Department of Electronics and Telecommunications, Universita degli Studi di Firenze, Florence, Italy.
IEEE Trans Ultrason Ferroelectr Freq Control. 2010 Jan;57(1):193-202. doi: 10.1109/TUFFC.2010.1398.
Ultrasound contrast agents (UCA) populations are typically polydisperse and contain microbubbles with radii over a given range. Although the behavior of microbubbles of certain sizes might be masked by the behavior of others, the acoustic characterization of UCA is typically made on full populations. In this paper, we have combined acoustic and optical methods to investigate the response of isolated lipid-shelled microbubbles to low-pressure (49 and 62 kPa peak negative pressure) ultrasound tone bursts. These bursts induced slow deflation of the microbubbles. The experimental setup included a microscope connected to a fast camera acquiring one frame per pulse transmitted by a single-element transducer. The behavior of each bubble was measured at multiple frequencies, by cyclically changing the transmission frequency over the range of 2 to 4 MHz during subsequent pulse repetition intervals. The bubble echoes were captured by a second transducer and coherently recorded. More than 50 individual microbubbles were observed. Microbubbles with radii larger than 3 mum did not experience any size reduction. Smaller bubbles slowly deflated, generally until the radius reached a value around 1.4 microm, independent of the initial microbubble size. The detected pressure amplitude backscattered at 2.5 cm distance was very low, decreasing from about 5 Pa down to 1 Pa at 2 MHz as the bubbles deflated. The resonant radius was evaluated from the echo amplitude normalized with respect to the geometrical cross section. At 2-MHz excitation, deflating microbubbles showed highest normalized echo when the radius was 2.2 microm while at higher excitation frequencies, the resonant radius was lower. The relative phase shift of the echo during the deflation process was also measured. It was found to exceed pi/2 in all cases. A heuristic procedure based on the analysis of multiple bubbles of a same population was used to estimate the undamped natural frequency. It was found that a microbubble of 1.7 microm has an undamped natural frequency of 2 MHz. The difference between this size and the resonant radius is discussed as indicative of significant damping.
超声造影剂（UCA）通常是多分散的，包含半径在给定范围内的微泡。尽管某些尺寸的微泡的行为可能会被其他微泡的行为所掩盖，但 UCA 的声学特性通常是在整个群体中进行的。在本文中，我们结合了声学和光学方法来研究孤立的脂质壳微泡对低压（49 和 62 kPa 峰值负压）超声脉冲的响应。这些脉冲会导致微泡缓慢放气。实验装置包括一个显微镜，显微镜连接到一个高速摄像机，每秒拍摄一个脉冲，由单个换能器传输。通过在随后的脉冲重复间隔期间周期性地将传输频率在 2 到 4 MHz 的范围内改变，对每个气泡的行为进行了多次频率测量。第二个换能器捕获气泡回波并进行相干记录。观察到超过 50 个单个微泡。半径大于 3 µm 的微泡没有经历任何尺寸减小。较小的气泡缓慢放气，通常直到半径达到约 1.4 µm 的值，与初始微泡尺寸无关。在 2.5 cm 距离处检测到的背散射压力幅度非常低，随着气泡放气，从约 5 Pa 下降到 2 MHz 时的 1 Pa。从回波幅度归一化到几何横截面的角度评估了共振半径。在 2 MHz 激励下，放气微泡的归一化回波最大，当半径为 2.2 µm 时，而在较高的激励频率下，共振半径较低。在放气过程中还测量了回波的相对相移。在所有情况下，它都超过了 pi/2。使用基于同一群体中多个气泡的分析的启发式程序来估计无阻尼固有频率。发现 1.7 µm 的微泡的无阻尼固有频率为 2 MHz。该尺寸与共振半径之间的差异被认为是显著阻尼的指示。
