人体组织传热及应用研究

概述了人体组织热传导的基本理论以及生物传热方程和传热模型的发展状况,介绍了Pennes方程、Weinbaum-Jiji方程和Stolwijk模型,并综述了生物传热学在临床医学中的应用,包括生物热物性参数的测量、温度场的无损重构、临床热疗以及在工程生理学中的应用。     

基于多岛遗传算法的生物组织三维温度场重构研究

首次提出了基于多岛遗传算法进行生物组织三维温度场无损重构的重要思想,把复杂的生物热传导反问题的求解转换为正问题的求解过程,并通过实验对该重构思想的可行性和可靠性进行了验证。以生物组织内点热源的位置P(x,y,z)和温度t为优化变量,把同一表面各个对应点的实验温度值和计算温度值相减并取绝对值之和,以此为目标函数逐次迭代。目标值越小,则当前变量,即热源位置和温度值最优。多岛遗传算法可以很好地应用于生物组织三维温度场的重构,无需提取生物组织全部的表面温度数据,为热传导反问题的研究提供了一条具有借鉴性的方法与思路。     

中医隔物灸的传热学研究

运用有限元软件ANSYS进行数值建摸模拟中医隔物灸产生的热在人体组织中的传导过程,并给出定性和定量分析.同时运用高灵敏度红外热像仪记录艾炷隔附子饼灸对生物组织热作用的过程,对比实验和计算结果验证了实验和数值模拟方法的合理性.然后建立简化的人体两维模型,得到穴位点横截面上热传导过程的两维温度分布图和热在生物组织中传播的整个过程.红外热像技术和数值建模方法为临床各种灸法的运用提供了理论和实验基础支持.     

离体猪胸主动脉的热传导率测试及分析

为进一步丰富血管组织热物性参数数据,以降低双极电外科血管闭合手术中高频电流产生的热量对手术装置加热点临近组织的热损伤,本研究基于准稳态平板法实验原理,采用电热膜和微型NTC热敏电阻分别作为加热元件和测温元件搭建实验装置;然后以家猪离体主动脉为研究对象,利用给电热膜通电加热组织、热敏电阻采集其温度数据,通过实验得到其热传导率数据;最后对比了血管组织热传导率和生物组织阻抗随温度变化特性。研究结果显示离体家猪胸主动脉在26.5℃时,热传导率为0.4150 W/m-K。在温度升高至63℃时之前,热传导率增加随温度升高变化不明显;当温度高于63℃之后,热传导率呈下降趋势;当温度高于79℃后,随温度的上升,因组织内含水量持续减少,血管组织热传导率随温度升高而快速下降,温度接近90℃时,热传导率下降到0.201 W/m-K。研究发现家猪主动脉热传导率的变化特性与生物组织阻抗随温度变化特性在温度区间非常吻合,表明组织热传导率和阻抗特性存在一定相关性,为深入揭示电外科手术过程中生物传热机理提供重要参考。     

高强度聚焦超声治疗中热波效应和声热耦合研究进展

高强度聚焦超声(HIFU)治疗剂量学研究是HIFU技术的一个重要科学问题。由于HIFU治疗是一种高热通量、加热时间短的治疗方式,通过非线性声传播与生物传热相耦合的模型来研究HIFU治疗的温场分布必须考虑如下因素:(1)由于靶区组织的温升具有明显的热波效应,需要对经典生物热传导方程进行修正;(2)HIFU治疗中的组织声热耦合效应变得更为复杂,这包括生物组织温度上升使得生物组织的声学性质发生动态变化(即热声透镜),以及液体汽化所产生的蒸汽泡对温度场产生影响。本文综述了影响HIFU治疗温场分布的上述因素,以期对HIFU治疗的生物物理机制有更深的认识。     

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.     

水冷式微波偶极子辐射器在生物介质中的温度分布研究

偶极子辐射天线在生物介质中近场辐射及温度特性对腔道微波热疗有重要实用意义。本文在腔内水冷式微波偶极子辐射器近场辐射模型和比吸收率计算了工作基础上，求解相应的生物介质热传导方程，得到了水冷式腔腔道辐射器在生物介质中的温度分布。计算表明，水冷可改善热区温度分布，主要使径向最高温度点由无水冷时贴近辐射器壁向组织深部迁移；径向治疗深度比无水冷时向组织内部延伸；相应的治疗体积则比无水冷时增加。若采取先用小功     

一种通过六边间隙式阵列的天线相位调制所产生的加热模式

本文介绍一种采用六边形阵列的微波天线所产生的热治疗癌症的方法。其目的是通过对六边形阵列的每一天线上的信号进行相位调制,既可产生均匀的,又可产生非均匀的可控的加热模式而作用于生物组织。该阵列包括位于六边形的角上,两根天线对角线的距离是4cm。在阵列内通过热传导模拟的方法可计算出每单位体积吸收的功率以及所转换的热量。本文还提出了有关吸收率和温度关系的模型,包括     

在组织间隙传导性热疗法中控制最低肿瘤温度的理论基础

本文叙述了由一种简单的局部热疗法所得到的稳态内部肿瘤温度的模拟:用温热的传导性加热元件组成的平行阵列进行组织间隙治疗。在“传导性加热”过程中,能量只加到间质探针上。邻近的组织通过热传导而被加热。组织间隙传导性加热过程的模拟与在数字计算机上用所要治疗的组织的有限差模型能生物热传输方程所得到的解有关。这些模拟提示了当仔细检查传导性间质温度过高时的完整的温度分布时,温度分布均匀的物质是明显的。除了     

超声引起的双层生物组织中的温度场研究

超声热疗在临床上得到了越来越广泛的应用,成为目前临床治疗的一种有效手段,但治疗中人体组织的温度测量是决定治疗成败的重要因素.基于超声的温度分布的无损测量是其中的一种方案,也成为医学超声研究的一个热点.采用有限差分的离散方法求解Pennes方程,从而得到了超声作为热源的情况下双层生物组织中的温度场.通过实验验证,说明该方法能较为准确地计算出超声热源情况下双层组织中的温升分布情况.同时讨论了各个参数对双层组织温度的影响,实验和计算结果发现初始声压和组织的热传导率对焦点温升的影响较大,组织的热容对焦点温升的影响很小,只有热传导率对焦点的位置有较大的影响.     

Weinbaum-Jiji生物传热方程在低温外科手术中的应用

皮肤低温外科手术中的一个关键问题是对体表受冷刀作用后组织温度的准确预测和调控,而生物组织内真实血管的空间分布会对这一相变传热过程产生重要的影响,本文将常温情况下建立的Weinbaum-Jiji生物传热方程拓展应用于该问题的分析,由此考察血管空间分布结构对皮肤相变传热过程的影响,以一维问题为例,采用变时间步长的数值计算方法得到了体表受冷刀作用下的组织冻结温度变化的情况,作为对比,本文还求解了经典Pennes生物传热方程及普通导热方程的相应相应相变传热问题,三类方程的比较表明在相同冷冻和物性条件下,血管空间分布效应会对一定部位的组织瞬态温度的应产生很大影响,这提示了在低温外科手术中预示相变过程时考虑血管传热机制的重要性,文中还考察了相变温度和大血管人口速度对温度预示的影响情况,就作者所知,本文是考虑血管分布效应后对皮肤低温相变传热进行分析的首次尝试。     

Dual-phase lag effects on thermal damage to biological tissues caused by laser irradiation

A dual-phase lag (DPL) bioheat conduction model, together with the broad beam irradiation method and the rate process equation, is proposed to investigate thermal damage in laser-irradiated biological tissues. It is shown that the DPL bioheat conduction model could predict significantly different temperature and thermal damage in tissues from the hyperbolic thermal wave and Fourier's heat conduction models. It is also found that the DPL bioheat conduction equations can be reduced to the Fourier heat conduction equations only if both phase lag times of the temperature gradient (τ_T) and the heat flux (τ_q) are zero. This is different from the DPL model for pure conduction materials, for which it can be reduced to the Fourier's heat conduction model provided that τ_q=τ_T. Effects of laser parameters and blood perfusion on the thermal damage simulated in tissues are also studied. The result shows that the overall effects of the blood flow on the thermal response and damage are similar to those of the time delay τ_T.     

在体生物组织热物性测试

本文通过应用Ｐｅｎｎｅｓ生物组织方程和半导体热敏电阻探针，运用热脉冲技术的理论与实验相比较方法，对在全生物组织刊物物性测试。     

A Green's function approach to local rf heating in interventional MRI

Current safety regulations for local radiofrequency (rf) heating, developed for externally positioned rf coils, may not be suitable for internal rf coils that are being increasingly used in interventional MRI. This work presents a two-step model for rf heating in an interventional MRI setting: (1) the spatial distribution of power in the sample from the rf pulse (Maxwell's equations); and (2) the transformation of that power to temperature change according to thermal conduction and tissue perfusion (tissue bioheat equation). The tissue bioheat equation is approximated as a linear, shift-invariant system in the case of local rf heating and is fully characterized by its Green's function. Expected temperature distributions are calculated by convolving (averaging) transmit coil specific absorption rate (SAR) distributions with the Green's function. When the input SAR distribution is relatively slowly varying in space, as is the case with excitation by external rf coils, the choice of averaging methods makes virtually no difference on the expected heating as measured by temperature change (deltaT). However, for highly localized SAR distributions, such as those encountered with internal coils in interventional MRI, the Green's function method predicts heating that is significantly different from the averaging method in current regulations. In our opinion, the Green's function method is a better predictor since it is based on a physiological model. The Green's function also elicits a time constant and scaling factor between SAR and deltaT that are both functions of the tissue perfusion rate. This emphasizes the critical importance of perfusion in the heating model. The assumptions made in this model are only valid for local rf heating and should not be applied to whole body heating.     

The theoretical and experimental evaluation of the heat balance in perfused tissue

Accurate treatment planning is necessary for the successful application of hyperthermia in the clinic. The validity of four different bioheat models or combinations of models is evaluated: the conventional bioheat transfer equation, the limited effective conductivity model, a mixed heat sink-effective conductivity model and a discrete vessel model. The heat balance for the heated volume, and especially the ratio between conductive heat removal and heat escape through the veins, is different for each of these models. Model predictions were compared with results from experiments on isolated perfused bovine tongues. Tongues were suspended in a water-filled container and heated by conduction. The steady state temperature distribution and heat balance were determined at various blood flow rates. Increased blood flow was found to lower the mean tissue temperature and to enhance both conductive and venous heat removal. This result agrees only with the mixed heat sink-effective conductivity and the discrete vessel model predictions. At low flow rates a modified heat sink term should be used because the venous efflux temperature was significantly lower than the mean tissue temperature.     

A three-dimensional description of heating patterns in vascularised tissues during hyperthermic treatment

A three-dimensional finite difference computer model has been developed to calculate temperature distributions in vascularised tissues in hyperthermia. Besides offering the possibility of calculating temperature distributions according to the conventional ' bioheat transfer' method, this model allows the introduction of several discrete blood vessels and can describe their influence on the temperature distribution. The model can be used to evaluate all types of heating techniques. First calculations on discrete large vessels show inhomogeneities caused by these vessels in the temperature distribution in tissue in hyperthermia. The theory and model presented can form the basis of a new bioheat transfer theory, with a vessel-temperature related bioheat transfer heat-sink term able to describe the small-scale temperature variation problems in local hyperthermia.     

Fast FFT-based bioheat transfer equation computation

This paper describes a modeling method of the tissue temperature evolution over time in hyper or hypothermia. The tissue temperature evolution over time is classically described by Pennes’ bioheat transfer equation which is generally solved by a finite difference method. In this paper we will present a method where the bioheat transfer equation can be algebraically solved after a Fourier transformation over the space coordinates. As an example, we implemented this method for the simulation of a percutaneous high intensity ultrasound hepatocellular carcinoma curative treatment and compared it with the finite difference method and experimental data.     

Calculation of change in brain temperatures due to exposure to a mobile phone.

In this study we evaluated for a realistic head model the 3D temperature rise induced by a mobile phone. This was done numerically with the consecutive use of an FDTD model to predict the absorbed electromagnetic power distribution, and a thermal model describing bioheat transfer both by conduction and by blood flow. We calculated a maximum rise in brain temperature of 0.11 degrees C for an antenna with an average emitted power of 0.25 W, the maximum value in common mobile phones, and indefinite exposure. Maximum temperature rise is at the skin. The power distributions were characterized by a maximum averaged SAR over an arbitrarily shaped 10 g volume of approximately 1.6 W kg(-1). Although these power distributions are not in compliance with all proposed safety standards, temperature rises are far too small to have lasting effects. We verified our simulations by measuring the skin temperature rise experimentally. Our simulation method can be instrumental in further development of safety standards.     

Determination of heterogeneous thermal parameters using ultrasound induced heating and MR thermal mapping

In this paper, a method for the determination of spatially varying thermal conductivity and perfusion coefficients of tissue is proposed. The temperature evolution in tissue is modelled with the Pennes bioheat equation. The main motivation here is a model-based optimal control for ultrasound surgery, in which the tissue properties are needed when the treatment is planned. The overview of the method is as follows. The same ultrasound transducers, which are eventually used for the treatment, are used to inflict small temperature changes in tissue. This temperature evolution is monitored using MR thermal imaging, and the tissue properties are then estimated on the basis of these measurements. Furthermore, an approach to choose transducer excitations for the determination procedure is also considered. The purpose of this paper is to introduce a method and therefore simulations are used to verify the method. Furthermore, computations are accomplished in a 2D spatial domain.     

Dominant factors affecting temperature rise in simulations of human thermoregulation during RF exposure

Numerical models of the human thermoregulatory system can be used together with realistic voxel models of the human anatomy to simulate the body temperature increases caused by the power absorption from radio-frequency electromagnetic fields. In this paper, the Pennes bioheat equation with a thermoregulatory model is used for calculating local peak temperatures as well as the body-core-temperature elevation in a realistic human body model for grounded plane-wave exposures at frequencies 39, 800 and 2400 MHz. The electromagnetic power loss is solved by the finite-difference time-domain (FDTD) method, and the discretized bioheat equation is solved by the geometric multigrid method. Human thermoregulatory models contain numerous thermophysiological and computational parameters--some of which may be subject to considerable uncertainty--that affect the simulated core and local temperature elevations. The goal of this paper is to find how greatly the computed temperature is influenced by changes in various modelling parameters, such as the skin blood flow rate, models for vasodilation and sweating, and clothing and air movement. The results show that the peak temperature rises are most strongly affected by the modelling of tissue blood flow and its temperature dependence, and mostly unaffected by the central control mechanism for vasodilation and sweating. Almost the opposite is true for the body-core-temperature rise, which is however typically greatly lower than the peak temperature rise. It also seems that ignoring the thermoregulation and the blood temperature increase is a good approximation when the local 10 g averaged specific absorption rate is smaller than 10 W kg(-1).