Slice-selective Zero Echo Time imaging of ultra-short T2 tissues based on spin-locking

Purpose: To expand the capabilities of Zero Echo Time (ZTE) pulse sequences with a slice selection method suitable for the shortest-lived tissues in the body. Methods: We introduce two new sequences that integrate spin-locking pulses into standard ZTE imaging to achieve slice selection: one for moderately short $T_2$ (DiSLoP), the other for ultra-short $T_2$ samples (PreSLoP). These methods exploit the slower signal decay (at $T_{1\rho}\gg T_2$) to retain the magnetization in the slices during the selection process, which is otherwise comparable to or even much longer than $T_2$. Results: We demonstrate control over the slice profiles and positions for 2D imaging. We measure magnetization decay times during spin-locking ($T_{1\rho}$) as a function of pulse amplitude, showing significant lifetime enhancement for amplitudes as low as 10 uT. We show imaging of slice-selected samples with $T_2$ characteristic times in the range of single milliseconds with DiSLoP and PreSLoP, and with the latter for sub-millisecond $T_2$ tissues. As compared to standard 3D ZTE sequences, PreSLoP achieves the same signal-to-noise ratio (SNR) in 2-5 times shorter scan times, and we argue that this is due to the filling scheme of the finite gap at the center of $k$-space unavoidable with ZTE sequences. Finally, we discuss a combination of DiSLoP with a dynamical decoupling sequence to avoid this central gap, leading to further scan time accelerations. Conclusions: The proposed sequences are capable of slice-selected 2D imaging of tissues with $T_2$ as low as 275 us with good SNR within clinically acceptable scan times.