NMR imaging in solids by multiple-quantum resonance

Abstract

NMR imaging is now a well-established technique for studying biological systems (1). In its most general form, an imaging method uses a magnetic field gradient to encode the positions of the nuclear spins with a spatially varying Larmor frequency. Once the variations in resonant frequency have been decoded appropriately, an image of the nuclear spin density or, more generally, of any mix of NMR parameters can be created. In a linear magnetic field gradient, g, the spread of frequencies across a thickness AZ is ghz. If features on the order of AZ are to be resolved, then the externally imposed field, gAz, must itself be resolved relative to any background or internal field. For solids, the dominant background field is usually the local dipolar field, BL. In biological systems familiar from ‘H imaging, rapid isotropic molecular motion often averages these internal dipolar fields to zero. However, in a strongly protonated solid, where molecular motion is restricted, a typical value for BL might be 5 G so that a gradient greater than 50 G/cm (0.5 T/m) would be needed to achieve a resolution of 1 mm. One approach (2-4) to this problem is to reduce the effective local field by a multiple-pulse line-narrowing sequence (5, 6). The alternative approach is to leave the local field untouched, but to impose a gradient large enough to meet the condition g % &/AZ. In this communication, we demonstrate a prototype imaging experiment for solids based in spirit on this “brute force” method of increasing the gradient, but which relies instead on the properties of multiplequantum NMR transitions (7, 8) to increase the effective gradient strength by an order of magnitude. Specifically, we intensify the effect of the gradient upon the evolution of the spin system by creating high-order multiple-quantum coherences and following their development in the static field gradient. A multiple-quantum coherence of order n = Mi Mj, where Mi and Mj are the magnetic-quantum numbers for high-field states Ii) and lj), evolves n times more rapidly in an inhomogeneous field than the usual single-quantum coherence. That is, if a singlequantum transition in the presence of a field gradient appears with resonance offset AU, then an n-quantum transition appears at nAo. The effective local dipolar fields are, however, roughly comparable for highand low-order coherences in very large