Line-narrowing approaches to solid state NMR imaging: pulsed gradients and second averaging

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Abstract
There are questions in materials sciences which can be addressed by NMR imaging. However, not all materials exhibit sufficient molecular motion to reduce dipolar couplings to the level at which the familiar techniques of medical NMR imaging are appropriate. For the more general case of these less mobile materials, line-narrowing methods, specialized to imaging, are appropriate. We have earlier demonstrated that such imaging sequences adequately overcome the fundamental dipolar broadening in engineering polymers; however, these sequences are subject to certain practical limitations which we address. We present a class of homonuclear line-narrowing imaging methods which intercalate short gradient pulses ( ca. 5 ps) into the RF pulse sequence. Such a strategy improves spatial resolution and signal sensitivity by reducing extraneous broadening from off-resonance effects. In a further implementation, the chemical shift and susceptibility terms in the spin hamiltonian are suppressed by ‘second averaging’ about a particular axis, while the hamiltonian for the gradient pulses is aligned along that axis: the result is a further improvement in spatial resolution. Prospects for the future of solid state NMR imaging are considered.