Pulse encoding for ZTE imaging: RF excitation without dead-time penalty

Purpose To overcome limitations in the duration of RF excitation in zero-TE (ZTE) MRI by exploiting intrinsic encoding properties of RF pulses to retrieve data missed during the dead time caused by the pulse. Methods An enhanced ZTE signal model was developed using multiple RF pulses, which enables accessing information hidden in the pulse-induced dead time via encoding intrinsically applied by the RF pulses. Such ZTE with pulse encoding was implemented by acquisition of two ZTE data sets using excitation with similar frequency-swept pulses differing only by a small off-resonance in their center frequency. In this way, the minimum scan time is doubled but each acquisition contributes equally to the SNR, as with ordinary averaging. The method was demonstrated on long-T-2 and short-T-2 phantoms as well as in in vivo experiments. Results ZTE with pulse encoding provided good image quality at unprecedented dead-time gaps, demonstrated here up to 6 Nyquist dwells. In head imaging, the ability to use longer excitation pulses led to approximately 2-fold improvements in SNR efficiency as compared with conventional ZTE and allowed the creation of T-1 contrast. Conclusion Exploiting intrinsic encoding properties of RF pulses in a new signal model enables algebraic reconstruction of ZTE data sets with large dead-time gaps. This permits larger flip angles, which can be used to achieve enhanced T-1 contrast and significant improvements in SNR efficiency in case the Ernst angle can be better approached, thus broadening the range of application of ZTE MRI.