Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.
Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, Massachusetts, USA.
Magn Reson Med. 2023 Dec;90(6):2572-2591. doi: 10.1002/mrm.29827. Epub 2023 Sep 5.
Developing a general framework with a novel stochastic offset strategy for the design of optimized RF pulses and time-varying spatially non-linear ΔB shim array fields for restricted slice excitation and refocusing with refined magnetization profiles within the intervals of the fixed voxels.
Our framework uses the decomposition property of the Bloch equations to enable joint design of RF-pulses and shim array fields for restricted slice excitation and refocusing with auto-differentiation optimization. Bloch simulations are performed independently on orthogonal basis vectors, Mx, My, and Mz, which enables designs for arbitrary initial magnetizations. Requirements for refocusing pulse designs are derived from the extended phase graph formalism obviating time-consuming sub-voxel isochromatic simulations to model the effects of crusher gradients. To refine resultant slice-profiles because of voxelwise optimization functions, we propose an algorithm that stochastically offsets spatial points at which loss is computed during optimization.
We first applied our proposed design framework to standard slice-selective excitation and refocusing pulses in the absence of non-linear ΔB shim array fields and compared them against pulses designed with Shinnar-Le Roux algorithm. Next, we demonstrated our technique in a simulated setup of fetal brain imaging in pregnancy for restricted-slice excitation and refocusing of the fetal brain.
Our proposed framework for optimizing RF pulse and time-varying spatially non-linear ΔB shim array fields achieve high fidelity restricted-slice excitation and refocusing for fetal MRI, which could enable zoomed fast-spin-echo-MRI and other applications.
开发一种通用框架，提出一种新的随机偏移策略，用于设计优化的射频脉冲和时变空间非线性 ΔB 匀场阵列场，以在固定体素间隔内对受限切片进行激发和重聚焦，并在磁化率分布内实现精细的磁化率分布。
我们的框架利用布洛赫方程的分解特性，通过自动微分优化，实现了 RF 脉冲和匀场阵列场的联合设计，用于受限切片的激发和重聚焦。布洛赫模拟分别在正交基矢量 Mx、My 和 Mz 上进行，这使得任意初始磁化率的设计成为可能。从扩展相位图形式主义中推导出重聚焦脉冲设计的要求，避免了耗时的亚体素等频线模拟来模拟破碎机梯度的影响。为了因为体素优化函数而细化得到的切片轮廓，我们提出了一种算法，即在优化过程中随机偏移计算损耗的空间点。
我们首先将我们提出的设计框架应用于不存在非线性 ΔB 匀场阵列场的标准切片选择激发和重聚焦脉冲，并将其与 Shinnar-Le Roux 算法设计的脉冲进行了比较。接下来，我们在妊娠胎儿脑成像的模拟设置中展示了我们的技术，用于胎儿脑的受限切片激发和重聚焦。
我们提出的用于优化射频脉冲和时变空间非线性 ΔB 匀场阵列场的框架实现了高保真的胎儿 MRI 受限切片激发和重聚焦，这可能使变焦快速自旋回波-MRI 和其他应用成为可能。
