Single-scan acquisition of full restricted diffusion NMR data

Restricted diffusion of adsorbed liquids and gases provides access to key microstructural parameters, such as surface to volume ratio and tortuosity, through the diffusion time (Δ) dependence of the apparent diffusion coefficient (D). Conventional pulsed field gradient restricted diffusion NMR measurements, however, require repeated acquisitions over multiple gradient strengths and diffusion times, leading to prohibitively long experiment durations that hinder studies of evolving or time dependent systems. Here, we introduce a Single scan sPatially encoded Ultrafast ResTricted diffusion (SPURT) method that captures the full D(Δ) dependence in a single acquisition by combining spatial diffusion encoding with a train of low flip angle readout pulses.

The method was demonstrated using water diffusion in thermally modified pine wood, a heterogeneous porous material with well characterized restricted diffusion behaviour. Apparent diffusion coefficients obtained with SPURT agree with those from reference measurements within experimental uncertainty. Structural parameters extracted from the SPURT data, including surface to volume ratio, pore width, and tortuosity, also match values obtained from conventional experiments and reported in the literature. Notably, SPURT reduces the total acquisition time for restricted diffusion measurements by two to three orders of magnitude, decreasing measurement times from hours to seconds.

By transforming a fundamentally multi experiment protocol into a single scan measurement, SPURT enables rapid characterization of restricted diffusion and microstructure in porous materials. This capability opens new opportunities for studying systems undergoing dynamic structural changes, such as hydration, curing, drying, or degradation, and is particularly advantageous for experiments employing hyperpolarization, as the time consuming polarization step needs to be performed only once.