Dual-pathway multi-echo sequence for simultaneous frequency and T₂ mapping

Purpose: To present a dual-pathway multi-echo steady state sequence and reconstruction algorithm to capture T₂, T₂^∗ and field map information. Methods: Typically, pulse sequences based on spin echoes are needed for T₂ mapping while gradient echoes are needed for field mapping, making it difficult to jointly acquire both types of information. A dual-pathway multi-echo pulse sequence is employed here to generate T₂ and field maps from the same acquired data. The approach might be used, for example, to obtain both thermometry and tissue damage information during thermal therapies, or susceptibility and T₂ information from a same head scan, or to generate bonus T₂ maps during a knee scan. Results: Quantitative T₂, T₂^∗ and field maps were generated in gel phantoms, ex vivo bovine muscle, and twelve volunteers. T₂ results were validated against a spin-echo reference standard: A linear regression based on ROI analysis in phantoms provided close agreement (slope/R² = 0.99/0.998). A pixel-wise in vivo Bland–Altman analysis of R₂ = 1/T₂ showed a bias of 0.034 Hz (about 0.3%), as averaged over four volunteers. Ex vivo results, with and without motion, suggested that tissue damage detection based on T₂ rather than temperature-dose measurements might prove more robust to motion. Conclusion: T₂, T₂^∗ and field maps were obtained simultaneously, from the same datasets, in thermometry, susceptibility-weighted imaging and knee-imaging contexts.