Cao Zhipeng, Donahue Manus J, Ma Jun, Grissom William A
Vanderbilt University Institute of Imaging Science, Nashville, Tennessee, USA.
Department of Biomedical Engineering, Vanderbilt University, Nashville, Tennessee, USA.
Magn Reson Med. 2016 Mar;75(3):1198-208. doi: 10.1002/mrm.25739. Epub 2015 Apr 27.
To design multichannel large-tip-angle kT-points and spokes radiofrequency (RF) pulses and gradient waveforms for transmit field inhomogeneity compensation in high field magnetic resonance imaging.
An algorithm to design RF subpulse weights and gradient blip areas is proposed to minimize a magnitude least-squares cost function that measures the difference between realized and desired state parameters in the spin domain, and penalizes integrated RF power. The minimization problem is solved iteratively with interleaved target phase updates, RF subpulse weights updates using the conjugate gradient method with optimal control-based derivatives, and gradient blip area updates using the conjugate gradient method. Two-channel parallel transmit simulations and experiments were conducted in phantoms and human subjects at 7 T to demonstrate the method and compare it to small-tip-angle-designed pulses and circularly polarized excitations.
The proposed algorithm designed more homogeneous and accurate 180° inversion and refocusing pulses than other methods. It also designed large-tip-angle pulses on multiple frequency bands with independent and joint phase relaxation. Pulses designed by the method improved specificity and contrast-to-noise ratio in a finger-tapping spin echo blood oxygen level dependent functional magnetic resonance imaging study, compared with circularly polarized mode refocusing.
A joint RF and gradient waveform design algorithm was proposed and validated to improve large-tip-angle inversion and refocusing at ultrahigh field.
设计多通道大翻转角kT点和辐条射频（RF）脉冲以及梯度波形，用于高场磁共振成像中发射场不均匀性补偿。
提出一种设计RF子脉冲权重和梯度尖峰面积的算法，以最小化一个幅度最小二乘代价函数，该函数测量自旋域中实际状态参数与期望状态参数之间的差异，并对积分RF功率进行惩罚。通过交错目标相位更新、使用基于最优控制导数的共轭梯度法更新RF子脉冲权重以及使用共轭梯度法更新梯度尖峰面积，迭代求解最小化问题。在7T的体模和人体受试者中进行了两通道并行发射模拟和实验，以演示该方法并将其与小翻转角设计的脉冲和圆极化激励进行比较。
与其他方法相比，所提出的算法设计出了更均匀、更精确的180°反转和重聚焦脉冲。它还在具有独立和联合相位弛豫的多个频带上设计了大翻转角脉冲。在一项手指轻敲自旋回波血氧水平依赖性功能磁共振成像研究中，与圆极化模式重聚焦相比，该方法设计的脉冲提高了特异性和对比噪声比。
提出并验证了一种联合RF和梯度波形设计算法，以改善超高场下的大翻转角反转和重聚焦。
