NMR imaging of solids with multiple-pulse line narrowing and radiofrequency gradients

摘要

The usual methods of magnetic resonance imaging fail in rigid solids due to the line shape contributions of dipolar coupling, chemical shift dispersion and anisotropy, and bulk magnetic susceptibility. This dissertation presents a new method of solid-state imaging by nuclear magnetic resonance which averages away these contributions with multiple-pulse line-narrowing and encodes spatial information with pulsed radiofrequency field gradients. This method is closely related to simultaneously developed methods utilizing pulsed DC gradients, and offers similar improvements in sensitivity and resolution. The advantage of rf gradients is that they can be rapidly switched without inducing eddy currents in the probe or the magnet. In addition, the phases and amplitudes of the rf gradients can be switched by equipment which is already part of an NMR spectrometer capable of solid-state spectroscopy. The line-narrowing and gradient pulses originate in separate rf circuits tuned to the same frequency. Interactions between the circuits have been minimized by a method of active Q-switching which employs PIN diodes in the matching networks of these circuits.Both one- and two-dimensional images are presented. The latter are obtained by a novel method in which the two dimensions of imaging transverse to the static magnetic field are encoded by two orthogonal components of a single rf gradient. A [pi]/2 phase shift of the rf phase relative to that of the line-narrowing pulses selects one component or the other. This arrangement allows the solid-state analogs of versatile imaging sequences based on Fourier imaging and eliminates the need for sample rotation and back-projection methods.Coherent averaging theory is used to analyze this imaging technique and exact numerical simulations on several coupled spins are discussed. These lend insight to the residual linewidth and its dependence on pixel position as well as to the range of applicability of this technique.