Academic literature on the topic 'Dielectric properties of human tissues'

This work investigates the in vivo dielectric properties of healthy and benign rat mammary tissues in an attempt to expand the dielectric property knowledge of animal models. The outcomes of this study can enable testing of microwave medical technologies on animal models and interpretation of tissue alteration-dependent in vivo dielectric properties of mammary tissues. Towards this end, in vivo dielectric properties of healthy rat mammary tissues and chemically induced benign rat mammary tumors including low-grade adenosis, sclerosing adenosis, and adenosis were collected with open-ended coaxial probes from 500 MHz to 18 GHz. The in vivo measurements revealed that the dielectric properties of benign rat mammary tumors are higher than the healthy rat mammary tissues by 9.3% to 35.5% and 19.6% to 48.7% for relative permittivity and conductivity, respectively. Furthermore, to our surprise, we found that the grade of the benign tissue affects the dielectric properties for this study. Finally, a comparison with ex vivo healthy human mammary tissue dielectric properties revealed that the healthy rat mammary tissues best replicate the dielectric properties of healthy medium density human samples.

Abstract. RF transmission properties of human tissues were investigated in the frequency range from 50 MHz to 1 GHz. This work was motivated by the increasing interest in communication links between medically active implants and external interrogator units. We investigated theoretically and experimentally the transmission loss between an implant and an external interrogator unit. We assumed that due to the size of the implant a maximum area of only 1 cm2 is available for the printed circuit antenna. The size of the external interrogator antenna is less restricted. The maximum depth of the implant beneath the surface of the body was assumed to be 10 cm. For the simulations we took the dielectric properties of skin, fat and muscle as published in the literature. For the measurements, an artificial muscle dielectric proposed in the literature was used consisting mainly of a mixture of water, sugar and salt. In simulation and measurements the reactive part of the impedance of the antennas was compensated numerically. In simulations and measurements we obtained a transmission loss between 30 dB around 100 MHz and 65 dB around 900 MHz.

As new algorithms for microwave imaging emerge, it is important to have standard accurate benchmarking tests. Currently, most researchers use homogeneous phantoms for testing new algorithms. These simple structures lack the heterogeneity of the dielectric properties of human tissue and are inadequate for testing these algorithms for medical imaging. To adequately test breast microwave imaging algorithms, the phantom has to resemble different breast tissues physically and in terms of dielectric properties. We propose a systematic approach in designing phantoms that not only have dielectric properties close to breast tissues but also can be easily shaped to realistic physical models. The approach is based on regression model to match phantom's dielectric properties with the breast tissue dielectric properties found in Lazebnik et al. (2007). However, the methodology proposed here can be used to create phantoms for any tissue type as long asex vivo,in vitro, orin vivotissue dielectric properties are measured and available. Therefore, using this method, accurate benchmarking phantoms for testing emerging microwave imaging algorithms can be developed.

INTRODUCTION: This article deals with measurement of dielectric properties (relative permittivity and conductivity) of phantoms, specifically a tumor phantom and a breast phantom. We focused on the waveguide and resonance methods for the measurement of dielectric properties. The article describes the principle of these methods, and also the production process of a breast phantom and a tumor phantom. These phantoms can be used for measurements in the microwave frequency range, 8–12 GHz.OBJECTIVE: The study’s objective was to design a tumor phantom and a breast phantom, and to measure their dielectric properties. These properties must simulate human tissue.METHODS: To measure dielectric properties of human tissue, phantoms were designed using the waveguide Hippel`s method and the resonance method with a cavity resonator.RESULTS: The aim of this work was to create the phantoms that would have properties comparable to those of real tissues. Results of measurement are shown as frequency dependence of relative permittivity and conductivity for breast, breast phantom, tumor, and tumor phantom.

Abstract. In the recent years, the task of estimating the currents induced within the human body by environmental electromagnetic fields has received increased attention from scientists around the world. While important progress was made in this direction, the unpredictable behaviour of living biological tissue made it difficult to quantify its reaction to electromagnetic fields and has kept the problem open. A successful alternative to the very difficult one of performing measurements is that of computing the fields within a human body model using numerical methods implemented in a software code. One of the difficulties is represented by the fact that some tissue types exhibit an anisotropic character with respect to their dielectric properties. Our work consists of computing currents induced by extremely low frequency (ELF) electric fields in anisotropic muscle tissues using in this respect, a human body model extended with muscle fibre orientations as well as an extended version of the Finite Integration Technique (FIT) able to compute fully anisotropic dielectric properties.

In biomedical applications realistic phantoms are becoming more useful for validation and testing of precursor systems. These artificial phantoms require stable and flexible tissue-mimicking materials with realistic dielectric properties in order to properly model human tissues. We have fabricated a low-water-content, low cost, mechanically and electrically stable, good shelf life and multi-layered heterogeneous phantom consisting of skin, fat and muscle tissues. We have chosen semi-solid type phantom for each tissue layer. The size and thickness of the each layer is chosen based on the average thickness of human tissue. Altering the ingredient composition wisely we can alter its dielectric properties also. By reason of no osmosis occurrence, the tissues can be employed to construct heterogeneous phantoms or even anthropomorphic phantoms without causing any changes in geometry or electrical properties. The performance of the fabricated phantom is carried out using an open-ended coaxial slim probe system by Agilent Technologies. Nearly all previous studies were based on very high frequency( VHF), so we present ultra-wide band (UWB), 500MHz-20GHz multilayered phantoms. We have measured our phantom after 2 month time period and we got quite good results for the dielectric properties without having significant variations. Thus, our fabricated sets of ATE phantom have good long lasting properties with good physical and dielectric stability.

Knowledge of the microwave dielectric properties of human tissues is essential for the understanding and development of medical microwave techniques. In particular, microwave thermography relies on processes fundamentally determined by the high frequency electromagnetic properties of human tissues. The specific aim of this work was to provide detailed information on the dielectric properties of female human breast tissue at 3-3.5GHz, the frequency of operation of the Glasgow microwave thermography equipment. At microwave frequences the frequency variation of the dielectric properties of biological tissues is thought to be determined mainly by the dipolar relaxation of tissue water. Water exists in different states of binding within the tissue; the relaxation of each component of this water may be parameterised by the Debye or Cole-Cole equations. At a single frequency an average relaxation frequency may be calculated for a given tissue type. Mixture equations may be used to describe the dielectric properties of two-phase mixtures in terms of the dielectric properties and volume fractions of the component phases. Biological tissues are very much more complex than these two phase models. However, comparisons of the observed dielectric properties as a function of water content, with models calculated from mixture theory allow some qualitative conclusions to be drawn regarding tissue structure. Human and animal dielectric data at frequencies between 0.1 and 10GHz have been collected from the literature and are displayed in tabular form. These comprehensive tables were used to examine the widely-held assumption an animal tissue is representative of the corresponding human tissue. This assumption was concluded to be uncertain in most cases because of lack of available data, and perhaps wrong for certain tissue types. The tables were also used to compare in vivo and in vitro dielectric data. These may be expected to be different because the tissue is in a physiologically abnormal state in vitro. However at microwave frequencies in vitro data was found to be representative of the tissue in vivo provided gross deterioration of the tissue is avoided. A new resonant cavity perturbation technique was designed for dielectric measurements of small volumes of lossy materials at a fixed frequency of 3.2GHz. This technique may be used to measure materials of a wide range of permittivities and conductivities with accuracies of 3-4%. The major sources of error were found to be tissue heterogeneity and sample preparation procedures. Using this technique in vitro dielectric measurements were made on human female breast tissues. A large number of data were gathered on fat and normal breast tissues, and on benign and malignant breast tumours. Each data set was parameterised using the Debye equation. Results from this suggest that all breast tissues measured in this work contain a component of bound water. A smaller proportion of water is bound in fat than is bound in other tissues. Comparisons were made of the dielectric properties of breast tissues with values calculated from mixture theories. Permittivity data largely fall within bounds set by mixture theory: conductivity data often fall outside these limits. This may imply that physiological saline is not a good approximation to tissue waters; or it may imply that another relaxation process is occurring in addition to the dipolar relaxation of saline. Comparisons of tissue type indicate that a dielectric imaging system could be designed which would detect breast diseases, but that severe problems could arise in distinguishing disease types from dielectric imaging alone.

Thesis (Ph.D.) - University of Glasgow, 1998.
Includes bibliographical references (p.127-139). Print version also available. Mode of access : World Wide Web. System requirements : Adobe Acrobat reader required to view PDF document.

Thesis (M.S.)--Worcester Polytechnic Institute.
Keywords: basis functions; perfectly matched layers; PML; neck model; parallel plate resonator; finite element; circulator; glottal waveform; multi-transmission line; dielectric properties of human tissues; radiation currents; weighted residuals; non-acoustic sensor. Includes bibliographical references (p. 104-107).

The work reported in this thesis is motivated by the need for wireless powering of a miniaturized implantable device for neurophysiological research and possible clinical applications. The antenna used in such applications must be studied in the context of biological tissue media. In this thesis, we perform a preliminary study of antenna behaviour in the complex tissue environment. Our test cases are the wire dipole antenna chosen for its structural simplicity and the spiral antenna, selected for its wide bandwidth. The simulation tool SEMCAD-X, is based on the Finite-Difference Time-Domain (FDTD) method and is used throughout this work. To have an in-depth understanding of the characteristics of different solvers implemented in SEMCAD-X and relevant for our applications, we first simulate the antenna structures in the free-space region using both SEMCAD-X and HFSS (a Finite-Element Method (FEM) simulation software). The cross-platform comparison between these two simulation tools helps us identify the advantages of using conformal FDTD solver over the conventional staircase FDTD solver in SEMCAD-X. We then embed the antennas in tissue-like non-homogeneous lossy media to observe the terminal voltages induced by an impinging plane-wave. These numerical experiments will help us with the assessment of the following: variations of antenna properties with the in-tissue locations, and more importantly the dependence of the induced voltage on the depth of the implant.

Bones are multifunctional passive organs of movement that supports soft tissue and directly attached muscles. They also protect internal organs and are a reserve of calcium, phosphorus and magnesium. Each bone is covered with periosteum, and the adjacent bone surfaces are covered by articular cartilage. Histologically, the bone is an organ composed of many different tissues. The main component is bone tissue (cortical and spongy) composed of a set of bone cells and intercellular substance (mineral and organic), it also contains fat, hematopoietic (bone marrow) and cartilaginous tissue. Bones are a tissue that even in adult life retains the ability to change shape and structure depending on changes in their mechanical and hormonal environment, as well as self-renewal and repair capabilities. This process is called bone turnover. The basic processes of bone turnover are: • bone modeling (incessantly changes in bone shape during individual growth) following resorption and tissue formation at various locations (e.g. bone marrow formation) to increase mass and skeletal morphology. This process occurs in the bones of growing individuals and stops after reaching puberty • bone remodeling (processes involve in maintaining bone tissue by resorbing and replacing old bone tissue with new tissue in the same place, e.g. repairing micro fractures). It is a process involving the removal and internal remodeling of existing bone and is responsible for maintaining tissue mass and architecture of mature bones. Bone turnover is regulated by two types of transformation: • osteoclastogenesis, i.e. formation of cells responsible for bone resorption • osteoblastogenesis, i.e. formation of cells responsible for bone formation (bone matrix synthesis and mineralization) Bone maturity can be defined as the completion of basic structural development and mineralization leading to maximum mass and optimal mechanical strength. The highest rate of increase in pig bone mass is observed in the first twelve weeks after birth. This period of growth is considered crucial for optimizing the growth of the skeleton of pigs, because the degree of bone mineralization in later life stages (adulthood) depends largely on the amount of bone minerals accumulated in the early stages of their growth. The development of the technique allows to determine the condition of the skeletal system (or individual bones) in living animals by methods used in human medicine, or after their slaughter. For in vivo determination of bone properties, Abstract 10 double energy X-ray absorptiometry or computed tomography scanning techniques are used. Both methods allow the quantification of mineral content and bone mineral density. The most important property from a practical point of view is the bone’s bending strength, which is directly determined by the maximum bending force. The most important factors affecting bone strength are: • age (growth period), • gender and the associated hormonal balance, • genotype and modification of genes responsible for bone growth • chemical composition of the body (protein and fat content, and the proportion between these components), • physical activity and related bone load, • nutritional factors: – protein intake influencing synthesis of organic matrix of bone, – content of minerals in the feed (CA, P, Zn, Ca/P, Mg, Mn, Na, Cl, K, Cu ratio) influencing synthesis of the inorganic matrix of bone, – mineral/protein ratio in the diet (Ca/protein, P/protein, Zn/protein) – feed energy concentration, – energy source (content of saturated fatty acids - SFA, content of polyun saturated fatty acids - PUFA, in particular ALA, EPA, DPA, DHA), – feed additives, in particular: enzymes (e.g. phytase releasing of minerals bounded in phytin complexes), probiotics and prebiotics (e.g. inulin improving the function of the digestive tract by increasing absorption of nutrients), – vitamin content that regulate metabolism and biochemical changes occurring in bone tissue (e.g. vitamin D3, B6, C and K). This study was based on the results of research experiments from available literature, and studies on growing pigs carried out at the Kielanowski Institute of Animal Physiology and Nutrition, Polish Academy of Sciences. The tests were performed in total on 300 pigs of Duroc, Pietrain, Puławska breeds, line 990 and hybrids (Great White × Duroc, Great White × Landrace), PIC pigs, slaughtered at different body weight during the growth period from 15 to 130 kg. Bones for biomechanical tests were collected after slaughter from each pig. Their length, mass and volume were determined. Based on these measurements, the specific weight (density, g/cm3) was calculated. Then each bone was cut in the middle of the shaft and the outer and inner diameters were measured both horizontally and vertically. Based on these measurements, the following indicators were calculated: • cortical thickness, • cortical surface, • cortical index. Abstract 11 Bone strength was tested by a three-point bending test. The obtained data enabled the determination of: • bending force (the magnitude of the maximum force at which disintegration and disruption of bone structure occurs), • strength (the amount of maximum force needed to break/crack of bone), • stiffness (quotient of the force acting on the bone and the amount of displacement occurring under the influence of this force). Investigation of changes in physical and biomechanical features of bones during growth was performed on pigs of the synthetic 990 line growing from 15 to 130 kg body weight. The animals were slaughtered successively at a body weight of 15, 30, 40, 50, 70, 90, 110 and 130 kg. After slaughter, the following bones were separated from the right half-carcass: humerus, 3rd and 4th metatarsal bone, femur, tibia and fibula as well as 3rd and 4th metatarsal bone. The features of bones were determined using methods described in the methodology. Describing bone growth with the Gompertz equation, it was found that the earliest slowdown of bone growth curve was observed for metacarpal and metatarsal bones. This means that these bones matured the most quickly. The established data also indicate that the rib is the slowest maturing bone. The femur, humerus, tibia and fibula were between the values of these features for the metatarsal, metacarpal and rib bones. The rate of increase in bone mass and length differed significantly between the examined bones, but in all cases it was lower (coefficient b <1) than the growth rate of the whole body of the animal. The fastest growth rate was estimated for the rib mass (coefficient b = 0.93). Among the long bones, the humerus (coefficient b = 0.81) was characterized by the fastest rate of weight gain, however femur the smallest (coefficient b = 0.71). The lowest rate of bone mass increase was observed in the foot bones, with the metacarpal bones having a slightly higher value of coefficient b than the metatarsal bones (0.67 vs 0.62). The third bone had a lower growth rate than the fourth bone, regardless of whether they were metatarsal or metacarpal. The value of the bending force increased as the animals grew. Regardless of the growth point tested, the highest values were observed for the humerus, tibia and femur, smaller for the metatarsal and metacarpal bone, and the lowest for the fibula and rib. The rate of change in the value of this indicator increased at a similar rate as the body weight changes of the animals in the case of the fibula and the fourth metacarpal bone (b value = 0.98), and more slowly in the case of the metatarsal bone, the third metacarpal bone, and the tibia bone (values of the b ratio 0.81–0.85), and the slowest femur, humerus and rib (value of b = 0.60–0.66). Bone stiffness increased as animals grew. Regardless of the growth point tested, the highest values were observed for the humerus, tibia and femur, smaller for the metatarsal and metacarpal bone, and the lowest for the fibula and rib. Abstract 12 The rate of change in the value of this indicator changed at a faster rate than the increase in weight of pigs in the case of metacarpal and metatarsal bones (coefficient b = 1.01–1.22), slightly slower in the case of fibula (coefficient b = 0.92), definitely slower in the case of the tibia (b = 0.73), ribs (b = 0.66), femur (b = 0.59) and humerus (b = 0.50). Bone strength increased as animals grew. Regardless of the growth point tested, bone strength was as follows femur > tibia > humerus > 4 metacarpal> 3 metacarpal> 3 metatarsal > 4 metatarsal > rib> fibula. The rate of increase in strength of all examined bones was greater than the rate of weight gain of pigs (value of the coefficient b = 2.04–3.26). As the animals grew, the bone density increased. However, the growth rate of this indicator for the majority of bones was slower than the rate of weight gain (the value of the coefficient b ranged from 0.37 – humerus to 0.84 – fibula). The exception was the rib, whose density increased at a similar pace increasing the body weight of animals (value of the coefficient b = 0.97). The study on the influence of the breed and the feeding intensity on bone characteristics (physical and biomechanical) was performed on pigs of the breeds Duroc, Pietrain, and synthetic 990 during a growth period of 15 to 70 kg body weight. Animals were fed ad libitum or dosed system. After slaughter at a body weight of 70 kg, three bones were taken from the right half-carcass: femur, three metatarsal, and three metacarpal and subjected to the determinations described in the methodology. The weight of bones of animals fed aa libitum was significantly lower than in pigs fed restrictively All bones of Duroc breed were significantly heavier and longer than Pietrain and 990 pig bones. The average values of bending force for the examined bones took the following order: III metatarsal bone (63.5 kg) <III metacarpal bone (77.9 kg) <femur (271.5 kg). The feeding system and breed of pigs had no significant effect on the value of this indicator. The average values of the bones strength took the following order: III metatarsal bone (92.6 kg) <III metacarpal (107.2 kg) <femur (353.1 kg). Feeding intensity and breed of animals had no significant effect on the value of this feature of the bones tested. The average bone density took the following order: femur (1.23 g/cm3) <III metatarsal bone (1.26 g/cm3) <III metacarpal bone (1.34 g / cm3). The density of bones of animals fed aa libitum was higher (P<0.01) than in animals fed with a dosing system. The density of examined bones within the breeds took the following order: Pietrain race> line 990> Duroc race. The differences between the “extreme” breeds were: 7.2% (III metatarsal bone), 8.3% (III metacarpal bone), 8.4% (femur). Abstract 13 The average bone stiffness took the following order: III metatarsal bone (35.1 kg/mm) <III metacarpus (41.5 kg/mm) <femur (60.5 kg/mm). This indicator did not differ between the groups of pigs fed at different intensity, except for the metacarpal bone, which was more stiffer in pigs fed aa libitum (P<0.05). The femur of animals fed ad libitum showed a tendency (P<0.09) to be more stiffer and a force of 4.5 kg required for its displacement by 1 mm. Breed differences in stiffness were found for the femur (P <0.05) and III metacarpal bone (P <0.05). For femur, the highest value of this indicator was found in Pietrain pigs (64.5 kg/mm), lower in pigs of 990 line (61.6 kg/mm) and the lowest in Duroc pigs (55.3 kg/mm). In turn, the 3rd metacarpal bone of Duroc and Pietrain pigs had similar stiffness (39.0 and 40.0 kg/mm respectively) and was smaller than that of line 990 pigs (45.4 kg/mm). The thickness of the cortical bone layer took the following order: III metatarsal bone (2.25 mm) <III metacarpal bone (2.41 mm) <femur (5.12 mm). The feeding system did not affect this indicator. Breed differences (P <0.05) for this trait were found only for the femur bone: Duroc (5.42 mm)> line 990 (5.13 mm)> Pietrain (4.81 mm). The cross sectional area of the examined bones was arranged in the following order: III metatarsal bone (84 mm2) <III metacarpal bone (90 mm2) <femur (286 mm2). The feeding system had no effect on the value of this bone trait, with the exception of the femur, which in animals fed the dosing system was 4.7% higher (P<0.05) than in pigs fed ad libitum. Breed differences (P<0.01) in the coross sectional area were found only in femur and III metatarsal bone. The value of this indicator was the highest in Duroc pigs, lower in 990 animals and the lowest in Pietrain pigs. The cortical index of individual bones was in the following order: III metatarsal bone (31.86) <III metacarpal bone (33.86) <femur (44.75). However, its value did not significantly depend on the intensity of feeding or the breed of pigs.

Human walking requires sophisticated coordination of muscles, tendons, and ligaments working together to provide a constantly changing combination of force, stiffness and damping. In particular, the human knee joint acts as a variable damper, dissipating greater amounts of energy when the knee undergoes large rotational displacements during walking, running or hopping. Typically, this damping results from the dissipation, or loss, of metabolic energy. It has been proven to be possible however; to collect this otherwise wasted energy through the use of electromechanical transducers of several different types which convert mechanical energy to electrical energy. When properly controlled, this type of device not only provides desirable structural damping effects, but the energy generated can be stored for use in a wide range of applications. A novel approach to an energy harvesting knee joint damper is presented using a dielectric elastomer (DE) smart material based electromechanical transducer. Dielectric elastomers are extremely elastic materials with high electrical permittivity which operate based on electrostatic effects. By placing compliant electrodes on either side of a dielectric elastomer film, a specialized capacitor is created, which couples mechanical and electrical energy using induced electrostatic stresses. Dielectric elastomer energy harvesting devices not only have a high energy density, but the material properties are similar to that of human tissue, making it highly suitable for wearable applications. A theoretical framework for dielectric elastomer energy harvesting is presented along with a mapping of the active phases of the energy harvesting to the appropriate phases of the walking stride. Experimental results demonstrating the energy harvesting capability of a DE generator undergoing strains similar to those experienced during walking are provided for the purpose of verifying the theoretical results. The work presented here can be applied to devices for use in rehabilitation of patients with muscular dysfunction and transfemoral prosthesis as well as energy generation for able-bodied wearers.

The anisotropic properties of human skin tissues are important for many applications, such as medical device/patient interface modeling. Typically, uniaxial [1] and biaxial [2] tests are used for mechanical testing of skin. The inflation method, less commonly used for skin, presents the advantages of imposing a biaxial deformation state more similar to that found in vivo. For methods that incorporate full-field displacement measurements, the material directions can be determined from the test rather than pre-selected prior to testing [3]. Furthermore, the mechanical response measured by the inflation method has been shown to be insensitive to preconditioning [4].