Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2000-01-12
2002-02-26
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S319000
Reexamination Certificate
active
06351125
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns an iterative method for homogenizing the magnetic field in the working volume of the main field magnet of a magnetic resonance apparatus using ferromagnetic homogenizing elements which are mounted at predetermined positions on one or more support bodies.
BACKGROUND OF THE INVENTION
A method of this type is known e.g. from U.S. Pat. No. 5,959,454.
Similar homogenizing devices with homogenizing elements of ferromagnetic material are known e.g. from U.S. Pat. No. 5,045,794 or U.S. Pat. No. 5,485,088 or U.S. Pat. No. 5,959,454 U.S. Pat. No. 3,622,869 discloses e.g. alternative methods of homogenizing using correction coils through which adjustable electric currents flow.
U.S. Pat. No. 5,045,794 describes a device and a method of homogenizing the magnetic field of the main field magnet of a typical magnetic resonance tomograph for medical diagnosis with a tubular cylindrical bore for the reception of a patient, and a nearly spherical working volume in the region of the geometric center of the bore which uses ferromagnetic homogenizing elements. Guiding rails are mounted in a longitudinal direction on the surface of the tubular bore into which e.g strip-shaped mechanical support bodies are inserted which can be provided with homogenizing elements of iron having e.g. disc-shaped design. The mechanical support bodies in the guiding rails thereby provide a plurality of predetermined locations or positions for fixing the homogenizing elements. In the magnetic field of the main field magnet, the homogenizing elements of iron are magnetized, approximately to their point of saturation, in a direction parallel to the axis of the bore. The relatively weak additional magnetic field of the magnetized homogenizing elements is superimposed on the relatively strong magnetic field of the main field magnet. The magnetic field in the working volume of the main field magnet is, in general, not sufficiently homogeneous for use in the magnetic resonance apparatus due to the mechanical tolerances in the structural components of the main field magnet coil. Through the choice of the geometric arrangement and the amount of homogenizing elements in the region surrounding the working volume, it is possible to create a spatial profile of the additional magnetic field generated by the homogenizing elements which largely compensates for the original inhomogeneity of the magnetic field of the main field magnet. With main field magnets of a bore diameter of 90 cm in a spherical working volume having a diameter of approximately 50 cm, typical values of the remaining field disturbances are e.g. on the order of magnitude of a few ppm. In the event that soft-magnetic homogenizing elements e.g. of iron are used, magnetization only occurs in the direction of the main field at the center of the magnet bore, however, not in the opposite direction. When using disc-shaped homogenizing elements whose normal direction extends perpendicularly to the axis of the magnet bore, one obtains an nearly fixed value for the magnetization, corresponding to saturated magnetization, even at relatively small magnetic field strengths of the main field corresponding to magnetic inductances in excess of 0.4 Tesla, due to the shape anisotropy of the homogenizing elements. In the region of the working volume of a main field magnet having a bore diameter of approximately 90 cm, the magnetic field profile generated by a small homogenizing element with dimensions of e.g. 40×40×0.3 mm
3
is almost identical to that of a magnetic dipole with known dipole moment and is therefore easy to calculate, Homogenization of the magnetic field in the working volume of the main field magnet using the ferromagnetic homogenizing elements is basically effected by initially measuring the generally still inhomogeneous profile of the magnetic field in the working volume at a plurality of locations. Subsequently, the measured values are used as input data record for a numerical calculation program which, in a first calculation step, permits calculation of a geometric arrangement of the ferromagnetic homogenizing elements at predetermined locations, whose additional magnetic field profile largely compensates the original inhomogeneities of the magnetic field of the main field magnet in the working volume. When disc-shaped homogenizing elements are used, several homogenizing elements can be stacked on top of one another at a plurality of predetermined locations.
The homogenizing elements are then mounted on the mechanical support bodies at the predetermined locations within the bore of the main field magnet corresponding to the geometric arrangement determined by the numerical calculation program. Subsequently, the profile of the magnetic field in the working volume of the main field magnet, which is in general more homogeneous, is re-measured in a second measuring step. The measured values can be used as input data to calculate changes in the geometric arrangement of the homogenizing elements in the numerical calculation program, and geometric arrangement of the homogenizing elements can be changed accordingly. This enables an iterative improvement of the geometric arrangement of the homogenizing elements for achieving a homogeneous magnetic field in the working volume of the main field magnet of the magnetic resonance apparatus.
U.S. Pat. Nos. 5,485,088 and 5,959,454, e.g., disclose the use of permanent magnetic material for the ferromagnetic homogenizing elements. Homogenizing elements of permanent magnetic material must be magnetized in a magnetic field which is stronger than the coercive field strength of the material, before actually being used in the main field magnet in a magnetic resonance apparatus. In principle, they can then be used with any direction of magnetization relative to the direction of the magnetic field of the main field magnet and even with the opposite direction. Furthermore, with disc-shaped homogenizing elements, it is possible to generate a direction of magnetization parallel to the normal direction of the disc. This is required, e.g., with main field magnets having the geometry described in U.S. Pat. No. 5,959,454. In order to permit calculation of the profile of the magnetic field strength generated by the homogenizing elements of permanent magnetic material in the working volume of the main field magnet, thereby making them of practical use for homogenizing methods, the state of magnetization must remain constant or change only slightly during installation of the homogenizing elements in the main field magnet. This is the case with only very few materials, such as NdFeB or Smco alloys.
Homogenizing methods using ferromagnetic homogenizing elements have the fundamental advantage that the large number of possible positions for the homogenizing elements create a corresponding large number of degrees of freedom for compensating even complicated field profiles of the still somewhat inhomogenous magnetic field of the main field magnet compared to methods using correction coils with adjustable currents having a relatively small number of degrees of freedom corresponding to the number of correction coils.
The fundamental disadvantage of homogenizing methods using ferromagnetic homogenizing elements is that the homogenizing elements have, for practical reasons, a minimum size which limits the precision with which deviations from the homogeneous profile of the magnetic field can be compensated. In contrast thereto, methods using correction coils with adjustable currents permit precise adjustment of the profiles of magnetic fields for compensation of inhomogeneous magnetic fields.
A basic precondition for the applicability of homogenizing methods using ferromagnetic homogenizing elements is that suitable distributions of the homogenizing elements at the predetermined positions be capable of determination using the profile of the magnetic field determined in a previous measuring step via theoretical means only, i,e. using numerical calculation programs, Subsequently, these methods can be use
Bruker Analytik GmbH
Patidar Jay
Shrivastav Brij B.
Vincent Paul
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