Modelling and correcting the impact of RF pulses for continuous monitoring of hyperpolarized NMR
Abstract. Monitoring the build-up or decay of hyperpolarization in nuclear magnetic resonance requires radio-frequency (RF) pulses to generate observable nuclear magnetization. However, the pulses also lead to a depletion of the polarization and, thus, alter the spin dynamics. To simulate the effects of RF pulses on the polarization build-up and decay, we propose a first-order rate-equation model describing the dynamics of the hyperpolarization process through a single source and a relaxation term. The model offers a direct interpretation of the measured steady-state polarization and build-up time constant. Furthermore, the rate-equation model is used to study three different methods to correct for the errors introduced by RF pulses: (i) a 1/ cosn θ correction, which is only applicable to decays, (ii) an analytic formula to correct for the build-up and decay times and (iii) a newly proposed iterative, self-consistent correction. The corrections are first tested in low signal-to-noise ratio (SNR) simulations (SNR around 40 for 2.5° pulses), predicting accurate results (±10 % error) up to 25° pulses. The correction methods are then tested on experimental data obtained with dynamic nuclear polarization (DNP) using 4-oxo-TEMPO in 1H glassy matrices, resulting in high SNR acquisitions (around 1000 for 2.4° pulses). It is experimentally demonstrated that the rate-equation model allows to obtain build-up times and steady-state polarization (enhancement) even for large RF flip angles (25°) during build-up yielding results within ±10 % error when compared to data acquired with small RF flip angles (< 3°). For decay experiments, corrections are shown to be accurate for up to 12° RF flip angles with discrepancies to the simulations attributed to the low experimental acquisition SNR. In conclusion, corrections based on a rate-equation description offer fast and accurate estimations of achievable polarization levels and build-up time constants in hyperpolarization experiments for a wide range of samples.
Gevin von Witte et al.
Status: final response (author comments only)
CC1: 'Comment on mr-2023-5', Norbert Mueller, 27 Apr 2023
- AC1: 'Reply on CC1', Gevin von Witte, 02 May 2023
- RC1: 'Comment on mr-2023-5', Anonymous Referee #1, 02 May 2023
- RC2: 'Comment on mr-2023-5', Anonymous Referee #2, 07 May 2023
Gevin von Witte et al.
Model code and software
Model code for the experimental RF correction https://gitlab.ethz.ch/gvwitte/rfcorrection
Gevin von Witte et al.
Viewed (geographical distribution)
Comment on the preprint (https://doi.org/10.5194/mr-2023-5) of
„Modelling and correcting the impact of RF pulses for continuous monitoring of hyperpolarized NMR“
by Gevin von Witte, Matthias Ernst, and Sebastian Kozerke
I just want to add a comment relating to the experimental approaches for monitoring hyperpolarization in NMR. Some time ago in collaboration with other labs my research group investigated the potential of spin noise detection for monitoring buildup of hyperpolarization (see references at the end of this text). Given that the buildup of hyperpolarization is a slow process, in many cases, spin noise measurements should be possible concurrently to the buildup. While being much less sensitive than pulsed excitation based approaches (but sensitivity should not be an issue here) spin noise measurements appear to be virtually non-invasive and will also work for negative polarization, where a pulse might trigger coherent emission (M/RASER). At high polarization levels both pulse and noise based approaches are of course subject to feedback effects involving the rf-receiver circuit, i.e. radiation damping. So the circuit’s quality factor may have an impact on the build-up rates determined by either method. This may be of relevance in the context of this preprint and thus might be considered by the autors.
I apologize if this comment contains too much self-promotion.
Pöschko MT, Peat D, Owers-Bradley J, and Müller N. (2016) Use of Nuclear Spin Noise Spectroscopy to Monitor Slow Magnetization Buildup at Millikelvin Temperatures, ChemPhysChem 17, 3035-3039. https://doi.org/10.1002/cphc.201600323
Pöschko MT, Vuichoud B, Milani J, Bornet A, Bechmann M, Bodenhausen G, Jannin S, and Müller N. (2015) Spin Noise Detection of Nuclear Hyperpolarization at 1.2K, ChemPhysChem 16, 3859-3864. https://doi.org/10.1002/cphc.201500805