PGSE, OGSE, and sensitivity to axon diameter in diffusion MRI: Insight from a simulation study.

Area of Science:

• Neuroimaging
• Diffusion MRI
• White Matter Microstructure

Background:

• Diffusion MRI is crucial for non-invasively probing white matter microstructure.
• Axon diameter is a key parameter influencing white matter properties and function.
• Optimizing diffusion sequences is essential for accurate and sensitive measurements of axon diameter.

Purpose of the Study:

• To determine optimal pulsed gradient spin-echo (PGSE) and oscillating gradient spin-echo (OGSE) sequence parameters.
• To maximize sensitivity to axon diameter under idealized and practical conditions.

Main Methods:

• Simulations of a two-compartment white matter model with nonpermeable cylinders.
• Investigation of clinically plausible PGSE and OGSE parameters using trapezoidal diffusion gradient waveforms.
• Assessment of signal sensitivity as the derivative of the signal with respect to axon diameter.

Main Results:

• PGSE yields maximum sensitivity in idealized conditions with parallel fibers.
• Low-frequency OGSE demonstrates higher sensitivity in practical scenarios with dispersed fiber orientations.
• Current clinical scanners (60 mT/m) can only resolve axon diameters down to 6 µm, while advanced systems (300 mT/m) can detect diameters as small as 2-3 µm.

Conclusions:

• Low-frequency OGSE enhances sensitivity beyond PGSE in practical diffusion MRI applications.
• OGSE sequences are particularly beneficial for MRI systems equipped with high-performance gradients.
• Improved sensitivity allows for probing a wider range of axon diameters, enhancing microstructure analysis.