Distortions in multiple-pulse solid state NMR imaging: Gradient decoupling, time-sequenced second averaging, and over-sampling

Three approaches to reducing image artifacts are described that are specific to multiple-pulse line-narrowing methods of NMR imaging. Gradient decoupling avoids excess line broadening from off-resonance gradient phase evolution by restricting the gradient to selected windows in which the gradient Hamiltonian commutes with the toggling frame state, and where the averaged Hamiltonian between gradient pulses is either cyclic or anti-cyclic. This forces the residual averaged dipolar Hamiltonian to be independent of the gradient evolution. Time-sequenced second averaging addresses the on-resonance broadening, where residual error terms dominate the spin dynamics (a lack of second averaging), by adding a second coherent averaging that retains part of the modulation associated with off-resonance terms, and thus smoothes out the line-narrowing efficiency with spatial offset. Over-sampling is useful to increase both the resolution and sensitivity of an image, but it introduces a sampling modulation that produces sidebands. These are eliminated by a series of prepulses in a fashion reminiscent of CYCLOPS phase cycling.