Revisiting classical diffusion magnetic resonance methods as a means to measure time-dependent diffusion.

Area of Science:

• Physics
• Biophysics
• Materials Science

Background:

• Diffusion microstructural magnetic resonance (MR) aims to measure time-dependent diffusion, where mean-squared displacement (MSD) is non-linear with time.
• Current MR methods often require non-standard sequences or make non-physical assumptions to quantify this time-dependence.

Purpose of the Study:

• To demonstrate that standard spin echo and stimulated echo MR sequences can directly probe time-dependent diffusion (MSD).
• To propose a novel framework for quantitative MSD measurement using readily available MR techniques.

Main Methods:

• Utilizing a log-signal ratio from pairs of measurements with different inter-pulse spacings (Δ).
• Applying the Gaussian phase approximation (GPA) for quantitative analysis with short, finite-duration gradient pulses.
• Validating the framework using simulations of one-dimensional diffusion between parallel planes (impermeable and permeable).

Main Results:

• The proposed framework shows that the log-signal ratio is proportional to the difference in MSD between two time points.
• Excellent agreement was achieved between estimated and ground truth MSD values in simulated diffusion environments.
• The framework's applicability is broad, as the GPA can be adapted to various microstructural properties.

Conclusions:

• Standard spin echo and stimulated echo MR sequences are sufficient for directly probing time-dependent diffusion (MSD).
• The developed framework provides a quantitative and widely applicable method for microstructural analysis using conventional MR hardware.
• This approach simplifies the measurement of complex diffusion dynamics, enhancing insights into microenvironmental properties.