Institute of Neuroscience and Medicine 4, Forschungszentrum Jülich, INM-4, 52428, Jülich, Germany.
RWTH Aachen University, Aachen, Germany.
Sci Rep. 2023 Jan 20;13(1):1138. doi: 10.1038/s41598-023-28265-0.
To quantify T2*, multiple echoes are typically acquired with a multi-echo gradient echo sequence using either monopolar or bipolar readout gradients. The use of bipolar readout gradients achieves a shorter echo spacing time, enabling the acquisition of a larger number of echoes in the same scan time. However, despite their relative time efficiency and the potential for more accurate quantification, a comparative investigation of these readout gradients has not yet been addressed. This work aims to compare the performance of monopolar and bipolar readout gradients for T2* quantification. The differences in readout gradients were theoretically investigated with a Cramér-Rao lower bound and validated with computer simulations with respect to the various imaging parameters (e.g., flip angle, TR, TE, TE range, and BW). The readout gradients were then compared at 3 T using phantom and in vivo experiments. The bipolar readout gradients provided higher precision than monopolar readout gradients in both computer simulations and experimental results. The difference between the two readout gradients increased for a lower SNR and smaller TE range, consistent with the prediction made using Cramér-Rao lower bound. The use of bipolar readout gradients is advantageous for regions or situations where a lower SNR is expected or a shorter acquisition time is required.
为了量化 T2*，通常使用多回波梯度回波序列采集多个回波，采用单极或双极读出梯度。使用双极读出梯度可以实现更短的回波间隔时间，从而在相同的扫描时间内采集更多的回波。然而，尽管它们在时间效率方面具有相对优势，并且在定量方面具有更大的潜力，但对于这些读出梯度的比较研究尚未得到解决。本工作旨在比较单极和双极读出梯度在 T2*定量方面的性能。通过克拉美罗下界理论研究了读出梯度的差异，并结合不同成像参数（例如翻转角、TR、TE、TE 范围和 BW）的计算机模拟进行了验证。然后在 3T 下使用体模和体内实验对读出梯度进行了比较。在计算机模拟和实验结果中，双极读出梯度的精度均高于单极读出梯度。随着 SNR 的降低和 TE 范围的减小，两种读出梯度之间的差异增大，这与克拉美罗下界的预测一致。在 SNR 较低或采集时间较短的区域或情况下，使用双极读出梯度具有优势。
