Information Systems and Radiological Sciences Laboratories, Stanford University, Stanford, CA 94305, USA.
IEEE Trans Med Imaging. 2009 Oct;28(10):1548-59. doi: 10.1109/TMI.2009.2020064. Epub 2009 May 12.
Large-tip-angle multidimensional radio-frequency (RF) pulse design is a difficult problem, due to the nonlinear response of magnetization to applied RF at large tip-angles. In parallel excitation, multidimensional RF pulse design is further complicated by the possibility for transmit field patterns to change between subjects, requiring pulses to be designed rapidly while a subject lies in the scanner. To accelerate pulse design, we introduce a fast version of the optimal control method for large-tip-angle parallel excitation. The new method is based on a novel approach to analytically linearizing the Bloch equation about a large-tip-angle RF pulse, which results in an approximate linear model for the perturbations created by adding a small-tip-angle pulse to a large-tip-angle pulse. The linear model can be evaluated rapidly using nonuniform fast Fourier transforms, and we apply it iteratively to produce a sequence of pulse updates that improve excitation accuracy. We achieve drastic reductions in design time and memory requirements compared to conventional optimal control, while producing pulses of similar accuracy. The new method can also compensate for nonidealities such as main field inhomogeneties.
大角度多维射频（RF）脉冲设计是一个难题，因为在大角度时，磁化对施加的 RF 的非线性响应。在并行激发中，多维 RF 脉冲设计变得更加复杂，因为在不同的受试者之间可能会改变发射场模式，这要求在受试者躺在扫描仪中时快速设计脉冲。为了加速脉冲设计，我们引入了一种针对大角度并行激发的快速最优控制方法。该新方法基于一种新的方法，该方法在大角度 RF 脉冲周围对 Bloch 方程进行解析线性化，从而为在大角度脉冲上添加小角度脉冲而产生的微小脉冲创建了一个近似线性模型。可以使用非均匀快速傅立叶变换快速评估线性模型，并且我们将其迭代应用以产生一系列改进激发准确性的脉冲更新。与传统的最优控制相比，我们实现了设计时间和内存要求的急剧减少，同时产生了类似精度的脉冲。该新方法还可以补偿主磁场不均匀等非理想因素。
