Eckersley Robert J, Tang Meng-Xing, Chetty Kevin, Hajnal Joseph V
Imaging Sciences Department, Imperial College London, London, UK.
Ultrasound Med Biol. 2007 Nov;33(11):1787-95. doi: 10.1016/j.ultrasmedbio.2007.05.015. Epub 2007 Jul 16.
Real-time visualization of microbubbles in the microvasculature of deep tissues remains a challenge for existing nonlinear microbubble imaging techniques. A technique with high sensitivity to nonlinear signals is required to compensate for the effects of limited power used to avoid bubble destruction and the high attenuation of the overlying tissues for deeper targets. The use of coded pulses in ultrasound imaging is well established as a means of improving the signal-to-noise ratio (SNR) within B-mode ultrasound imaging, but the feasibility of this approach for detecting microbubbles has not been well studied. In this work we investigate the use of binary phase encoding together with phase and amplitude modulation (PIAM) for the detection of nonlinear signals from microbubbles. A series of simulation experiments were conducted using a modified Rayleigh-Plesset model together with Golay and Barker coding techniques to investigate (i) the ability of binary encoded PIAM to detect nonlinear signals, (ii) the effect of the SNR and insonating pressure on the detection process, (iii) the sensitivity of different pulse encoding approaches and (iv) the effects of bubble resonance behavior on the detection process. The results show that the binary encoding approach combined with PIAM is able to detect nonlinear signals from microbubbles. It was found that nonlinear scattering from the microbubbles degrades the sensitivity of the binary encoded approach such that at high SNR there is no advantage in using these pulses over existing short-pulse PIAM. However, at lower SNR (<20 dB) the increased pulse length provides improved sensitivity without significant loss of spatial resolution, even under conditions in which the detection failed completely for existing approaches. The results also show that both the insonating acoustic pressure and resonance behavior of bubbles have an effect on the detection sensitivity and spatial resolution for the binary encoded approach.
对于现有的非线性微泡成像技术而言，在深部组织的微血管中实现微泡的实时可视化仍是一项挑战。需要一种对非线性信号具有高灵敏度的技术，以补偿为避免气泡破坏而使用的有限功率的影响，以及针对更深目标的上层组织的高衰减。在超声成像中使用编码脉冲作为提高B模式超声成像中信噪比（SNR）的一种手段已得到充分确立，但这种方法用于检测微泡的可行性尚未得到充分研究。在这项工作中，我们研究了使用二进制相位编码以及相位和幅度调制（PIAM）来检测来自微泡的非线性信号。使用改进的瑞利 - 普莱斯模型以及格雷码和巴克码编码技术进行了一系列模拟实验，以研究（i）二进制编码的PIAM检测非线性信号的能力，（ii）SNR和入射压力对检测过程的影响，（iii）不同脉冲编码方法的灵敏度，以及（iv）气泡共振行为对检测过程的影响。结果表明，二进制编码方法与PIAM相结合能够检测来自微泡的非线性信号。研究发现，微泡的非线性散射会降低二进制编码方法的灵敏度，以至于在高SNR时，使用这些脉冲相对于现有的短脉冲PIAM并无优势。然而，在较低的SNR（<20 dB）时，增加的脉冲长度可提高灵敏度，且不会显著损失空间分辨率，即使在现有方法完全无法检测的条件下也是如此。结果还表明，气泡的入射声压和共振行为均会对二进制编码方法的检测灵敏度和空间分辨率产生影响。
