Field Strength-Dependent White Matter R₁ and R₂ Anisotropy of Phase-Cycled Balanced Steady-State Free Precession...

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Neuroimaging
• Biophysics

Background:

• Phase-cycled balanced steady-state free precession (pc-bSSFP) is a valuable MRI technique.
• Understanding white matter (WM) microstructure is crucial for diagnosing neurological conditions.
• Relaxation rates (R1, R2) and asymmetry indices (AI) provide insights into tissue properties.

Purpose of the Study:

• To investigate the orientation dependence of R1, R2, and AI in WM fiber tracts.
• To determine how this dependence changes with magnetic field strength (3T and 9.4T).
• To explore the underlying mechanisms, including susceptibility effects and magic angle effects.

Main Methods:

• Acquired pc-bSSFP data in the healthy human brain at 3T and 9.4T.
• Processed data using motion-insensitive rapid configuration relaxometry (MIRACLE) and frequency response analysis.
• Estimated fractional anisotropy (FA) and fiber-to-field angle (θ) from diffusion tensor imaging (DTI).

Main Results:

• R2 and AI exhibited significant orientation dependence, while R1 showed a weaker but noticeable dependence.
• Anisotropy increased systematically from 3T to 9.4T.
• Susceptibility effects were found to be the primary driver of R2 anisotropy at ultra-high fields (9.4T).

Conclusions:

• Microstructure-driven relaxation anisotropy significantly impacts pc-bSSFP relaxometry, especially R2.
• R2 anisotropy is predominantly driven by susceptibility at ultra-high fields.
• Additional mechanisms likely contribute to R2 anisotropy at lower field strengths.