HYFI: Hybrid filling of the dead-time gap for faster zero echo time imaging

Purpose To improve the SNR efficiency of zero echo time (ZTE) MRI pulse sequences for faster imaging of short- T 2 components at large dead-time gaps. Methods HYFI is s strategy of retrieving inner k-space data missed during the dead-time gap arising from radio-frequency excitation and switching in ZTE imaging. It performs hybrid filling of the inner k-space with a small single-point-imaging core surrounded by a stack of shells acquired on radial readouts in an onion-like fashion. The exposition of this concept is followed by translation into guidelines for parameter choice and implementation details. The imaging properties and performance of HYFI are studied in simulations as well as phantom, in-vitro and in-vivo imaging, with an emphasis on comparison with the PETRA technique (pointwise encoding time reduction with radial acquisition). Results Simulations predict higher SNR efficiency of HYFI compared to PETRA at preserved image quality with the advantage increasing with the size of the k-space gap. These results are confirmed by imaging experiments with gap sizes of 25 to 50 Nyquist dwells, in which scan times for similar SNR could be reduced by 25 to 60%. Conclusion The HYFI technique provides both high SNR efficiency and image quality, thus outperforming previously known ZTE-based pulse sequences in particular for large k-space gaps. Promising applications include direct imaging of ultra-short T 2 components, such as the myelin bilayer or collagen, T 2 mapping of ultra-fast relaxing signals, and ZTE imaging with reduced chemical shift artifacts.