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

To investigate how the relaxation rates (R₁, R₂) and asymmetry indices (AI), derived from phase-cycled balanced steady-state free precession (pc-bSSFP) data, depend on the orientation of white matter (WM) fiber tracts at different field strengths.

Phase-cycled bSSFP data acquired at 3 and 9.4T in the healthy human brain were processed using motion-insensitive rapid configuration relaxometry (MIRACLE) and a frequency response analysis to derive R₁, R₂, and AI values, respectively. Fractional anisotropy (FA) and fiber-to-field angle (θ) were estimated based on 3T diffusion tensor imaging. The orientation dependence of R₁, R₂, and AI in WM was characterized using literature model fits as well as Monte Carlo random walk simulations to explore the influence of field strength and susceptibility effects.

R₂ and AI exhibited a pronounced orientation dependence while the influence of anisotropy on R₁ was weaker, but noticeable. The observed anisotropy increased systematically from 3 to 9.4T. Literature models assuming either a susceptibility or a generalized magic angle effect described the R₂ and AI anisotropy to a high degree (R² ≥ 0.99). The calculated partial contributions of susceptibility to R₂ anisotropy increased from 24.0%-39.0% at 3T to 77.0%-87.1% at 9.4T. The Monte Carlo simulations were able to reproduce the characteristics of R₂ anisotropy, but not its strength.

Microstructure-driven relaxation anisotropy considerably affects pc-bSSFP relaxometry, in particular R₂. The findings indicate that R₂ anisotropy is driven by susceptibility at ultra-high fields whereas additional mechanisms likely contribute at lower field strengths.