Balchandani Priti, Qiu Deqiang
Department of Radiology, Icahn School of Medicine at Mount Sinai, New York, NY, USA; Department of Radiology, Stanford University, Stanford, CA, USA.
Department of Radiology, Stanford University, Stanford, CA, USA; Department of Radiology and Imaging Sciences, Emory University School of Medicine, Atlanta, GA, USA.
Magn Reson Imaging. 2014 Sep;32(7):804-12. doi: 10.1016/j.mri.2014.04.003. Epub 2014 Apr 13.
The adiabatic Shinnar-Le Roux (SLR) algorithm for radiofrequency (RF) pulse design enables systematic control of pulse parameters such as bandwidth, RF energy distribution and duration. Some applications, such as diffusion-weighted imaging (DWI) at high magnetic fields, would benefit from RF pulses that can provide greater B1 insensitivity while adhering to echo time and specific absorption rate (SAR) limits. In this study, the adiabatic SLR algorithm was employed to generate 6-ms and 4-ms 180° semi-adiabatic RF pulses which were used to replace the refocusing pulses in a twice-refocused spin echo (TRSE) diffusion-weighted echo planar imaging (DW-EPI) sequence to create two versions of a twice-refocused adiabatic spin echo (TRASE) sequence. The two versions were designed for different trade-offs between adiabaticity and echo time. Since a pair of identical refocusing pulses is applied, the quadratic phase imposed by the first is unwound by the second, preserving the linear phase created by the excitation pulse. In vivo images of the human brain obtained at 7Testa (7T) demonstrate that both versions of the TRASE sequence developed in this study achieve more homogeneous signal in the diffusion-weighted images than the conventional TRSE sequence. Semi-adiabatic SLR pulses offer a more B1-insensitive solution for diffusion preparation at 7T, while operating within SAR constraints. This method may be coupled with any EPI readout trajectory and parallel imaging scheme to provide more uniform coverage for diffusion tensor imaging at 7T and 3T.
用于射频（RF）脉冲设计的绝热辛纳尔 - 勒鲁（SLR）算法能够系统地控制诸如带宽、RF能量分布和持续时间等脉冲参数。一些应用，例如高磁场下的扩散加权成像（DWI），将受益于能够在遵守回波时间和比吸收率（SAR）限制的同时提供更高B1不敏感性的RF脉冲。在本研究中，采用绝热SLR算法生成6毫秒和4毫秒的180°半绝热RF脉冲，这些脉冲被用于替换两次重聚焦自旋回波（TRSE）扩散加权回波平面成像（DW - EPI）序列中的重聚焦脉冲，以创建两次重聚焦绝热自旋回波（TRASE）序列的两个版本。这两个版本针对绝热性和回波时间之间的不同权衡进行设计。由于应用了一对相同的重聚焦脉冲，第一个脉冲施加的二次相位被第二个脉冲解缠，从而保留了由激发脉冲产生的线性相位。在7特斯拉（7T）下获得的人脑活体图像表明，本研究中开发的TRASE序列的两个版本在扩散加权图像中均比传统TRSE序列实现了更均匀的信号。半绝热SLR脉冲为7T下的扩散准备提供了一种对B1更不敏感的解决方案，同时在SAR限制范围内运行。该方法可与任何EPI读出轨迹和平行成像方案相结合，为7T和3T下的扩散张量成像提供更均匀的覆盖。
