Surface and Gradiometer Coils near a Conducting Body:
The Lift-Off Effect
B. H. Suits, A. N. Garroway, and J. B. Miller
The use of surface coils in magnetic resonance is widespread.
Examples include MRI, detection of subsurface aquifers by NMR,
and, more recently, landmine detection by nuclear quarupole
resonance. In many of these cases a finite-sized sample to be
examined is contained within a larger medium that is a poor
electrical conductor, and eddy currents induced by the RF
fields provide a loss mechanism that reduces the effective
quality factor, Q, of the transmitter and receiver coils. Here
the losses induced in a circular surface coil (a horizontal
loop antenna) separated a distance from a dissipative medium are
calculated and compared to measurements. It is shown that
often the overall efficiency of the coil for magnetic resonance
can be improved by displacing the coil away from the conducting
medium a prescribed "lift-off" distance. The use of a
gradiometer as a surface coil is also examined, and it is
shown by theory and experiment that in certain circumstances such
a gradiometer can be more efficient than a conventional surface
coil for inspection of conducting media.
Journal of Magnetic Resonance 135, 373-379 (1998).
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