MISSTEC-S: A fast 1H pulse calibration from spectra simultaneously produced by a spin echo and a stimulated echo

Abstract

Radio-Frequency (RF) pulse calibration is an essential step in guaranteeing both optimum acquisition quality in multi-pulse NMR and accurate results in quantitative experiments. Most existing methods are based on a series of spectra for which the flip angle of one or more pulses is progressively incremented, implying a significant experiment time. In order to circumvent this drawback, we have previously proposed an approach based on the acquisition of a spin echo and a stimulated echo − the MISSTEC sequence − which requires only 8 s to determine the ${\mathrm{PW}}_{90}$-¹H, while it is several minutes in the case of the use of a nutation curve.

In this work, a new sequence for RF calibration is presented: MISSTEC-S. It is derived from the previously proposed MISSTEC sequence, but the observation of echoes in presence of magnetic field gradient is replaced by the observation of FIDs.

This modification allows both spectra to be phased, while imposing a strong constraint on the Mixing Time (TM). However, the relationship used to calculate the flip angle is only correct when TM is small enough to neglect longitudinal relaxation during this delay. In order to reduce TM, the first FID is truncated during acquisition and subsequently lengthened using points from the second FID. Results obtained with MISSTEC-S were compared to those obtained from a complete nutation curve and an excellent correlation was observed, although the experimental time to obtain the ${\mathrm{PW}}_{90}$ is dramatically reduced.