Tissue Dielectric Properties Research Articles

A method for in vivo detection of abnormal subepidermal tissues based on dielectric properties.

In this paper, a convenient and noninvasive scanning imaging method using microwave frequency band for abnormal subepidermal tissues detection is proposed in the aim to improve diagnosis and prognosis in clinical environments. This method is based on the reflective detection technology with coaxial probe that is used to measure the dielectric properties of tissues. An improved equivalent circuit and simulated annealing algorithm (SA) were used in this work to analyze the dielectric properties of tissues. Computational simulations incorporating a simplified model of subcutaneous hemorrhage described in this work were used to evaluate this method. The dielectric properties data of tissues in the model of simulation is derived from the literature. The simulation results demonstrated the potential of this method to detect abnormal subepidermal tissues conveniently and expose them in the image accurately.

Are Children More Exposed to Radio Frequency Energy From Mobile Phones Than Adults?

There has been long-standing controversy, both among scientists and in the public, about whether children absorb more radio frequency (RF) energy in their heads than adults when using a mobile telephone. This review summarizes the current understanding of this issue, and some of the complexities in comparing the absorption of RF energy in different individuals from use of mobile phones. The discussion is limited to dosimetric issues, i.e., possible age-related differences in absorption of RF energy in the heads of mobile phone users. For most metrics of exposure, in particular those relevant to assessing the compliance of handsets with regulatory limits, there is no clear evidence for age-related differences in exposure. For two metrics of exposure, there is a clear evidence that age can play a factor: 1) the local specific absorption rate (SAR), in particular anatomically defined locations within the brain, will vary with head size and hence with age and 2) the SAR, in particular tissues, (e.g., bone marrow in the skull) can vary with age due to age-related differences in the dielectric properties of tissue. However, these differences involve SAR levels that are below the 1-g or 10-g peak spatial SAR (psSAR averaged over 1 or 10 g of tissue) and have no significance for compliance assessment. Age-related differences observed in worst case simulations such as presently considered are difficult to generalize to human populations under real-world exposure conditions due to many variables that determine SAR during realistic usages.

Development of Anatomically Realistic Numerical Breast Phantoms Based on T1- and T2-Weighted MRIs for Microwave Breast Cancer Detection

Anatomically realistic computational electromagnetics models are important tools for testing the feasibility and robustness of image algorithms in microwave breast cancer detection. In this letter, we present the structural development process of a novel anatomically realistic 2-D numerical breast phantom. This phantom is derived from T1- and T2-weighted MRIs of patients. T1 and T2 images for the same slice position are synthesized and transformed into uniform grids mapping to the corresponding dielectric properties of tissues via using edge-detection calibration algorithm and tissue deviation masks. The output grids are compatible with the widely used piecewise-linear mapping from voxel intensities to dielectric properties. The 2-D finite-difference time-domain (2D-FDTD) simulation results of this phantom verify the feasibility of this model. Compared to the existing phantoms, the proposed method is able to export anatomically realistic skin, suspected tumor, and chest wall, which contributes to the improvement of model accuracy.

Review of temperature dependence of thermal properties, dielectric properties, and perfusion of biological tissues at hyperthermic and ablation temperatures.

The application of supraphysiological temperatures (>40°C) to biological tissues causes changes at the molecular, cellular, and structural level, with corresponding changes in tissue function and in thermal, mechanical and dielectric tissue properties. This is particularly relevant for image-guided thermal treatments (e.g. hyperthermia and thermal ablation) delivering heat via focused ultrasound (FUS), radiofrequency (RF), microwave (MW), or laser energy; temperature induced changes in tissue properties are of relevance in relation to predicting tissue temperature profile, monitoring during treatment, and evaluation of treatment results. This paper presents a literature survey of temperature dependence of electrical (electrical conductivity, resistivity, permittivity) and thermal tissue properties (thermal conductivity, specific heat, diffusivity). Data of soft tissues (liver, prostate, muscle, kidney, uterus, collagen, myocardium and spleen) for temperatures between 5 to 90°C, and dielectric properties in the frequency range between 460 kHz and 3 GHz are reported. Furthermore, perfusion changes in tumors including carcinomas, sarcomas, rhabdomyosarcoma, adenocarcinoma and ependymoblastoma in response to hyperthmic temperatures up to 46°C are presented. Where appropriate, mathematical models to describe temperature dependence of properties are presented. The presented data is valuable for mathematical models that predict tissue temperature during thermal therapies (e.g. hyperthermia or thermal ablation), as well as for applications related to prediction and monitoring of temperature induced tissue changes.

INVESTIGATION OF CLASSIFIERS FOR TUMOR DETECTION WITH AN EXPERIMENTAL TIME-DOMAIN BREAST SCREENING SYSTEM

In this work we examine, for the flrst time, the use of classiflcation algorithms for early- stage tumor detection with an experimental time-domain microwave breast screening system. The experimental system contains a 16-element antenna array, and testing is done on breast phantoms that mimic breast tissue dielectric properties. We obtain experimental data from multiple breast phantoms with two possible tumor locations. In this work, we investigate a method for detecting the tumors within the breast but without the usual complexity inherent to image-generation methods, and conflrm its feasibility on experimental data. The proposed method uses machine learning techniques, namely Support Vector Machines (SVM) and Linear Discriminant Analysis (LDA), to determine whether the current breast being scanned is tumor-free. Our results show that both SVM and LDA methods have promise as algorithms supporting early breast cancer microwave screening.

Quantitative Thermoacoustic Tomography for ex vivo Imaging Conductivity of Breast Tissue

We assess the feasibility of our developed quantitative thermoacoustic tomography (qTAT) system for breast tumor characterization using mastectomy specimens before the clinical investigation. A circular scanning TAT system coupled with a finite-element based reconstruction algorithm is used to recover the dielectric property distribution of normal and tumor tissues from three female subjects who underwent mastectomy. Statistical method is used to analyze the tissue dielectric properties obtained. The recovered qTAT images reveal large contrast in conductivity between tumor and normal breast tissues. In addition, significant difference in conductivity exists among all the specimens examined. Finally, the recovered tumor size for these specimens agrees well with their exact size. This preclinical evaluation suggests that it is feasible to detect and characterize a breast tumor quantitatively with our qTAT method.

Improved circuit model of open-ended coaxial probe for measurement of the biological tissue dielectric properties between megahertz and gigahertz

This note describes an improved equivalent circuit analysis model for open-ended coaxial probes for measurement of the dielectric properties of biological tissues below the gigahertz level. Some parameters in the conventional model that influence the measurement results were found to be still relative to the dielectric properties of the test sample and the terminal admittance of the probe was found to be dependent on the frequency. This was not found to be the case with the conventional model. According to the simulation results in frequency range from 30 MHz to 1 GHz, a polynomial expression was found to fit the frequency–admittance curve for terminal admittance and the equivalent circuit expression of probe terminal admittance was finally modified. The simulated annealing algorithm was used to calculate the dielectric properties of the new expression. The accuracy of the improved model was validated through a simulation test and experiment based on a series of solutions over 30 MHz–1 GHz. The new model was compared to the conventional model and was found to provide more accurate permittivity estimation over a wider frequency range than the conventional model if said range was between megahertz and gigahertz.

Study of the Effects of Changing Physiological Conditions on Dielectric Properties of Breast Tissues

This paper addresses the changes in the physical characteristics (temperature and water/blood content) of breast tissue under different physiological conditions. We examined ex vivo specimens of breast tissue excised at the time of surgery to study the effects of physiological conditions on dielectric properties. We observed that the dielectric properties strongly depend on tissue physiological state. When the biological tissues undergo physiological changes, such as those due to disease or those induced by external changes such as variations in the environmental temperature, the microscopic processes deviate from their normal state and impact the overall dielectric properties. This suggests that microwave imaging might be used to monitor the physiological conditions of the body.

The role of skin conductivity in a low frequency exposure assessment for peripheral nerve tissue according to the ICNIRP 2010 guidelines

Based on numerical computations using commercially available finite difference time domain code and a state-of-the art anatomical model of a 5-year old child, the influence of skin conductivity on the induced electric field strength inside the tissue for homogeneous front-to-back magnetic field exposure and homogeneous vertical electric field exposure was computed. Both ungrounded as well as grounded conditions of the body model were considered. For electric field strengths induced inside CNS tissue the impact of skin conductivity was found to be less than 15%. However, the results demonstrated that the use of skin conductivity values as obtainable from the most widely used data base of dielectric tissue properties and recommended by safety standards are not suitable for exposure assessment with respect to peripheral nerve tissue according to the ICNIRP 2010 guidelines in which the use of the induced electric field strengths inside the skin is suggested as a conservative surrogate for peripheral nerve exposure. This is due to the fact that the skin conductivity values derived from these data bases refer to the stratum corneum, the uppermost layer of the skin, which does not contain any nerve or receptor cells to be protected from stimulation effects. Using these skin conductivity values which are approximately a factor 250–500 lower than skin conductivity values used in studies on which the ICNIRP 2010 guidelines are based on, may lead to overestimations of the induced electric field strengths inside the skin by substantially more than a factor of 10. However, reliable conductivity data of deeper skin layers where nerve and preceptor cells are located is very limited. It is therefore recommended to include appropriate background information in the ICNIRP guidelines and the dielectric tissue property databases, and to put some emphasis on a detailed layer-specific characterization of skin conductivity in near future.

Dielectric properties of muscle and liver from 500 MHz–40 GHz

Dielectric properties are the most important parameters determining energy deposition when biological tissues are exposed to radio frequency and microwave fields. Energy absorption is determined by the specific absorption rate (SAR). SAR distributions can be computed accurately only if the complex relative permittivity of the target tissue is known to a sufficiently high accuracy, and currently there is a lack of data on the dielectric properties of biological tissues at high frequencies. In this study, tissue dielectric properties are measured using an open-ended coaxial probe technique from 500 MHz up to 40 GHz. We present dielectric data for ex vivo bovine and porcine muscle and liver tissues at 37 °C. One-pole Cole–Cole model is used to fit the measured data as a function of frequency and the dispersion parameters are presented. This data is supported by an accurate study on reference liquids such as methanol and ethanediol.

Swedish review strengthens grounds for concluding that radiation from cellular and cordless phones is a probable human carcinogen

With 5.9 billion reported users, mobile phones constitute a new, ubiquitous and rapidly growing exposure worldwide. Mobile phones are two-way microwave radios that also emit low levels of electromagnetic radiation. Inconsistent results have been published on potential risks of brain tumors tied with mobile phone use as a result of important methodological differences in study design and statistical power. Some studies have examined mobile phone users for periods of time that are too short to detect an increased risk of brain cancer, while others have misclassified exposures by placing those with exposures to microwave radiation from cordless phones in the control group, or failing to attribute such exposures in the cases. In 2011, the World Health Organization, International Agency for Research on Cancer (IARC) advised that electromagnetic radiation from mobile phone and other wireless devices constitutes a “possible human carcinogen,” 2B. Recent analyses not considered in the IARC review that take into account these methodological shortcomings from a number of authors find that brain tumor risk is significantly elevated for those who have used mobile phones for at least a decade. Studies carried out in Sweden indicate that those who begin using either cordless or mobile phones regularly before age 20 have greater than a fourfold increased risk of ipsilateral glioma. Given that treatment for a single case of brain cancer can cost between $100,000 for radiation therapy alone and up to $1 million depending on drug costs, resources to address this illness are already in short supply and not universally available in either developing or developed countries. Significant additional shortages in oncology services are expected at the current growth of cancer. No other environmental carcinogen has produced evidence of an increased risk in just one decade. Empirical data have shown a difference in the dielectric properties of tissues as a function of age, mostly due to the higher water content in children's tissues. High resolution computerized models based on human imaging data suggest that children are indeed more susceptible to the effects of EMF exposure at microwave frequencies. If the increased brain cancer risk found in young users in these recent studies does apply at the global level, the gap between supply and demand for oncology services will continue to widen. Many nations, phone manufacturers, and expert groups, advise prevention in light of these concerns by taking the simple precaution of “distance” to minimize exposures to the brain and body. We note than brain cancer is the proverbial “tip of the iceberg”; the rest of the body is also showing effects other than cancers.

Microwave imaging for neoadjuvant chemotherapy monitoring: initial clinical experience

IntroductionMicrowave tomography recovers images of tissue dielectric properties, which appear to be specific for breast cancer, with low-cost technology that does not present an exposure risk, suggesting the modality may be a good candidate for monitoring neoadjuvant chemotherapy.MethodsEight patients undergoing neoadjuvant chemotherapy for locally advanced breast cancer were imaged longitudinally five to eight times during the course of treatment. At the start of therapy, regions of interest (ROIs) were identified from contrast-enhanced magnetic resonance imaging studies. During subsequent microwave examinations, subjects were positioned with their breasts pendant in a coupling fluid and surrounded by an immersed antenna array. Microwave property values were extracted from the ROIs through an automated procedure and statistical analyses were performed to assess short term (30 days) and longer term (four to six months) dielectric property changes.ResultsTwo patient cases (one complete and one partial response) are presented in detail and demonstrate changes in microwave properties commensurate with the degree of treatment response observed pathologically. Normalized mean conductivity in ROIs from patients with complete pathological responses was significantly different from that of partial responders (P value = 0.004). In addition, the normalized conductivity measure also correlated well with complete pathological response at 30 days (P value = 0.002).ConclusionsThese preliminary findings suggest that both early and late conductivity property changes correlate well with overall treatment response to neoadjuvant therapy in locally advanced breast cancer. This result is consistent with earlier clinical outcomes that lesion conductivity is specific to differentiating breast cancer from benign lesions and normal tissue.

A Feasibility Study for Microwave Breast Cancer Detection Using Contrast-Agent-Loaded Bacterial Microbots

We propose a new approach to microwave breast tumor sensing and diagnosis based on the use of biocompatible flagellated magnetotactic bacteria (MTB) adapted to operate in human microvasculature. It has been verified experimentally by Martel et al. that externally generated magnetic gradients could be applied to guide the MTB along preplanned routes inside the human body, and a nanoload could be attached to these bacterial microbots. Motivated by these useful properties, we suggest loading a nanoscale microwave contrast agent such as carbon nanotubes (CNTs) or ferroelectric nanoparticles (FNPs) onto the MTB in order to modify the dielectric properties of tissues near the agent-loaded bacteria. Subsequently, we propose a novel differential microwave imaging (DMI) technique to track simultaneously multiple swarms of MTB microbots injected into the breast. We also present innovative strategies to detect and localize a breast tissue malignancy and estimate its size via this DMI-trackable bacterial microrobotic system. Finally, we use an anatomically realistic numerical breast phantom as a platform to demonstrate the feasibility of this tumor diagnostic method.

Dielectric sensor for in vivo tissue characterisation

The design of a simple dielectric sensor for in vivo tissue dielectric property measurement is provided. The design is based on an impedance sensor.

Dielectric properties of porcine glands, gonads and body fluids

Dielectric properties of porcine glandular tissues and gonads (in vivo) and body fluids (in vitro) have been obtained in the frequency range of 50 MHz to 20 GHz. The experimental data were fitted to a two term Cole–Cole expression. The data presented complement the available dielectric properties of tissues in the literature and can be used in numerical simulations of the exposure of people to electromagnetic fields.

A genetic algorithm for optimizing multi-pole Debye models of tissue dielectric properties

Models of tissue dielectric properties (permittivity and conductivity) enable the interactions of tissues and electromagnetic fields to be simulated, which has many useful applications in microwave imaging, radio propagation, and non-ionizing radiation dosimetry. Parametric formulae are available, based on a multi-pole model of tissue dispersions, but although they give the dielectric properties over a wide frequency range, they do not convert easily to the time domain. An alternative is the multi-pole Debye model which works well in both time and frequency domains. Genetic algorithms are an evolutionary approach to optimization, and we found that this technique was effective at finding the best values of the multi-Debye parameters. Our genetic algorithm optimized these parameters to fit to either a Cole–Cole model or to measured data, and worked well over wide or narrow frequency ranges. Over 10 Hz–10 GHz the best fits for muscle, fat or bone were each found for ten dispersions or poles in the multi-Debye model. The genetic algorithm is a fast and effective method of developing tissue models that compares favourably with alternatives such as the rational polynomial fit.

Design and dosimetric analysis of a 385 MHz TETRA head exposure system for use in human provocation studies

A new head exposure system for double-blind provocation studies investigating possible effects of terrestrial trunked radio (TETRA)-like exposure (385 MHz) on central nervous processes was developed and dosimetrically analyzed. The exposure system allows localized exposure in the temporal brain, similar to the case of operating a TETRA handset at the ear. The system and antenna concept enables exposure during wake and sleep states while an electroencephalogram (EEG) is recorded. The dosimetric assessment and uncertainty analysis yield high efficiency of 14 W/kg per Watt of accepted antenna input power due to an optimized antenna directly worn on the subject's head. Beside sham exposure, high and low exposure at 6 and 1.5 W/kg (in terms of maxSAR10g in the head) were implemented. Double-blind control and monitoring of exposure is enabled by easy-to-use control software. Exposure uncertainty was rigorously evaluated using finite-difference time-domain (FDTD)-based computations, taking into account anatomical differences of the head, the physiological range of the dielectric tissue properties including effects of sweating on the antenna, possible influences of the EEG electrodes and cables, variations in antenna input reflection coefficients, and effects on the specific absorption rate (SAR) distribution due to unavoidable small variations in the antenna position. This analysis yielded a reasonable uncertainty of <±45% (max to min ratio of 4.2 dB) in terms of maxSAR10g in the head and a variability of <±60% (max to min ratio of 6 dB) in terms of mass-averaged SAR in different brain regions, as demonstrated by a brain region-specific absorption analysis.

Changes in the dielectric properties of ex vivo bovine liver during microwave thermal ablation at 2.45 GHz

In microwave thermal ablation (MTA) therapy, the dielectric properties of the target tissue play an important role in determining the radiation properties of the microwave ablation antenna. In this work, the ex vivo dielectric properties of bovine liver were experimentally characterized as a function of the temperature during MTA at the frequency of 2.45 GHz. The obtained data were compared with measurements performed at the end of the MTA treatment, and considering the heating achieved with a temperature-controlled water bath. Finally, measured data were used to perform a numerical study evaluating the effects of changes in tissue's dielectric properties during the MTA treatment on the radiation properties of a microwave interstitial ablation antenna, as well as on the obtained thermal lesion. Results evidenced a significant decrease of both relative permittivity (about 38%) and electric conductivity (about 33%) in the tissue during treatment as the temperature increased to over 60 °C, with a dramatic drop when the temperature approached 100 °C. Moreover, the numerical study evidenced that changes in tissue's dielectric properties during the MTA treatment affect the distribution of the power absorbed by the tissue (specific absorption rate—SAR, W kg−1) surrounding the microwave interstitial ablation antenna, leading to a peak SAR up to 20% lower, as well as to a thermal lesion up to 8% longer. This work may represent a preliminary step towards the future development of a procedure for MTA treatment planning.

Microwave Imaging and Emerging Applications

Microwave Imaging and Emerging Applications

THREE-DIMENSIONAL FDTD ANALYSIS OF THE DUAL-BAND IMPLANTABLE ANTENNA FOR CONTINUOUS GLUCOSE MONITORING

The flnite difierence time domain (FDTD) method is widely used as a computational tool to simulate the electromagnetic wave propagation in biological tissues. When expressed in terms of Debye parameters, dispersive biological tissues dielectric properties can be e-ciently incorporated into FDTD codes. In this paper, FDTD formulation with nonuniform grid is presented to simulate a dual medical implant communications service (MICS) (402{405MHz) and industrial, scientiflc, and medical (ISM) (2.4{2.48GHz) band implantable antenna for continuous glucose-monitoring applications. In addition, we present computationally simpler two-pole Debye models that retain the high accuracy of the Cole-Cole Model for dry skin in MICS and ISM bands. The re∞ection coe-cient simulation result with Debye dispersion is presented and compared with the published results. FDTD was also applied to analyze antenna's far- fleld.