Maximizing spectral sensitivity without compromising resolution in phase-incremented, steady-state solution NMR.

Abstract

NMR acquisitions based on Ernst-angle excitations are widely used for maximizing spectral sensitivity without compromising bandwidth or resolution. However, if relaxation times T₁, T₂ are long and similar, as is often the case in liquids, steady-state free-precession (SSFP) experiments could provide higher sensitivity per <math xmlns="http://www.w3.org/1998/Math/MathML"> <msqrt><mrow><mi>acquisition</mi></mrow> </msqrt> <mo>_</mo> <mi>time</mi></math> (SNR_t). Although strong offset dependencies and poor spectral resolutions have impeded SSFP's analytical applications, this study reexplores if, when and how can phase-incremented (PI) SSFP schemes overcome these drawbacks. It is found that PI-SSFP can indeed provide a superior SNR_t than Ernst-angle FT-NMR acquisitions, but that achieving this requires using relatively large flip angles. This, however, can restrict PI-SSFP's spectral resolution and lead to distorted line shapes; to deal with this we introduce here a new SSFP outlook that overcomes this dichotomy. This outlook also leads to a new processing pipeline for PI-SSFP acquisitions, providing high spectral resolution even when utilizing relatively the large flip angles. The enhanced SNR_t that the ensuing method can provide over FT-based NMR counterparts, is demonstrated with a series of ¹³C and ¹⁵N investigations on organic compounds.