Alternating unbalanced SSFP for 3D R2' mapping of the human brain.

ABSTRACT

PURPOSE: Measuring the transverse-relaxation rate R2' provides valuable information in quantitative evaluation of tissue microstructure, for example, in terms of oxygenation levels. Here, we propose an alternating unbalanced SSFP pulse sequence for rapid whole-brain 3D R2' mapping. METHODS: Unlike currently practiced, spin echo-based R2' measurement techniques, the proposed method alternates between SSFP-FID and SSFP-ECHO modes for rapid 3D encoding of transverse relaxation rates expressed as R2 + R2' and R2-R2' . Z-shimming gradients embedded into multi-echo trains of each SSFP module are designed to achieve relative immunity to large-scale magnetic-field variations (ΔB0 ). Appropriate models for the temporal evolution of the two groups of SSFP signals were constructed with ΔB0 -induced modulations accounted for, leading to ΔB0 -corrected estimation of R2 , R2' , and R2∗ (= R2 + R2' ). Additionally, relative magnetic susceptibility (Δχ) maps were obtained by quantitative susceptibility mapping of the phase data. Numerical simulations were performed to optimize scan parameters, followed by in vivo studies at 3 T in 7 healthy subjects. Measured parameters were evaluated in six brain regions, and subjected to interparameter correlation analysis. RESULTS: The resultant maps of R2' and additionally derived R2 , R2∗ , and Δχ all demonstrated the expected contrast across brain territories (eg, deep brain structures versus cortex), with the measured values in good agreement with previous reports. Furthermore, regression analyses yielded strong linear relationships for the transverse relaxation parameters ( R2' , R2 , and R2∗ ) against Δχ. CONCLUSION: Results suggest feasibility of the proposed method as a practical and reliable means for measuring R2' , R2 , R2∗ , and Δχ across the entire brain.