2001270911A1/en
Publication of WO2002029431A2 publication Critical patent/WO2002029431A2/fr
Publication of WO2002029431A3 publication Critical patent/WO2002029431A3/fr

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Classifications

    • 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
    • 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

Definitions

  • the present invention relates to a permanent magnet assembly capable of producing a homogenous magnetic field in an air gap.
  • a permanent magnet assembly capable of producing a homogenous magnetic field in an air gap.
  • Such an assembly may be used in for example magnetic resonance imaging (MRI).
  • MRI magnetic resonance imaging
  • Permanent magnets have been used for many years to generate magnetic flux in air gaps. Permanent magnet materials have been developed which enable the generation of relatively high flux densities in air gaps, particularly in assemblies in which permanent magnet material is used together with high permeability magnetic steel or iron.
  • MRI requires the generation of a high magnetic flux density in an air gap into which that part of a patient which is to be examined must be inserted.
  • the magnetic flux density must have a very high homogeneity throughout that part of the air gap occupied by the patient. Homogeneities of the order of 10 ppm or better are required.
  • the image resolution which can be achieved with MRI systems is a function of the magnitude of the flux density, high fields giving better resolution.
  • MRI systems are large permanent installations.
  • Super-conducting magnets are often used, generating flux densities of the order of one or two T or higher.
  • the largest magnets define air gaps which are sufficiently large to accommodate the whole body of the patient.
  • Given the size of current MRI systems, such systems generally are made available only in large hospitals in major cities. If a relatively small portable MRI facility could be provided, this would allow the examination of for example the limb of a person or animal (for veterinary use).
  • Such portable systems could be made available at many more centres, extending orthopaedic services to local communities and thus avoiding the need for patients to travel to major centres.
  • Permanent magnet assemblies show considerable advantages if considered for use in portable MRI facilities as they require no cooling nor high current, high stability, ultra low ripple DC power supplies. To be readily usable however, it would be necessary for a portable MRI facility to be transportable in a medium sized commercial vehicle.
  • a permanent magnet assembly capable of generating a flux density of the order of 0.6T or more, providing high field homogeneity (equal or better than 10 ppm), defining an air gap adequate to enclose at least a single human limb with ease of access for the patient either lying or standing alongside the magnet, and having a size and weight suitable for mounting on a medium size commercial vehicle.
  • Figure 1 of that document shows a simple assembly comprising two spaced apart permanent magnets which extend between two plates of high permeability material such that the permanent magnets and plates define a frame extending around an air gap.
  • the permanent magnets define planar end surfaces which bear against mating planar surfaces of the plates.
  • the magnets are arranged such that magnetic poles of the same polarity face the same plate. This structure ensures a highly homogenous distribution of magnetic induction up to the side surfaces of the magnets facing the gap because the magnetic field distribution across the gap is uniform up to the surfaces.
  • a permanent magnet assembly comprising two spaced apart permanent magnets which extend between two plates of high permeability material such that the permanent magnets and plates define a frame extending around an air gap, the permanent magnets defining end surfaces which bear against mating surfaces of the plates, and the permanent magnets being arranged such that poles of the same polarity face the same plate, characterised in that the end surfaces of the permanent magnets are shaped such that the lengths of inner side surfaces of the permanent magnets that form sides of the air gap are less than the lengths of outer side surfaces of the permanent magnets that are remote from the air gap.
  • the invention makes it possible to achieve relatively high flux densities by providing more permanent magnet material than is provided in a conventional rectangular-section magnet assembly without having to increase either the height of the air gap or the cross-sectional area of the permanent magnet material.
  • the total volume of permanent magnetic material is increased without increasing the height of the air gap by adding to the height of the magnets only in parts of the magnets which are not adjacent the air gap.
  • permanent magnet assemblies can be provided which are of a size and weight suitable for transportation and which define relatively accessible air gaps into which a patient's limb can be inserted.
  • the invention provides a method of using permanent magnetic material to generate magnetic induction, in a suitably dimensioned air gap, which has a magnitude significantly above the level normally generated by this material. It also provides high homogeneity and utilises a geometry which allows the permanent magnet material to be operated at optimum values of field and flux density, thus minimising cost, weight and volume.
  • each end surface of the permanent magnets comprises a first end surface portion extending away from the inner side surface towards the outer side surface, a second end surface portion extending away from the outer side surface, and the third end surface portion defining a step extending between the first and second end surface portion.
  • the third end surface portion may be substantially parallel to the inner and outer side surfaces.
  • the first end surface portion may extend from the inner side surface in a direction which is inclined away from the second end surface portion.
  • At least one of the plates has a surface remote from the air gap which defines an indentation such that the thickness of a portion of the plate between the air gap and the indentation is less than the thickness of portions of the plate adjacent the indentation.
  • surfaces of the plates facing the air gap are profiled to produce a uniform flux density across the air gap.
  • Figure 1 is a schematic representation of a conventional permanent magnet assembly
  • FIG. 1 illustrates characteristics of permanent magnetic material
  • Figure 3 illustrates one quarter of a structure such as that shown in Figure 1;
  • Figure 4 illustrates the field homogeneity of the assembly shown in Figure 3;
  • Figure 5 illustrates a modification of the structure shown in Figure 3 capable of producing higher flux densities
  • Figure 6 shows the induction quality of the assembly of Figure 5
  • FIG. 7 schematically illustrates an assembly in accordance with the present invention
  • Figure 8 illustrates in greater detail one quarter of the assembly of Figure 7;
  • Figure 9 illustrates the gap flux density quality achieved with the assembly shown in Figure 8.
  • Figure 10 illustrates a further geometry giving the efficiency and flux density magnitude of Figure 9 but with improved quality.
  • Figure 11 illustrates in greater detail the geometry of the design of Figure 10
  • Figure 12 illustrates the gap induction quality for the design shown in Figure 10;
  • Figure 13 represents the air gap quality of the design shown in Figure 10;
  • Figure 14 shows an alternative embodiment of the invention designed to allow easier patient access
  • Figure 15 illustrates the gap induction quality for the design shown in Figure 14.
  • Figure 16 represents the air gap quality of the design shown in Figure 14.
  • this shows a conventional permanent magnet assembly comprising a first permanent magnet 1, a second permanent magnet 2, a first steel end plate 3, and a second steel end plate 4.
  • the ends of the permanent magnets 1 and 2 are planar and bear against the mating planar faces of the plates 3 and 4.
  • the permanent magnets 1, 2 and plates 3, 4 define an air gap 5. It is difficult to achieve high flux densities in the air gap without using large permanent magnets 1, 2.
  • Figure 2 illustrates the characteristics of permanent magnetic material.
  • Permanent magnetic materials operate in the second quadrant of the induction (B)/field (H) curve with a negative field H for a positive induction B.
  • the hysteresis loop crosses the H axis at the value of the coercive field He and the B axis at the remnant induction Br.
  • the values of B and H in the permanent magnetic material depend on the point where a line 6 (the so called load line) crosses the hysteresis curve.
  • the gradient of the load line 6 is dependent upon the volume and geometry of the permanent magnetic material and the size of the air gap.
  • a very small air gap will result in a load line with a large magnitude negative gradient.
  • the flux in the material and in the gap will approach Br, but with very low field H generated by the material.
  • a large gap will result in a low value of induction B in the air and the material, with a large field H being required to drive the flux through the air gap.
  • the energy density B.H available from the permanent magnetic material is low and the material is not being used efficiently.
  • There is an optimum gradient for the load line 6 where the product B.H in the material is a maximum. Efficient use of the permanent magnetic material demands that the material is operated in conditions which correspond as closely as possible to this optimum condition.
  • Figure 3 illustrates in detail one quarter of a conventional permanent magnet assembly of the type shown in Figure 1.
  • the diagram shows the magnet geometry used for a finite element analysis. One quarter of the dipole is shown, with appropriate dipole symmetries defined at the outer boundaries. Thus the upper rectangle 7 represents one half of the plate 3 of Figure 1, the rectangle 8 represents one quarter of the air gap 5, and the rectangle 9 represents one half of the permanent magnet 2.
  • This material has a maximum energy density at approximately:
  • the cross-sectional area of the permanent magnetic material on one side of the assembly as shown in Figure 3 is 5000 mm 2 (50 x 100 mm).
  • Figure 3 shows lines of flux density (contours of constant vector potential).
  • This diagram also gives the numerical value of By at (0,0) as 0.535 T.
  • the vertical component of induction varies by up to four in ten thousand over the full air gap. This is far larger than the allowed tolerance for MRI applications. This variability results from the finite permeability of the top and bottom plates.
  • the flux paths through the steel vary in length depending on the point on the magnet from which the lines emanate. Longer path lengths result in a slightly higher MMF drop, producing a reduced field.
  • the cross-sectional area of the permanent magnets required for the assembly as partially illustrated in Figure 5 is 25,000 mm 2 (50 x 500 mm). This is a factor of five increase in the cross section as compared with Figure 3.
  • the flux density in the permanent magnet material is in the order of 0.3 T. This demonstrates the inefficiency which results when the working point of the permanent magnetic material is far from optimum.
  • the thickness of the steel plate against which the magnetic material bears has been significantly increased in the embodiment illustrated in Figure 5 as compared with the embodiment of Figure 3. This was necessary to conduct the higher flux through the steel sheet.
  • the quality of the induction is similar to that obtained in the case of the embodiment of Figure 3, but at the expense of a large increase in size and weight.
  • I and 2 define end surfaces which are non-planar.
  • the permanent magnets are relatively short on the side which faces the air gap 5 and relatively long on the side which is remote from the air gap.
  • I I of the permanent magnets that form sides of the air gap 5 are less than the length of outer side surfaces 12 and 13 of the permanent magnets that are remote from the air gap 5.
  • the faces of the plates 3 and 4 against which the end surfaces of the magnets 1 and 2 bear are shaped to be a close fit to the magnets.
  • the efficiency of the use of the permanent magnet material is improved by providing an increased volume of permanent magnetic material by allowing a greater height of this material remote from the air gap. In this way, the height of the air gap is not changed.
  • the structure schematically illustrated in Figure 7 is shown in greater detail in Figure 8 which shows one quarter of the assembly in a manner analogous to that of Figure 3.
  • the numerals 7, 8 and 9 are used to represent the same features as in Figure 3.
  • the end surface of the permanent magnet 9 comprises a first end surface portion 14 extending from the inner side surface 11 towards the outer side surface 13, a second end surface portion 15 extending away from the outer side surface 13, and a third end surface portion 16 defining a step extending between the first and second end portions 14 and 15.
  • the surface portion 14 extends for a distance of 40 mm from the air gap, and then the overall length of the permanent magnet 9 is increased as a result of a 40 mm step 16.
  • the efficiency of the permanent magnetic material is improved as compared with for example the conventional structure of Figure 3 by providing additional permanent magnetic material which is not immediately adjacent to the air gap.
  • the resultant cross section of the side face of the permanent magnetic material is 19,100 nun 2 (50 x 40 mm) + (90 x 190mm).
  • the resulting quality and magnitude of flux density is shown in Figure 9.
  • the homogeneity of the air gap flux density can be improved by careful profiling of the surfaces of the end plates facing the air gap.
  • the geometry of Figure 8 results in a good, though not perfect, flux density distribution in the air gap. This is because the arrangement provides some auto-compensation for the reduced height of permanent magnetic material immediately adjacent to the air gap. The flux density in this region is strongly reduced by the influence of the relatively remote larger volume of permanent magnetic material, and the resulting larger magnitude negative magnetic field is close to that required to produce a fully homogenous flux density in the gap.
  • Figure 10 shows one configuration capable of producing improved flux density distributions in the air gap.
  • Figure 11 illustrates the surface shapes in greater detail.
  • the surface 15 is identical to that of Figure 8 but the surface 14 although planar is inclined away from the surface 15 as compared with Figure 8 and as a result the height of the step surface 16 is increased.
  • the increase in height of the step 16 is 3 mm, a relatively small dimensional adjustment which nevertheless provides a significant improvement in quality as is apparent from Figure 12 which shows the gap induction quality predicted for the design shown in Figure 10.
  • the thickness of the plate 7 is a maximum of 230 mm, tapering down to a minimum of 110 mm.
  • Figure 15 The flux density quality achieved with the structure of Figure 14 is shown in Figure 15 (x > 0 ⁇ 150 mm) and Figure 16 (x > 0 ⁇ 110 mm).
  • Figures 15 and 16 should be compared with Figures 12 and 13. It will be seen that the removal of the steel to form the indentation 17 has produced negligible deterioration to either flux density magnitude or quality.
  • the flux density distribution in Figure 16 shows a maximum deviation from a constant field of 75 parts per million. As before, this can be corrected for by careful profiling of the plate surfaces facing the air gap.
  • the described embodiments of the invention provide a viable and economic design for a permanent MRI magnet assembly producing flux densities of the order of 1 T with high homogeneity.
  • the dimensions used in the simulations of the designs as represented in the above drawings may be too small for practical MRI magnet but the described assemblies can be increased in size as necessary, scaling all dimensions in the horizontal and vertical plane as required, without loss of either flux density magnitude or quality.

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

Abstract

L'invention concerne un ensemble aimants permanents s'utilisant par exemple dans un dispositif d'imagerie par résonance magnétique, qui comporte deux aimants (1,2) permanents espacés s'étendant entre deux plaques (3,4) constituées d'une matière à perméabilité élevée. Les aimants permanents et les plaques définissent une structure s'étendant autour d'un entrefer (5) dans lequel un flux magnétique homogène doit être produit. Les aimants (1,2) permanents définissent des surfaces d'extrémité qui appuient contre des surfaces correspondantes des plaques, et les pôles des aimants permanents de même polarité se situent face à la même plaque. Les surfaces d'extrémité des aimants permanents ne sont pas planes, et sont façonnées de sorte que les longueurs des surfaces (10,11) latérales intérieures des aimants permanents, qui forment les côtés de l'entrefer (5), sont inférieures à celles des surfaces (12,13) latérales extérieures.
PCT/GB2001/003297 2000-10-06 2001-07-23 Ensemble aimants permanents WO2002029431A2 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU2001270911A AU2001270911A1 (en) 2000-10-06 2001-07-23 Permanent magnet assembly

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB0024495.4 2000-10-06
GB0024495A GB0024495D0 (en) 2000-10-06 2000-10-06 Permanent magnet assembly

Publications (2)

Publication Number Publication Date
WO2002029431A2 true WO2002029431A2 (fr) 2002-04-11
WO2002029431A3 WO2002029431A3 (fr) 2002-05-10

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PCT/GB2001/003297 WO2002029431A2 (fr) 2000-10-06 2001-07-23 Ensemble aimants permanents

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AU (1) AU2001270911A1 (fr)
GB (1) GB0024495D0 (fr)
WO (1) WO2002029431A2 (fr)

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4706057A (en) * 1985-05-23 1987-11-10 Siemens Aktiengesellschaft Magnet of a nuclear spin tomograph
WO1994020971A1 (fr) * 1993-03-09 1994-09-15 Commissariat A L'energie Atomique Structure d'aimant permanent a haute efficacite et a faibles fuites

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4706057A (en) * 1985-05-23 1987-11-10 Siemens Aktiengesellschaft Magnet of a nuclear spin tomograph
WO1994020971A1 (fr) * 1993-03-09 1994-09-15 Commissariat A L'energie Atomique Structure d'aimant permanent a haute efficacite et a faibles fuites

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
JONES R.E. ET AL.: "Directionally Homogenous Magnets for Vacuum Deposition Apparatus. October 1976." IBM TECHNICAL DISCLOSURE BULLETIN, vol. 19, no. 5, 1 October 1976 (1976-10-01), pages 1856-1857, XP002179705 New York, US *

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Publication number Publication date
AU2001270911A1 (en) 2002-04-15
WO2002029431A3 (fr) 2002-05-10
GB0024495D0 (en) 2000-11-22

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