Development of a novel 10-echo multi-contrast sequence based on EPIK to deliver simultaneous quantification of T₂ and T₂^* with application to oxygen extraction fraction

Purpose: The simultaneous quantification of T₂ and T₂ * maps based on fast sequences for combined GE and SE acquisition has rekindled research and clinical interest by offering a wide range of attractive applications, e.g., dynamic tracking of oxygen extraction fraction (OEF). However, previously published methods based on EPI-readouts have been hindered by resolution and the number of acquired echoes.

Methods: This work presents a novel 10-echo GE-SE EPIK (EPI with keyhole) sequence for the rapid quantification of T₂ '. T₂ /T₂ * maps from the GE-SE EPIK sequence were validated using three phantoms and 15 volunteers at 3T. The incorporation of a sliding window approach, combined with the full sampling of the k-space center inherent to EPIK, enables a high effective temporal resolution. That is, for an eight-slice breath-hold experiment, a temporal sampling rate of eight reconstructed slices per 1.1 s.

Results: In comparison with repeated single-echo SE, multi-echo GE, and spectroscopy methods, the GE-SE EPIK sequence shows good agreement in quantifying T₂ /T₂ * values, while the gray matter/white matter separation yielded the expected contrast differentiation. The OEF was calculated with a view to an initial application with clinical relevance, producing results comparable to those in the literature and with good sensitivity in breath-hold experiments.

Conclusions: GE-SE EPIK provides increased resolution and more echoes, including two SEs, than comparable sequences. Moreover, GE-SE EPIK achieves this within an acquisition time of 57 s for 20 slices (matrix size = 128×128; FOV = 24 cm) and with a reasonably short TE for the final echo (114 ms). The sequence can dynamically track OEF changes in a breath-hold experiment.