Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Hvidovre, Hvidovre, Denmark. Center for Magnetic Resonance, Department of Health Technology, Technical University of Denmark, Kgs. Lyngby, Denmark. These authors contributed equally to the work.
J Neural Eng. 2020 Jul 13;17(4):046010. doi: 10.1088/1741-2552/ab98dc.
Low-intensity transcranial ultrasound stimulation (TUS) is emerging as a non-invasive brain stimulation technique with superior spatial resolution and the ability to reach deep brain areas. Medical image-based computational modeling could be an important tool for individualized TUS dose control and targeting optimization, but requires further validation. This study aims to assess the impact of the transducer model on the accuracy of the simulations.
Using hydrophone measurements, the acoustic beam of a single-element focused transducer (SEFT) with a flat piezoelectric disc and an acoustic lens was characterized. The acoustic beam was assessed in a homogeneous water bath and after transmission through obstacles (3D-printed shapes and skull samples). The acoustic simulations employed the finite-difference time-domain method and were informed by computed tomography (CT) images of the obstacles. Transducer models of varying complexity were tested representing the SEFT either as a surface boundary condition with variable curvature or also accounting for its internal geometry. In addition, a back-propagated pressure distribution from the first measurement plane was used as source model. The simulations and measurements were quantitatively compared using key metrics for peak location, focus size, intensity and spatial distribution.
While a surface boundary with an adapted, 'effective' curvature radius based on the specifications given by the manufacturer could reproduce the measured focus location and size in a homogeneous water bath, it regularly failed to accurately predict the beam after obstacle transmission. In contrast, models that were based on a one-time calibration to the homogeneous water bath measurements performed substantially better in all cases with obstacles. For one of the 3D-printed obstacles, the simulated intensities deviated substantially from the measured ones, irrespective of the transducer model. We attribute this finding to a standing wave effect, and further studies should clarify its relevance for accurate simulations of skull transmission.
Validated transducer models are important to ensure accurate simulations of the acoustic beam of SEFTs, in particular in the presence of obstacles such as the skull.
低强度经颅超声刺激（TUS）作为一种具有优越空间分辨率和能够到达深部脑区的非侵入性脑刺激技术正在兴起。基于医学图像的计算建模可能是 TUS 剂量控制和靶向优化的重要工具，但需要进一步验证。本研究旨在评估换能器模型对模拟准确性的影响。
使用水听器测量，对具有平面压电盘和声透镜的单元素聚焦换能器（SEFT）的声束进行了表征。在均匀水浴中和经过障碍物（3D 打印形状和颅骨样本）传输后，评估了声束。声学模拟采用有限差分时域法，并根据障碍物的计算机断层扫描（CT）图像进行。测试了不同复杂程度的换能器模型，这些模型将 SEFT 表示为具有可变曲率的表面边界条件，或者还考虑了其内部几何形状。此外，还使用来自第一测量平面的反向传播压力分布作为源模型。使用关键指标（峰值位置、焦点大小、强度和空间分布）对模拟和测量结果进行定量比较。
虽然基于制造商给出的规格的适配“有效”曲率半径的表面边界可以在均匀水浴中重现测量的焦点位置和大小，但它经常无法准确预测障碍物传输后的光束。相比之下，基于对均匀水浴测量的一次性校准的模型在所有情况下表现都要好得多，无论障碍物如何。对于其中一个 3D 打印障碍物，无论换能器模型如何，模拟强度与测量强度都有很大偏差。我们将此发现归因于驻波效应，进一步的研究应阐明其对颅骨传输的准确模拟的相关性。
验证后的换能器模型对于确保 SEFT 声束的准确模拟非常重要，尤其是在存在颅骨等障碍物的情况下。
