EP0272411B1 - Passive Anordnung von Ausgleichskörpern und Verfahren zur Bestimmung der Lage der Ausgleichskörper für einen Magneten der magnetischen Resonanz - Google Patents

Passive Anordnung von Ausgleichskörpern und Verfahren zur Bestimmung der Lage der Ausgleichskörper für einen Magneten der magnetischen Resonanz Download PDF

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Publication number
EP0272411B1
EP0272411B1 EP19870115844 EP87115844A EP0272411B1 EP 0272411 B1 EP0272411 B1 EP 0272411B1 EP 19870115844 EP19870115844 EP 19870115844 EP 87115844 A EP87115844 A EP 87115844A EP 0272411 B1 EP0272411 B1 EP 0272411B1
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EP
European Patent Office
Prior art keywords
magnet
shim
bore
locations
tube
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Expired
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EP19870115844
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English (en)
French (fr)
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EP0272411A1 (de
Inventor
Mark Ernest Vermilyea
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General Electric Co
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General Electric Co
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Publication date
Priority claimed from US06/937,297 external-priority patent/US4698611A/en
Priority claimed from US06/937,299 external-priority patent/US4771244A/en
Application filed by General Electric Co filed Critical General Electric Co
Publication of EP0272411A1 publication Critical patent/EP0272411A1/de
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F7/00—Magnets
    • H01F7/06—Electromagnets; Actuators including electromagnets
    • H01F7/20—Electromagnets; Actuators including electromagnets without armatures

Definitions

  • the present invention relates to passively shimming magnetic resonance magnets to obtain imaging quality homogeneity in the bore of the magnet.
  • correction coils are typically used. These coils are capable of creating different field shapes which can be superimposed on an inhomogeneous main magnetic field to perturb the main magnetic field in a manner which increases the overall field uniformity. Unfortunately, many sets of such coils are typically required.
  • a state of the art magnetic resonance (MR) imaging magnet has between ten and twenty independent sets of correction coils, each with its own power supply to provide the correct current flow. Naturally, these coils add significantly to the cost and complexity of the magnet.
  • One way of removing the need for correction coils is to shim the magnet passively, using only pieces of iron to bring an initially inhomogeneous field to within imaging homogeneity specifications. With the iron placed inside the bore of the magnet a minimal addition to the size and weight would be required. A passively shimmed magnet would be cheaper and more reliable than the typical set of correction coils presently used.
  • Electromagnetic coils are generally designed to produce certain terms of a spherical harmonic expansion. Such a design criteria is difficult to implement with passive shims because the permeability of iron cannot be reversed, whereas a current reversal through a coil can be used to obtain a field reversal in a correction coil. Additionally, the size and complexity of the groups of shim pieces which would be required to produce a single harmonic would not make this approach feasible. Since magnetic coupling between the shims is also a complicating factor, shimming with large pieces which inevitably become physically close to one another increases the difficulty in properly shimming the magnet.
  • Passive shimming is currently used to correct large deviations in magnetic fields that cannot be corrected by the available correction coils alone.
  • the passive shimming is accomplished by placing a piece of iron in an appropriate place outside the magnet. The desired level of field uniformity can then be achieved by the correction coils.
  • EP-A-0 167 059 in Figure 3 discloses a tube of synthetic material on which can be fixed magnetic correcting pieces of iron in the form of a plurality of iron plates, said tube being placed within the central bore of a MR magnet to compensate for inhomogeneity within said bore.
  • the iron plates are arcuate in circumferential direction, they are demountable and how their locations on the tube are determined.
  • a method of passively shimming a magnet having a central bore and using shims placed in the bore of the magnet comprises the steps of measuring the initial field homogeneity in the bore of the magnet.
  • the magnetic field effect of a shim at each of the predetermined permissible shim locations in the bore of the magnet is checked independently of one another, to determine the shim strength needed to improve magnetic field inhomogeneity in the magnet bore.
  • the locations wherein positive shim strengths were found to be beneficial are selected and used to determine shim strength at each selected location, considering all selected locations simultaneously. Locations found to require negative shim strengths are eliminated and with the new selected locations shim strengths are again determined until all selected locations remaining require positive shim strengths.
  • the shims of predicted positive strengths are placed in their selected locations in the bore of the magnet.
  • the present invention provides a method for passively shimming a magnet and a passive shim assembly as set forth in claims 1 and 5, respectively.
  • a passive shim assembly comprising a nonmagnetic thin wall tube 11 is shown.
  • the tube is fabricated of fiberglass creating a tube with a 3.175 mm (1/8") wall thickness.
  • a plurality of longitudinally extending nonmagnetic channel pieces 13 are equally circumferentially spaced about the interior of the tube 11.
  • the channel pieces extend the length of the tube and are secured thereto by screws threadingly engaging the fiberglass tube.
  • the channel pieces each have two projecting edges 13a extending on either side of the channel. The edges are parallel to the tube and spaced away therefrom. The edges extend the longitudinal length of the tube.
  • the channel pieces can be fabricated by extruding aluminum to the desired shape or if eddy currents are a problem, the channel pieces can be pultruded from composite material. Pultrusion is a process in which continuous filaments are drawn through an orifice, which also meters out encapsulating resin such as a thermoplastic.
  • the ferromagnetic strips can comprise 0.254 mm (.010 inch) thick low carbon steel, cut to an axial width of 2 cm. and a circumferential extent at their mean radius of 30 degrees for a 1 meter bore magnet. The strips are secured the arcuate carrier pieces such as by threaded fasteners 19.
  • the carrier pieces are anchored in their longitudinal position by clamps 21 which can be fabricated from aluminum.
  • the clamps which can be more easily seen in Fig. 2, when tightened by bolts 23 to the arcuate carrier piece, captures a portion of the edge 13a of channel 13 between the clamps and the arcuate carrier piece fixing the longitudinal position of the ferromagnetic strips.
  • the radial thickness of the entire assembly is kept to a minimum to minimize interference with precious bore space, which is occupied by gradient and RF coils and the patient table (which are not shown).
  • the axial position of the ferromagnetic strips are infinitely adjustable and very fine adjustment of the shim strength is available by changing the stack height of the strips. Thinner steel strips may be used to provide finer strength adjustment.
  • the channels are placed every 45 degrees around the inner circumference of the bore allowing eight discrete circumferential locations for the ferromagnetic strips.
  • the maximum axial force on a 1 cm. thick shim with the magnet energized is about 9 kg (20 pounds) in a 0.5 T magnet.
  • the shim carrier could be moved when the clamps are loosened while making adjustment of the axial locations.
  • a handle could readily be made which would allow easy control of the shim carrier with the clamps loose.
  • a passive shim assembly comprises a nonferromagnetic thin wall tube 31, which in the preferred embodiment is fabricated of fiberglass material 3.175 mm (1/8") thick.
  • a plurality of channel pieces 33 are equally spaced about the exterior of the tube. The channel pieces extend the length of the tube and are secured thereto by screws threadingly engaging the fiberglass tube 31. Some of the screws 35 located at the ends of the channel extend above the channel pieces surface to position the tube concentrically in the bore of the magnet. This can be more clearly seen in Fig. 4.
  • the channel pieces have projecting edges 33a on either side of the channel, extending away from the channel. The edges are parallel to the tube and spaced away therefrom. The edges extend the longitudinal length of the tube.
  • the channel pieces can be fabricated by extruding aluminum to the desired shape or if eddy currents are a problem, the channel can be pultruded from composite material. Increasing the number of circumferential locations provides greater flexibility in eliminating tesseral (axiperiodic) harmonics.
  • Arcuate shaped drawer pieces 37 fit between adjacent edges 33a and extend the length of the tube.
  • Ferromagnetic strips 41 which serve as the shims, are stacked to the desired height on the drawers and secured to the drawers selected predrilled holes 42 the more axial holes provided the finer axial field adjustability.
  • the sliding axial adjustability of the shims in the Figure 1 embodiment provides infinite adjustment which might be desirable in some situations.
  • the number of ferromagnetic strips provides an adjustment of strength.
  • the radial thickness of the shims is kept to a minimum so that the shims fit in the space provided by the adjustable height of the extended screws 35.
  • the drawers can slide out of the bore of the magnet to permit adjusting the axial position and thickness of the ferromatic strips.
  • the drawers can be removed with the magnet energized and the ferromagnetic strip positions adjusted.
  • the position and height of the ferromagnetic strips in the bore of the magnet are used to create magnetic fields shapes which correct for inhomogeneities in the field created by the magnet. Flexibility of positioning the steel strips is therefore important so that all the field shapes needed to counteract fields which could be precludes imaging quality homogeneity can be obtained. Flexibility in positioning is also important since it is doubtful that the initial prediction of the locations of the ferromagnetic strips will be perfect. Arcuate shims in specific locations are not intended to eliminate specific harmonics. Rather the combination of all the shims together are intended to increase the field homogeneity. In situations where required shim height would interfere with available bore space wider shims can be used in both embodiments.
  • the first step in block 45 is to determine the initial inhomogeneity in the bore of the magnet to be shimmed.
  • the magnetic field is measured in the energized magnet on a imaginary grid 46.
  • a grid having 314 points which lie on the periphery of thirteen circles and on 2 points 20 cm on either side of the center of the bore on the z-axis as shown in Figs. 7 and 8 can be used.
  • the PLAS3D code determines for each permissible shim location the field effect of an arc shaped steel shim of given axial, radial and circumferential dimensions at each of the 314 field measurement points.
  • the axial and circumferential positions of the arc shaped steel shims is a variable in the shimming procedure. For example, if the allowable domain of arcs is from -90 to 90 cm along the z-axis, the field effects can be determined with an arc every 10 cm along the z-axis. With higher densities the algorithm will take longer, but result in more possible shim locations and so provide generally better homogeneity.
  • the magnetic field of magnetized material may be represented as a series of spherical harmonics expanded about the origin of the magnet coordinate system.
  • the equations for the magnetic field harmonics are: where the coefficients A(n,m) represent the volume integral over the shim, a(n,m) the transducer functions defined by Schenck et al., and P(n,m) the associated Legendre polynomial.
  • the number of terms required to accurately represent the magnetic field depends on the size of the volume of interest, for the present shimming purposes, expansion through order and degree eight is sufficient.
  • the magnetization in the steel shims may be calculated or assumed.
  • arc field effects need only be done for one circumferential location at each chosen axial location, and the arc field indexed in fifteen degree increments to represent the field of any of twenty four circumferential locations.
  • Typical circumferential arc densities are only 8 to 12 per circle, so that indexing gives accurate results.
  • the effect of each arc location individually on the chosen grid is evaluated for its optimum strength, defined as that strength which yields the minimum inhomogeneity on the imaging volume.
  • ⁇ is the chosen measure of field inhomogeneity
  • Bz m represents the measured field at point m
  • C m is the coefficient representing the field per unit thickness created by a shim at the location in question at field point m.
  • the SHIMPSV algorithm determines where the arcuate shims are to be positioned and their thicknesses. While the PLAS3D program may come up with many dozens of locations which require positive strength of shims, only 20-25 of these locations will be required for shimming. Therefore the algorithm must decide which ones to eliminate.
  • the SHIMPSV algorithm starts with all the locations needing positive strength shims which is approximately half the locations initially checked by the PLAS3D algorithm. A linear least squares optimization is then performed on all the positive shim strengths simultaneously. The result of the initial run will contain negative strength shims, these locations are eliminated from consideration. Negative strengths result because the effect of all the shim locations found to have positive strengths individually is not the same as the effect of all these shim considered simultaneously.
  • the remaining locations requiring positive strength shims are then taken, and the least square optimization run again.
  • the process of eliminating negative strength locations is repeated until a solution with all positive strength is obtained.
  • the predicted inhomogeneity of the solution is compared to the desired inhomogeneity in block 57.
  • the field homogeneity attainable with a given group of positive strength shim locations generally depends inversely on the number of shim locations being worked with, so the more shims the better. If a solution with all positive strengths with a predicted inhomogeneity within specification is not possible, the parameters are altered in block 61 to increase the number of allowable shim locations tried by the PLAS3D code.
  • the field in the bore is again measured in the 314 locations on the grid in block 65. If the inhomogeneity deviates from the prediction more than desired, the SHIMPSV algorithm is run again with arc locations fixed and with the field values obtained with the shims in place. A least squares routine is then used to adjust the thicknesses of the arcs in block 67. These thickness changes should be small fractions of the initial thicknesses, and once implemented should reduce the inhomogeneity to within the desired range.

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

Claims (14)

  1. Verfahren zum passiven Anordnen von Ausgleichskörpern bei einem Magneten mit einer zentralen Bohrung unter Verwendung von Ausgleichskörpern bzw. Lehren (17), die in der Bohrung des Magneten angeordnet sind, wobei das Verfahren die folgenden Schritte aufweist:
    (a) Messen der anfänglichen Feld-Inhomogenität in der Bohrung des Magneten,
    (b) Wählen zulässiger Lehrenpositionen in der Bohrung des Magneten,
    (c) Prüfen der Magnetfeldwirkung von einer Lehre an jedem der vorbestimmten zulässigen Lehrenpositionen in der Bohrung des Magneten unabhängig voneinander, um die erforderliche Lehrenfestigkeit zu ermitteln, um die Magnetfeld-Inhomogenität in der Magnetbohrung zu verbessern,
    (d) Selektieren der Positionen, wo positive Lehrenfestigkeiten vorteilhaft gefunden wurden,
    (e) Ermitteln der notwendigen Lehrenfestigkeiten, um die Magnetfeld-Inhomogenität an jeder gewählten Position zu verbessern, wobei alle gewählten Positionen gleichzeitig betrachtet werden,
    (f) Eliminieren von Positionen, an denen eine erforderliche negative Lehrenfestigkeit gefunden wurden, und Wiederholen der Schritte (e) und (f), bis alle übrigen gewählten Positionen positive Lehrenfestigkeiten erfordern, und
    (g) Anordnen von von Lehren vorbestimmter Dicke an ihren gewählten Positionen in der Bohrung des Magneten.
  2. Verfahren nach Anspruch 1, wobei nach dem Schritt (f) in Anspruch 1 die folgenden Schritte durchgeführt werden
       Vorhersagen der Feld-Inhomogenität mit den Lehren vorbestimmter Dicke an ihren gewählten Positionen in der Bohrung des Magneten,
       Vergleichen der vorhergesagten Inhomogenität mit einer gewünschten Inhomogenität,
       Vergrößern der Anzahl vorbestimmter zulässiger Positionen in Schritt (b), um die Differenz zwischen der vorhergesagten und der gewünschten Inhomogenität zu verkleinern, und Wiederholen der Schritte (c), (d), (e) und (f).
  3. Verfahren nach Anspruch 1, wobei ferner der folgende Schritt durchgeführt wird:
       Wiedereinführung einer Position in Schritt (f), die zuvor in einer früheren Iteration eliminiert wurde, weil eine negative Lehrenfestigkeit erforderlich war, um die Anzahl gewählter Positionen vergrößern zu helfen, die für positive Lehrenfestigkeiten als erforderlich gefunden wurden.
  4. Verfahren nach Anspruch 1, ferner enthaltend die Schritte:
    (h) Messen der Feld-Inhomogenität in der Magnetbohrung mit den in ihrer Lage befindlichen Lehren,
    (i) Ermitteln inkrementaler Änderungen in der Lehrenfestigkeit an den gewählten Positionen, um die Differenz zwischen einer gewünschten Inhomogenität und einer vorhergesagten Inhomogenität zu verkleinern auf der Basis der Feld-Inhomogenität, die mit den in ihrer Lage befindlichen Lehren gemessen wird, und
    (j) inkrementelles Ändern der Lehrendicken an den gewählten Positionen in der Magnetbohrung um die Beträge, die in dem vorherigen Schritt ermittelt wurden.
  5. Passive Anordnung von Lehren bzw. Ausgleichskörpern für einen Magneten mit einer zentralen Bohrung, enthaltend:
       eine nicht-magnetische Röhre (11), die koaxial in der Bohrung des Magneten angeordnet ist,
       mehrere bogenförmige Streifen (17) vorbestimmter Länge aus einem ferromagnetischen Material, dadurch gekennzeichnet, daß die Streifen lösbar an dem Rohr befestigt sind, die Streifen sich in Umfangsrichtung um das Rohr herum erstrecken und an verschiedenen Positionen auf dem Rohr angeordnet sind, die unter Verwendung des Verfahrens nach einem der Ansprüche 1 bis 4 gewählt sind.
  6. Anordnung nach Anspruch 5, wobei die Streifen (17) an der Außenseite des Rohres (11) befestigt sind.
  7. Anordnung nach Anspruch 5, wobei die Streifen (17) an der Innenseite des Rohres (11) befestigt sind.
  8. Anordnung nach Anspruch 5, wobei ferner Mittel zum koaxialen Anordnen des Rohres in der Bohrung vorgesehen sind.
  9. Anordnung nach Anspruch 6 oder 7, wobei alle Streifen (17) die gleiche Länge haben.
  10. Anordnung nach Anspruch 9, wobei die Streifen (17) als Streifenstapel mit unterschiedlichen Höhen befestigt sind.
  11. Anordnung nach Anspruch 5, ferner enthaltend:
       mehrere Kanalteile (13), die an der Innenseite des Rohres (11) befestigt sind und die im gleichen Abstand um den Umfang herum angeordnet sind und sich in einer Längsrichtung erstrecken,
       mehrere bogenförmige Trägerstücke (15), die zwischen benachbarten Kanalstücken an den gewählten Positionen gleitend angebracht sind,
       Klemmittel (21), die den Täger (15) an den Kanälen befestigen, um eine Gleitbewegung zu verhindern, und
       einen Stapel von Streifen (17) aus ferromagnetischem Material, der an jedem der Trägerstücke lösbar befestigt ist.
  12. Anordnung nach Anspruch 11, wobei die Trägerstücke (15), die Kanalteile (13) und die Klemmittel (21) jeweils nicht-magnetische Materialien aufweisen.
  13. Anordnung nach Anspruch 5, ferner enthaltend:
       mehrere Kanalteile (33), die an der Außenseiten des Rohres (31) befestigt sind und die im gleichen Abstand in Umfangsrichtung angeordnet sind und sich in einer Längsrichtung erstrecken,
       mehrere bogenförmige Ziehstücke (37), die zwischen benachbarten Kanalstücken gleitend angebracht sind, und
       mehrere Stapel der Streifen (41) aus ferromagnetischem Material, die an jedem der Ziehstücke lösbar befestigt sind.
  14. Anordnung nach Anspruch 13, wobei ferner Mittel (35) zum koaxialen Anordnen des Rohres in der Bohrung des Magneten vorgesehen sind.
EP19870115844 1986-12-03 1987-10-28 Passive Anordnung von Ausgleichskörpern und Verfahren zur Bestimmung der Lage der Ausgleichskörper für einen Magneten der magnetischen Resonanz Expired EP0272411B1 (de)

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
US06/937,297 US4698611A (en) 1986-12-03 1986-12-03 Passive shimming assembly for MR magnet
US937297 1986-12-03
US06/937,299 US4771244A (en) 1986-12-03 1986-12-03 Method of passively shimming magnetic resonance magnets
US937299 1986-12-03

Publications (2)

Publication Number Publication Date
EP0272411A1 EP0272411A1 (de) 1988-06-29
EP0272411B1 true EP0272411B1 (de) 1992-10-07

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EP19870115844 Expired EP0272411B1 (de) 1986-12-03 1987-10-28 Passive Anordnung von Ausgleichskörpern und Verfahren zur Bestimmung der Lage der Ausgleichskörper für einen Magneten der magnetischen Resonanz

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EP (1) EP0272411B1 (de)
JP (1) JP2602513B2 (de)
DE (1) DE3782150T2 (de)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE19922652A1 (de) * 1999-05-18 2001-01-11 Bruker Analytik Gmbh Einrichtung zum Homogenisieren eines Magnetfeldes

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Publication number Priority date Publication date Assignee Title
EP0303880B1 (de) * 1987-08-14 1991-11-06 Siemens Aktiengesellschaft Elektrischer Magnet für Kernspin-Thomographen
US5235284A (en) * 1989-07-07 1993-08-10 Mitsubishi Denki Kabushiki Kaisha Passive shim arrangement for nuclear magnetic resonance
JPH0339676A (ja) * 1989-07-07 1991-02-20 Mitsubishi Electric Corp 磁場補正装置
US5045794A (en) * 1989-12-04 1991-09-03 General Electric Company Method of optimizing passive shim placement in magnetic resonance magnets
US5006804A (en) * 1989-12-04 1991-04-09 General Electric Company Method of optimizing shim coil current selection in magnetic resonance magnets
US5343183A (en) * 1990-11-09 1994-08-30 Mitsubishi Denki Kabushiki Kaisha Magnetic field correction device
JPH04328477A (ja) * 1991-04-30 1992-11-17 Mitsubishi Electric Corp 電磁石装置
US5418462A (en) * 1994-05-02 1995-05-23 Applied Superconetics, Inc. Method for determining shim placement on tubular magnet
DE19901331B4 (de) * 1999-01-15 2006-10-26 Bruker Biospin Gmbh Einrichtung und Verfahren zum Homogenisieren eines Magnetfeldes
GB2414080B (en) * 2004-05-14 2006-07-26 Oxford Magnet Tech Tool and method for shimming a magnet
US9778334B2 (en) 2014-05-07 2017-10-03 Scott Technology Nz Limited Magnetic shimming and magnet arrangements

Family Cites Families (4)

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Publication number Priority date Publication date Assignee Title
JPS60189905A (ja) * 1984-03-09 1985-09-27 Mitsubishi Electric Corp 高均一磁界発生装置
DE8419763U1 (de) * 1984-07-02 1986-03-20 Siemens AG, 1000 Berlin und 8000 München Kernspin-Tomographiegerät
JPH03966Y2 (de) * 1985-03-01 1991-01-14
JPS61201145A (ja) * 1985-03-04 1986-09-05 Sanyo Electric Co Ltd 核磁気共鳴撮像装置

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE19922652A1 (de) * 1999-05-18 2001-01-11 Bruker Analytik Gmbh Einrichtung zum Homogenisieren eines Magnetfeldes
DE19922652C2 (de) * 1999-05-18 2001-08-02 Bruker Analytik Gmbh Einrichtung zum Homogenisieren eines Magnetfeldes

Also Published As

Publication number Publication date
DE3782150D1 (de) 1992-11-12
JP2602513B2 (ja) 1997-04-23
JPS63177506A (ja) 1988-07-21
EP0272411A1 (de) 1988-06-29
DE3782150T2 (de) 1993-06-03

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