Hurley Aaron C, Al-Radaideh Ali, Bai Li, Aickelin Uwe, Coxon Ron, Glover Paul, Gowland Penny A
Sir Peter Mansfield Magnetic Resonance Centre, Department of Computer Science, University of Nottingham, UK.
Magn Reson Med. 2010 Jan;63(1):51-8. doi: 10.1002/mrm.22167.
The radiofrequency (RF) transmit field is severely inhomogeneous at ultrahigh field due to both RF penetration and RF coil design issues. This particularly impairs image quality for sequences that use inversion pulses such as magnetization prepared rapid acquisition gradient echo and limits the use of quantitative arterial spin labeling sequences such as flow-attenuated inversion recovery. Here we have used a search algorithm to produce inversion pulses tailored to take into account the heterogeneity of the RF transmit field at 7 T. This created a slice selective inversion pulse that worked well (good slice profile and uniform inversion) over the range of RF amplitudes typically obtained in the head at 7 T while still maintaining an experimentally achievable pulse length and pulse amplitude in the brain at 7 T. The pulses used were based on the frequency offset correction inversion technique, as well as time dilation of functions, but the RF amplitude, frequency sweep, and gradient functions were all generated using a genetic algorithm with an evaluation function that took into account both the desired inversion profile and the transmit field inhomogeneity.
由于射频穿透和射频线圈设计问题，在超高场时射频（RF）发射场严重不均匀。这尤其会损害使用反转脉冲的序列（如磁化准备快速采集梯度回波）的图像质量，并限制了定量动脉自旋标记序列（如流动衰减反转恢复）的使用。在此，我们使用了一种搜索算法来生成经过定制的反转脉冲，以考虑7T时RF发射场的不均匀性。这产生了一个切片选择性反转脉冲，在7T时头部通常获得的RF幅度范围内效果良好（切片轮廓良好且反转均匀），同时在7T时大脑中仍保持实验上可实现的脉冲长度和脉冲幅度。所使用的脉冲基于频率偏移校正反转技术以及函数的时间扩展，但RF幅度、频率扫描和梯度函数均使用遗传算法生成，其评估函数同时考虑了所需的反转轮廓和发射场不均匀性。
