Expanded modeling of temperature-dependent dielectric properties for microwave thermal ablation

Microwaves are a promising source for thermal tumor ablation due to their ability to rapidly heat dispersive biological tissues, often to temperatures in excess of 100 °C. At these high temperatures, tissue dielectric properties change rapidly and, thus, so do the characteristics of energy delivery. Precise knowledge of how tissue dielectric properties change during microwave heating promises to facilitate more accurate simulation of device performance and helps optimize device geometry and energy delivery parameters. In this study, we measured the dielectric properties of liver tissue during high-temperature microwave heating. The resulting data were compiled into either a sigmoidal function of temperature or an integration of the time-temperature curve for both relative permittivity and effective conductivity. Coupled electromagnetic-thermal simulations of heating produced by a single monopole antenna using the new models were then compared to simulations with existing linear and static models, and experimental temperatures in liver tissue. The new sigmoidal temperature-dependent model more accurately predicted experimental temperatures when compared to temperature-time integrated or existing models. The mean percent differences between simulated and experimental temperatures over all times were 4.2% for sigmoidal, 10.1% for temperature-time integration, 27.0% for linear and 32.8% for static models at the antenna input power of 50 W. Correcting for tissue contraction improved agreement for powers up to 75 W. The sigmoidal model also predicted substantial changes in heating pattern due to dehydration. We can conclude from these studies that a sigmoidal model of tissue dielectric properties improves prediction of experimental results. More work is needed to refine and generalize this model.     

Modeling of the dielectric properties of trabecular bone samples at microwave frequency

In this paper, the dielectric properties of human trabecular bone are evaluated under physiological condition in the microwave range. Assuming a two components medium, simulation and experimental data are presented and discussed. A special experimental setup is developed in order to deal with inhomogeneous samples. Simulation data are obtained using finite difference time domain from a realistic sample. The bone mineral density of the samples are also measured. The simulation and experimental results of the present study suggest that there is a negative relation between bone volume fraction (BV/TV) and permittivity/conductivity: the higher the BV/TV, the lower the permittivity/conductivity. This is in agreement with the recently published in vivo data.     

Testing of dielectric properties of biological tissues in microwave range for solving matching problems.

Since the employment of microwave energy for defrosting biological tissues and for microwave-aided diagnosis in cryosurgery is very promising, the problem of ensuring the match between the contact antennas (applicators) and the frozen biological object has become a pressing one. It is for this reason that investigations were aimed at the study of the dielectric properties of the dog's muscular and fatty tissues in a temperature range of +20 to -30 degrees C including the phase transition of the samples to the solid state. The investigations were performed in a waveguide device using a short-circuit method. During the investigation possible error was estimated. The experiment provides for the reduction of the error caused by the waveguide linear expansion.     

A quick accurate method for measuring the microwave dielectric properties of small tissue samples.

A resonant cavity perturbation technique has been developed which provides a quick, simple and accurate method of measuring the microwave dielectric properties of small, easily prepared samples of tissues, tissue-simulating materials and biological fluids. The method gives dielectric properties averaged over the sample volume. The measurement accuracy with the apparatus used is +/- 2.2% for relative permittivity and +/- 3.5% for the loss factor for most tissue and biological material samples. With appropriate basic microwave equipment the method can be applied over the 1.5-6 GHz range of frequencies of interest for active and radiometric tissue microwave imaging.     

The dielectric properties of human pineal gland tissue and RF absorption due to wireless communication devices in the frequency range 400-1850 MHz

In order to enable a detailed analysis of radio frequency (RF) absorption in the human pineal gland, the dielectric properties of a sample of 20 freshly removed pineal glands were measured less than 20 h after death. Furthermore, a corresponding high resolution numerical model of the brain region surrounding the pineal gland was developed, based on a real human tissue sample. After inserting this model into a commercially available numerical head model, FDTD-based computations for exposure scenarios with generic models of handheld devices operated close to the head in the frequency range 400-1850 MHz were carried out. For typical output power values of real handheld mobile communication devices, the obtained results showed only very small amounts of absorbed RF power in the pineal gland when compared to SAR limits according to international safety standards. The highest absorption was found for the 400 MHz irradiation. In this case the RF power absorbed inside the pineal gland (organ mass 96 mg) was as low as 11 microW, when considering a device of 500 mW output power operated close to the ear. For typical mobile phone frequencies (900 MHz and 1850 MHz) and output power values (250 mW and 125 mW) the corresponding values of absorbed RF power in the pineal gland were found to be lower by a factor of 4.2 and 36, respectively. These results indicate that temperature-related biologically relevant effects on the pineal gland induced by the RF emissions of typical handheld mobile communication devices are unlikely.     

Model for the dielectric properties of human lung tissue against frequency and air content

Electrical impedance tomographic spectroscopy measurements of the lungs are taken from nine normal subjects, in the frequency range 9.6 kHz–1.2 MHz. The results show that resistivity ρ′_FRC relative to functional residual capacity increases almost linearly with inspiration volume V, with the slope of the curve increasing with frequency f. Resistivity ρ′_{9.6 kHz} relative to 9.6 kHz decreases with f. ρ′_{9.6 kHz} increases with V, at any given frequency. Curves for ρ′_{9.6 kHz} show a roughly linear trend with log₁₀(f). Based on a discussion of the measurement results, a mathematical lung tissue model is designed that involves extra-capillary blood vessels and alveoli, the walls of which consist of blood-filled capillaries, epithelial cells and intercellular liquid. Using this model, the increase in ρ′_FRC with V is explained by the thinning of alveolar walls with increasing air content. The almost linear shape of curves for ρ′_{9.6 kHz} is attributed to four partly overlapping main dispersions caused by extra-capillary blood vessels, epithelial cells, blood and the capillary network.     

Changes in the dielectric properties of rat tissue as a function of age at microwave frequencies.

The dielectric properties of ten rat tissues at six different ages were measured at 37 degrees C in the frequency range of 130 MHz to 10 GHz using an open-ended coaxial probe and a computer controlled network analyser. The results show a general decrease of the dielectric properties with age. The trend is more apparent for brain, skull and skin tissues and less noticeable for abdominal tissues. The variation in the dielectric properties with age is due to the changes in the water content and the organic composition of tissues. The percentage decrease in the dielectric properties of certain tissues in the 30 to 70 day old rats at cellular phone frequencies have been tabulated. These data provide an important input in the provision of rigorous dosimetry in lifetime-exposure animal experiments. The results provide some insight into possible differences in the assessment of exposure for children and adults.     

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.     

A large-scale study of the ultrawideband microwave dielectric properties of normal breast tissue obtained from reduction surgeries

The efficacy of emerging microwave breast cancer detection and treatment techniques will depend, in part, on the dielectric properties of normal breast tissue. However, knowledge of these properties at microwave frequencies has been limited due to gaps and discrepancies in previously reported small-scale studies. To address these issues, we experimentally characterized the wideband microwave-frequency dielectric properties of a large number of normal breast tissue samples obtained from breast reduction surgeries at the University of Wisconsin and University of Calgary hospitals. The dielectric spectroscopy measurements were conducted from 0.5 to 20 GHz using a precision open-ended coaxial probe. The tissue composition within the probe's sensing region was quantified in terms of percentages of adipose, fibroconnective and glandular tissues. We fit a one-pole Cole-Cole model to the complex permittivity data set obtained for each sample and determined median Cole-Cole parameters for three groups of normal breast tissues, categorized by adipose tissue content (0-30%, 31-84% and 85-100%). Our analysis of the dielectric properties data for 354 tissue samples reveals that there is a large variation in the dielectric properties of normal breast tissue due to substantial tissue heterogeneity. We observed no statistically significant difference between the within-patient and between-patient variability in the dielectric properties.     

In vitro measurements of temperature-dependent specific heat of liver tissue

We measured the specific heat of liver tissue in vitro by uniformly heating liver samples between two electrodes. We insulated the samples by expanded polystyrene, and corrected for heat loss and water loss. The specific heat of the liver is temperature-dependent, and increases by 17% at 83.5 degrees C (p < 0.05), compared to temperatures below 65 degrees C. The average specific heat was 3411 J kg(-1)K(-1) at 25 degrees C, and 4187 J kg(-1)K(-1) at 83.5 degrees C. Water loss from the samples was significant above 70 degrees C, with approximately 20% of reduction in sample mass at 90 degrees C.     

正常人血小板射频段介电响应测量

目的在射频段测量正常人血小板介电谱,建立血小板对交流电场介电响应的数据特征。方法利用频域阻抗技术测量人血小板交流阻抗,经过介电谱、Cole-Cole图、介电损耗因子Δε”、导电率虚部Δκ”、介电损耗角正切Δtgδ等频谱,明确人血小板介电频响的数据特征。结果在射频电场中,人血小板的介电常数和电导率具有频率依赖性;血小板介电响应具有两个中心特征频率:第一中心特征频率fC1=6.66MHz,第二中心特征频率fC2=9.81MHz。结论交流阻抗技术能够测量血小板射频段介电响应。     

Non-linear electrical properties of skin in the low frequency range

This paper describes the non-linear electrical properties of the skin during the passage of a sinusoidal current from the standpoint of the constant-current method. The non-linearity occurs in the current dependency of the skin impedance and in Lissajous figures. It exhibits both rapid and slow variations. The concept of a non-linear impedance and its equivalent circuit are introduced for a sinusoidal voltage. With regard to the current dependency of the impedance it can be said that both the starting point and the degree of the dependency vary with frequency and impedance. The nonlinearity is more apparent with a larger current, a lower frequency, and a higher impedance. with an increasing current, the ionic conductance increases and the polarisation admittance decreases. Lissajous figures are formed when the voltage is distorted notably from the sinusoidal wave form. Detailed investigations are undertaken for the elliptic figures, third harmonics, rectification, and breakdown of the skin appearing with a decrease of the frequency and increase of the current. Finally, the special mechanism of the ionic conduction in the keratin layer is indicated as one of the causes for non-linearity.     

正常人血小板射频段介电响应测量

目的 在射频段测量正常人血小板介电谱,建立血小板对交流电场介电响应的数据特征.方法 利用频域阻抗技术测量人血小板交流阻抗,经过介电谱、Cole-Cole图、介电损耗因子△ε"、导电率虚部△κ"、介电损耗角正切△tgδ等频谱,明确人血小板介电频响的数据特征.结果 在射频电场中,人血小板的介电常数和电导率具有频率依赖性;血小板介电响应具有两个中心特征频率:第一中心特征频率fC1=6.66 MHz,第二中心特征频率fC2=9.81 MHz.结论 交流阻抗技术能够测量血小板射频段介电响应.     

Age dependence of dielectric properties of bovine brain and ocular tissues in the frequency range of 400 MHz to 18 GHz

In order to identify possible age-dependent dielectric properties of brain and eye tissues in the frequency range of 400 MHz to 18 GHz, measurements on bovine grey and white matter as well as on cornea, lens (cortical) and the vitreous body were performed using a commercially available open-ended coaxial probe and a computer-controlled vector network analyser. Freshly excised tissues of 52 animals of two age groups (42 adult animals, i.e. 16-24 month old and 10 young animals, i.e. 4-6 month old calves) were examined within 8 min (brain tissue) and 15 min (eye tissue), respectively, of the animals' death. Tissue temperatures for the measurements were 32+/-1 degrees C and 25+/-1 degrees C for brain and eye tissues, respectively. Statistical analysis of the measured data revealed significant differences in the dielectric properties of white matter and cortical lens tissue between the adult and the young group. In the case of white matter the mean values of conductivity and permittivity of young tissue were 15%-22% and 12%-15%, respectively, higher compared to the adult tissue in the considered frequency range. Similarly, young cortical lens tissue was 25%-76% higher in conductivity and 27%-39% higher in permittivity than adult cortical lens tissue.     

Tumorous necrotic nodule in the liver: unexpected effect of the microwave tissue coagulator.

The microwave tissue coagulator (MTC) is used in hepatectomy because it provides excellent haemostasis during the procedure. A 59 year old man underwent partial hepatic lobectomy with MTC, for metastasis from colon cancer. A tumorous necrotic nodule was discovered in the liver. The nodule measured 2.5 cm at its largest diameter. Microscopically, it showed extensive coagulation necrosis and massive sinusoidal dilatation. To date, such a necrotic mass clinically mimicking neoplasm has not been reported as a complication of hepatectomy using MTC. Although it is unknown how the rounded necrotic nodule was formed in this case, clinicians should be aware of this phenomenon to avoid unnecessary operations. Likewise, pathologists should recognise such histological changes and review the clinical history of the patient when coagulation necrosis with massive sinusoidal dilatation is observed in a biopsy or hepatectomy specimen.     

B超引导下微波热毁损治疗肝脏海绵状血管瘤

目的观察微波热毁损治疗肝脏海绵状血管瘤的疗效及安全性。方法87例肝脏海绵状血管瘤患者，其中男性28例，女性59例，平均年龄42．6岁。113个病灶，瘤体直径5～10cm。在B超引导下采用微波治疗肝脏单发及多发海绵状血管瘤。结果3个月后复查B超，68个病灶（≤8cm）瘤体体积缩小50％以上，瘤体血供全部消失，病灶完全毁损；其余病灶瘤体体积缩小25％以上，病灶内血流明显减少。6个月后复查，仅3个直径≥8cm的病灶体积增大，经第2次治疗后血流消失。全部病例无出血等并发症。结论B超引导下经皮肝穿刺微波热毁损治疗肝脏血管瘤，安全有效，无并发症；对≥8cm的病灶需2次以上治疗才能达到满意疗效。     

Radiofrequency dielectric properties of animal tissues as a function of time following death

The dielectric properties of three bovine tissues, liver, kidney and spleen, as a function of time following death, were measured in the frequency range from 20 kHz to 100 MHz using an automatic network analyser and an end-of-the-line sensor. The dielectric constant of kidney and spleen decreases as a function of time following death, particularly at frequencies below 1 MHz. However, all tissues measured show a characteristic increase in the frequency-independent ionic conductivity. This is believed to reflect changes in the conductivity of the extracellular region of tissues after death. The dielectric parameters, i.e. the static dielectric constant, the relaxation time and the coefficient of the relaxation time distribution, obtained by a curve-fitting process, do not change within the first 10 h following death in the case of liver, whereas early changes occur for both kidney and spleen. High initial values of the static dielectric constant for these tissues decrease significantly within a few hours following death. Similarly, the relaxation time which is relatively long for kidney and spleen, as compared with liver, decreases with time. Our data compare favourably with those reported by several investigators for similar tissues in other species (dog, cat, swine and cattle).     

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.     

新型各向异性组织介电特性在体测量射频探头仿真研究

本文提出了一种新型探头设计方案。该探头可通过一次操作,完成射频段各向异性的生物组织其介电特性在正交方向上的在体测量。通过仿真实验,本文在1~1 000 MHz频段内针对探头尺寸参数对极板间能量耦合度和探头测量灵敏度的影响进行了研究。针对该频段内的实际测量需求,给出了探头的具体设计方案,并验证了该探头的测量效果。仿真结果显示,在200~400 MHz内,该探头能够将极板间能量耦合度控制在–12 d B以下,并能保持对各向异性生物组织其介电特性的测量具有较高的灵敏度。仿真结果表明,该探头在射频段内对各向异性的生物组织其介电特性有良好的测量能力,避免了传统测量方法因多次操作而造成的测量误差,为各向异性的生物组织其介电特性在体测量技术研究提供了新思路。