| The electromagnetic properties of biological tissue, is the response characteristics of the biological tissue as a substance to electromagnetic field. Magnetic property, namely the permeability of biological tissue, is close to the vacuum magnetic permeability, so it can be regarded as constant; electrical characteristics, mainly refers to the dielectric properties of biological tissue, including two parts of conductivity and dielectric constant, respectively expressed by macroscopic parameters σ and ε. When the biological tissue is put into the time harmonic electric field, the dielectric properties of biological tissue are related to the frequency of electric field, the complex relative permittivity of biological tissue can be expressed as εr=ε’-jε", the real part ε’is the relative permittivity and the imaginary part ε" is the loss factor. The loss factor ε" describes the loss characteristics of tissues, therefore it usually be classified as a part of the equivalent frequency-dependent conductivity κ(ω)=σ+ωε0ε", ε0 is the vacuum permittivity. Numerous research results indicates that, the dielectric properties between different types of normal tissue vary greatly, the range of relative permittivity and conductivity of the healthy biological tissue respectively are 5.7-98.1 and 0.17-2.11 S m-1 when measuring frequency is 100MHz. With the same type of tissue, the dielectric properties of the tumor tissue are 30%-50% larger than that of normal tissu, the dielectric properties of tumor tissue also change in different position. The relative permittivity and conductivity of low-water-content tissue are one order of magnitude lower than that of high-water-content tissue. The relative permittivity changes a little with temperature, the variation of conductivity with temperature is relatively larger. In summary, the dielectric properties of biological tissue are relate to the frequency, the type of tissue, the structure of tissue and temperature.Because of the difference between the dielectric properties of tumor tissue and normal tissue, the images of dielectric properties of living tissue can reflect the physiological and pathological state of tissue and organ and provide valuable information for medical diagnosis. The images even can be used for the early diagnosis of cancer, tracking and monitoring the whole changing process that normal tissue turns into tumor tissue enabling early intervention therapy on tumor, so research on the dielectric properties of biological tissue is remarkable for the research and treatment of cancer. Furthermore, in field of microwave hyperthermia, the temperature can be imaged using the characteristic that dielectric properties change with temperature, temperature monitoring has important applications in the treatment of tumors, the relationships between the dielectric properties of human tissues and the applied electromagnetic power, frequency and duration of treatment also affect the treatment effect. If the dielectric properties of human tissues are not intensively studied, it will bring disastrous consequences in clinical application. So the reasearch on the dielectric properties of biological tissue has very important significance for the development of biological theory basis and clinical applications.Study on the dielectric properties of biological tissue is divided into two directions:study on the parameters of dielectric properties of biological tissue and study on the measurement method of dielectric properties of biological tissue. Study on the dielectric properties of biological tissue is mainly about the reasearch of relationships between the dielectric properties of biological tissue and influencing factors, i.e., frequency, temperature, structure of tissue, using a variety of measurement technology. Within the frequency range of 10Hz-100GHz, the dielectric properties of 55 human healthy tissues can be estimated using parameter model given by C.Gabriel et al., the parameter model is based on the dielectric properties of tissues in vitro, therefore it cannot reflect the dielectric properties of tissues in vivo. In addition, the present research on dielectric properties of human tissue is all about healthy tissue, only few tumor tissues,such as breast and liver, have been intensively studied. In order to study the dielectric properties of tissues in vivo and tumor tissues, it must pay attention to measurement methods of the dielectric properties of biological tissue.The dielectric properties of biological tissues, especially the tissues in vivo, are much more complicated than that of non-biological materials, so the measurement method of dielectric properties of biological tissues must have the following characteristics:(1) the method shluld cover the frequency range of few MHz to several GHz; (2) it is convenient to control and monitor the temperature; (3) it is convenient to add polarized electric field and magnetic field; (4) the biological tissue samples required for the measurement cannot be too much; (5) it is best that the method is able to measure the dielectric properties of biological tissues in vivo.The content of this thesis is the measurement of dielectric properties within the frequency range of 40-500MHz. The open-ended coaxial line method can be used for ultrawideband measurement of dielectric properties, the structure is open, the composition of measurement system is simple. In addition, the method does not have requirements, such as the sample size, shape and physical form, so it is simple for the preparation of samples. This paper analyzed open-ended coaxial line method from the components of the measuring system, the equivalent model and dielectric properties calculation etc. Furthermore, error analysis is carried out through the comparasion of the measurments of standard liquid using this methods. Finally the dielectric properties of porcine tissue are measured at different frequencies.First, the measurement system consists of a portable computer, a vector network analyzer connected with an S-parameters component and a semi-rigid coaxial cable, the thermostatic water bath can be added if necessary. The function of the thermostatic water bath is to maintain the temperature of tissue in the process of measurement preventing changes of temperature affect the measurement result. Combined with the structure characteristics of biological tissues, and compare the measurement results using different diameter and different materials filled coaxial line, the low loss semi-rigid coaxial cable that outer diameter is 0.358cm and filled with PTFE is selected as the measuring probe. Secondly, the coaxial line cannot be simply regarded as an ideal transmission line within the frequency range of 40-500MHz, the measurement system is equivalent to a two port network, so the actual reflection coefficient and the reflection coefficient measured by network analyzer can be link with the scattering matrix. Thirdly, the open circuit, short circuit and standard solution are measured, so the solution of the equations can be simplified using the reflection coefficients obtained. Then the dielectric properties of methanol, ethanol and n-propanol are measured and compared with the literature value, takiing analysis and solution of measurement error. Finally, the relative permittivity and conductivity of the liver, muscle and fat of porcine at 37℃ are measured at the frequencies of 42.58MHz,64MHz,128MHz,170MHz,298MHz,400MHz and 468MHz。At present, the dielectric properties of biological tissue are generally measured in vitro or invasive measurement in vivo, the open-ended coaxial line measurement method also has such shortcoming. Non-invasive dielectric properties measurement in vivo has great application prospect, but it still has no perfect method for non-invasive dielectric properties measurement. The electrical impedance tomography (EIT), magnetic induction tomography (MIT), magnetic resonance electrical impedance tomography (MR EIT) and magnetic resonance electrical properties of tomography (MR EPT) are intensively studied relatively. Unlike other non-invasive imaging technique of dielectric properties, the MR EPT technique has several advantages. It does not need additional measuring electrode and it also do not need to inject energy to the human body, the hardware needed to use is just the standard MRI system and supporting the RF coil. The imaging quality of the dielectric properties jointly depend upon the MRI system, B1-mapping technique, MR image quality and EPT algorithm.However, the image obtained using the current MR EPT technique does not have enough resolution and cannot be used in clinical disease research and diagnosis. At present, the MR EPT algorithm not only makes approximations, but also make assumptions according to the anatomical features of human body, such as human brain were symmetrical, and changing characteristics of dielectric properties, such as the hypothesis that the gradient of complex dielectric properties were zero. Furthermore, the core algorithm of EPT are based on the two order differential operation of Maxwell electromagnetic equations, which makes the noise amplified in the process of calculation and leads to the calculation results very sensitive to noise. To analyze the measurement error of the EPT technique, the tissue-mimicking materials that have precise dielectric properties are needed. However, the tissue-mimicking materials at the frequency typically used for MRI, such as 42.58MHz (IT),64MHz (1.5T),128MHz (3T),170MHz (4T),298MHz (7T), 400MHz (9.4T) and 468MHz (11T) are not studied in previous literatures and the dielectric properties are frequency-dependent. At the same time, the dielectric properties of the tissue-mimicking material are related to its component, making it a tedious work to simulate various human tissues (about hundreds of types, including healthy and tumor tissues).In this paper, the materails based on oil-in-gelatin dispersions and sodium chloride are used to simulate biological tissues. First, a group of mimicking materials, name as samples, are prepared. Then the dielectric properties of samples are measured at different frequencies to establish discrete database that describes the relationship between dielectric properties and its composition. Finally, based on the discrete base, the empirical formulae that describe the relationship between sample composition and dielectric properties are obtained using nonlinear least-squares method. According to the empirical formulae, it is easy to deduce the compositions of materials with arbitrary raletive permittivity and conductivity. The materials can mimic various human tissues within a wide range of frequencies and can be used to form heterogeneous anthropomorphic phantoms. The dielectric properties of the materials were measured at room temperature (25℃), so it is necessary to keep the materials at the same temperature in order to obtain precise mimicking of the temperature-dependent dielectric properties. Furthermore, the materials should be placed in the plastic containers and wrapped with preservative films to prevent the evaporation of water, so that these materials could remain the dielectric properties over extended periods. |
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