Field Strength‐Dependent White Matter R 1 and R 2 Anisotropy of Phase‐Cycled Balanced Steady‐State Free Precession Relaxometry

ABSTRACT Purpose To investigate how the relaxation rates ( R 1 , R 2 ) 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. Methods 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 R 1 , R 2 , 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 R 1 , R 2 , 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. Results R 2 and AI exhibited a pronounced orientation dependence while the influence of anisotropy on R 1 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 R 2 and AI anisotropy to a high degree ( R 2 ≥ 0.99). The calculated partial contributions of susceptibility to R 2 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 R 2 anisotropy, but not its strength. Conclusion Microstructure‐driven relaxation anisotropy considerably affects pc‐bSSFP relaxometry, in particular R 2 . The findings indicate that R 2 anisotropy is driven by susceptibility at ultra‐high fields whereas additional mechanisms likely contribute at lower field strengths.