Study of Dielectric Properties of Biological Tissues in the Microwave Frequency Range

The complex dielectric constants at room temperature of various goat tissues (liver, muscle, kidney, heart and brain) and corn syrup were measured in the frequency range 1 to 10 GHz with the help of a HP Network Analyzer N5230A. Open ended coaxial cable method was employed for the measurement. The system imperfections are completely avoided by calibrating the system with four known materials and their reflection coefficients were used in the calculation along with the reflection coefficient of the sample. The relaxation frequency in the δ region, spread of relaxation, volume fraction of protein present in tissues are calculated from the measured dielectric data. The dielectric constant of corn syrup samples suggests the feasibility of using corn syrup as a tissue equivalent for microwave imaging applications. Introduction and Scope Electrical properties of biological materials and their interaction with electromagnetic waves have attracted the attention of researchers working in the field of medicine and Electromagnetics. Studies indicate that microwaves have tremendous potential especially in the diagnostic and therapeutical medical applications. A number of innovative technologies employing radiowaves and microwaves are under investigation for the purpose of developing new techniques to improve detection, diagnosis and treatment of cancer. In order to understand the interaction of electromagnetic fields in tissues, it is important to know accurately its complex dielectric constant. 254 Elizabeth Rufus and Zachariah C Alex The published data in the literature shows a substantial contrast that exists between different types of tissues [4-7]. Even though some of the tissues electrical property is available in the microwave frequency range, it is not sufficient for its applications in medical field. Therefore we conducted a study to measure the complex dielectric property of various goat tissues (invitro) in the frequency range 1GHz to 10GHz with the help of a HP Network Analyzer 8510B. The dielectric property of corn syrup also in various concentrations were calculated and compared with that of the tissues to study the feasibility of it being used as a tissue equivalent. We adopted the open ended coaxial probe method to measure the dielectric properties since the coaxial probe technique does not require tissue manipulation or preparation. The end of the probe is simply placed in contact with the tissue sample and the complex input reflection coefficient is measured as a function of frequency using a vector network analyzer. The measured reflection coefficient is then converted to a complex permittivity using numerical models [10-15]. The present work is not only aimed at measuring the dielectric property information of various normal tissues, but to find out its more specific characteristics like relaxation frequency, spread of relaxation, volume fraction solid/water fraction in tissues, volume fraction of protein from this information. Materials and Method The dielectric constant measurement of biological samples at microwave frequencies is a difficult task. Different methods are reported for the accurate measurement of high loss samples. But most of the methods are suited only for a certain range of frequency [7]. In this work, we adopted the coaxial cable method, which can be used, for a wide range of frequencies with a single probe. Network analyzer HP8510B was used for the measurement. Theory of the technique is as follows. The lumped element approach was selected for measuring the reflection, which is related to the characteristic impedance of the transmission line and that of the sensor probe. The impedance of the sensor is a function of the frequency and relative permittivity of the test sample. Under quasi-static approximation the stationary formula for the aperture admittance of an open-ended coaxial line terminated by a semi-infinite medium on a ground plane reduces to