Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Patent
1992-11-17
1994-06-28
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
G01V 300
Patent
active
053250605
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to coil circuits and more particularly to multi-mode resonant gradient coil circuits for imaging and spectroscopy in Nuclear Magnetic Resonance (NMR) systems.
Ultra high speed imaging techniques like echo-planar imaging, P. Mansfield, J. Phys. C. 10, L55 (1977) and echo-volumar imaging, P. Mansfield, A. M. Howseman and R. J. Ordidge, J. Phys. E. 22, 324-330 (1989) require at least one gradient which is either sinusoidally or cosinusoidally modulated, or preferably trapezoidally modulated. Sinusoidal or cosinusoidal modulation is straight forward to achieve in practice by using a series or parallel resonant circuit in which the gradient coil assembly forms the inductance. Data acquisition in such an imaging arrangement can be achieved by varying the data sampling rate in order to get equal spin phase increments between sampling points. A disadvantage of this method is the fact that the peak gradient amplitude is .pi./2 greater than the amplitude of an equivalent square wave modulation. This means that the gradient driver amplifier must be capable of supplying .pi./2 more current than in the square or trapezoidal modulation case.
Another factor which militates against using sinusoidal modulation is patient safety. Induced currents within the body due to rapidly varying magnetic field gradients present a potential hazard if the induced current levels approach the neural stimulation threshold level. Neurone models suggest that the important factors determining neural stimulation are concerned with the charging and discharging of nodal capacitance in the process of achieving the neurone firing potential. This leads to a formula which involves the product of the rate of change of magnetic field within the body multiplied by the duration over which this time variation applies. When this function is suitably integrated, (it has been shown that P. Mansfield [to be published]) that the critical factor in calculating stimulation threshold levels for gradient switching is not the rate of change of magnetic field but the maximum field excursion experienced by the tissue. Indeed, neurone stimulation turns out to be independent of the rate of change of the gradient field. In this circumstance it is, therefore, more efficient, and in general, safer, to consider fast rise trapezoidal or square wave modulation of currents rather than sinusoidal gradients since for a given imaging time square wave gradients are 2/.pi. lower that those with an equivalent sinusoidal modulation.
The practical problem is how to generate a fast rise square wave for use in a whole body imaging machine. It is clear that for the most efficient square wave modulation system, the circuit arrangement must be energy conserving. Such energy conserving non-linear switches have been developed and described (British Patent GB 2184625B). A difficulty with non-linear circuitry is that the switch is an active device and will have a maximum current carrying capacity and at the same time be required to have a high inverse voltage requirement. High power switches capable of carrying 500 to 1000 amps are not available with the requisite peak inverse voltage requirements of up to 10 kV. Such devices may be developed in the future but are likely to be very expensive and temperamental.
The present invention provides an entirely passive circuit approach to the generation of trapezoidal and square wave modulated gradients for use in high speed NMR imaging. The circuit arrangement according to the present invention operates in a series drive arrangement which is eminently suitable for high current, low voltage audio amplifiers currently available commercially. A parallel multi-modal circuit has been described A. Macovski and C. Meyer, Proceedings of the 6th Annual Meeting, Society of Magnetic Resonance in Medicine, New York, 1, 499 (1987) but is not as valuable because high voltage driver amplifiers are required.
The present invention provides a multi-mode resonant coil circuit comprising passive components for generation of a square, trape
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A. Macovski, et al. "Gradient Power and T.sub.2 Considerations in Rapid Imaging", Society of Magnetic Resonance in Medicine, Sixth Annual Meeting and Exhibition, Aug. 17-21, 1987 p. 449.
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Coxon Ronald J.
Mansfield Peter
British Technology Group Limited
Tokar Michael J.
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