IEEE Trans Ultrason Ferroelectr Freq Control. 2018 Feb;65(2):178-187. doi: 10.1109/TUFFC.2017.2781190.
Contrast-enhanced ultrasound (CEUS) imaging has great potential for use in new ultrasound clinical applications such as myocardial perfusion imaging and abdominal lesion characterization. In CEUS imaging, contrast agents (i.e., microbubbles) are used to improve contrast between blood and tissue because of their high nonlinearity under low ultrasound pressure. However, the quality of CEUS imaging sometimes suffers from a low signal-to-noise ratio (SNR) in deeper imaging regions when a low mechanical index (MI) is used to avoid microbubble disruption, especially for imaging at off-resonance transmit frequencies. In this paper, we propose a new strategy of combining CEUS sequences with the recently proposed multiplane-wave (MW) compounding method to improve the SNR of CEUS in deeper imaging regions without increasing MI or sacrificing frame rate. The MW-CEUS method emits multiple Hadamard-coded CEUS pulses in each transmission event (i.e., pulse-echo event). The received echo signals first undergo fundamental bandpass filtering (i.e., the filter is centered on the transmit frequency) to eliminate the microbubble's second-harmonic signals because they cannot be encoded by pulse inversion. The filtered signals are then Hadamard decoded and realigned in fast time to recover the signals as they would have been obtained using classic CEUS pulses, followed by designed recombination to cancel the linear tissue responses. The MW-CEUS method significantly improved contrast-to-tissue ratio and SNR of CEUS imaging by transmitting longer coded pulses. The image resolution was also preserved. The microbubble disruption ratio and motion artifacts in MW-CEUS were similar to those of classic CEUS imaging. In addition, the MW-CEUS sequence can be adapted to other transmission coding formats. These properties of MW-CEUS can potentially facilitate CEUS imaging for many clinical applications, especially assessing deep abdominal organs or the heart.
超声造影（CEUS）成像在心肌灌注成像和腹部病变特征等新的超声临床应用中具有很大的应用潜力。在 CEUS 成像中，对比剂（即微泡）由于在低超声压力下具有高非线性，因此被用于提高血液和组织之间的对比度。然而，当使用低机械指数（MI）以避免微泡破裂时，CEUS 成像的质量在较深的成像区域中有时会受到低信噪比（SNR）的影响，尤其是在离频发射频率下进行成像时。在本文中，我们提出了一种新的策略，即将 CEUS 序列与最近提出的多平面波（MW）合成方法相结合，以在不增加 MI 或牺牲帧率的情况下提高 CEUS 在较深成像区域中的 SNR。MW-CEUS 方法在每次发射事件（即脉冲回波事件）中发射多个 Hadamard 编码的 CEUS 脉冲。接收的回波信号首先经过基本带通滤波（即滤波器位于发射频率中心），以消除微泡的二次谐波信号，因为它们不能通过脉冲反转进行编码。滤波后的信号随后进行 Hadamard 解码并在快速时间中重新对齐，以恢复使用经典 CEUS 脉冲获得的信号，然后进行设计重组以消除线性组织响应。MW-CEUS 方法通过发射更长的编码脉冲显著提高了 CEUS 成像的对比-组织比和 SNR。图像分辨率也得到了保留。MW-CEUS 中的微泡破裂比和运动伪影与经典 CEUS 成像相似。此外，MW-CEUS 序列可以适应其他发射编码格式。MW-CEUS 的这些特性可能有助于 CEUS 成像在许多临床应用中，特别是评估深部腹部器官或心脏。
