Accelerated Thermal Ablation of Biological Tissue Volumes using HIFU beams with Shock Fronts

Acoustical Physics - The paper presents the results of a numerical experiment comparing the rates of volumetric thermal ablation of bovine liver tissue ex vivo, generated by a multielement...     

Thermo-mechanical laser ablation of soft biological tissue: modeling the micro-explosions

Characteristics of thermo-mechanical laser ablation process are investigated using an original numerical model. In contrast with previous models, it is based on a microscopic physical model of the micro-explosion process, which combines thermodynamic behavior of tissue water with elastic response of the solid tissue components. Diffusion of dissipated heat is treated in one dimension, and the amount of thermal damage is assessed using the Arrhenius model of the protein denaturation kinetics. Influence of the pulse fluence and duration on temperature profile development, ablation threshold, and depth of thermal damage is analyzed for the case of Er:YAG laser irradiation of human skin. Influence of mechanical properties on the ablation threshold of soft tissue is predicted theoretically for the first time. In addition, feasibility of deep tissue coagulation with a repetitively pulsed Er:YAG laser is indicated from the model. Received: 9 July 1998 / Revised version: 26 February 1999 / Published online: 26 May 1999     

Improved thermal model and its application in UV high-power pulsed laser ablation of metal target

An improved thermal model describing UV high-power nanosecond laser ablation of metal target is presented. The vaporization effect, the plasma shielding effect, as well as the absorption coefficient and absorptivity dependence of temperature are considered in this model. Take iron target as an example, the numerical solutions are obtained from the heat flow equations (before and after melting) using a finite difference method. The space and time dependence of temperature of the target, the time dependence of temperature at two different positions for a certain laser fluence, and the ablation rate as a function of laser fluence are also presented. The numerical results that agreed well with the experimental data are much better than the one without, which indicates that the above two effects and the temperature dependence of absorption coefficient and absorptivity in UV high-power laser ablation of metal target should not be neglected. We hope the present model will be useful for further experimental investigation of metal thin films prepared by pulsed laser deposition.     

Laser ablation parameters of atherosclerotic arteries

Summary The threshold of atherosclerotic plaque ablation in water by the XeCl laser radiation was measured to be of (1.5±0.2) J/cm². Within the simple thermal model this value corresponds to the mean temperature of about 373 K in the irradiated volume as for other angioplasty lasers. Time-of-flight probing of ablation in air by the XeCl and CO₂ lasers revealed microsecond lifetimes of ablated volumes that confirm the mechanism of explosive boiling of superheated tissue water. So the local temperature must be about 582 K which can be due to inhomogeneous light distribution and absorption. The short lifetime leads to the absence of heat diffusion and, hence, to the ablative character of tissue destruction.     

The effect of perfusion on the temperature distribution during thermotherapy: Study on perfused porcine kidneys

The effect of perfusion on the temperature distribution during radio-frequency hyperthermia and laser-induced thermotherapy was investigated with the perfused porcine kidney model. The phase shift-based proton resonance frequency shift method was used to map the temperature distribution. In experiments with modulated perfusion rates it was demonstrated that perfusion dissipates a significant amount of the absorbed energy and, therefore, the resulting heat distribution is strongly dependent on the perfusion rate. The measured time course of the temperature distribution was used to estimate the thermal conductivity, local perfusivity and heat absorption rate of the tissue. These parameters were in a good agreement with literature data. This approach can also be extended to measure heat absorption and heat transfer parameters in vivo, which can significantly improve the accuracy of thermotherapy session planning.     

Finite difference modelling of the temperature rise in non-linear medical ultrasound fields

Non-linear propagation of ultrasound can lead to increased heat generation in medical diagnostic imaging due to the preferential absorption of harmonics of the original frequency. A numerical model has been developed and tested that is capable of predicting the temperature rise due to a high amplitude ultrasound field. The acoustic field is modelled using a numerical solution to the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation, known as the Bergen Code, which is implemented in cylindrical symmetric form. A finite difference representation of the thermal equations is used to calculate the resulting temperature rises. The model allows for the inclusion of a number of layers of tissue with different acoustic and thermal properties and accounts for the effects of non-linear propagation, direct heating by the transducer, thermal diffusion and perfusion in different tissues. The effect of temperature-dependent skin perfusion and variation in background temperature between the skin and deeper layers of the body are included. The model has been tested against analytic solutions for simple configurations and then used to estimate temperature rises in realistic obstetric situations. A pulsed 3 MHz transducer operating with an average acoustic power of 200 mW leads to a maximum steady state temperature rise inside the foetus of 1.25 degrees C compared with a 0.6 degree C rise for the same transmitted power under linear propagation conditions. The largest temperature rise occurs at the skin surface, with the temperature rise at the foetus limited to less than 2 degrees C for the range of conditions considered.     

电子束辐照产生的热激波的数值模拟

 采用能量沉积解析法、材料低压本构关系及一维应变弹塑性流体动力学模型,对“闪光二号”电子束辐照硬铝产生的热激波的传播规律进行了数值模拟。结果表明,计算值与实测值基本符合。     

激光诱导间质肿瘤热疗的数值模拟和实验研究

本文在考虑生物组织物性动态变化的情况下建立了激光诱导间质肿瘤热疗(LITT)的物理数学模型,采用MonteCarlo方法数值模拟了LITT中激光能量在生物组织内的传输过程,基于Pennes生物传热方程和Arrhenius方程数值求解了组织内的温度分布和热损伤体积的变化,分析了热物性及血液灌注率的动态变化对LITT过程的影响,并与相应的离体实验结果进行了对比。数值模拟结果表明,组织的热物性及血液灌注率的动态变化对于热损伤体积的变化具有重要的影响。因此在激光诱导间质肿瘤热疗的数值模拟中应该考虑热物性及血液灌注率的动态变化以期为临床治疗方案的制定提供更为准确的依据。     

热阻塞效应对激光辐照靶材烧蚀过程的影响分析

研究了百兆瓦级激光烧蚀碳/碳复合材料靶材产生的等离子体吸收激光束能量引起的热阻塞效应。首先,基于逆轫致吸收理论,建立了激光在烧蚀靶材产生的等离子体中的传播模型;然后,基于磁流体理论,得到了等离子体在百兆瓦级激光形成的电磁场中的波动方程,建立了等离子体吸收激光能量引起热阻塞效应的模型。最后,对烧蚀过程中粒子的总密度、吸收系数、靶材表面等效热流随激光持续时间的变化规律以及是否考虑热阻塞效应时,靶面垂直方向的温度场进行了数值模拟。结果表明:等离子体的形成,对激光形成了明显的热阻塞效应,削弱了激光对靶材的烧蚀作用,使粒子总密度、吸收系数、靶材表面等效热流以及靶面垂直方向温度场的变化均呈现为非线性。     

Numerical Investigation Into the Highly Nonlinear Heat Transfer Equation with Bremsstrahlung Emission in the Inertial Confinement Fusion Plasmas

A highly nonlinear parabolic partial differential equation that models the electron heat transfer process in laser inertial fusion has been solved numerically. The strong temperature dependence of the electron thermal conductivity and heat loss term (Bremsstrahlung emission) makes this a highly nonlinear process. In this case, an efficient numerical method is developed for the energy transport mechanism from the region of energy deposition into the ablation surface by a combination of the Crank‐Nicolson scheme and the Newton‐Raphson method. The quantitative behavior of the electron temperature and the comparison between analytic and numerical solutions are also investigated. For more clarification, the accuracy and conservation of energy in the computations are tested. The numerical results can be used to evaluate the nonlinear electron heat conduction, considering the released energy of the laser pulse at the Deuterium‐Tritium (DT) targets and preheating by heat conduction ahead of a compression shock in the inertial confinement fusion (ICF) approach. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Theoretical model of CO2 laser ablation of soft-tissue phantoms

Summary A model of thermal laser ablation of soft tissues is developed taking into consideration two mechanisms: evaporation and liquid moving, due to vapour pressure gradient. Usually a soft tissue is modelled as a single-component material with thermal and optical properties very similar to those of water. We examined the non-stable kinetics of the evaporation process, for short-pulse infrared laser ablation of soft tissues, and we also calculated the average liquid velocity and the ablation rates under vapour pressure gradient. The theoretical results are in good agreement with previous reported experimental data on gelatin and polyacrylamide tissue phantoms The authors of this paper have agreed to not receive the proofs for correction     

A Volume of Fluid Based Method for Fluid Flows with Phase Change

This paper presents a numerical method directed towards the simulation of flows with mass transfer due to changes of phase. We use a volume of fluid (VOF) based interface tracking method in conjunction with a mass transfer model and a model for surface tension. The bulk fluids are viscous, conducting, and incompressible. A one-dimensional test problem is developed with the feature that a thin thermal layer propagates with the moving phase interface. This test problem isolates the ability of a method to accurately calculate the thermal layers responsible for driving the mass transfer in boiling flows. The numerical method is tested on this problem and then is used in simulations of horizontal film boiling.     

Laser-induced convenient fabrication of CdS nanocages with super-adsorption capability for methyl blue solution

We report on the successful synthesis of cadmium sulfide(CdS) nanocages by laser ablation of bulk Cd target in thioacetamide(TAA) solution. The CdS nanocages exhibit obvious interior hollow spaces and distinctive porous-shell structures. After laser ablation of Cd target in liquid condition, the unique structure should be attributed to the initial formation of Cd micro-gas bubble via a model of micro-explosive boiling model. Surprisingly, the obtained CdS nanocages can provide a super-adsorption of methyl blue(MB) solution. The maximum adsorption capacity reaches up to 11813.3 mg/g,which is much higher than that reported in many previous researches. Without using any complicated stabilizers or soft directing agents, the pure CdS nanocages fabricated by laser ablation will serve as advanced absorbents in further research.     

Nanosecond laser ablation for pulsed laser deposition of yttria

A thermal model to describe high-power nanosecond pulsed laser ablation of yttria (Y₂O₃) has been developed. This model simulates ablation of material occurring primarily through vaporization and also accounts for attenuation of the incident laser beam in the evolving vapor plume. Theoretical estimates of process features such as time evolution of target temperature distribution, melt depth and ablation rate and their dependence on laser parameters particularly for laser fluences in the range of 6 to 30 J/cm² are investigated. Calculated maximum surface temperatures when compared with the estimated critical temperature for yttria indicate absence of explosive boiling at typical laser fluxes of 10 to 30 J/cm². Material ejection in large fragments associated with explosive boiling of the target needs to be avoided when depositing thin films via the pulsed laser deposition (PLD) technique as it leads to coatings with high residual porosity and poor compaction restricting the protective quality of such corrosion-resistant yttria coatings. Our model calculations facilitate proper selection of laser parameters to be employed for deposition of PLD yttria corrosion-resistive coatings. Such coatings have been found to be highly effective in handling and containment of liquid uranium.     

Nonlinear absorption in biological tissue for high intensity focused ultrasound

In recent years the propagation of the high intensity focused ultrasound (HIFU) in biological tissue is an interesting area due to its potential applications in non-invasive treatment of disease. The base principle of these applications is the heat effect generated by ultrasound absorption. In order to control therapeutic efficiency, it is important to evaluate the heat generation in biological tissue irradiated by ultrasound. In his paper, based on the Khokhlov-Zabolotkaya-Kuznetsov (KZK) equation in frequency-domain, the numerical simulations of nonlinear absorption in biological tissues for high intensity focused ultrasound are performed. We find that ultrasound thermal transfer effect will be enhanced with the increasing of initial acoustic intensity due to the high harmonic generation. The concept of extra absorption factor is introduced to describe nonlinear absorption in biological tissue for HIFU. The theoretical results show that the heat deposition induced by the nonlinear theory can be nearly two times as large as that predicated by linear theory. Then, the influence of the diffraction effect on the position of the focus in HIFU is investigated. It is shown that the sound focus moves toward the transducer compared with the geometry focus because of the diffraction of the sound wave. The position of the maximum heat deposition is shifted to the geometry focus with the increase of initial acoustic intensity because the high harmonics are less diffraction. Finally, the temperature in the porcine fat tissue changing with the time is predicated by Pennes' equation and the experimental results verify the nonlinear theoretical prediction.     

基于三点法的柱状生物组织热特性参数估计

针对柱状生物活体组织,提出了通过测量组织表面三个测量点的温度变化,同时确定多个热特性参数的新的非稳态无损测量方法。基于Pennes生物传热方程,建立了描述三点法测量系统传热过程的数学模型,进行了正问题的求解。根据数值计算以及参数灵敏度分析的结果,采用改进的高斯方法,对生物活体组织的热特性参数估计进行了模拟计算及分析。结果表明,采用三点测温同时确定活体生物组织的导热系数、血液灌注率、综合换热系数的测量方法是有效和可行的。     

Controlled tissue emulsification produced by high intensity focused ultrasound shock waves and millisecond boiling

In high intensity focused ultrasound (HIFU) applications, tissue may be thermally necrosed by heating, emulsified by cavitation, or, as was recently discovered, emulsified using repetitive millisecond boiling caused by shock wave heating. Here, this last approach was further investigated. Experiments were performed in transparent gels and ex vivo bovine heart tissue using 1, 2, and 3 MHz focused transducers and different pulsing schemes in which the pressure, duty factor, and pulse duration were varied. A previously developed derating procedure to determine in situ shock amplitudes and the time-to-boil was refined. Treatments were monitored using B-mode ultrasound. Both inertial cavitation and boiling were observed during exposures, but emulsification occurred only when shocks and boiling were present. Emulsified lesions without thermal denaturation were produced with shock amplitudes sufficient to induce boiling in less than 20 ms, duty factors of less than 0.02, and pulse lengths shorter than 30 ms. Higher duty factors or longer pulses produced varying degrees of thermal denaturation combined with mechanical emulsification. Larger lesions were obtained using lower ultrasound frequencies. The results show that shock wave heating and millisecond boiling is an effective and reliable way to emulsify tissue while monitoring the treatment with ultrasound.     

Theoretical determination of the ablation rate of metals in multiple-nanosecond laser pulses irradiation regime

A detailed understanding of the physical determinants of the ablation rate in multiple nanosecond laser pulses regime is of key importance for technological applications such as patterning and pulsed-laser deposition. Here, theoretical modeling is employed to investigate the ablation of thick metallic plates by intense, multiple nanosecond laser pulses. A new photo-thermal model is proposed, in which the complex phenomena associated to the ablation process are accounted for as supplementary terms of the classical heat equation. The pulsed laser ablation in the nanosecond regime is considered as a competition between thermal vapourization and melt ejection under the action of the plasma recoil pressure. Computer simulations using the photo-thermal model presented here and the comparison of the theoretical results with experiment indicate two different mechanisms that contribute to the decrease of the ablation efficiency. First, during the ablation process the vapour/plasma plume expanding above the irradiated target attenuates the laser beam that reaches the sample, leading to a marked decrease of the ablation efficiency. Additional attenuation of the laser beam incident on the sample is produced due to the heating of the plasma by the absorption of the laser beam into the plasma plume. The second mechanism by which the ablation efficiency decreases consists of the reduction of the incident laser intensity with the lateral area, and of the melt ejection velocity with the depth of the hole.     

Transient 3D/2D simulation of laser-induced ablation of silicon

A numerical software has been developed to simulate heating, enthalpy-based phase changes and ablation of silicon during pulsed or continuous-wave laser irradiation. The unsteady heat transfer equation is solved by finite differences in two or three dimensions with full resolution of the thin liquid layer. An intelligent adaptive grid refinement and a semi-analytic treatment of the surface elements have been implemented to simulate laser cuts with lots of laser pulses in moderate computing time. The code has been successfully verified by comparisons with an analytic solution and with experimental data. Details of the mathematical model, the implementation in Matlab^®and comparisons with experimental laser cuts are presented in this paper.     

一种基于双波长的光声测温技术

光声测温是一种利用光声效应来进行温度监控的新方法,具有非侵入式、高灵敏度和探测深度较深等优点.但现有的单波长光声测温方法极易受到系统及测量环境干扰而导致测量精度降低.为了解决这一问题,本文提出了一种双波长光声温度测量方法.在光声测温理论的基础上,分析推导了双波长光声测温的基本原理,并进行了仿体及离体组织样品的双波长光声测温实验.实验结果显示,与传统单波长模式相比,双波长模式下的光声温度测量误差明显减小,测量精度平均提高35%以上.研究结果表明双波长光声测温方法能够有效提高光声温度测量的精度和稳定性,可作为一种更精准的光声温度监控方法应用于医疗手术等领域.