Jianhua Zhou

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 Fouriers 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 (tau(T)) and the heat flux (tau(q)) are zero. This is different from the DPL model for pure conduction materials, for which it can be reduced to the Fouriers heat conduction model provided that tau(q)=tau(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 tau(T).

Non-Fourier Heat Conduction Effect on Laser-Induced Thermal Damage in Biological Tissues

To ensure personal safety and improve treatment efficiency in laser medical applications, one of the most important issues is to understand and accurately assess laser-induced thermal damage to biological tissues. Biological tissues generally consist of nonhomogeneous inner structures, in which heat flux equilibrates to the imposed temperature gradient via a relaxation phenomenon characterized by a thermal relaxation time. Therefore, it is naturally expected that assessment of thermal damage to tissues could be inaccurate when a classical bioheat conduction model is employed. However, little attention has been given to studying the impact of the bioheat non-Fourier effect. In this article, a thermal wave model of bioheat transfer, together with a seven-flux model for light propagation and a rate process equation for tissue damage, is presented to investigate thermal damage in biological tissues. It is shown that the thermal damage assessed with the thermal wave bioheat model may differ significantly from that assessed with the classical bioheat model. Without including the bioheat non-Fourier effect, the assessment of thermal damage to biological tissue may not be reliable.

Numerical Simulation of Thermal Damage to Living Biological Tissues Induced by Laser Irradiation Based on a Generalized Dual Phase Lag Model

A generalized dual phase lag (DPL) bioheat model based on the nonequilibrium heat transfer in living biological tissues is applied to investigate thermal damage induced by laser irradiation. Comparisons of the temperature responses and thermal damages between the generalized and classical DPL bioheat model, derived from the constitutive DPL model and Pennes bioheat equation, are carried out in this study. It is shown that the generalized DPL model could predict significantly different temperature and thermal damage from the classical DPL model and Pennes bioheat conduction model. The generalized DPL equation can reduce to the classical Pennes heat conduction equation only when the phase lag times of temperature gradient (τ T ) and heat flux vector (τ q ) are both zero. The effects of laser parameters such as laser exposure time, laser irradiance, and coupling factor on the thermal damage are also studied.

Numerical Simulation of Random Packing of Spherical Particles for Powder-Based Additive Manufacturing

Powder-based additive manufacturing is an efficient and rapid manufacturing technique because it allows fabrication of complex parts that are often unobtainable by traditional manufacturing processes. A better understanding of the packing structure of the powder is urgently needed for the powder-based additive manufacturing. In this study, the sequential addition packing algorithm is employed to investigate the random packing of spherical particles with and without shaking effect. The 3D random packing structures are demonstrated by illustrative pictures and quantified in terms of pair distribution function, coordination number, and packing density. The results are presented and discussed aiming to produce the desirable packing structures for powder-based additive manufacturing.

A Boundary Element Method for Evaluation of the Effective Thermal Conductivity of Packed Beds

The problem of evaluating the effective thermal conductivity of random packed beds is of great interest to a wide-range of engineers and scientists. This study presents a boundary element model (BEM) for the prediction of the effective thermal conductivity of a twodimensional packed bed. The model accounts for four heat transfer mechanisms: (1) conduction through the solid; (2) conduction through the contact area between particles; (3) radiation between solid surfaces; and (4) conduction through the fluid phase. The radiation heat exchange between solid surfaces is simulated by the net-radiation method. Two regular packing configurations, square array and hexagonal array, are chosen as illustrative examples. The comparison between the results obtained by the present model and the existing predictions are made and the agreement is very good. The proposed BEM model provides a new tool for evaluating the effective thermal conductivity of the packed beds. DOI: 10.1115/1.2430721

Inverse Heat Conduction Using Measured Back Surface Temperature and Heat Flux

In the high-energy laser heating of a target, the temperature and heat flux at the heated surface are not directly measurable, but they can be estimated by solving an inverse heat conduction problem based on the measured temperature and/or the heat flux at the accessible (back) surface. In this study, the one-dimensional inverse heat conduction problem in a finite slab is solved by the conjugate gradient method, using measured temperature and heat flux at the accessible (back) surface. Simulated measurement data are generated by solving a direct problem, in which the front surface of the slab is subjected to high-intensity periodic heating. Two cases are simulated and compared, with the temperature or heat flux at the heated front surface chosen as the unknown function to be recovered. The results show that the latter choice (i.e., choosing back surface heat flux as the unknown function) can give better estimation accuracy in the inverse heat conduction problem solution. The front surface temperature can be computed with high precision as a by-product of the inverse heat conduction problem algorithm. The robustness of this inverse heat conduction problem formulation is tested by different measurement errors and frequencies of the input periodic heating flux.

Inverse Estimation of Surface Heating Condition in a Finite Slab With Temperature-Dependent Thermophysical Properties

Temperature and heat flux at the heated surface can be estimated by solving an inverse heat conduction problem (IHCP) based on measured temperature and/or heat flux at the accessible locations (e.g., back surface). Most of the previous studies used temperature measurement data in the objective function, and little work has been done for the inverse numerical algorithm based on heat flux measurement data. In this study, a one-dimensional IHCP in a finite slab is solved by using the conjugate gradient method. The heat flux measurement data are, for the first time, incorporated into the objective function for a nonlinear heat conduction problem with temperature-dependent thermophysical properties. The results clearly show that the inverse approach of using heat flux measurement data in the objective function can provide much better predictions than the traditional approaches in which the temperature measurements are employed in the objective function. Parametric studies are performed to demonstrate the robustness of the formulated IHCP algorithm by testing it for two different materials under different frequencies of the imposed heat flux along with random errors of the measured heat flux at the back surface.

Numerical Study on the Thawing Process of Biological Tissue Induced by Laser Irradiation

Most of the laser applications in medicine and biology involve thermal effects. The laser-tissue thermal interaction has therefore received more and more attentions in recent years. However, previous works were mainly focused on the case of laser heating on normal tissues (37 degrees C or above). To date, little is known on the mechanisms of laser heating on the frozen biological tissues. Several latest experimental investigations have demonstrated that lasers have great potentials in tissue cryopreservation. But the lack of theoretical interpretation limits its further application in this area. The present paper proposes a numerical model for the thawing of biological tissues caused by laser irradiation. The Monte Carlo approach and the effective heat capacity method are, respectively, employed to simulate the light propagation and solid-liquid phase change heat transfer. The proposed model has four important features: (1) the tissue is considered as a nonideal material, in which phase transition occurs over a wide temperature range; (2) the solid phase, transition phase, and the liquid phase have different thermophysical properties; (3) the variations in optical properties due to phase-change are also taken into consideration; and (4) the light distribution is changing continually with the advancement of the thawing fronts. To this end, 15 thawing-front geometric configurations are presented for the Monte Carlo simulation. The least-squares parabola fitting technique is applied to approximate the shape of the thawing front. And then, a detailed algorithm of calculating the photon reflection/refraction behaviors at the thawing front is described. Finally, we develop a coupled light/heat transport solution procedure for the laser-induced thawing of frozen tissues. The proposed model is compared with three test problems and good agreement is obtained. The calculated results show that the light reflectance/transmittance at the tissue surface are continually changing with the progression of the thawing fronts and that lasers provide a new heating method superior to conventional heating through surface conduction because it can achieve a uniform volumetric heating. Parametric studies are performed to test the influences of the optical properties of tissue on the thawing process. The proposed model is rather general in nature and therefore can be applied to other nonbiological problems as long as the materials are absorbing and scattering media.

Simulation of Laser-Induced Thermotherapy Using a Dual-Reciprocity Boundary Element Model With Dynamic Tissue Properties

This paper presents a nonlinear dual-reciprocity boundary element method (DRBEM) for bioheat transfer in laser-induced thermotherapy. The nonlinearity stems from the dynamic changes of tissue thermophysical and optical properties and the blood perfusion rate during laser heating. The proposed DRBEM is coupled with a modified Monte Carlo method and the Arrhenius rate equation to investigate laser light propagation, bioheat transfer, and irreversible thermal damage in tumors. The computer code is justified by comparing the DRBEM results with the finite-difference results. The photothermal processes in interstitial laser thermotherapy with single or double laser fiber scattering applicators are chosen as the demonstrative examples. The dynamic nature, together with the unique advantages of ¿boundary-only¿ and excellent adaptability to complex anatomical geometries that the DRBEM method offers, makes the present nonlinear DRBEM a powerful tool for analysis and optimization of the parameters in laser surgical procedure.

Numerical Simulation of Random Packing of Spherical Particles for Selective Laser Sintering Applications

Selective Laser Sintering (SLS) is an efficient and rapid manufacturing technique because it allows for making complex parts that are often unobtainable by traditional manufacturing processes. However, the application of such technique is quite limited by the balling phenomenon, for which it largely reduces the manufacturing quality. Eliminating the structural defects is crucial to overcome the balling phenomenon. Therefore, a better understanding of the packing structure details is urgently needed for the SLS applications. In this study, the sequential addition packing algorithm is employed to investigate the random packing of spherical particles with and without shaking effect. The 3-D random packing structures are demonstrated by illustrative pictures and quantified in terms of pair distribution function, coordination number and packing density. The results are presented and discussed aiming to produce the optimal packing parameters for the SLS manufacturing process.Copyright