writer：Fenfen Wang; Sanath K. Ramakrishna; Pingchuan Sun; Riqiang Fu*
keywords：Direct polarization; Single-pulse excitation; Triple-pulse excitation; Acoustic ringing; Background suppression
source：期刊
specific source：The Journal of Magnetic Resonance, 2021, accepted
Issue time：2021年
Direct polarization using a single pulse is the simplest excitation scheme in nuclear magnetic resonance (NMR) experiments, capable of quantifying various compositions in many materials applications. However, this single-pulse excitation generally gives rise to NMR spectra with a severely distorted baseline due to the background signals arising from probe components and/or due to the radiofrequency (RF) acoustic ringing, especially in low-γ nuclei and wide-line NMR. In this work, a triple-pulse excitation scheme is proposed to simultaneously suppress the background signals and eliminate the RF acoustic ringing. The acoustic ringing is cancelled through subtraction in any two consecutive scans by alternating the receiver phase while keeping the phase of the pulse right before acquisition the same. While the triple-pulse scheme generates an additional flip-angle dependent scaling to the traditional single-pulse excitation profile in such a way that the scaling is one when the flip-angle is ~ 90° but becomes almost zero when the flip-angle is very small. Therefore, the background signals arising from the materials outside the sample coil experiencing a very small fraction of the RF flip-angles can be effectively suppressed. Various samples containing ¹H and quadrupolar nuclei (¹⁷O, ²⁵Mg, and ²³Na) have been used to demonstrate the effectiveness of this newly proposed triple-pulse excitation in terms of suppressing the background signals and eliminating the acoustic ringing effects.