Multiple-pulse methods of1H N.M.R. imaging of solids: second-averaging

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Abstract
Multiple-pulse cycles composed of short (approximately 2–4 μs) gradient pulses intercalated between solid echoes are examined to find useful approaches to solid-state imaging. The usefulness of a cycle is judged on how well it fulfils requirements that the dipolar terms in the Magnus expansion of the average Hamiltonian of the cycle vanish to zeroth order for finite radio-frequency pulse lengths, and to first order in a delta-function pulse approximation; that all important first-order dipolar cross-terms also vanish; and that chemical-shift evolution is suppressed. For imaging in solids, we demonstrate a phase-shifted version of the MREV-8 sequence with gradient pulses of alternating polarity applied in the long windows. The addition of phase shifts to certain pulses eliminates resonance offset and chemical-shift interactions via ‘second-averaging’, while the gradient is unaffected by this averaging since it is applied along the axis of second averaging. An image is shown of a poly(methyl methacrylate) phantom with resolution of approximately 100 μm for a 6·4 mm diameter sample. Methods are suggested for further increasing the resolution by a factor of four to six.