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

PURPOSE

Measuring the transverse-relaxation rate 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 mapping.

METHODS

Unlike currently practiced, spin echo-based measurement techniques, the proposed method alternates between SSFP-FID and SSFP-ECHO modes for rapid 3D encoding of transverse relaxation rates expressed as R + and R . Z-shimming gradients embedded into multi-echo trains of each SSFP module are designed to achieve relative immunity to large-scale magnetic-field variations (ΔB ). Appropriate models for the temporal evolution of the two groups of SSFP signals were constructed with ΔB -induced modulations accounted for, leading to ΔB -corrected estimation of R , , and (= R + ). 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 and additionally derived R , , 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 ( , R , and ) against Δχ.

CONCLUSION

Results suggest feasibility of the proposed method as a practical and reliable means for measuring , R , , and Δχ across the entire brain.