Orientationally invariant metrics of apparent compartment eccentricity from double pulsed field gradient diffusion...

Area of Science:

• Magnetic Resonance Imaging
• Biophysics
• Materials Science

Background:

• Pulsed field gradient (PFG) diffusion sequences offer advanced microstructural tissue information.
• Current PFG methods lack rotational invariance, leading to orientation-dependent metrics in anisotropic tissues.
• Detecting nonspherical compartments is challenging with standard diffusion experiments.

Purpose of the Study:

• To develop a rotationally invariant framework for estimating double wave vector diffusion (d-PFG) metrics.
• To introduce the d-PFG 5-design for theoretically exact determination of orientationally averaged signals.
• To demonstrate the utility of d-PFG for characterizing microstructural properties like compartment eccentricity.

Main Methods:

• Proposed a new framework, the d-PFG 5-design, utilizing 60 pairs of diffusion wave vectors.
• Employed orientational averaging of signals to emulate powder-averaged data, removing macroscopic anisotropy.
• Leveraged exact numerical integration theory for polynomials on the rotation group.

Main Results:

• Demonstrated rotational invariance in estimating compartment eccentricity using the d-PFG 5-design.
• Showcased compartment eccentricity as a novel microstructural measure complementary to fractional anisotropy (FA).
• Validated the method with numerical simulations and ex vivo diffusion MRI in a nonhuman primate brain.

Conclusions:

• The d-PFG 5-design enables rotationally invariant estimation of d-PFG metrics.
• Compartment eccentricity derived from d-PFG provides valuable microstructural information.
• This approach advances the characterization of tissue microstructure using diffusion MRI.