Surface and gradiometer coils near a conducting body: The lift-off effect

The use of surface coils in magnetic resonance is widespread, Examples incl ude MRI, detection of subsurface aquifers by NMR, and, more recently, landm ine detection by nuclear quadrupole resonance. In many of these cases a fin ite-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 pr ovide a loss mechanism that reduces the effective quality factor Q of the t ransmitter and receiver coils. Here the losses induced in a circular surfac e 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 b y displacing the coil away from the conducting medium a prescribed "lift-of f" distance. The use of a gradiometer as a surface coil is also examined, a nd it is shown by theory and experiment that in certain circumstances such a gradiometer can be more efficient than a conventional surface coil for in spection of conducting media. (C) 1998 Academic Press.