Institute for Biomedical Engineering, ETH Zurich and University of Zurich, Zurich, Switzerland.
Magn Reson Med. 2022 Mar;87(3):1360-1374. doi: 10.1002/mrm.29056. Epub 2021 Nov 14.
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.
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 and short-T phantoms as well as in in vivo experiments.
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 contrast.
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 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.
通过利用射频脉冲的固有编码特性来获取在脉冲引起的死区时间内丢失的数据，克服零回波时间（ZTE）MRI 中射频激发持续时间的限制。
使用多个射频脉冲开发了增强的 ZTE 信号模型，该模型通过固有地由射频脉冲施加的编码来访问隐藏在脉冲引起的死区时间内的信息。通过使用具有相似频率扫描的激励来采集两个 ZTE 数据集来实现具有脉冲编码的 ZTE，这些脉冲仅在其中心频率处有一个小的离频。这样，最小扫描时间加倍，但每个采集对 SNR 的贡献与普通平均一样。该方法在长 T 和短 T 体模以及体内实验中进行了演示。
在前所未有的死区时间间隔下，具有脉冲编码的 ZTE 提供了良好的图像质量，这里展示的最大死区时间间隔为 6 个奈奎斯特间隔。在头部成像中，使用较长的激发脉冲的能力与传统的 ZTE 相比，提高了约 2 倍的 SNR 效率，并允许创建 T 对比度。
在新的信号模型中利用射频脉冲的固有编码特性，可以对具有较大死区时间间隔的 ZTE 数据集进行代数重建。这允许更大的翻转角，可用于实现增强的 T 对比度和 SNR 效率的显著提高，前提是可以更好地接近 Ernst 角，从而拓宽 ZTE MRI 的应用范围。
