频率对高强度聚焦超声在层状生物组织中形成损伤的影响研究

发布时间：2018-01-20 12:52

本文关键词： 高强度聚焦超声层状组织热损伤频率选择　出处：《重庆医科大学》2015年硕士论文　论文类型：学位论文

【摘要】：高强度聚焦超声(High Intensity Focused Ultrasound, HIFU)是一种非侵入式的新型治疗技术,目前己应用于多种肿瘤以及非肿瘤疾病的治疗。HIFU治疗依赖于组织对超声能量的吸收,但更大的组织声吸收也意味着更快的衰减和有限的组织穿透距离。因此,在临床治疗深部的肿瘤时,选择合适的超声频率以平衡组织热吸收效率和超声穿透距离这两个因素,从而达到最大的组织热损伤是一个非常重要的问题。本文基于KZK方程和分层介质模型计算了超声在层状生物组织中的非线性传播,利用HIFU换能器声场扫描实验来验证计算结果。基于Pennes生物热传导方程求解了组织中的温度分布,并考虑组织声速和衰减系数随温度的变化；利用等效热剂量模型计算组织中的热剂量,并选择30EM作为产生热损伤的阈值。利用HIFU辐照离体牛肝组织形成凝固性坏死的实验来验证数值计算热损伤范围的方法。本文计算模型中使用开口外径22 cm内径8 cm焦距16 cm的球壳式自聚焦换能器,在声输出总功率为400 W条件下,声束透过皮肤、脂肪、肌肉三层组织后聚焦于靶区肝组织中。选取了不同的靶区深度即焦点到皮肤表面深度(5,7,10,15 cm),不同的超声频率(0.4,0.6,0.8,1.0,1.2,1.4,1.6MHz),计算了每个靶区深度条件下不同的超声频率对于焦域尺寸、最大声强、最大热吸收率和损伤体积的影响,分析了这些量随频率变化的原因。同时还计算了焦点处谐波滋生系数和机械指数随频率的变化,分析了机械效应对于组织损伤的影响。声场扫描实验结果与仿真计算结果吻合较好；离体牛肝实验中,在靶区声强较低的情况下,实验结果与仿真计算吻合较好,在靶区声强较高的情况下,实验结果大于仿真计算,这是由于本文仿真计算模型未考虑空化和微泡对于组织机械及热损伤的贡献。对于同一靶区深度,随着超声频率的增加,焦域-6dB尺寸剧烈减小；对于同一超声频率,随着靶区深度的增加,焦域-6dB尺寸几乎不变。随着超声频率的增加,焦域处最大声强、最大热吸收率、损伤体积均出现了先增大后减小；对于不同靶区深度,达到各个量最大值的最优频率均不一样。在不同靶区深度(5,7,10,15 cm)达到最大的热吸收的最佳频率分别为1.5 MHz、1.3 MHz.1.1 MHz和0.9MHz,达到最大损伤体积的最佳频率分别为0.8 MHz、0.7 MHz.0.7 MHz和0.6 MHz。随着超声频率的增加,谐波滋生系数先增大后减小,在某一频率取得最大值；机械指数线性下降,发生空化的几率减小。综合考虑靶区组织的热吸收效率、组织热损伤体积和机械指数等因素,在治疗深部肿瘤时,需要选择较低的超声频率,以获得较大的热损伤和机械作用,且这个超声频率可以使用仿真模型计算得到。
[Abstract]:High intensity focused ultrasound (HIFU) is a new non-invasive treatment technique. At present, HIFU has been used in the treatment of various tumor and non-tumor diseases. HIFU therapy depends on the absorption of ultrasonic energy by tissues. But greater tissue acoustic absorption also means faster attenuation and limited tissue penetration. Therefore, in clinical treatment of deep tumors. The appropriate ultrasonic frequency was chosen to balance the heat absorption efficiency of tissue and the ultrasonic penetration distance. In this paper, the nonlinear propagation of ultrasound in layered biological tissue is calculated based on KZK equation and layered medium model. The HIFU transducer acoustic field scanning experiment is used to verify the calculation results. The temperature distribution in the tissue is solved based on the Pennes biological heat conduction equation, and the variation of the tissue sound velocity and attenuation coefficient with the temperature is considered. The equivalent heat dose model was used to calculate the heat dose in the tissue. 30EM was chosen as the threshold to produce thermal damage. The method of numerical calculation of thermal damage range was verified by using HIFU irradiation in vitro bovine liver tissue to form coagulant necrosis. In this paper, the open end was used in the calculation model. Diameter 22. A spherical shell type self-focusing transducer with an internal diameter of 8 cm and a focal length of 16 cm. When the total power of sound output is 400 W, the sound beam passes through the skin, fat and muscle and then focuses on the liver tissue of the target area. The different target depth, that is, the focal point to the skin surface depth, is selected. 10 ~ 15 cm ~ (-1), with different ultrasonic frequencies of 0.4 ~ (0.6) ~ 0.81.0 ~ 1.2m ~ (1. 4) ~ (1.6) MHz). The effects of different ultrasonic frequencies on focal area size, maximum sound intensity, maximum heat absorption rate and damage volume of each target were calculated. The reasons for the variation of these quantities with frequency are analyzed, and the variation of harmonic breeding coefficient and mechanical index with frequency are also calculated. The effect of mechanical effect on tissue damage is analyzed. In the isolated bovine liver experiment, the experimental results are in good agreement with the simulation under the condition of low sound intensity in the target area, and the experimental results are larger than the simulation calculation in the case of higher sound intensity in the target area. This is due to the fact that the contribution of cavitation and microbubbles to the mechanical and thermal damage is not taken into account in this simulation model. For the same target depth, the focal region -6dB size decreases dramatically with the increase of ultrasonic frequency. For the same ultrasonic frequency, with the increase of the depth of the target area, the focal region -6dB size is almost unchanged. With the increase of the ultrasonic frequency, the maximum sound intensity and the maximum heat absorption rate are obtained at the focal region. The damage volume increased first and then decreased. For different target depth, the optimal frequency of reaching the maximum value of each quantity is different. The optimum frequency of thermal absorption is 1. 5 MHz and 1. 3 MHz.1.1 MHz, respectively. The optimum frequencies to reach the maximum damage volume were 0. 8 MHz 0. 7 MHz.0.7 MHz and 0. 6 MHz, respectively, with the increase of ultrasonic frequency. The harmonic breeding coefficient first increases and then decreases, and reaches the maximum value at a certain frequency. The mechanical index decreases linearly, and the probability of cavitation decreases. Considering the heat absorption efficiency of target tissue, tissue thermal damage volume and mechanical index, etc., in the treatment of deep tumor. It is necessary to select a lower ultrasonic frequency to obtain larger thermal damage and mechanical action, and this ultrasonic frequency can be calculated by using the simulation model.
【学位授予单位】：重庆医科大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：R454.3

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