Department of Biomedical Engineering, Duke University, Durham, North Carolina 27708, USA.
J Acoust Soc Am. 2018 May;143(5):2801. doi: 10.1121/1.5036733.
Ultrasound beamforming relies on models of propagation to convert samples of the backscattered field through time into spatial image samples. The most common model is straight-line propagation of a focused wave, assuming a narrow steered and focused beam that propagates along a selected direction. The reconstructed image suffers from defocusing, reduced signal-to-noise ratio (SNR), and contrast loss away from the focus. "Virtual source" methods coherently combine the recorded data from multiple transmissions to form a synthetic transmit focus by making geometric assumptions about the transmissions. These also include diverging waves (virtual source behind the array) and plane waves (virtual source at infinity). Retrospective encoding for conventional ultrasound sequences (REFoCUS) beamforming has been proposed to instead model transmission as the superposition of the responses of individual transmit elements on the transducer array and to efficiently estimate the "complete data set"-individual element transmit and receive responses. In addition to isolating individual element contributions, the result of this unifying framework is a high-SNR, two-way focused image from focused plane wave or diverging transmissions. No significant differences were observed for either SNR or image quality measured by contrast-to-noise ratio between the appropriate virtual source method and REFoCUS beamforming in simulation and experimental imaging.
超声束形成依赖于传播模型，将反向散射场的样本通过时间转换为空间图像样本。最常见的模型是聚焦波的直线传播，假设窄的定向和聚焦波束沿着选定的方向传播。重建图像会出现散焦、信号噪声比（SNR）降低以及远离焦点的对比度损失。“虚拟源”方法通过对传输进行几何假设，相干地组合来自多个传输的记录数据，通过对传输进行几何假设，形成合成发射焦点。这些假设还包括发散波（阵列后面的虚拟源）和平行波（无穷远处的虚拟源）。已经提出了用于常规超声序列的回溯编码（REFoCUS）波束形成，以将传输建模为换能器阵列上各个发射元件响应的叠加，并有效地估计“完整数据集”-各个元件的发射和接收响应。除了隔离单个元件的贡献之外，该统一框架的结果是来自聚焦平面波或发散传输的高 SNR、双向聚焦图像。在模拟和实验成像中，通过对比噪声比测量，适当的虚拟源方法和 REFoCUS 波束形成在 SNR 或图像质量方面没有观察到显著差异。
