Department of Radiology, Brigham and Women's Hospital, Harvard Medical School, Boston, MA, United States of America.
Phys Med Biol. 2018 May 10;63(10):105001. doi: 10.1088/1361-6560/aabe37.
Transcranial focused ultrasound (tFUS) is emerging as a non-invasive brain stimulation modality. Complicated interactions between acoustic pressure waves and osseous tissue introduce many challenges in the accurate targeting of an acoustic focus through the cranium. Image-guidance accompanied by a numerical simulation is desired to predict the intracranial acoustic propagation through the skull; however, such simulations typically demand heavy computation, which warrants an expedited processing method to provide on-site feedback for the user in guiding the acoustic focus to a particular brain region. In this paper, we present a multi-resolution simulation method based on the finite-difference time-domain formulation to model the transcranial propagation of acoustic waves from a single-element transducer (250 kHz). The multi-resolution approach improved computational efficiency by providing the flexibility in adjusting the spatial resolution. The simulation was also accelerated by utilizing parallelized computation through the graphic processing unit. To evaluate the accuracy of the method, we measured the actual acoustic fields through ex vivo sheep skulls with different sonication incident angles. The measured acoustic fields were compared to the simulation results in terms of focal location, dimensions, and pressure levels. The computational efficiency of the presented method was also assessed by comparing simulation speeds at various combinations of resolution grid settings. The multi-resolution grids consisting of 0.5 and 1.0 mm resolutions gave acceptable accuracy (under 3 mm in terms of focal position and dimension, less than 5% difference in peak pressure ratio) with a speed compatible with semi real-time user feedback (within 30 s). The proposed multi-resolution approach may serve as a novel tool for simulation-based guidance for tFUS applications.
经颅聚焦超声（tFUS）作为一种非侵入性脑刺激方式正在兴起。声波压力波与骨组织之间复杂的相互作用给通过颅骨准确靶向声焦点带来了许多挑战。希望通过图像引导和数值模拟来预测颅骨内的颅内声传播；然而，这种模拟通常需要大量的计算，这就需要一种快速的处理方法，以便为用户提供现场反馈，指导声焦点到达特定的脑区。在本文中，我们提出了一种基于有限差分时域公式的多分辨率模拟方法，用于模拟来自单个元件换能器（250 kHz）的经颅声波传播。多分辨率方法通过提供调整空间分辨率的灵活性来提高计算效率。通过图形处理单元的并行化计算，也可以加速模拟。为了评估该方法的准确性，我们通过不同的超声入射角测量了离体羊颅骨中的实际声场。将测量的声场与模拟结果在焦点位置、尺寸和压力水平方面进行了比较。还通过比较不同分辨率网格设置组合下的模拟速度来评估所提出方法的计算效率。由 0.5 和 1.0 毫米分辨率组成的多分辨率网格具有可接受的精度（焦点位置和尺寸的偏差在 3 毫米以内，峰值压力比的差异小于 5%），并且与半实时用户反馈兼容（在 30 秒内）。所提出的多分辨率方法可以作为基于模拟的 tFUS 应用引导的新工具。
