针对限制幅值磁共振脉冲的优化设计

在磁共振脉冲优化领域,优化脉冲普遍存在幅值过大的问题,这极大地限制了优化脉冲的使用范围。为了限制优化脉冲的幅值,扩大其应用范围,提出了一种基于L-BFGS-B数值算法的脉冲优化设计方法。首先基于Liouville-von Neuman方程,使用最优控制思想构建优化模型；然后使用L-BFGS-B算法,在限制幅值的条件下对优化模型进行数值迭代求解；最后以脉冲的激发效率以及激发轮廓的均匀性作为衡量优化脉冲优劣的标准对该方法进行仿真和实验验证。结果表明,采用该方法获得的优化脉冲在幅值被限制的前提下,仍能获得较传统磁共振脉冲更好的共振激发效果,进而增强信号的灵敏度,提高图像的质量。

Optimal Pulse Design with Limited radiofrequency Amplitude in Nuclear Magnetic Resonance

In the field of MR (magnetic resonance) pulse optimization, the amplitude of optimal pulse is pervasively excessive, which greatly limits the use of optimized pulse. In order to limit the amplitude of the optimal pulse, expanding its scope of application, a new L-BFGS-B numerical algorithm based optimization method is proposed. First, based on Liouville-von Neuman equation, using the optimal control theory to build optimization models; then use the L-BFGS-B algorithm to calculation of optimization model, within the limits of the magnitude of amplitude; finally, verify this method by simulation and experiment, where the excitation efficiency and uniformity are the standard to judge the optimal pulse. Simulation and experiment results indicate that the optimized pulse with limited RF amplitude can obtain better excitation profile, compared with the conventional rectangle pulses, thus enhancing the sensitivity of the signal and improving the quality of the image.