Spatially‐localized NMR spectroscopy employing an inhomogeneous surface‐spoiling magnetic field gradient 1. Phase coherence spoiling theory and gradient coil design

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Abstract
The use of a current‐generated, inhomogeneous, surface‐spoiling magnetic field gradient for enhancing magnetic resonance spatial localization, by rapidly inducing spin phase incoherence in surface lying regions of a sample is examined theoretically. A geometrically simple surface‐spoiling magnetic field gradient coil design is presented and its gradient field characterized via computer simulations. Mathematical expressions describing the time dependence of net sample spin phase coherence under influence of the spoiling gradient are developed for application with homogeneous (B1) volume coils. The dependence of spoiling efficiency on the magnetogyric ratio of the nuclide under investigation and on the current driving the gradient field is described. Spoiling periods of ca 1–2 ms with driving currents of ca 0.5–1.0 A are predicted to be adequate for surface‐spoiling experiments with rat, e.g., for noninvasive monitoring of liver. A companion article W. Chen and J. J. H. Ackerman, NMR Biomed., 3, 158–165 (1990)) describes implementation of the surface‐spoiling technique with multicompartment models (phantoms) and with rat in vivo.