Echoes and imaging in solids

Abstract

The broad spectral lines usually encountered in solid state NMR present considerable difficulties for imaging. One successful approach to the problem is to artificially narrow the line by multipulse or sample spinning methods. An alternative is to apply sufficiently large magnetic field gradients that they dominate the line broadening and seek ways to deal with bandwidth and power requirements thereby introduced. This paper explores the second route and demonstrates that spin-echo techniques help to solve several of the inherent problems. Gradient echoes produced by periodic reversal of the field gradients have significant advantages. The addition of synchronous RF pulses can produce an extended train of ‘solid’ echoes which overcomes, at least to some extent, the bandwidth limitation of this approach and permits rapid imaging in two dimensions. Slice selection and three-dimensional back projection have also been achieved in solid-like samples. Comparison with linenarrowing methods and relative advantages of the different approaches are addressed.