Dielectric Properties Of Biological Tissues Research Articles

Dielectric properties of muscle and adipose tissue-mimicking solutions for microwave medical imaging applications

The aim of the present study was to synthesize mixture solutions which can accurately mimic the dielectric properties of biological tissues, specifically muscle and adipose tissues between 500 MHz and 50 GHz. Mixtures utilizing concentrations of bovine serum albumin (BSA) dissolved in phosphate buffered saline (PBS) and Ringer’s solutions were synthesized to mimic in vivo and ex vivo muscle tissues. Solutions consisting of concentrations of peanut oil and Triton X-100 (TX) in PBS and Ringer’s solutions were also synthesized to mimic in vivo and ex vivo adipose tissue. Results were then analysed and compared to measured dielectric properties of in vivo and ex vivo biological tissues from another previous study. Good agreement with said dielectric property measurements were obtained since the solutions from this study yielded low RMSE and RMSRE values. This implied that such solutions can be utilized in the construction of human body phantoms for narrowband and ultra-wideband microwave devices for near field breast cancer imaging.

Important Aspects of Human Blood Exposure in the Radio and Microwave Field

Abstract To expose humans to microwave emissions damages biological welfare. For measuring the dielectric properties of biological tissues, it was studied whether current safety standard recommendations are inappropriate as far as human blood function is concerned. Thirty age and gender matched blood samples will serve as control. For increased accuracy detection of any possible alteration and to help orientate towards the details that are to be studied ultra-structurally, a special stain was used. An electronic microscope analysis will be performed on all the blood samples after specific preparation to see whether such microwave exposure might affect the ultra-structure of the blood cells. After modeling and sectioning the blocks were contrasted using acetate-uranyl and lead-citrate and study them with an electronic microscope. The obtained images are processed using Viewer Imaging Analysis software for further analysis. In the end, scanning microscopy must be used in order to detect any surface dimorphisms that might appear due to the microwave exposure.

Characterization of the dielectric properties of biological tissues using mixture equations and correlations to different states of hydration

The aim of this study was to characterize the hydration fractions of biological tissues and to model these accurately from mixture equations. Hydration fractions, better known as volume fractions, are based on quantification of tissue hydration and accurate knowledge about the physiological composition of tissue fluids. Data on weight loss percentages for excised muscle and adipose tissue from a previous study were utilized for this purpose. The Bruggeman and Maxwell Garnett equations were then used to characterize the dielectric properties of the tissues in terms of mixtures of dry biological constituents and physiological saline solutions. It is shown that these models are accurate in modelling in vivo and ex vivo tissue in different states of hydration. This is based on precise knowledge of the physiological composition of biological fluids and their corresponding percentage contents.

Feasibility of water content-based dielectric characterisation of biological tissues using mixture models

This study quantitatively examines the validity of mixture formulae as models to describe the microwave-range dielectric properties of biological tissue of varying water content. Mixture formulae, specifically the Maxwell Garnett and Bruggeman models, are used to predict the dielectric properties of ex-vivo bovine muscle and liver tissue samples varying in water content. The tissues are modelled as comprising of cell and macromolecule inclusions in a water matrix. The model predictions are compared to dielectric measurements performed with a network analyser and dielectric probe in the 0.5–8.5 GHz band. There was a poor match between the properties predicted by the models and the measured results at most frequency points, for both tissue types. However, the overall predicted and measured trends over the measured band correlated well. The Maxwell Garnett and Bruggeman models may prove a valuable tool aiding in the characterisation of the dielectric properties of materials with different water contents, however, currently, direct application of the model with the assumption of solid inclusions in a water matrix is not feasible without substantial improvement to the models. The dielectric properties of biological tissue are of fundamental importance for many medical technologies ranging from diagnostic to therapeutic. There is a need for continuous improvements to be made to the techniques used to measure and characterise the dielectric properties of tissues. Mixture models are investigated in this study as potentially a valuable candidate modelling technique for the dielectric profiling of tissues based on water content.

Simplified parametric models of the dielectric properties of brain and muscle tissue during electrical stimulation

Parametric models are commonly used to describe the dispersive dielectric properties of biological tissues. While distinct regions of dispersion have been identified, the relative contribution of each during electrical stimulation is unknown. This study quantified the contribution of individual poles in parametric models of brain and muscle dielectric properties during electrical stimulation. The effect on the extracellular voltage waveform and threshold current for nerve stimulation of selectively removing subsets of poles from Cole-Cole and Debye models was examined. Errors were introduced when dispersions below 100 kHz were removed in both brain and muscle tissue. Poles below 1 kHz influenced the amplitude of the extracellular voltage waveform and the predicted minimum stimulation current. Poles between 1 kHz and 100 kHz influenced the waveform shape, with a minor effect on stimulus amplitude. The results confirm that low frequency dispersion in conductivity and permittivity can fundamentally influence the electric field and neural response during stimulation and provide insight into the relative contribution of the different dispersive regimes. Furthermore, they provide justification for for simplifying parametric models of dielectric properties through the removal of high frequency poles above 100 kHz which could improve the efficiency of time-domain solvers for simulations involving time-varying or aperiodic stimuli as may be required for certain closed-loop stimulation paradigms.

Dielectric Characterization of <italic>In Vivo</italic> Abdominal and Thoracic Tissues in the 0.5–26.5 GHz Frequency Band for Wireless Body Area Networks

The dielectric properties of biological tissues are of utmost importance in the development of wireless body area networks (WBANs), especially for implanted devices. The early design stages of medical devices like capsule endoscopy, pacemakers, or physiological sensors rely on precise knowledge of the dielectric properties of the tissues present in their surrounding medium. Many of these applications make use of electromagnetic phantoms, which are software or physical models that imitate the shape and the electromagnetic properties of the tissues. They are used for designing devices in software simulations and for testing them in laboratory trials, aiding in both the development of WBAN antennas or in communication link evaluations. The existing reports about dielectric in vivo properties are limited and have drawbacks like: low variety of characterized tissues, lacking some relevant ones, and limitations and inhomogeneity in the measured frequency range. This paper aims at filling that gap by providing a new database of dielectric properties of biological tissues measured in vivo . In particular, it is focused on the tissues of the thoracic and the abdominal regions, measured at the same wide frequency band, on the same animal specimen, and under the same conditions. The properties have been obtained by measuring porcine tissues in the 0.5–26.5 GHz band with the open-ended coaxial technique. In this paper, we focus on those tissues that have been scarcely characterized so far in the literature, like heart, esophagus, stomach, and pancreas. The Cole–Cole fitting parameters of the measured tissues and their uncertainties are provided.

Characterization of the dielectric properties of biological tissues and their correlation to tissue hydration

The aim of this paper was to investigate the correlation between the dielectric properties of biological tissues and their different states of hydration. Measurements were conducted on rat muscle and fat tissues both in vivo and ex vivo, in different states of dehydration through a loss on drying technique. Dielectric parameters were observed as a function of frequency from 500 MHz to 50 GHz by utilizing an open-ended coaxial probe at constant room temperature of 25° C. By observing which dehydration fractions fall within in vivo values, it was deduced that ex vivo samples are reliable in representing the dielectric parameters of in vivo samples, under certain controlled hydration conditions whereas, further analysis should be conducted for tissue characterized by low water content.

Development of Water Content Dependent Tissue Dielectric Property Models.

We propose dielectric tissue property models dependent on both water and air content covering the microwave frequency range. Water is the largest constituent of biological tissues and its effect on the dielectric properties of biological tissue has been studied. However, dehydration effects due to thermal heating have not been fully characterized. We combined 1) Maxwell-Fricke mixture theory with a four-pole Cole-Cole equation to include water and air content dependency and as the second approach a different 2) Maxwell mixture model was coupled with a Debye function. The proposed approaches (1 and 2) were able to predict the permittivity (ε') and conductivity (σ) of bovine liver and swine lung tissues at different hydration and inflation states from 1-15 GHz. A second approach coupling Maxwell and Debye models required fewer assumptions and modelled tissue properties with higher accuracy (less than 15% mean percent error in all tissue types). These models may help improve the accuracy of microwave ablation simulation when tissue water content changes as a result of vaporization, and may facilitate personalized treatment planning.

Intercomparison of methods for measurement of dielectric properties of biological tissues with a coaxial sensor at millimeter-wave frequencies

Coaxial sensors are effective for measurement of dielectric properties of biological tissues. Several measurement methods used to derive dielectric properties have been investigated for the measurement with a coaxial sensor at microwave frequencies. While the measurement accuracy depends on the method used, there has been insufficient intercomparison of these methods and their model approximation errors. On the other hand, we have developed a coaxial sensor for the measurement of complex permittivity at millimeter-wave (MMW) frequencies of up to 100 GHz. However, the scarcity of reference data at MMW frequencies makes the validation of the measurement system difficult. Thus, it is essential to clarify the model approximation error of the method used in the measurement system, particularly at MMW frequencies. This study aims to clarify the model approximation errors of methods for dielectric property measurement using a coaxial sensor at MMW frequencies. The model approximation errors were assessed by comparing results obtained by the methods with those based on the theoretical formula of the full-wave modal expression of Maxwell’s equations. The measurement uncertainty for the theoretical formula was estimated for a standard liquid sample to clarify the contribution of the model approximation errors to the uncertainty. Furthermore, the methods were applied to the measurement of porcine tissues at body temperature, and the measurement accuracy and usability for measurement at MMW frequencies are discussed.

Dielectric properties of colon polyps, cancer, and normal mucosa: Ex vivo measurements from 0.5 to 20GHz.

Colorectal cancer is highly preventable by detecting and removing polyps, which are the precursors. Currently, the most accurate test is colonoscopy, but still misses 22% of polyps due to visualization limitations. In this paper, we preliminary assess the potential of microwave imaging and dielectric properties (e.g., complex permittivity) as a complementary method for detecting polyps and cancer tissue in the colon. The dielectric properties of biological tissues have been used in a wide variety of applications, including safety assessment of wireless technologies and design of medical diagnostic or therapeutic techniques (microwave imaging, hyperthermia, and ablation). The main purpose of this work is to measure the complex permittivity of different types of colon polyps, cancer, and normal mucosa in exvivo human samples to study if the dielectric properties are appropriate for classification purposes. The complex permittivity of freshly excised healthy colon tissue, cancer, and histological samples of different types of polyps from 23 patients was characterized using an open-ended coaxial probe between 0.5 and 20GHz. The obtained measurements were classified into five tissue groups before applying a data reduction step with a frequency dispersive single-pole Debye model. The classification was finally compared with pathological analysis of tissue samples, which is the gold standard. The complex permittivity progressively increases as the tissue degenerates from normal to cancer. When comparing to the gold-standard histological tissue analysis, the sensitivity and specificity of the proposed method is the following: 100% and 95% for cancer diagnosis; 91% and 62% for adenomas with high-grade dysplasia; 100% and 61% for adenomas with low-grade dysplasia; and 100% and 74% for hyperplastic polyps, respectively. In addition, complex permittivity measurements were independent of the lesion shape and size, which is also an interesting property comparing to current colonoscopy techniques. The contrast in complex permittivities between normal and abnormal colon tissues presented here for the first time demonstrate the potential of these measurements for tissue classification. It also opens the door to the development of a microwave endoscopic device to complement the outcomes of colonoscopy with functional tissue information.

Open-Ended Coaxial Probe Technique for Dielectric Measurement of Biological Tissues: Challenges and Common Practices.

Electromagnetic (EM) medical technologies are rapidly expanding worldwide for both diagnostics and therapeutics. As these technologies are low-cost and minimally invasive, they have been the focus of significant research efforts in recent years. Such technologies are often based on the assumption that there is a contrast in the dielectric properties of different tissue types or that the properties of particular tissues fall within a defined range. Thus, accurate knowledge of the dielectric properties of biological tissues is fundamental to EM medical technologies. Over the past decades, numerous studies were conducted to expand the dielectric repository of biological tissues. However, dielectric data is not yet available for every tissue type and at every temperature and frequency. For this reason, dielectric measurements may be performed by researchers who are not specialists in the acquisition of tissue dielectric properties. To this end, this paper reviews the tissue dielectric measurement process performed with an open-ended coaxial probe. Given the high number of factors, including equipment- and tissue-related confounders, that can increase the measurement uncertainty or introduce errors into the tissue dielectric data, this work discusses each step of the coaxial probe measurement procedure, highlighting common practices, challenges, and techniques for controlling and compensating for confounders.

Investigation of histology radius for dielectric characterisation of heterogeneous materials

Knowledge of the dielectric properties of biological tissues is fundamental in the design of novel electromagnetic-based medical devices. Tissue dielectric properties are typically measured using the open-ended coaxial probe technique, which is designed for homogeneous samples. Histological analysis may be conducted to associate the measured dielectric properties to different tissue types within heterogeneous samples. However, the histology radius (i.e., the radial extent of the tissue sample that undergoes histological analysis) has not been consistently defined in the literature; therefore, this parameter may be a source of error in dielectric data. For this reason, we investigate the histology radius of various heterogeneous samples. Dielectric measurements were conducted over the frequency range of 0.5 to 20 GHz on radially heterogeneous tissue-mimicking materials and biological tissues, with different dielectric properties and contrasts. The experimental results were validated with numerical simulations and indicate that: i) the histology radius does not exceed the probe radius; ii) the dielectric properties of radially heterogeneous tissues depend on the spatial distribution of each material within the histology radius; and iii) the bulk dielectric properties of concentric heterogeneous tissues highly depend on the properties of each constituent material within the histology radius. This study supports consistent identification of the histology radius and provides a basis for rigorous interpretation of the dielectric properties of heterogeneous tissues.

An anatomically accurate dielectric profile of the porcine kidney

Microwave ablation is a promising treatment for kidney cancer. Accurate knowledge of the dielectric properties of biological tissues is vital for quantifying the safety, reliability and accuracy of ablation, among other microwave medical treatments and diagnostic technologies. In dielectric studies to date, the heterogeneity within the kidney has not been considered, and the kidney has been treated as a fully homogeneous organ. Therefore, the available dielectric data of the kidney is not as thorough and accurate as it could be. For this reason, in this study, dielectric measurements are performed over a broad frequency range to quantitatively investigate the difference between the dielectric properties of various regions of the kidney and to develop an anatomically accurate dielectric profile of the kidney. All measurements are conducted on freshly excised porcine kidney samples, and confounders impacting dielectric data are controlled and related metadata recorded. The results demonstrate a considerable difference of up to 49% between the dielectric properties of different regions of the kidney. The findings in this paper suggest that the heterogeneity within the kidney should be taken into consideration in order to obtain an accurate representation of the actual dielectric properties. Additionally, a two-pole Cole-Cole model is fitted to the measured data of the different regions of the kidney and the model parameters presented for reference. The anatomically accurate dielectric profile of kidney provided in this paper will support the development of more effective and reliable microwave medical treatments.

Dielectric Properties of <italic>Ex Vivo</italic> Porcine Liver Tissue Characterized at Frequencies Between 5 and 500 kHz When Heated at Different Rates

The released energy during radio frequency thermal ablation therapy changes the dielectric properties of biological tissues. Understanding changes of dielectric properties of biological tissues during heating is fundamental to suitably model the medical procedure. The aim of this work is to obtain the thermal dependences of conductivity and permittivity of ex vivo porcine liver tissue at six frequencies from 5 to 500kHz, during heating from 37 °C to 100 °C at three heating rates of approximately 0.1, 3, and 10 °C/min. Two experimental setups using different heating sources and a four-needle electrode connected to an impedance analyzer were developed to evaluate the thermal dependencies. The results at a body temperature of 37 °C show a good agreement with the data reported in the literature. The conductivity initially shows an increase followed by a decrease, whereas the permittivity increases before a subsequent sharp decrease. Above 60°C, different trends are observed for the three heating rates studied. The electric conductivity and permittivity show a similar behavior at all evaluated frequencies and heating rates. The observed abrupt change of the slope near 45 °C at a slow heating rate may be used to identify the region of reversible changes in the tissue. These results confirm the connection among tissue dielectric properties, working frequency, and exposure time with thermal damage during heating.

A study of the effects of preservative solutions on the dielectric properties of biological tissue

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 of fat was observed compared with a decrease in that measured for muscle, when both preserved in TES. Moreover, the imaginary part of permittivity 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.

Minimum information for dielectric measurements of biological tissues (MINDER): A framework for repeatable and reusable data

The dielectric properties of biological tissues characterise the interaction of human tissues with electromagnetic (EM) fields. Accurate knowledge of the dielectric properties of tissues are vital in EM-based therapeutic and diagnostic techniques, and for assessing the safety of wireless devices. Despite the importance of these properties, the field has suffered from inconsistencies in reported data. The dielectric measurement process for tissues is known to be affected by both measurement confounders and clinical confounders; however, adequate metadata is often lacking in the literature. For this reason, this work proposes a standard, called Minimum Information for Dielectric Measurements of Biological Tissues (MINDER). In the MINDER model, the minimum types of raw data and metadata needed to interpret or replicate a dielectric study are identified and described. Alongside the minimum information model, a controlled vocabulary for metadata parameters is proposed. We also provide an example of this model applied to a dielectric measurement scenario on a biological tissue sample. The MINDER model enables reproducibility of measurements, ease of interpreting and re-using data, and comparison of data across studies. Further, this standard framework will support dielectric databases, with data searchable through metadata parameters such as temperature, frequency range, tissue type, and tissue state.

Modeling of the dielectric properties of biological tissues within the histology region

The dielectric properties of biological tissues characterize the interaction of electromagnetic fields with the human body. As such, accurate knowledge of these properties is vital to the design and development of electromagnetic medical technologies. Despite their importance, the reported dielectric properties of key tissues have been inconsistent across studies. The natural heterogeneity of tissue has been identified as a contributing factor to these inconsistencies. In order to attribute dielectric properties to heterogeneous tissues, histological analysis is conducted to determine which tissue types contribute to the dielectric measurement. However, the Histology Region, i.e., the volume of the tissue sample that undergoes histological analysis, and the region corresponding to the dielectric measurement, has not been well-defined. Thus, instead of reducing uncertainties, more questions have been raised about the accuracy of data: if the Histology Region is not identified correctly, then the corresponding dielectric measurement does not represent the actual tissues involved. In this work, we examine the longitudinal extent of the Histology Region (i.e., the histology depth) for various heterogeneous samples composed of phantoms and biological tissues. This study highlights the fact that the relationship between the volume of tissue in a sample and the contribution of that tissue to the measured dielectric properties is not linear. Assuming that this relationship is linear may be a significant source of error in dielectric data. Further, we model, for the first time, the nonlinear relationship between the contribution of individual tissues to the dielectric measurement and the volume that each tissue occupies within the bulk sample. This work enables prediction of the permittivity of a sample with longitudinal heterogeneities, given knowledge of the constituent tissues of the sample, and provides the basis for modeling of all types of heterogeneities. These results will contribute to the minimization of uncertainties in the dielectric measurement of heterogeneous tissues.

Near-field interference microwave diagnostics

The article explores the dimensions of the probing region of two coaxial probes during the measurement of the dielectric properties of biological tissues and media at microwave radiation. This region is formed in the overlapping evanescent fields of the probes.

Closed-form equation to estimate the dielectric properties of biological tissues as a function of age.

Developing microwave systems for biomedical applications requires accurate dielectric properties of biological tissues for reliable modeling before prototyping and subject testing. Dielectric properties of tissues decrease with age due to the change in their water content, but there are no detailed age-dependent data, especially for young tissue-like newborns, in the literature. In this article, an age-dependent formula to predict the dielectric properties of biological tissues was derived. In the proposed method, the variation of water concentration in each type of tissue as a function of age was used to calculate its relative permittivity and conductivity. The derived formula shows that the concentration of water in each tissue type can be modeled as a negative exponential function of age. The dielectric properties of each tissue type can then be calculated as a function of the dielectric properties of water and dielectric properties of the organ forming the tissue and its water concentration. The derived formula was used to generate the dielectric properties of several types of human tissues at different ages using the dielectric properties of a human adult. Moreover, the formula was validated on pig tissues of different ages. A close agreement was achieved between the calculated and measured data with a maximum difference of only 2%. Bioelectromagnetics. 38:474-481, 2017. © 2017 Wiley Periodicals, Inc.

Applying Broadband Dielectric Spectroscopy (BDS) for the Biophysical Characterization of Mammalian Tissues under a Variety of Cellular Stresses.

The dielectric properties of biological tissues can contribute non-invasively to a better characterization and understanding of the structural properties and physiology of living organisms. The question we asked, is whether these induced changes are effected by an endogenous or exogenous cellular stress, and can they be detected non-invasively in the form of a dielectric response, e.g., an AC conductivity switch in the broadband frequency spectrum. This study constitutes the first methodological approach for the detection of environmental stress-induced damage in mammalian tissues by the means of broadband dielectric spectroscopy (BDS) at the frequencies of 1–106 Hz. Firstly, we used non-ionizing (NIR) and ionizing radiation (IR) as a typical environmental stress. Specifically, rats were exposed to either digital enhanced cordless telecommunication (DECT) radio frequency electromagnetic radiation or to γ-radiation, respectively. The other type of stress, characterized usually by high genomic instability, was the pathophysiological state of human cancer (lung and prostate). Analyzing the results of isothermal dielectric measurements provided information on the tissues’ water fraction. In most cases, our methodology proved sufficient in detecting structural changes, especially in the case of IR and malignancy. Useful specific dielectric response patterns are detected and correlated with each type of stress. Our results point towards the development of a dielectric-based methodology for better understanding and, in a relatively invasive way, the biological and structural changes effected by radiation and developing lung or prostate cancer often associated with genomic instability.