Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Patent
1998-01-16
1999-10-26
Oda, Christine K.
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
324318, 324319, 324320, G01V 300
Patent
active
059734957
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and apparatus for elimination of mutual coupling in magnetic coils and more particularly to magnetic coils suitable for use in NMR or MRI systems.
There are many applications in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) where two or more coils are used to receive and/or transmit RF signals or pulses. In such situations it is desirable to have the two coils magnetically decoupled so that transmission or reception on one coil does not interfere with the transmission or reception on the second coil. The traditional way of dealing with the coil design problem is to have both coils geometrically orthogonal. A more recent development is one where two coaxial coils are made magnetically orthogonal by having the inner of the coaxial pair actively magnetically screened (P Mansfield and B Chapman, Active magnetic screening of gradient coils in NMR imaging. J Magn Reson 66, 573-576 (1986)); (P Mansfield and B Chapman, Active magnetic screening of coils for static and time dependent magnetic field generation in NMR imaging. J Phys E 19, 540-545 (1986)) and (P Mansfield and B Chapman, Multi-shield active magnetic screening of coil structures in NMR. J Magn Reson 72, 211-233 (1987)). However, with either magnetic or geometric orthogonality there is, in practice, a residual flux leakage from one coil to the other creating a mutual inductance which although small can lead to undesirable effects. For example, if both coils are tuned to a common resonant frequency, residual mutual coupling between the two coils will cause a splitting of the resonance response of the coil pair. This effect is particularly troublesome when the two coils are required to receive simultaneously signal from the same source (P Mansfield, A Freeman and R Bowtell, Active screening in RF coil design. MAGMA 2, 391-392 (1994)). Theoretical signal reception from the same source on two or more coils which have zero mutual inductance means that the signals can be independently received, amplified and co-added to give a signal-to-noise (S/R) enhancement. For a carefully designed system it may be possible to dispense with amplification of individual signals from each coil by co-adding the signals at the coil stage into a single signal with improved S/R which is then amplified in a single amplifier channel. This arrangement would be possible only if the relative phases of the two or more RF signals were 2.pi.n where n=0,1,2 . . .
It is an object of the present invention to provide a way of eliminating mutual inductance effects in coil assemblies comprising two or more separate tuneable coils. The method uses passive electronic components to achieve the necessary decoupling and works best when one or more coils are tuned to a common frequency.
The present invention provides a method of eliminating mutual inductance effects in resonant coil assemblies in which a plurality of coils are situated in sufficiently close proximity to create small mutual inductances between the coils, the method comprising evaluating the mutual inductances using a T, star or other transformation of the relevant parts of the circuit thereby isolating the mutual inductances in such a way that series capacitances may be introduced to tune out the mutual inductances at a common frequency thereby reducing the coil array to a synchronously tuned circuit.
The invention also provides a method of eliminating mutual inductance effects in coil assemblies comprising three or more coils in which for a non-symmetric coil arrangement at least two non-equal capacitive elements are connected to respective coils. For two or more coils the method also comprises the steps of independently and optionally amplifying the signals received from the coils, phase compensating the signals and co-adding the signals.
The present invention also provides a coil system comprising two or more coils each coil output being tuned to the same operating frequency, the coils being positioned in sufficiently close proximity to create small mutual inductan
REFERENCES:
patent: 4769605 (1988-09-01), Fox et al.
patent: 5477146 (1995-12-01), Jones
Oda Christine K.
Shrivastav Brij B.
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