当前位置:首页 >> 物理 >>

书刊参考文献


5
Propagation and Absorption in Tissue Media

The propagation of electromagnetic waves in biological materials is governed by the dielectric constant, conductivity, source configuration, and the geometrical factors that describe the tissue structure. These parameters also determine the quantity of energy a given biological body extracts from the propagating wave. When the radius of curvature of the body surface is large compared to the wavelength and beam width of the impinging radiation, planar tissue models may be used to estimate the absorbed energy and its distribution inside the body. Otherwise, the absorbed energy will be dictated by the size of the body, the curvature of its surface, the ratio of body size to wavelength, and the source characteristics. The purpose of this chapter is to present a concise account of electromagnetic wave propagation in biological media, with special emphasis on the energy coupling and distribution characteristics in models of biological structures. Such information is essential for analyzing the interrelationships among various observed biological effects, for separating known and substantiated effects from those that are speculative and unsubstantiated, for assessing therapeutic effectiveness of electromagnetic waves, and for extracting diagnostic information from field effects. It should be noted that the whole-body absorption of electromagnetic energy by humans and laboratory animals is of interest because it is related to the energy required to alter the thermoregulatory system of the exposed subject, and because it may serve as an index for extrapolating experimental results to human exposures. The distribution of absorbed electromagnetic waves within an irradiated body is important because it relates to specific responses of the body, because it facilitates understanding of phenomena, and because it contributes to definition of mechanisms of interaction.
137

S. M. Michaelson et al., Biological Effects and Health Implications of Radiofrequency Radiation ? Springer Science+Business Media New York 1987

138

CHAPTER 5

5.1. PLANAR TISSUE GEOMETRIES

5. 1. 1. Reflection and Transmission

The fate of plane waves at a planar tissue interface depends on the frequency, polarization, and angle of incidence of the wave, and on the dielectric constant and conductivity of the tissue. For a linearly polarized plane wave impinging normallyon a boundary separating two semiinfinite media, the reftection and transmission coefficients are respectively given by equations (2.55) and (2.56). When the dielectric properties of the media are approximately equal, there is no reftection and the transmission is maximum. In contrast, there is complete reftection if the second medium is perfectly conductive. Table 5-1 summarizes the magnitude of the reftection coefficient at the boundary separating various tissues. The fraction of normal incident power reftected by the discontinuity is given by R 2 ? Clearly, about one-haH of the incident power is reftected at these boundaries. Further, the reftection coefficients for tissue-tissue interfaces range from a low of about 5 for muscle-blood to high of about 50 for bone-biological ftuid interfaces. This suggests that the closer the dielectric properties on both sides of the interface, the smaller is the power reftection. The fraction of transmitted power is related to the power transmission coefficient (1 - R 2 ). It is readily apparent from Table 5-1 that the transmitted power at air-tissue interfaces is quite substantial at radio and microwave frequencies. Moreover, Fig. 5-1 shows that the power transmission coefficient is highly frequency dependent, especially at lower frequencies, while the transmitted power for an air-fat interface is about twice as great as for an air-muscle boundary (about 40% at 1 GHz); it is nearly the same as that for a fat-muscle interface. Clearly, power transmission is highest when the dielectric properties of the adjacent media are similar. As the transmitted wave propagates in the tissue medium, energy is extracted from the wave and absorbed by the medium. This absorption will result in a progressive reduction of the wave's power density as it advances in the tissue. This reduction is quantified by the depth of penetration 6, which is the distance in which the power density decreases by a factor of e- 2 ? Table 5-2 presents the calculated depth of penetration in selected tissues using typical dielectric constants and conductivities provided in Chapter 4. A graphical representation of penetration depth versus frequency for blood, muscle, and fat is given in Fig. 5-2. It is seen that 6 is frequency dependent and takes on different values for different tissues. In particular, the penetration depth for fat and bone is nearly five times greater than for higher-water-content tissues, and has values that range from a few millimeters to several centimeters.


赞助商链接
相关文章:
参考文献期刊格式
参考文献期刊格式 - 参考文献规范格式 一、参考文献的类型 参考文献(即引文出处)的类型以单字母方式标识,具体如下: M——专著 J——期刊文章 C——论文集 D—...
常用的一些参考文献
常用的一些参考文献_计算机软件及应用_IT/计算机_专业资料。软件工程类常用的一些...[1] 万峰科技.ASP.NET 网站开发四[酷]全书[J].图书情报知识,2005,(5). ...
参考文献、书籍
参考文献书籍_理学_高等教育_教育专区。课题开展主要参考文献 1、 《大学生朋辈心理辅导》 永铎 作者: 王冰蔚,杨宾峰,王 出版社:清华大学出版社 作者:颜农秋...
文内引用和参考文献格式
5、引用无作者文献 引用无作者文献, 如果文献标题没有出现在正文里, 则括号...词用大写;对于参考书籍,包括百科全书、字典和词汇表,在书籍标题前须加上―In‖...
论文引用参考文献格式
论文引用参考文献格式_教育学/心理学_人文社科_专业资料。很多人写论文不知道怎么写参考文献的格式 参考文献格式发布者:实践与培养科 发布时间:2008-1-3 点击:...
参考文献标准格式
参考文献标准格式 - 参考文献标准格式来源: 赵现勇的日志 参考文献类型:专著[M],论文集[C],报纸文章[N],期刊文章[J], 学位论文[D],报告[R],标准[S],...
论文参考书籍撰写格式
论文参考书籍撰写格式_高等教育_教育专区。毕业论文参考文献规范格式 一、参考文献的类型 参考文献(即引文出处)的类型以单字母方式标识,具体如下: M——专著 C—...
译著参考文献标注方法
译著参考文献标注方法_经济/市场_经管营销_专业资料。译著参考文献标注方法参考文献格式在不同的学校和机构有不同的要求, 一般来说译著的参考文献格式一般如下: [序...
论文参考文献标准格式,写法
论文参考文献标准格式,写法_制度/规范_工作范文_实用文档。论文后参考文献怎么写...起止 页码(可选) [1]刘国钧,陈绍业.图书馆目录[M].北京:高等教育出版社,...
参考文献书写格式
毕业设计(论文)规范化管理与培养学生综合素质[EB/OL]:中国高等教育网教学研究, . 2005-2-2: 附:参考文献著录中的文献类别代码 普通图书:M 标准:S 会议录:C...
更多相关标签: