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

Purpose: Measuring the transverse-relaxation rate (Formula presented.) 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 (Formula presented.) mapping. Methods: Unlike currently practiced, spin echo–based (Formula presented.) measurement techniques, the proposed method alternates between SSFP-FID and SSFP-ECHO modes for rapid 3D encoding of transverse relaxation rates expressed as R₂ + (Formula presented.) and R₂ (Formula presented.) (Formula presented.). 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, (Formula presented.), and (Formula presented.) (= R₂ + (Formula presented.)). 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 (Formula presented.) and additionally derived R₂, (Formula presented.), 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 ((Formula presented.), R₂, and (Formula presented.)) against Δχ. Conclusion: Results suggest feasibility of the proposed method as a practical and reliable means for measuring (Formula presented.), R₂, (Formula presented.), and Δχ across the entire brain.