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首页> 外文期刊>Archives of acoustics >Thermal Effects Induced in Liver Tissues by Pulsed Focused Ultrasonic Beams from Annular Array Transducer
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Thermal Effects Induced in Liver Tissues by Pulsed Focused Ultrasonic Beams from Annular Array Transducer

机译：环形阵列换能器的脉冲聚焦超声束在肝脏组织中引起的热效应

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Many therapeutic applications of pulsed focused ultrasound are based on heating of detected lesions which may be localized in tissues at different depths under the skin. In order to concentrate the acoustic energy inside tissues at desired depths a new approach using a planar multi-element annular array transducer with an electronically adjusted time-delay of excitation of its elements, was proposed. The 7-elements annular array transducer with 2.4 MHz center operating frequency and 20 mm outer diameter was produced. All its elements (central disc and 6 rings) had the same radiating area. The main purpose of this study was to investigate thermal fields induced in bovine liver in vitro by pulsed focused ultrasonic beams with various acoustic properties and electronically steered focal plane generated from the annular array transducer used. The measurements were performed for the radiating beams with the 20 mm focal depth. In order to maximize nonlinear effects introducing the important local temperature rise, the measurements have been performed in two-layer media comprising of a water layer, whose thickness was specific for the transducer used and equal to 13 mm, and the second layer of a bovine liver with a thickness of 20 mm. The thickness of the water layer was determined numerically as the axial distance where the amplitude of the second harmonics started to increase rapidly. The measurements of the temperature rise versus time were performed using a thermocouple placed inside the liver at the focus of the beam. The temperature rise induced in the bovine liver in vitro by beams with the average acoustic power of 1 W, 2 W and 3 W and duty cycle of 1/5, 1/15 and 1/30, respectively, have been measured. For each beam used the exposure time needed for the local tissue heating to the temperature of 43℃ (used in therapies based on ultrasonic enhancement of drug delivery or in therapies involving stimulation of immune system by enhancement of the heat shock proteins expression) and to the temperature of 56℃ (used in HIFU therapies) was determined. Two sets of measurements were done for each beam considered. First, the thermocouple measurement of the temperature rise was done and next, the real-time monitoring of dynamics of growth of the necrosis area by using ultrasonic imaging technique, while the sample was exposed to the same acoustic beam. It was found that the necrosis area becomes visible in the ultrasonic image only for beams with the average acoustic power of 3 W, although after cutting the sample the thermo ablated area was visible with the naked eye even for the beams with lower acoustic power. The quantitative analysis of the obtained results allowed to determine the exposure time needed to get the necrosis area visible in the ultrasonic image.
机译：脉冲聚焦超声的许多治疗应用都是基于加热检测到的病变，这些病变可能位于皮肤下不同深度的组织中。为了将声能集中在组织内所需的深度，提出了一种使用平面多元件环形阵列换能器的新方法，该换能器具有电子调节其元素的激励延时。生产了具有2.4 MHz中心工作频率和20 mm外径的7元件环形阵列换能器。它的所有元素（中心盘和6个环）具有相同的辐射面积。这项研究的主要目的是研究由具有各种声学特性的脉冲聚焦超声束和由使用的环形阵列换能器产生的电子控制焦平面在体外在牛肝中引起的热场。对焦点深度为20 mm的辐射束进行测量。为了使引入重要的局部温度升高的非线性效应最大化，已经在两层介质中进行了测量，该介质包括水层和第二层牛，该水层的厚度对于所使用的传感器而言是特定的，等于13 mm，第二层是牛肝脏厚度为20毫米。将水层的厚度数值确定为二次谐波的振幅开始迅速增加的轴向距离。使用放置在光束焦点处肝脏内部的热电偶进行温度升高随时间的测量。已经测量了平均声功率分别为1 W，2 W和3 W以及占空比分别为1 / 5、1 / 15和1/30的光束在牛肝中引起的温升。对于使用的每个光束，将局部组织加热至43℃的温度（用于基于超声增强药物递送的疗法或涉及通过增强热休克蛋白表达来刺激免疫系统的疗法中）使用的暴露时间。确定温度为56℃（用于HIFU治疗）。对所考虑的每个光束进行了两组测量。首先，进行温度升高的热电偶测量，其次，当样品暴露于相同的声束时，通过使用超声成像技术实时监测坏死区域的生长动态。已发现，仅对于平均声功率为3 W的光束，坏死区域才在超声图像中可见，尽管在切割样品后，即使对于声功率较低的光束，肉眼也可以看到热烧蚀区域。对获得的结果进行定量分析，可以确定使坏死区域在超声图像中可见所需的曝光时间。

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