Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Patent
1997-05-13
1999-11-23
Arana, Louis
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
600422, G01V 300
Patent
active
059906804
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to acoustically quiet gradient coil design in magnetic resonance imaging (MRI).
Magnetic gradient coils are a prerequisite for NMR imaging (P. Mansfield P. and P. G. Morris, NMR Imaging in Biomedicine. Academic Press, N.Y. (1982)) and also for use in a range of NMR applications including diffusion studies and flow. In NMR imaging the acoustic noise associated with rapid gradient switching combined with higher static magnetic field strengths is at best an irritant and at worst could be damaging to the patient. Some degree of protection can be given to adults and children by using ear defenders. However, for foetal scanning and in veterinary applications, acoustic protection is difficult if not impossible.
Several attempts have been made to ameliorate the acoustic noise problem. For example, by lightly mounting coils on rubber cushions, by increasing the mass of the total gradient assembly and by absorptive techniques in which acoustic absorbing foam is used to deaden the sound. Acoustic noise cancellation techniques have also been proposed which rely on injection of antiphase noise in headphones to produce a localised null zone. These methods are frequency and position dependent and could possibly lead to accidents where, rather than cancel the noise, the noise amplitude is doubled.
The present invention relates to a novel method for active control of acoustic output in quiet magnetic gradient coil design henceforth referred to simply as active acoustic control in magnetic coils and magnetic gradient coils which readdresses the cause of the acoustic noise problem over that achieved in uncontrolled active acoustic shielding (P. Mansfield, B. Chapman, P. Glover and R. Bowtell, International Patent Application, No. PCT/GB94/01187; Priority Data 9311321.5, Jun. 2, (1993); P. Mansfield, P. Glover and R. Bowtell, Active acoustic screening: design principles for quiet gradient coils in MRI. Meas. Sci. Technol. 5, 1021-1025 (1994); P. Mansfield, B. L. W. Chapman, R. Bowtell, P. Glover, R. Coxon and P. R. Harvey, Active acoustic screening: Reduction of noise in gradient coils by Lorentz force balancing. Magn. Reson. Med. 33, 276-281 (1995)) and offers considerable improvement in noise suppression.
The present invention provides an active acoustically controlled magnetic coil system which is adapted to be placed in a static magnetic field, the coil comprising a plurality of first electrical conductors and a plurality of at least second electrical conductors, the first and at least the second conductors being mechanically coupled by means of at least one block of material with a predetermined acoustic transmission characteristic and in which the first and at least the second conductors are spaced at a predetermined distance apart, first electrical current supply means for supplying a first alternating current to said first electrical conductor, at least a second electrical current supply means for supplying at least a second alternating current to said at least second electrical conductor, said first and at least second currents characterised in that they have different and variable amplitudes and different and variable relative phases, both these features being determined by the acoustic characteristics of the material and by its geometry and the predetermined distance.
The first and second current supply means may comprise means for supplying current waveforms with controllable shape said waveforms being shaped to fit the wave propagation properties characteristics of the mechanical coupling material. The currents can be rectangular or trapezoidal waveforms and in these cases the second current waveform will be delayed with r respect to the first. The phrase "alternating" is therefore extended to include rectangular or trapezoidal waveforms and the phrase "variable relative phases" is extended to include a delayed second waveform.
The leading and trailing edges of the second waveform will be delayed in time and accordingly shaped to fit the wave propagation properties characteristic of
REFERENCES:
patent: 5427102 (1995-06-01), Shimode et al.
patent: 5572131 (1996-11-01), Rzedzian
"Quiet Gradient Coils . . . " Chapman et al. Meas. Sci. Technol. 6(1995) pp. 349-354 pub. Jan. 1995.
"Quiet Transverse Gradient Coils: Lorentz Force Balanced Designs . . . " Bowtell et al. MRM 34: 494-497 May 1995.
"Active Acoustic Srceening: Reduction of Noise in Gradient Coils by Lorentz Force Balancing" Mansfield et al. MRM 33:276-281 Jan. 1995.
"Active Acoustic Sreening" Mansfield et al. Mansfield et al. Magnetic Resonance Center May 1994.
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