Tailored Excitation by Shaped RF Pulses. Optimization and Implementation of Excitation Containing a Notch

Abstract

Design optimization and practical implementation of tailored π/2 pulses able to provide a flat null at a narrow band of resonance frequencies (several hundred hertz) with uniform excitation and minimum phase distortion of alt other resonance frequencies (±5000 Hz) are described. The self-refocusing pulses are capable of being programmed into typical commercial NMR spectrometers and possess a practical duration (1-3 ms). A conjugate gradient method is employed in shape-design optimization. For practical implementation on a 500 MHz spectrometer, the optimized pulses are approximated by a train of width-modulated DANTE pulses. Implementation of one such shaped DANTE pulse, termed a NODE-1 pulse, on a spectrometer and application to a DNA oligomer are demonstrated via a one-dimensional proton NMR spectrum of the DNA duplex in H 2O with suppression of the water proton resonance and flat excitation of all frequencies including the imino proton resonances. Further application to a two-dimensional nuclear Overhauser effect spectrum of the DNA duplex in deuterium oxide solution with null excitation at frequencies corresponding to H2′ and H2″ resonances is illustrated.