Pseudorandom Selective Excitation in NMR

TL;DR

This paper uses average Hamiltonian theory to analyze and develop pseudorandom-DANTE sequences for selective excitation in NMR, demonstrating improved selectivity and experimental validation of the approach.

Contribution

It introduces the p-DANTE sequence, a novel aperiodic excitation method with theoretical analysis and experimental validation for enhanced selectivity in NMR.

Findings

p-DANTE sequences can selectively excite a single resonance.

Averaging over multiple p-DANTE sequences improves selectivity.

Theoretical predictions align with experimental results.

Abstract

In this work, average Hamiltonian theory is used to study selective excitation in a spin-1/2 system evolving under a series of small flip-angle $\theta-$pulses $(\theta\ll 1)$ that are applied either periodically [which corresponds to the DANTE pulse sequence] or aperiodically. First, an average Hamiltonian description of the DANTE pulse sequence is developed; such a description is determined to be valid either at or very far from the DANTE resonance frequencies, which are simply integer multiples of the inverse of the interpulse delay. For aperiodic excitation schemes where the interpulse delays are chosen pseudorandomly, a single resonance can be selectively excited if the $\theta$-pulses' phases are modulated in concert with the time delays. Such a selective pulse is termed a pseudorandom-DANTE or p-DANTE sequence, and the conditions in which an average Hamiltonian description of p-DANTE is found to be similar to that found for the DANTE sequence. It is also shown that averaging over different p-DANTE sequences that are selective for the same resonance can help reduce excitations at frequencies away from the resonance frequency, thereby improving the apparent selectivity of the p-DANTE sequences. Finally, experimental demonstrations of p-DANTE sequences and comparisons with theory are presented.