Modelling and correcting the impact of RF pulses for continuous monitoring of hyperpolarized NMR

Abstract

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 the errors introduced by RF pulses: (i) a 1/cos⁡n-1θ correction (θ denoting the RF pulse flip angle), which is only applicable to decays; (ii) an analytical model introduced previously in the literature; and (iii) an iterative correction approach proposed here. The three correction methods are compared using simulated data for a range of RF flip angles and RF repetition times. The correction methods are also tested on experimental data obtained with dynamic nuclear polarization (DNP) using 4-oxo-TEMPO in 1H glassy matrices. It is demonstrated that the analytical and iterative corrections allow us to obtain accurate build-up times and steady-state polarizations (enhancements) for RF flip angles of up to 25∘ during the polarization build-up process within ±10 % error when compared to data acquired with small RF flip angles (<3∘). For polarization decay experiments, corrections are shown to be accurate for RF flip angles of up to 12∘. In conclusion, the proposed iterative correction allows us to compensate for the impact of RF pulses offering an accurate estimation of polarization levels, build-up and decay time constants in hyperpolarization experiments.

Relaxation enhancement by microwave irradiation may limit dynamic nuclear polarization

“…20,22 The absolute values of the polarization were calculated on a single thermal equilibrium measurement per sample as reported and discussed previously. 17…”

“…20,22 The absolute values of the polarization were calculated on a single thermal equilibrium measurement per sample as reported and discussed previously. 17 Electron paramagnetic resonance (EPR) spectra were acquired with our in-house developed longitudinal-detection (LOD) EPR setup at 7 T at a temperature of 5 K. 22 A home-made coil was inserted along the main magnetic field (the NMR coil is perpendicular to it) with the MW source irradiating along the NMR coil's direction (same MW wave guide as for NMR). MW irradiation leads to a saturation of the electron polarization which is inductively detected with the longitudinal coil.…”

“…The perturbing effects of monitoring RF pulses can be minimized by using few and small flip-angle pulses or correcting for the pulses. 17 In the following, relaxation enhancement by MW irradiation depending on the experimental conditions (temperature and The sample formulation was broken down to the radical together with its target spins and possible gadolinium (Gd) doping, although the exact sample formulations vary between the different works. The first three samples from this work refer to the natural abundance sample and the second three to the partially deuterated (DNP juice) sample (cf.…”

“…Rate equation or kinetic models offer the possibility to qualitatively and even quantitatively describe the balance between hyperpolarization injection and relaxation. 17 In this paper we use such rate equation models to determine the relaxation-rate constant of the nuclei during the DNP polarization build-up process. We find that in many cases the nuclear relaxation-rate constants during the hyperpolarization build-up under MW irradiation are much larger than the ones obtained without MW irradiation.…”

“…For the decays depcited in orange (3.3 K) and light blue (2.0 K), the MW is switched to the central EPR frequency (197.28 GHz) resulting in zero DNP, as can be seen in (d). The measured polarization in (a) can be simulated (solid black line) with a single compartment rate equation model 17. The only model input parameters are the thermal electron polarization, measured build-up time and steady-state polarization.…”