球面自聚焦换能器声场的焦域温度分布估计

利用声场叠加原理,对大口径球面自聚焦换能器的声场分布进行了模拟,以声强下降一半为界,确定了球面自聚焦换能器辐照组织的主要加热区域。从热量传递概念出发,对焦域内的分布和温度随时间的变化做了合理估计。结果表明：焦域内组织温度分布与声场分布类似;在一定时间和区域,组织温度可以迅速上升23℃以上,基本满足治疗要求。     

组织温度分布的传热模型

高强度聚焦超声对组织加热时，正确估计组织内的声场及场点分布温度分布和变化是非常重要的，从热量传递概念出发，建立了组织的传热模型，用其对大口径自聚焦换能器的声场在聚焦区城内的温度变化进行估计，了３ｄＢ聚焦区城的大小。     

活塞聚焦换能器引起温度提升的研究

由于高强度聚焦超声的出现,活塞聚焦换能器引起的温度场日益受到重视.本研究基于生物热传导方程(Pennes方程)及高斯声束叠加法理论,给出了由活塞聚焦超声引起的温度提升理论模型,并计算温度场的横向分布.实验采用猪的肝脏组织和脂肪组织,采用活塞聚焦换能器加热的方法,用热电偶测量可组织内的温度提升,并将计算结果与实验结果进行了对比分析.结果表明:在弱聚焦且声源强度小于3.0 wW/cm2的情况下,此模型可准确预测生物组织的温度分布,生物组织温度的提升主要决定于线性声场,并且与换能器参数有关,温度的提升随着换能器的声功率、频率、半径的增加而增加,随焦距的增加而减小,随媒质的灌注长度的增加而增加.     

线阵B超的声场分布

线阵B超的成象区域都是在近中场,用简单的Fraunhofer近似研究组织中的声场分布是不恰当的。然而,整个阵列的激励声场是各单个阵元的激励声场的迭加,而且单个阵元的声场在大部分成象区域内都满足远场条件,可以用Fraunhofer近似处理得到较简单的声场分布函数。用单阵元分布函数迭加所得的换能器声场分布的数值计算,比计算经典的Fresnel积分的运算量要小得多。计算结果表明:换能器阵列在成象区域内的非聚焦声场与矩形平板换能器声场的近中场分布规律相近,适当引入延时可以使阵列换能器的声束有效会聚而实現电子聚焦。当换能器口径一定时,电子聚焦的最小焦距受阵元宽度限制。超声仪器对组织后向散射信号的接收是发射的逆过程,可以用相同的函数描述。同样,也可以通过适当的延时实現电子聚焦,从而进一步提高成象的分辨力。     

一种消除浅表组织病灶的微型线聚焦换能器声场的数值计算与仿真方法

目的:本文涉及一种消除浅表组织病灶的微型线聚焦超声声场的数值计算与仿真,通过计算得出声场的三维分布,同时研究柱面自聚焦换能器各个参数对声场的影响,以指导微型自聚焦换能器的设计。方法:本文通过建立三维直角坐标系,应用Rayleigh积分得出微型线聚焦超声换能器聚焦区域内每一点的声强表达式,运用C语言和matlab采用直接数值积分的方法对柱面聚焦超声换能器的声场进行了数值计算和仿真,得出声场的三维分布。结果:从声场的三维分布可以看出:按照本文给定的条件设计,微型线聚焦超声换能器展现出良好的聚焦性能,形成明显的焦域。和普通的柱面超声换能器相比,小型的柱面超声换能器形成的声场在z轴上的声场分布更均匀,x,y方向上衰减更快。结论:通过不同的参数对声场三维分布影响可以看出:微型线聚焦超声换能器的厚度,半径,张角和长度对聚焦的效果有明显影响。与给定的条件相比,选择较厚、半径和张角较大的换能器,能提高聚焦性能,而换能器的长度对聚焦效果影响不大。     

开口柱面换能器声场分析

目的:聚焦超声治疗妇科浅表组织疾病中存在聚焦换能器和定位探头同时放置的问题,在实际的治疗头设计中必须考虑放置B超探头对聚焦声场的影响,本文涉及一种开口柱面换能器聚焦超声声场的数值计算与仿真,通过计算得出声场的三维空间分布,同时研究柱面自聚焦换能器中不同开口形状、位置、面积大小对声场的影响,以指导开口换能器的实际设计。方法:本文通过建立三维直角坐标系,应用Rayleigh积分得出柱面换能器聚焦区域内每一点的声强表达式,运用C语言和matlab采用变步长辛卜生二重积分的方法对开口柱面聚焦超声换能器的声场进行了数值计算和仿真,得出声场的三维分布。结果:对开口的不同形状、大小、位置的声场分布仿真结果表明:不同开口形状都会使聚焦区域声压幅值下降,开口半径增大,声压表现为下降的趋势,Z轴向的声压在换能器的边缘较大,中间小。随着开口中心位置距原点的距离越大,Z轴向的声压分布越不均匀,而对聚焦位置影响不大。结论:通过不同开口参数对声场三维分布影响可以看出:柱面超声换能器的开口形状、位置、面积对聚焦区域的声压均有明显影响。与给定的条件相比,选择位置居中、面积较小、的换能器,能使聚焦性能得到提高,而换能器的开口形状对聚焦效果影响不大。     

用于磁声耦合刺激的64阵元相控阵超声换能器的设计

本研究基于磁声耦合刺激技术(transcranial magneto-acoustic stimulation,TMAS)的需求,设计了用于线性64阵元相控阵的超声换能器,探究相控阵超声换能器压电晶体宽度及间距,优化超声换能器聚焦声场分布.采用MATLAB软件建立相控阵换能器模型,仿真聚焦声场分布.搭建三维声场分布测试...     

矩形超声换能器冲激响应分析

采用冲激响应方法,不需作近换能器轴线区域或远场区域近似,就可完整分析超声换能器的辐射声场。声场中某观察点的速度势由该点的冲激响应和激励脉冲卷积得到。提出了一种矩形超声换能器冲激响应数值计算的通用表达式,克服了目前算法由于矩形几何形态复杂性造成的空间不连续,对冲激响应作脉宽-幅度变换,解决了小孔径换能器的远场数值计算问题。采用不同带宽的激励脉冲,计算了矩形换能器中心轴线声场分布,计算了特定深度冲激响应分布,结果和以往方法一致。     

高强度聚焦超声换能器声场的数值仿真与实验测量

目的 高强度聚焦超声(HIFU)肿瘤治疗过程中,HIFU换能器形成的声场分布是决定其治疗效果的关键因素之一,为了提高HIFU肿瘤治疗的安全性和可靠性,需要对HIFU换能器形成的声场进行预测.方法 采用时域有限差分(FDTD)法数值仿真水体内形成的声压分布与实验测量结果对比的研究方法,分析讨论了HIFU换能器在不同激励功...     

用于浅表肿瘤热疗的二元聚焦超声换能器及其热场分布

介绍一种由两个压电圆片振子（直径２５ｍｍ）构成前向聚焦声场（焦距约为１１４ｍｍ）的超声波加热装置。体模实验测量表明：经该装置持续加热（超声功率约２×４０Ｗ）１０～１５分钟后可在体模内达稳定温度分布,使换能器中心附近６ｍｍ×６ｍｍ×１０ｍｍ范围内达４１～４５℃有效热疗高温（起始环境温度为２０℃时）,较适用于浅表肿瘤的加热治疗。     

Comparison of modelled and observed in vivo temperature elevations induced by focused ultrasound: implications for treatment planning

Two numerical models for predicting the temperature elevations resulting from focused ultrasound heating of muscle tissue were tested against experimental data. Both models use the Rayleigh-Sommerfeld integral to calculate the pressure field from a source distribution. The first method assumes a source distribution derived from a uniformly radiating transducer whereas the second uses a source distribution obtained by numerically projecting pressure field measurements from an area near the focus backward toward the transducer surface. Both of these calculated ultrasound fields were used as heat sources in the bioheat equation to calculate the temperature elevation in vivo. Experimental results were obtained from in vivo rabbit experiments using eight-element sector-vortex transducers at 1.61 and 1.7 MHz and noninvasive temperature mapping with MRI. Results showed that the uniformly radiating transducer model over-predicted the peak temperature by a factor ranging from 1.4 to 2.8, depending on the operating mode. Simulations run using the back-projected sources were much closer to experimental values, ranging from 1.0 to 1.7 times the experimental results, again varying with mode. Thus, a significant improvement in the treatment planning can be obtained by using actual measured ultrasound field distributions in combination with backward projection.     

Optimal configuration of multiple-focused ultrasound transducers for external hyperthermia.

External ultrasound hyperthermia is considered to be a very flexible modality for heating deep-seated tumors owing to its penetration and focusing ability. However, using this flexibility requires that many complicated, interacting decisions be made to obtain optimal treatment. This paper presents the feasibility of arranging multiple-focused ultrasound transducers to produce an appropriate heating pattern for a specific treatment, based on the optimal scan parameters obtained from an optimization algorithm. The variable scan parameters of the heating system optimized are the transducer tilt and rotation angles, focal depth, scan radius, and output acoustical power. After obtaining the optimal scan parameters, multiple transducers are systematically arranged according to these scan parameters. Three-dimensional ultrasound power deposition and temperature distribution for a specific treatment are calculated for this multiple ultrasound transducer system. A more uniform temperature distribution in the treatment region for a large, highly perfused tumor can be achieved by scanning the system with respect to the central scan axis and/or swinging the transducers inwards and outwards. The maximum heating depth of focused ultrasound transducers used in this heating system is also studied. Simulation results demonstrate that the optimal arrangement of this multiple-focused ultrasound transducer system is highly promising for heating deep, large, and highly perfused tumors.     

Intraluminal ultrasound applicator compatible with magnetic resonance imaging "real-time" temperature mapping for the treatment of oesophageal tumours: an ex vivo study

High intensity ultrasound has shown considerable ability to produce precise and deep thermal coagulation necrosis. Focused, cylindrical, spherical or plane transducers have been used to induce high temperatures in tissues to coagulate proteins and kill cells. Recently magnetic resonance imaging (MRI) has been used, with extracorporeal or intracavitary focused transducers and cylindrical interstitial applicators, to monitor temperature distribution and provide feedback during heating procedures. If intraluminal applicators are used, the active part is in contact with the region of interest and it is essential to provide an accurate view of heat deposition and the extent of coagulation necrosis close to the transducer. The purpose of this study was to develop a 10 mm diameter intraluminal ultrasound applicator, designed to treat oesophageal cancers and compatible with MRI "real-time" temperature mapping. The active part of the ultrasound applicator, covered by a latex balloon, is a 15 X 8 mm2 plane transducer, which is in contact with the tumours during treatment. Each ultrasound exposure generates coagulation necrosis, in an area with the approximate shape of a rectangular parallelepiped up to 10 mm deep. When the exposures were repeated by rotating the applicator on its axis, sector-based or cylindrical volumes of necrosis could be produced, matching the shape of oesophageal cancers. Ex vivo trials were performed to demonstrate the applicator's compatibility with a clinical MRI scanner (1.5 T). MRI signals were acquired without any magnetic susceptibility distortion, even close to the applicator. Fast (0.72 images per second) 2D temperature mapping was performed during ultrasound exposure, using temperature-related proton resonance frequency shift at a resolution of 0.5 degrees C. Coagulation necrosis viewed with inversion recovery sequences, were in good agreement with the qualitative macroscopic observations made for the few cases tested in this study.     

凹球面聚焦超声换能器用于热疗时在人体内引起的瞬态温度场

本文引入了虚拟热源的概念,分析了边界条件对升温的影响及加热时间及温度场点位置的关系,详细研究了声源参数及生物组织特性参数对治疗区升温快慢的影响,比较了聚焦参数相同的册球面换在和高斯换能器声焦点的瞬态温度,所得结果对超声热疗的实施具有指导作用。     

An investigation of the flow dependence of temperature gradients near large vessels during steady state and transient tissue heating.

Temperature distributions measured during thermal therapy are a major prognostic factor of the efficacy and success of the procedure. Thermal models are used to predict the temperature elevation of tissues during heating. Theoretical work has shown that blood flow through large blood vessels plays an important role in determining temperature profiles of heated tissues. In this paper, an experimental investigation of the effects of large vessels on the temperature distribution of heated tissue is performed. The blood flow dependence of steady state and transient temperature profiles created by a cylindrical conductive heat source and an ultrasound transducer were examined using a fixed porcine kidney as a flow model. In the transient experiments, a 20 s pulse of hot water, 30 degrees C above ambient, heated the tissues. Temperatures were measured at selected locations in steps of 0.1 mm. It was observed that vessels could either heat or cool tissues depending on the orientation of the vascular geometry with respect to the heat source and that these effects are a function of flow rate through the vessels. Temperature gradients of 6 degrees C mm(-1) close to large vessels were routinely measured. Furthermore, it was observed that the temperature gradients caused by large vessels depended on whether the heating source was highly localized (i.e. a hot needle) or more distributed (i.e. external ultrasound). The gradients measured near large vessels during localized heating were between two and three times greater than the gradients measured during ultrasound heating at the same location, for comparable flows. Moreover, these gradients were more sensitive to flow variations for the localized needle heating. X-ray computed tomography data of the kidney vasculature were in good spatial agreement with the locations of all of the temperature variations measured. The three dimensional vessel path observed could account for the complex features of the temperature profiles. The flow dependences of the transient temperature profiles near large vessels during the pulsed experiments were consistent with the temperature distributions measured in the steady state experiments and provided unique insights into the process of convective heat transfer in tissues. Finally, it was shown that even for very short treatment times (3-20 s), large vessels had significant effects on the tissue temperature distributions.     

Investigation of a scanned cylindrical ultrasound system for breast hyperthermia

This paper investigates the feasibility of a scanned cylindrical ultrasound system for producing uniform heating from the central to the superficial portions of the breast or localized heating within the breast at a specific location. The proposed system consists of plane ultrasound transducer(s) mounted on a scanned cylindrical support. The breast was immersed in water and surrounded by this system during the treatment. The control parameters considered are the size of the transducer, the ultrasound frequency, the scan angle and the shifting distance between the axes of the breast and the system. Three-dimensional acoustical and thermal models were used to calculate the temperature distribution. Non-perfused phantom experiments were performed to verify the simulation results. Simulation results indicate that high frequency ultrasound could be used for the superficial heating, and the scan angle of the transducer could be varied to obtain an appropriate high temperature region to cover the desired treatment region. Low frequency ultrasound could be used for deep heating and the high temperature region could be moved by shifting the system. In addition, a combination of low and high frequency ultrasound could result in a portion treatment from the central to the superficial breast or an entire breast treatment. Good agreement was obtained between non-perfused experiments and simulation results. The findings of this study can be used to determine the effects of the control parameters of this system, as well as to select the optimal parameters for a specific treatment.     

Feasibility of using ultrasound for real-time tracking during radiotherapy

This study was designed to examine the feasibility of utilizing transabdominal ultrasound for real-time monitoring of target motion during a radiotherapy fraction. A clinical Acuson 128/XP ultrasound scanner was used to image various stationary and moving phantoms while an Elekta SL25 linear accelerator radiotherapy treatment machine was operating. The ultrasound transducer was positioned to image from the outer edge of the treatment field at all times. Images were acquired to videotape and analyzed using in-house motion tracking algorithms to determine the effect of the SL25 on the quality of the displacement measurements. To determine the effect on the dosimetry of the presence of the transducer, dose distributions were examined using thermoluminescent dosimeters loaded into an Alderson Rando phantom and exposed to a 10 x 10 cm2 treatment field with and without the ultrasound transducer mounted 2.5 cm outside the field edge. The ultrasound images acquired a periodic noise that was shown to occur at the pulsing frequency of the treatment machine. Images of moving tissue were analyzed and the standard deviation on the displacement estimates within the tissue was identical with the SL25 on and off. This implies that the periodic noise did not significantly degrade the precision of the tracking algorithm (which was better than 0.01 mm). The presence of the transducer at the surface of the phantom presented only a 2.6% change to the dose distribution to the volume of the phantom. The feasibility of ultrasonic motion tracking during radiotherapy treatment is demonstrated. This presents the possibility of developing a noninvasive, real-time and low-cost method of tracking target motion during a treatment fraction.     

组织声学特性对高强度聚焦超声温度场的影响

目的 数值仿真组织声学特性对高强度聚焦超声（HIFU）焦域处温度场的影响,为HIFU治疗安全性和可靠性提供理论依据.方法 以实测新鲜离体猪肝组织不同温度下的声速和衰减系数为依据,利用时域有限差分（FDTD）法数值仿真研究HIFU治疗过程中组织内声速、衰减系数的变化和温度场的分布,分析讨论声速和衰减系数变化对60 ℃以上可治疗区域大小、位置的影响.结果 随着照射时间的延长,焦域处肝组织温升增大,声速下降,声衰减系数增大.随着声速的变化,形成的可治疗区域变大,焦点位置向远离换能器方向移动;随着声衰减系数的变化,焦域大小和焦点位置几乎不变.结论 猪肝组织内声速的变化对可治疗焦域的位置和大小影响较大;声衰减系数的变化对焦域的影响较小.