An investigation of the variation of dielectric properties of ovine lung tissue with temperature.

In this paper, the effects of coagulation and temperature on the dielectric properties of human blood are investigated over the frequency range of 400 MHz -20 GHz using freshly extracted blood samples. The dielectric properties are measured using blood in four different sample collection tubes (bottles): one containing pure whole blood, two containing different anticoagulant agents, and one containing clot activator and serum separator. The collected data indicates that additive agents can have a significant impact on the measured dielectric properties of blood, both immediately after the sample is taken, and over longer time periods. This is an important finding as it suggests that measurements of blood properties conducted on sample repositories, or tissue banks, may not be representative of natural blood properties. Further, the results demonstrate that the dielectric properties of normal blood vary over time due to coagulation. Different clotting rates lead to dielectric properties of female and male blood samples that vary distinctly over time. The results also show that the relative permittivity of the anti-coagulated blood decreases with increasing temperature, up to the cross-over point around 10 GHz where the trend reverses.

Medical devices making use of radio frequency (RF) and microwave (MW) fields have been studied as alternatives to existing diagnostic and therapeutic modalities since they offer several advantages. However, the lack of accurate knowledge of the complex permittivity of different biological tissues continues to hinder progress in of these technologies. The most convenient and popular measurement method used to determine the complex permittivity of biological tissues is the open-ended coaxial line, in combination with a vector network analyser (VNA) to measure the reflection coefficient (S11) which is then converted to the corresponding tissue permittivity using either full-wave analysis or through the use of equivalent circuit models. This paper proposes an innovative method of using artificial neural networks (ANN) to convert measured S11 to tissue permittivity, circumventing the requirement of extending the VNA measurement plane to the coaxial line open end. The conventional three-step calibration technique used with coaxial open-ended probes lacks repeatability, unless applied with extreme care by experienced persons, and is not adaptable to alternative sensor antenna configurations necessitated by many potential diagnostic and monitoring applications. The method being proposed does not require calibration at the tip of the probe, thus simplifying the measurement procedure while allowing arbitrary sensor design, and was experimentally validated using S11 measurements and the corresponding complex permittivity of 60 standard liquid and 42 porcine tissue samples. Following ANN training, validation and testing, we obtained a prediction accuracy of 5% for the complex permittivity.

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Dielectric properties of biological tissue are important in view of emerging medical applications. We report on dielectric properties of ex vivo porcine fat and muscle and the effect of 10% formalin and Thiel Embalming solution (TES) as preservatives on the dielectric properties. This study is important as understanding the effect of such preservative solutions would allow for measurements to be done even days after excision. In this study, dielectric measurements were conducted on fat and muscle samples before and after preservation. Measurements were conducted between 0.5 and 20 GHz. All results obtained were fitted to a Debye model and uncertainty limits analyzed carefully. We also present the percentage difference in dielectric properties of fresh tissue and preserved tissue. The results show that changes in the dielectric properties due to tissue preservation depend on the type of tissue studied, the technique used and the test frequency. In fact, an increase in the real part of permittivity ϵ′ of fat was observed compared with a decrease in that measured for muscle, when both preserved in TES. Moreover, the imaginary part of permittivity ϵ″ of muscle preserved in TES increases at low frequency but then decreases at frequencies higher than 10.8 GHz. The changes in the dielectric properties of fat and muscle when preserved in 10% formalin reach a constant value above 5 GHz.