Pseudorandom selective excitation in NMR

Abstract

In this work, average Hamiltonian theory is used to study selective excitation under a series of small flip-angle θ-pulses θ ≪ π 3 applied either periodically [corresponding to the DANTE pulse sequence] or aperiodically to a spin-1/2 system. First, an average Hamiltonian description of the DANTE pulse sequence is developed that is valid for frequencies either at or very far from integer multiples of 1 τ , where τ is the interpulse delay. For aperiodic excitation, a single resonance, ν sel, can be selectively excited if the θ-pulse phases are modulated in concert with the interpulse delays. The conditions where average Hamiltonian theory can be accurately applied to describe the dynamics under aperiodic selective pulses, which are referred to as pseudorandom-DANTE or p-DANTE sequences, are similar to those found for the DANTE sequence. Signal averaging over different p-DANTE sequences improves the apparent selectivity at ν sel by reducing the excitations at other frequencies. Experimental demonstrations of p-DANTE sequences and comparisons with the theory are presented.