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
  • G01R33/387—Compensation of inhomogeneities
  • G01R33/3875—Compensation of inhomogeneities using correction coil assemblies, e.g. active shimming
• • 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 invention relates to a method of providing a homogeneous magnetic field in an air gap between two pole shoes by means of coils compensat- ing for field distortions, such as pincushion distortions.
• EP publication No. 21 7,520 discloses a coil structure to be used in con ⁇ nection with magnetic resonance measurements where a very homoge ⁇ neous magnetic field is required.
• the coil structure comprises equi- distantly arranged coil members. This coil structure renders it possible to compensate for magnetic field gradients.
• the object of the invention is to provide a method of compensating for other field distortions in such a manner that it is possible to provide a homogeneous magnetic field (better than 0.1 ppm) in a relatively large volume.
• a method of the above type is according to the invention characterised by the pincushion distortions being compensated for by means of sub ⁇ stantially circular conductor paths, in which the currents are individually controlled, the pincushion distortions optionally being compensated for by means of corrections (current densities) of a substantially triangular waveform, where the triangular waveforms are of a wavelength of 1 /2 P of the diameter of the pole shoe and P is preferably an integer from 0 and up to a suitably high value.
• These triangular functions render it pos- sible to reestablish the original pincushion distortion which can thereby be deducted the natural distortion.
• the triangular curve shapes can ad ⁇ vantageously be used for compensating for field distortions because the associated coil structures are relatively easy to manufacture.
• the coil members may be used for compensating for barrel distortions.
• fig. 1 illustrates a magnetic yoke with an air gap for measuring of nu ⁇ clear-magnetic resonance
• Fig. 2 illustrates a modified correction coil
• Fig. 3 illustrates the field originating from the modified correction coil
• Fig. 4 illustrates a coil compensating for a field gradient in the Y-direct- ion
• Fig. 5 illustrates a coil compensating for a field gradient in the X-direct- ion.
• Fig. 6 illustrates a pincushion distortion
• Fig. 7 illustrates the area within which the pincushion distortion is about 2 gauss
• Fig. 8 illustrates the curve shape without compensation
• Fig. 9 illustrates a helical coil generating a field of a triangular waveform
• Fig. 10 illustrates the field of the triangular waveform deducted the orig ⁇ inal field
• Fig. 1 1 illustrates the effect of a number of coils for generating fields of a triangular waveform
• Fig. 1 2 illustrates the resulting and substantially homogeneous field gen ⁇ erated by controlling the current supply to the coils
• Fig. 1 3 illustrates the field resulting from a simplified control of the cur- rent supply to the coils
• Fig. 14 illustrates a barrel distortion
• Fig. 1 5 illustrates how the barrel distortion is compensated for by a con ⁇ trol of the current supply to the correction coils
• Fig. 1 6 illustrates the magnetic field resulting from a compensation for barrel distortion
• Figs. 17 to 19 illustrate a specific embodiment of a coil configuration compensating for pincushion distortion.
• Fig. 2 illu- strates a coil comprising 10 windings of a total conductor width of ap ⁇ proximately 7 mm. This coil provides a field gradient across an area of approximately 7 mm.
• Fig. 3 illustrates two curves of this coil. Curve a is measured immediately above the pole shoe where the windings are pro ⁇ vided and curve b is measured immediately above the opposing pole shoe. Curve a shows that the resulting field gradient in the X-direction has an effect across a substantially larger area substantially correspond ⁇ ing to the total conductor width. In this manner a gradient across a large area can be obtained.
• Curve b shows the effect of the field at the opposing pole. It appears that it it necessary to perform such a correction at both pole shoes in a symmetrical manner in order to provide a symmetrical field. If a correc ⁇ tion is carried out only at one pole shoe, undesired gradients are intro ⁇ pokerd in the Z-direction.
• a correction coil has been wound which corresponds to Fig. 2 with 70 windings and a conductor width of 28 mm. This coil is able to correct the field in the X-direction in an interval of almost 40 mm.
• Figs. 4 and 5 show correction coils with linearly and equidistantly ar ⁇ ranged coil members covering the entire pole shoe surface. These coils are able to compensate for field gradients in the Y and the X-direction, respectively.
• the pincushion distortion in the Z-direction is shown in Fig. 6, but has been greatly exaggerated.
• the pincushion distortion is estimated to be 2 gauss of 10,000 gauss across an area of approximately 25% of the total surface area, cf. Fig. 7.
• two coils of the 3rd order which are phase-shifted relative to one another. These coils are the coils 7 and 8. The effect of these eight coils is illustrated in from Fig. 1 1 .
• the current densities shown in Fig. 1 2 can be provided by means of the configuration shown in Fig. 1 9 of coils on a plate. Each coil comprises one winding. A conduit is connected to each end of the individual wind ⁇ ings in a superposed layer, said conduit being connected to the end of said winding through a plating, cf. Fig. 1 9.
• Fig. 1 8 shows the conduits connected to the frame. The current supply to the conduits of Fig. 1 9 is controlled by means of resistance elements in a matrix as well as a num ⁇ ber of potentiometers.
• the current supply to each coil is controlled by means of potentiome ⁇ ters. Attempts have been made at simplifying the setting of the poten- tiometers.
• the distortion is reduced by 1 /4 by the intro ⁇ duction of coils of the next order. This can be utilized for scaling the in ⁇ dividual coils relative to one another.
• the increase of the field along the rim of the pole shoes can be provided by means of the above coils.
• Fig. 1 6 shows how it is possible to obtain the desired effect.
• Fig. 1 7 shows the curve in the same scaling as the remaining enlarged curves.
• the above coils render it possible to compensate for both pincushion and barrel distortion in the air gap in up to half the area covered by the correction coils.

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• Physics & Mathematics (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Magnetic Treatment Devices (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (1)

Family

ID=8088771

Family Applications (1)

Country Status (3)

Citations (1)

• 1993
  • 1993-01-07 DK DK1493A patent/DK1493A/da not_active Application Discontinuation
• 1994
  • 1994-01-06 WO PCT/DK1994/000010 patent/WO1994016453A1/en active Application Filing
  • 1994-01-06 AU AU58572/94A patent/AU5857294A/en not_active Abandoned

Patent Citations (1)

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