Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Utility Patent
1998-12-18
2001-01-02
Oda, Christine K. (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S318000
Utility Patent
active
06169404
ABSTRACT:
BACKGROUND OF INVENTION
This invention relates to a C-shaped superconducting magnet assembly for a magnetic resonance imaging system (hereinafter called “MRI”), and more particularly to improved and simplified vibration cancellation in such an assembly.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold ensures that the magnet coils can be made superconducting, such that when a power source is initially connected to the coil (for a relatively short period) current continues to flow through the coils even after power is removed due to the absence of resistance, thereby maintaining a strong magnetic field. Superconducting magnets find wide application in the field of MRI.
Imaging quality is dependent on the production of a strong yet highly homogeneous and stabe magnetic field in the imaging region of the superconducting magnet.
In order to compensate for the inhomogeneities in MRI magnets, various arrangements including ferromagnetic shim materials and correction coils have been used. However, even after extensive shimming to provide the required magnetic field homogeneity for a particular installation, it has been found that superconducting magnets are subject to vibration which can adversely affect field homongeneity and imaging quality, particularly the so-called C-shaped configuration in which heavy magnet elements are supported in a cantilever manner. Even light vibrations induced by people walking in the vicinity or other normal vibrations such as vehicles passing outside the installation encountered during use can adversely affect the imaging quality, particularly if the vibrations induce resonant frequency vibrations in the magnet structure.
SUMMARY OF INVENTION
Thus, there is a particular need for a system to cancel vibrations of the superconducting magnet structure to avoid the aforementioned problems.
In accordance with one form of the subject invention, a vibration cancellation system for a cantilever supported magnet structure includes a vibration sensor at a remote unsupported end to provide a signal responsive to vibration which is phase inverted and amplified for application to an actuator which provides an out of phase force for application to a support member for the supported end of the magnet structure. The out of phase force is applied through a rigid structure interposed between the magnetoresistive actuator and separated portions of the support member. In the case of a C-shaped magnet, the vibration sensor is at the unsupported end of the separated pole pieces and the force can be applied to the connecting member between the opposite ends of the pole pieces.
REFERENCES:
patent: 5548653 (1996-08-01), Pla et al.
patent: 5731704 (1998-03-01), Schnur et al.
patent: 5942898 (1999-08-01), Petropoulos et al.
Cabou Christian G.
Freedman Irving M.
General Electric Company
Oda Christine K.
Price Phyllis Y.
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