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

## TL;DR

This study explores how magnetic field strength affects relaxation rates and anisotropy in white matter using brain imaging techniques.

## Contribution

The paper reveals that susceptibility effects dominate R2 anisotropy at ultra-high fields, unlike lower fields where other mechanisms may play a role.

## Key findings

- R2 and asymmetry indices show strong orientation dependence, increasing with higher field strength.
- Susceptibility contributes 77.0%–87.1% to R2 anisotropy at 9.4T, compared to 24.0%–39.0% at 3T.
- Monte Carlo simulations replicate R2 anisotropy characteristics but not their magnitude.

## Abstract

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.

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.

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.

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.