A review of studies investigating the dielectric properties of biological tissues for application in hyperthermia and microwave thermal ablation

Heating of biological tissues beyond 40 C has become an established method of treating a number of diseases, most notably tumours, where hyperthermia and thermal ablation are important modalities. In some interventions, tissue temperatures reached can even go beyond 100 C, and demand precise knowledge of tissue dielectric properties and how these vary with frequency and temperature in order to facilitate accurate computational simulations for preclinical planning. This paper reviews the available literature concerning dielectric properties of biological tissues and their temperature dependence, focusing on the frequencies of 915 MHz and 2.45 GHz, at which most of the studies reviewed investigate predominantly liver tissue. In this review a comparative analysis of the results obtained by different research groups are presented in the different studies is also made, indicating possible limiting factors in the different studies. These studies propose a number of different models which could be used to describe temperature dependence. Due to the prevalence of liver investigations, it would be ideal to conduct further studies on different biological tissues.peer-reviewe

Application of artificial neural networks for accurate determination of the complex permittivity of biological tissue

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.peer-reviewe

The synthesis of a liver tissue mimicking solution for microwave medical applications

This paper presents the synthesis of a mixture solution that is equivalent to ex-vivo liver tissue dielectric characteristics between 500 MHz and 5 GHz. The mimicking solution was synthesized using concentrations of two chemicals, the solute which is referred to as the inclusion phase and the solvent, referred to as the host phase. The inclusion phase consisted of bovine serum albumin (BSA) powder and the host phase consisted of a phosphate-buffered saline (PBS) solution with a concentration of Triton X-100 (TX-100). The dielectric properties of these two phases were substituted into Bruggemanʼs two-phase mixture equation to estimate the dielectric properties of excised liver. Furthermore, the study exploits Bruggeman’s equation to investigate the impact of tissue dehydration levels on the dielectric properties of an excised tissue. The effect of dehydration has been characterised as a function of time based on the loss-on-drying technique (a substance is heated until it is completely dry). Dielectric parameters were measured as a function of frequency using the Slim Form open-ended coaxial probe at a constant room temperature of circa 25°C. Measured dielectic data were fitted to the Cole-Cole model and good agreement with the mimicking solutions was obtained. These results indicate that these solutions can be used to model the human body phantoms for microwave medical applications.peer-reviewe

A numerical investigation of the dispersion law of materials by means of multi-length TDR data

In this paper, we propose a method for retrieving the dispersion law of a material under test from multi-length TDR measurements in reflection mode, repeated at several frequencies. By replacing the multi-frequency measurements with measurements using multi-length TDR probe, it is possible to retrieve the complex equivalent permittivity of the material in a frequency band of interest. The proposed procedure does not require a priori knowledge of the type of dispersion law of the material, which instead can possibly be inferred from the measured data. The algorithm is validated using numerically simulated data obtained with the commercial code CST Microstudio®.peer-reviewe

Effects of standard coagulant agents on the dielectric properties of fresh human blood

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.peer-reviewe

A method for extracting Debye parameters as a tool for monitoring watered and contaminated soils

Soil monitoring is a key topic from several perspectives, such as moisture level control for irrigation management and anti-contamination purposes. Monitoring the latter is becoming even more important due to increasing environmental pollution. As a direct consequence, there is a strong demand for innovative monitoring systems that are low cost, provide for quasi-real time and in situ monitoring, high sensitivity, and adequate accuracy. Starting from these considerations, this paper addresses the implementation of a microwave reflectometry based-system utilizing a customized bifilar probe and a miniaturized Vector Network Analyzer (m-VNA). The main objective is to relate frequency-domain (FD) measurements to the features of interest, such as the water content and/or the percentage of some polluting substances, through an innovative automatable procedure to retrieve the Debye dielectric parameters of the soil under different conditions. The results from this study confirm the potential of microwave reflectometry for moisture monitoring and contamination detection.peer-reviewe

Measurement and image-based estimation of dielectric properties of biological tissues — past, present, and future —

The dielectric properties of biological tissues are fundamental pararmeters that are essential for electromagnetic modeling of the human body. The primary database of dielectric properties compiled in 1996 on the basis of dielectric measurements at frequencies from 10 Hz to 20 GHz has attracted considerable attention in the research field of human protection from non-ionizing radiation. This review summarizes findings on the dielectric properties of biological tissues at frequencies up to 1 THz since the database was developed. Although the 1996 database covered general (normal) tissues, this review also covers malignant tissues that are of interest in the research field of medical applications. An intercomparison of dielectric properties based on reported data is presented for several tissue types. Dielectric properties derived from image-based estimation techniques developed as a result of recent advances in dielectric measurement are also included. Finally, research essential for future advances in human body modeling is discussed.peer-reviewe

Current Status and Emerging Techniques for Measuring the Dielectric Properties of Biological Tissues

The dielectric properties of biological tissues are key parameters that support the design and usability of a wide range of electromagnetic-based medical applications, including for diagnostics and therapeutics, and allow the determination of safety and health effects due to exposure to electromagnetic fields. While an extensive body of literature exists that reports on values of these properties for different tissue types under different measurement conditions, it is now evident that there are large uncertainties and inconsistencies between measurement reports. Due to varying measurement techniques, limited measurement validation strategies, and lack of metadata reporting and confounder control, reported dielectric properties suffer from a lack of repeatability and questionable accuracy. Recently, the American Society of Mechanical Engineers (ASME) Thermal Medicine Standards Committee was formed, which included a Tissue Properties working group. This effort aims to support the translation and commercialization of medical technologies, through the development of a standard lexicon and standard measurement protocols. In this work, we present initial results from the Electromagnetic Tissue Properties subgroup. Specifically, this paper reports a critical gap analysis facing the standardization pathway for the dielectric measurement of biological tissues. All established measurement techniques are examined and compared, and emerging ones are assessed. Perspectives on the importance and challenges in measurement validation, accuracy calculation, metadata collection, and reporting are also discussed

Dielectric properties of fresh human blood

The dielectric properties of blood are extremely important for electromagnetic medical applications such as imaging, hyperthermia and ablation. The aim of this paper is to investigate the dielectric properties of human blood and examine the effects of anti-coagulant agents over a broad frequency range. The measurements were performed using freshly extracted human blood samples with and without the added agents. The measurements were performed within 20 mins after extracting the blood sample. The obtained results demonstrate a significant variation between the dielectric properties of blood with and without anti-coagulant agents. The difference is larger in conductivity than in relative permittivity. The results also demonstrate clear variation in the dielectric properties of female and male blood samples. The findings in this paper suggest that measurements conducted on blood samples with the added agents may not represent the natural in-vivo and inter-patient variations in blood properties. In order to have an accurate representation of in-vivo blood dielectric properties, all factors should be considered carefully.peer-reviewe