Zhou Jianhua, Liu Jing, Yu Aibing
Cryogenics Laboratory, PO Box 2711, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, Peoples Republic China.
J Biomech Eng. 2005 Jun;127(3):416-31. doi: 10.1115/1.1894294.
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.
医学和生物学中的大多数激光应用都涉及热效应。因此，激光与组织的热相互作用近年来受到越来越多的关注。然而，以往的工作主要集中在激光加热正常组织（37摄氏度及以上）的情况。迄今为止，关于激光加热冷冻生物组织的机制知之甚少。一些最新的实验研究表明，激光在组织冷冻保存方面具有巨大潜力。但缺乏理论解释限制了其在该领域的进一步应用。本文提出了一个用于模拟激光辐照引起的生物组织解冻的数值模型。分别采用蒙特卡罗方法和有效热容法来模拟光传播和固液相变传热。所提出的模型有四个重要特点：（1）将组织视为一种非理想材料，其中相变在较宽的温度范围内发生；（2）固相、过渡相和液相具有不同的热物理性质；（3）还考虑了由于相变引起的光学性质变化；（4）光分布随着解冻前沿的推进而不断变化。为此，给出了15种解冻前沿几何构型用于蒙特卡罗模拟。应用最小二乘抛物线拟合技术来近似解冻前沿的形状。然后，描述了一种计算解冻前沿光子反射/折射行为的详细算法。最后，我们开发了一种用于激光诱导冷冻组织解冻的光/热传输耦合求解程序。将所提出的模型与三个测试问题进行了比较，得到了良好的一致性。计算结果表明，组织表面的光反射率/透射率随着解冻前沿的推进而不断变化，并且激光提供了一种优于传统表面传导加热的新加热方法，因为它可以实现均匀的体积加热。进行了参数研究以测试组织光学性质对解冻过程的影响。所提出的模型本质上相当通用，因此只要材料是吸收和散射介质，就可应用于其他非生物问题。
