Getting the Most out of Parahydrogen-Induced Signal Enhancement for MRI of Reacting Heterogeneous Systems

Methods based on magnetic resonance imaging (MRI) can be used for operando studies of heterogeneous catalytic processes in the gas phase. MRI can provide a detailed understanding of the heterogeneous reactor operation based on the information about spatial distribution of reactants and reaction products. However, low spin density, fast diffusion, and short relaxation times of gases along with magnetic field inhomogeneities associated with heterogeneous catalytic systems complicate such studies and compromise achievable sensitivity. Spin hyperpolarization techniques in general and parahydrogen-induced polarization (PHIP) in particular provide a major increase in the intensity of nuclear magnetic resonance (NMR) signals. At the same time, an antiphase lineshape of NMR signals associated with PHIP in high magnetic fields is disadvantageous for MRI experiments. This is because magnetic field gradients (both intrinsic and applied) lead to mutual cancelation of the positive and negative parts of such signals so that, for instance, frequency-encoding gradients of an MRI pulse sequence can significantly diminish or even eliminate the useful signal. In this study, we explore the effects of an antiphase NMR signal shape on MR images. We first demonstrate these effects for a homogeneous solution with thermal polarization of nuclear spins. We then address MRI of heterogeneous catalytic hydrogenation of a gas (propylene) with parahydrogen in a high magnetic field. The results demonstrate the importance of antiphase-to-inphase signal shape conversion when MRI is applied in such studies to use the signal enhancement provided by hyperpolarization to the maximum possible extent. This approach, which is implemented for the first time in an MRI study of a heterogeneous object (catalyst beads in an operating model reactor), allowed us to detect MR images of the gaseous reaction product in a model reactor and achieve a 10-fold improvement in the signal-to-noise ratio compared to the conventional three-dimensional MRI experiment performed on an antiphase NMR signal.

Automated summary

This study investigates the effects of an antiphase NMR signal shape on MR images and examines the use of parahydrogen for MRI of gas-phase heterogeneous catalytic hydrogenation. The results demonstrate the importance of converting the antiphase signal to inphase signal shape in order to maximize the signal enhancement provided by hyperpolarization. This approach, applied for the first time in an MRI study of a heterogeneous object, allows for improved signal-to-noise ratio compared to conventional antiphase NMR signal.