Dielectric properties of brain tissue between 0.01 and 10 GHz

Dielectric permittivity and conductivity are reported for grey and white matter from dog brain tissue between 0.01 and 10 GHz. Between 0.01 and approximately 1 GHz, the permittivity decreases and conductivity increases as a power law of frequency. Above 1 GHz, the conductivity increases quadratically with frequency due to dipolar reorientation of free water molecules in tissue; the apparent rotational relaxation frequency at 37 degrees C is 21--25 GHz, slightly below the 25 GHz characteristic frequency of pure water at that temperature. The microwave data are analysed using the Maxwell mixture theory applicable for a suspension of nonconducting, low permittivity spheres in bulk water. From the increase in conductivity above 1 GHz, and the tissue permittivity at 2--4 GHz, the apparent volume fraction of water is approximately 0.70 and 0.55 for grey and white matter, respectively, about 10--15% lower than respective values from the literature. This discrepancy is apparently due to a small fraction of water which does not contribute to the tissue permittivity above 1 GHz. Empirical equations are given to summarise the dielectric properties of 'average' brain tissue at 37 degrees C for future theoretical studies of microwave absorption in the head.     

Changes in dielectric properties of ex vivo bovine liver at 915 MHz during heating

We have developed a coaxial measurement system for determining the time and temperature dependence of the dielectric properties of bovine liver at 915 MHz during heating. Our data suggest that changes in dielectric properties due to heating are dominated by the relaxation response of two tissue components: tissue water and proteins. At temperatures above 60 degrees C, the effects of these two components contribute to increases of up to 100% and 5% in the values of conductivity and permittivity respectively. Changes due to tissue water content were found to be reversible with temperature, while changes due to protein denaturation were found to be permanent. The temperature coefficients for reversible changes were found to be 1.82 +/- 0.28% degrees C(-1) and -0.130 +/- (5.9 x 10(-2))% degrees C(-1) for conductivity and permittivity respectively. The critical temperatures and activation energies leading to irreversible changes in conductivity and permittivity were determined using Arrhenius analysis. Frequency factors of (1.14 +/- 0.27) x 10(43) s(-1) and (1.95 +/- 0.49) x 10(36) s(-1) were determined for permittivity and conductivity respectively. The activation energies were calculated to be 70.7 +/- 15.8 kcal mol(-1) for permittivity and 60.1 +/- 14.0 kcal mol(-1) for conductivity.     

Anisotropy in the dielectric properties of skeletal muscle

The dielectric permittivity and conductivity of freshly excised dog skeletal muscle were measured at frequencies between 20 Hz and 1 MHz, with the tissue samples oriented either parallel or perpendicular to the applied electric field. At frequencies below 100 kHz a four-electrode technique was employed; at frequencies above 100 kHz a conventional two-electrode technique was used in conjunction with a commercial admittance bridge. All measurements were performed with the tissue sample at body temperature. Over the entire frequency range of the study, the dielectric properties of the muscle were found to be significantly anisotropic. At audio frequencies, the conductivity of the muscle measured in a direction parallel to the fibre orientation was typically found to be ten times higher than that measured perpendicular to the fibre axis; at frequencies approaching 1 MHz the conductivities measured in both directions approached a common value, near 8 ms cm^?1. The dielectric permittivity in both orientations is strongly dependent on frequency. These data are analysed using the suspension equation, and comments are made on the mechanisms that are responsible for the observed properties. In an Appendix these results are related to some of the very early work on the subject.     

101例手术切除的结直肠癌组织介电特性的探讨

探究采用开端同轴线射频技术测量得到的结直肠癌组织和正常结直肠组织的介电特性是否具有显著差异,探讨能否应用该技术判断结直肠癌的病理类型。101例手术切除的新鲜离体结肠／直肠癌标本来自南方医科大学珠江医院手术室,应用开端同轴线仪器在170～500 MHz频率段分别检测癌组织和正常组织的相对介电常数和电导率。粗略选取170．32、212．90、255．48、298．06、383．22、468．38、500．00 MHz几个频率点的介电特性纳入统计分析。术后对标本行病理切片检查。结直肠癌组织的相对介电常数和电导率均比正常结直肠组织高（P＜0．01）,粘液腺癌组织的电导率在170．32、212．90、255．48、298．06、383．22 MHz时比腺癌组织高（P＜0．05）,结直肠癌高、中、低分化三种不同分化程度的介电特性无统计学意义（P＞0．05）。应用开端同轴线仪器可以快速而准确地辨别结直肠癌组织和正常结直肠组织,同时可以判断结直肠癌的病理类型。     

Dielectric properties of rat embryo and foetus as a function of gestation

The dielectric properties of rat embryos/foetuses have been acquired at several stages of gestation at 37 °C and in the frequency range of 40 MHz-20 GHz. Measurements were carried out on homogenized tissues, as trial experiments did not show any systematic difference between the dielectric data of intact and homogenized tissues at microwave frequencies. The results showed that dielectric properties of the foetus are generally higher than adult muscle and brain. The measured data also showed some decline for both permittivity and conductivity as the foetus grew from 18 to 20 days old; however, these changes were not statistically significant. Data were also collected for placenta and amniotic fluid which were in good agreement with those recently obtained from human tissues. Finally, tabulated numerical dielectric data for rat foetal tissues are presented for a wide range of medical and telecommunication frequencies.     

A large-scale study of the ultrawideband microwave dielectric properties of normal, benign and malignant breast tissues obtained from cancer surgeries

The development of microwave breast cancer detection and treatment techniques has been driven by reports of substantial contrast in the dielectric properties of malignant and normal breast tissues. However, definitive knowledge of the dielectric properties of normal and diseased breast tissues at microwave frequencies has been limited by gaps and discrepancies across previously published studies. To address these issues, we conducted a large-scale study to experimentally determine the ultrawideband microwave dielectric properties of a variety of normal, malignant and benign breast tissues, measured from 0.5 to 20 GHz using a precision open-ended coaxial probe. Previously, we reported the dielectric properties of normal breast tissue samples obtained from reduction surgeries. Here, we report the dielectric properties of normal (adipose, glandular and fibroconnective), malignant (invasive and non-invasive ductal and lobular carcinomas) and benign (fibroadenomas and cysts) breast tissue samples obtained from cancer surgeries. We fit a one-pole Cole-Cole model to the complex permittivity data set of each characterized sample. Our analyses show that the contrast in the microwave-frequency dielectric properties between malignant and normal adipose-dominated tissues in the breast is considerable, as large as 10:1, while the contrast in the microwave-frequency dielectric properties between malignant and normal glandular/fibroconnective tissues in the breast is no more than about 10%.     

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.     

Changes in dielectric properties at 460 kHz of kidney and fat during heating: importance for radio-frequency thermal therapy

We have developed a system to measure the changes due to heating to high temperatures in the dielectric properties of tissues in the radio-frequency range. A two-electrode arrangement was connected to a low-frequency impedance analyser and used to measure the dielectric properties of ex vivo porcine kidney and fat at 460 kHz. This frequency was selected as it is the most commonly used for radio-frequency thermal therapy of renal tumours. Tissue samples were heated to target temperatures between 48 and 78 degrees C in a hot water bath and changes in dielectric properties were measured during 30 min of heating and 15 min of cooling. Results suggest a time-temperature dependence of dielectric properties, with two separate components: one a reversible, temperature-dependent effect and the other a permanent effect due to structural events (e.g. protein coagulation, fat melting) that occur in tissues during heating. We calculated temperature coefficients of 1.3 +/- 0.1% degrees C(-1) for kidney permittivity and 1.6% degrees C(-1) for kidney conductivity, 0.9 +/- 0.1% degrees C(-1) for fat permittivity and 1.7 +/- 0.1% degrees C(-1) for fat conductivity. An Arrhenius model was employed to determine the first-order kinetic rates for the irreversible changes in dielectric properties. The following Arrhenius parameters were determined: an activation energy of 57 +/- 5 kcal mol(-1) and a frequency factor of (6 +/- 1) x 10(34) s(-1) for conductivity of kidney, an activation energy of 48 +/- 2 kcal mol(-1) and a frequency factor of 6 x 10(28) s(-1) for permittivity of kidney. A similar analysis led to an activation energy of 31 +/- 4 kcal mol(-1) and a frequency factor of (4.43 +/- 1) x 10(16) s(-1) for conductivity of fat, and an activation energy of 40 +/- 4 kcal mol(-1) and a frequency factor of 4 x 10(22) s(-1) for permittivity of fat. Structural events occurring during heating at different target temperatures as determined by histological analyses were correlated with the changes in the measured dielectric properties.     

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.     

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.     

Dielectric properties of ocular tissues in the supercooled and frozen states

Values of the relative permittivity and conductivity of the rabbit retina and lens have been measured between 10 MHz and 10 GHz over the temperature range 20 to -20 degrees C. As the temperature was lowered from 20 degrees C the dielectric parameters changed smoothly until -9 degrees C where freezing occurred. At this temperature a sharp transition was observed in both conductivity and relative permittivity, the latter falling to a value of below 10 over most of the frequency range. Analysis shows that this is due to the unfreezable water, from which a value of lens hydration is calculated. This value agrees with that obtained previously at 37 degrees C using conventional dielectric mixture theory, but the associated margin of error is much smaller.     

Dielectric properties of mammalian breast milk at radiofrequencies

The relative permittivity and AC conductivity of breast milk have been investigated in four different mammalian species, human, cow, goat and sheep, in the frequency range 0.1-100 MHz and at a room temperature of 26.5 +/- 0.5 degrees C. The results showed that the sheep milk exhibited the largest dielectric dispersion, followed in decreasing order by milks from the goat, cow and human. The dielectric data were fitted to the Debye and Cole-Cole structural equations and the fitted parameters have been presented for the different species. The curve-fitting analysis has shown that for all the milk samples the Cole-Cole model gave a better fit to the dielectric data than the Debye model, thus suggesting heterogeneity of structure in milk. On the basis of the Cole-Cole model, the relaxation times in the mammalian milks were found to be distributed about the mean values of 162 +/- 10, 171 +/- 9, 177 +/- 14 and 192 +/- 12 ns for human, cow, goat and sheep milks, respectively.     

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).     

2450MHz介质复介电常数的测量

目的测量2450MHz频率下蒸馏水和不同浓度NaCl溶液的复介电常数,用以判定系统的可靠性和稳定性.方法根据微扰法得到复介电常数的测量公式,然后通过实验对已知参数的蒸馏水和NaCl溶液进行测量,并计算其复介电常数和电导率,验证计算结果的可靠性和稳定性.结果对计算结果进行统计分析和误差分析,其均值、标准差和变异系数表明,最终结果可与M.I.T标准较好地吻合.结论这种系统可较好地测量2450MHz频率下高损耗介质的复介电常数,测量系统的主要误差是由介质体积的误差引起的,因此精确测量介质体积是测量生物组织等高损耗介质复介电常数的关键.     

30例健康人血液射频介电常数和电导率的测量

在10kHz～110MHz频率范围,利用4294A阻抗分析仪测量了30例正常健康人血液样本的介电常数和电导率,通过介电常数和电导率频谱、复数平面图、介电损耗频谱、电导率虚部频谱和损耗角正切频谱的数据分析,旨在建立正常人血液的电生理特征参数,为进一步的临床研究提供基础数据和研究方法。结果表明,正常健康人全血介电常数和电导率具有频率依从性,其介电行为具有两个特征频率：第一特征频率f1=0.59MHz;第二特征频率f2=2.12MHz。     

Correction of electrode polarization contributions to the dielectric properties of normal and cancerous breast tissues at audio/radiofrequencies

Spurious contributions from electrode polarization (EP) are a major nuisance in dielectric measurements of biological tissues and hamper accurate determination of tissue properties in the audio/radiofrequencies. Various electrode geometries and/or treatments have been employed traditionally to reduce EP contributions, although none succeeded to completely remove EP from measurements on tissues for all practical frequency ranges. A method of correction for contributions of EP to the dielectric properties of tissues is proposed. The method is based on modeling the electrode impedance with suitable functions and on the observation that certain parameters are only dependent on electrodes properties and can thus be determined separately. The method is tested on various samples with known properties, and its usefulness is demonstrated with samples of normal and cancerous human female breast tissue. It is observed that the dielectric properties of the tissues over the frequency range 40 Hz-100 MHz are significantly different among different types of breast tissue. This observation is used further to demonstrate that, by scanning the tip of the measuring dielectric probe (with modest spatial resolution) across a sample of excised breast tissue, significant variations in the electrical properties are detected at a position where a tumor is located. This study shows that dielectric spectroscopy has the potential to offer a viable alternative to the current methods for detection of breast cancer in vivo.     

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.     

Dielectric properties of human normal, malignant and cirrhotic liver tissue: in vivo and ex vivo measurements from 0.5 to 20 GHz using a precision open-ended coaxial probe

Hepatic malignancies have historically been treated with surgical resection. Due to the shortcomings of this technique, there is interest in other, less invasive, treatment modalities, such as microwave hepatic ablation. Crucial to the development of this technique is the accurate knowledge of the dielectric properties of human liver tissue at microwave frequencies. To this end, we characterized the dielectric properties of in vivo and ex vivo normal, malignant and cirrhotic human liver tissues from 0.5 to 20 GHz. Analysis of our data at 915 MHz and 2.45 GHz indicates that the dielectric properties of ex vivo malignant liver tissue are 19 to 30% higher than normal tissue. The differences in the dielectric properties of in vivo malignant and normal liver tissue are not statistically significant (with the exception of effective conductivity at 915 MHz, where malignant tissue properties are 16% higher than normal). Also, the dielectric properties of in vivo normal liver tissue at 915 MHz and 2.45 GHz are 16 to 43% higher than ex vivo. No statistically significant differences were found between the dielectric properties of in vivo and ex vivo malignant tissue (with the exception of effective conductivity at 915 MHz, where malignant tissue properties are 28% higher than normal). We report the one-pole Cole-Cole parameters for ex vivo normal, malignant and cirrhotic liver tissue in this frequency range. We observe that wideband dielectric properties of in vivo liver tissue are different from the wideband dielectric properties of ex vivo liver tissue, and that the in vivo data cannot be represented in terms of a Cole-Cole model. Further work is needed to uncover the mechanisms responsible for the observed wideband trends in the in vivo liver data.     

MRI拉莫尔频率范围内人体脑胶质瘤组织的介电特性

目的:研究人体脑胶质瘤组织在MRI拉莫尔频率范围内（50~500 MHz）的介电特性,建立人体脑胶质瘤组织介电参数频率谱图,为磁共振断层成像技术提供理论依据和数据参考。方法:以神经外科手术中切下的脑胶质瘤组织为标本,在温度为37 °C的恒温水箱中,利用开端同轴线法,在50~500 MHz频率范围内使用AV 3656A网络分析仪测量脑胶质瘤组织标本的介电特性。以四阶Cole-Cole模型为基础,利用最小二乘曲线拟合方法,提取人体脑胶质瘤组织的介电特征参数。同时,将实验测得的脑胶质瘤组织介电特性与健康人体组织介电特性数据库中的正常脑组织进行比较。结果:在测频率范围内,人体脑胶质瘤组织实测数据与Cole-Cole模型吻合良好,且有人体脑胶质瘤组织的相对介电常数比正常脑组织高29.5%~36.6%,电导率比正常脑组织高56.1%~64.8%。结论:本文报道了37 °C下人体脑胶质瘤组织在MRI拉莫尔频率范围内（50~500 MHz）的介电特性数据及相应的Cole-Cole模型介电特征参数,可为人体脑胶质瘤组织介电特性研究和磁共振断层成像技术提供基础数据。