Regional lung perfusion as determined by electrical impedance tomography in comparison with electron beam CT imaging

The aim of the experiments was to check the feasibility of pulmonary perfusion imaging by functional electrical impedance tomography (EIT) and to compare the EIT findings with electron beam computed tomography (EBCT) scans. In three pigs, a Swan-Ganz catheter was positioned in a pulmonary artery branch and hypertonic saline solution or a radiographic contrast agent were administered as boli through the distal or proximal openings of the catheter. During the administration through the proximal opening, the balloon at the tip of the catheter was either deflated or inflated. The latter case represented a perfusion defect. The series of EIT scans of the momentary distribution of electrical impedance within the chest were obtained during each saline bolus administration at a rate of 13/s. EBCT scans were acquired at a rate of 3.3/s during bolus administrations of the radiopaque contrast material under the same steady-state conditions. The EIT data were used to generate local time-impedance curves and functional EIT images showing the perfusion of a small lung region, both lungs with a perfusion defect and complete both lungs during bolus administration through the distal and proximal catheter opening with an inflated or deflated balloon, respectively. The results indicate that EIT imaging of lung perfusion is feasible when an electrical impedance contrast agent is used.     

Dielectric properties of breast carcinoma and the surroundingtissues

Relative permittivity of infiltrating breast carcinoma and the surrounding tissue was measured. The experiments were performed at frequencies from 20 kHz to 100 MHz at 37°C using an automatic network analyzer and an end-of-the-line capacitive sensor. Cole-Cole dielectric parameters were calculated by curve fitting using a computer program. Three main categories of tissues were considered: the central part of the tumor, the tumor surrounding tissue, and the peripheral tissue. Within each category, the large spread of the dielectric data for different specimens suggests structural and cellular inhomogeneities of the tumor tissue. However, certain consistency has been found in the dielectric relaxation time and the coefficient of the distribution of the relaxation time within each category. The results seem to indicate that RF impedance imaging can potentially be used as a diagnostic modality for the detection of human breast carcinoma     

Very wideband operation of twin-layer inset dielectric guide

An inset dielectric guide (IDG) with a multilayer dielectric filling is studied, using the transverse resonance diffraction, technique. The convergence and numeric efficiency of the analysis are good, and comparisons with experimental measurements of phase constant show the validity of the data produced. Those aspects of the IDG structure that limit its monomode bandwidth when it is filled with a single dielectric are investigated. It is shown how the monomode bandwidth of such an IDG may be made greater than that of rectangular metal waveguide by appropriate choice of aspect ratio. It is also shown how the simple use of two dielectric layers in the IDG slot can yield a guide whose monomode bandwidth is 66%, which is much greater than the typical operating bandwidth of a standard double ridge waveguide (40%). In the maximum bandwidth configuration, variation in the position of the boundary between the dielectrics produces little change in bandwidth. Hence, it appears that multilayer IDG is an attractive transmission media for wideband applications at both microwave and millimeter-wave frequencies     

Bound Waves in Compressible Plasma Gaps†

It is shown that a planar gap in a plasma medium is an appropriate idealized model for studying electromagnetic and plasma waves in a variety of situations, including plasma sheaths and ionospheric layers. A uniform, isotropic, lossless, warm plasma gap is investigated and is found to support both surface and interface waves which consist of an electromagnetic mode coupled to a plasma pressure mode. The surface waves are fast and they occur primarily at frequencies below the plasma frequency, whereas the interface waves are slow and are present at all frequencies ; in both cases, the waves may be either of the forward- or backward-travelling variety. In addition, the presence of leaky waves of the spectral type is demonstrated at frequencies above the plasma frequency. Dispersion curves are calculated for o large range of the parameters involved and the results are compared to those for surface waves along a dielectric slab which, in certain respects, represents a configuration analogous to the plasma gap if pressure effects are neglected.     

Propagation properties of ferromagnetic insulator guide

In order to realize new ferrite planar devices for applications at millimeter-wavelength frequencies, the authors have developed nonreciprocal ferrite devices schemes for incorporating ferromagnetic layers in dielectric insulator guide geometries. Their research program involves both the calculation and the measurement of device characteristics. For the calculation, a method of effective permeability, in which the permeability is a tensor, is introduced to characterize the ferrite material. The propagation properties of the insular guide are calculated by using a single-mode approach. The calculated results for dispersion, dielectric, and conductivity losses show resonant behavior with the application of a magnetic bias field for a guide configuration in which the ferrite replaces the insular dielectric. Ferrite phase shifters, filters, isolators, and circulators are potential applications of this guide configuration. For the experimental part of the study, wave dispersion and attenuation were measured in a purely dielectric insular guide from 26.5 to 40GHz. In addition to these experiments wave attenuation was measured as a function of magnetic bias field for the case where a hexagonal ferrite platelet was mounted on the ground plane near the insulator guide. General agreement is found between calculated and measured attenuation     

Conductivity measurements at the interface between the sinteredconductor and dielectric substrate at microwave frequencies

A novel measuring technique of the effective conductivity at microwave frequencies for both the sintered conductor surface and the interface between conductor and dielectric materials was developed. In the measurement, a dielectric rod resonator is placed between two dielectric plates, one side of which is coated with a sintered conductor. For measuring the surface conductivity, the dielectric rod is sandwiched by the conductor side of the plates. On the other hand, for measuring the interface conductivity, the dielectric rod is sandwiched by the dielectric side of the plates. By the configuration, only interface conductivity contributes to the conducting loss of the resonator, thus allowing the measurement of the interface conductivity. Using the new technique, the frequency dependence of both the surface and interface conductivity of a sintered copper, formed on a glass ceramic substrate by the co-firing technique, was investigated in the frequency range from 11 to 34 GHz. It was confirmed that the values of interface conductivity of the sintered copper were smaller than the values of the surface conductivity     

Dielectric spectroscopy of fresh fruit and vegetable tissues from 10 to 1800 MHz.

Dielectric spectroscopy data from measurements on tissue samples of nine fresh fruits and vegetables were used to study their dielectric behavior over the frequency range from 10 MHz to 1.8 GHz at 5 to 65 degrees C. Dielectric constant and loss-factor data are presented graphically for apple, avocado, banana, cantaloupe, carrot, cucumber, grape, orange, and potato, showing dielectric constants ranging from values of several hundred at 10 MHz to less than 100 at 1.8 GHz and loss factors on the order of one thousand at 10 MHz to less than 20 at 1.8 GHz. The dielectric loss factor increased consistently with increasing temperature at frequencies below 1 GHz. The dielectric constant increased with temperature at lower frequencies, but it decreased with temperature at the higher frequencies. This reversal of the sign of the temperature coefficient occurred at some point in the frequency range between 20 and 120 MHz where the temperature dependence of the dielectric constant was zero. At frequencies below this point, ionic conduction dominates the dielectric behavior, but above that point dipolar relaxation appears to control the behavior. Multiple linear regression provided equations for calculation of the loss factor in the frequency range from 10 to 300 MHz at temperatures from 5 to 65 degrees C. The data provide new information useful in understanding dielectric heating behavior and evaluating dielectric properties of such agricultural products for quality sensing applications.     

Numerical analysis of shielded dielectric resonators including substrate, support disc and tuning post

An efficient numerical technique and computer program has been developed for the determination of the resonant frequencies and field coefficients of cylindrical dielectric disc resonators in a configuration which models the various effects present in real technical applications. It accounts for the influences due to shielding, planar circuit substrate, dielectric support and a dielectric post used to tune out fabrication tolerances.     

Modal Inversion in Circular Waveguides---Part II: Application to Latching Nonreciprocal Phasers

The characteristics of the nonreciprocal phase shifter (phaser) configuration, consisting of a circumferentially magnetized circular toroid in circular waveguide, have been investigated theoretically. By suitably filling the toroid center with a material having a high dielectric constant, it is possible to reverse the normal order of the cutoff frequencies of the TE/sub 11/ and the operating TE/sub 01/ modes. The enhanced modal purity resulting from this inversion could yield significant improvements in the performance of practical phasers. The characteristic equation has been derived for the general case of a magnetized toroid in a circular waveguide, with dielectric material in both the central region and external to the toroid. The equation is valid for an arbitrary range of dielectric constants and magnetization. Differential phase shift calculations have been made specifically for operation within the modal inversion window. It is found that adequate differential phase shifts are obtained, even under the restrictions on geometric dimensions and range of dielectric constants imposed by the inversion requirements.     

Silicon as an advanced window material for high power gyrotrons

The absorptivity of high-purity grades of silicon (Si) and its reduction by subsequent doping procedures are investigated. The dielectric data are given for the wide range of frequencies (30 – 330 GHz) and temperatures (30 – 330 K) in comparison with the data set for sapphire. The advanced material performance in high power window applications is discussed taking into account both dielectric properties of the optimized silicon grades and thermal conductivity.     

Dielectric properties of human colostrum at microwave frequencies.

This article communicates the study of both the dielectric properties of human colostrums and breast milk at microwave frequencies. The colostrum samples were taken immediately after child birth and breast milk samples were collected at weekly intervals following the delivery. Rectangular cavity perturbation technique is used for the measurements of dielectric properties at the S-band of microwave frequency. The dielectric constants of the colostrums samples and breast milk samples are found to increase as weeks elapse, which is attributed to the reduced fat content and increased lactose concentration. The conductivity of these samples is similarly found to increase due to the increased dilution.     

The Dielectric Properties of the Bovine Eye Lens

The dielectric constant and electrical conductivity of the lens of the eye have been investigated throughout the frequency range from 0.5 to 200 Mc. The values obtained are frequency dependent. The frequency dependence is found consistent with the assumption of lens fiber membranes which are characterized by a membrane capacitance of very approximately 1?f/cm2. This is typical for biological membranes in general. Thus the existence of lens fiber membranes which are similar in molecular structure to the membranes of other biological cells is indicated. The dielectric data at very high frequencies are satisfactorily explained by the lens protein content.     

Some Characteristics of Dielectric Image Lines at Millimeter Wavelengths

The attenuation characteristics of several dielectric image lines have been calculated for the frequency range extending from 24 to 100 kmc and have been checked experimentally at 35 and 70 kmc. To obtain low attenuation at these high frequencies, dielectric materials with little loss and small size of cross section are required, while low values of the dielectric constant are also desirable. The effects of the size and shape of the dielectric cross section and of low dielectric constant are treated separately. To find proper materials with low dielectric constants several new foam plastics were investigated. Three types were found suitable for image line use, and in fact, these plastics have such good electrical and physical properties that they should be useful in many microwave applications. A qualitative measure of field extent is given for several image lines at 35 or 70 kmc, and various image lines and associated components are discussed. A new type of image line, called the tape line, is described.     

High-frequency, complex dielectric permittivity of saline solutionat elevated temperatures

The complex dielectric permittivity of saline solutions at elevated temperatures of up to 80°C has been measured at 1.1 GHz. The salinity of the solution was varied from 1000 to 5000 parts per million. The measurements were first made in a rectangular waveguide. The phase velocity and attenuation of the electromagnetic wave propagating in the waveguide were measured, and from those the real and imaginary parts of the dielectric permittivity were calculated. The complex permittivity of the heated saline solution was also measured in a parallel-plate radial waveguide. Details of the measurement techniques, together with measured data, are presented. It is found that the high-frequency conductivity and dielectric constant predicted by an empirical formula agree with the measured data in the temperature range of 20° to 40°C. However, for temperatures higher than 40°C, the measured conductivities and dielectric constant are both higher than the empirical data     

Coplanar waveguide excitation of dielectric resonator antennas

The circuit and radiation properties of a cylindrical dielectric resonator antenna excited by a coplanar waveguide feed are investigated. The coupling between the feed and the radiator was measured as a function of the position, dielectric constant, and height of the cylinder. The radiation patterns and resonant frequencies were also measured. These measured results are found to be in good agreement with theoretical calculations     

Output fluctuations of CW-pumped Nd:YAG lasers

The mechanisms that cause amplitude modulation in CW-pumped YAG lasers were investigated experimentally. The depth of modulation and frequency spectrum of the output fluctuations of a commercially available Nd:YAG laser were measured. It was found that after performing several modifications on the laser system, the output fluctuations were reduced by two orders of magnitude. At the conclusion of the program, the depth of modulation for frequencies below 10 Hz was 10^-2, and for frequencies between 10 Hz and 100 kHz, it was 10^-3. Above 100 kHz, the modulation decreased rapidly to 10^-5reaching shot noise level above 500 kHz.     

Frequency and gate voltage effects on the dielectric properties of Au/SiO2/n-Si structures

To determine the dielectric constant (ε′), dielectric loss (ε″), loss tangent (tan δ), the ac electrical conductivity (σ_ac) and the electric modulus of Au/SiO₂/n-Si structure, the measurement admittance technique was used. Experimental results show that the values of ε′, ε″, tan δ, σ_ac and the electric modulus show fairly large frequency and gate bias dispersion especially at low frequencies due to the interface charges and polarization. An increase in the values of the ε′ and ε″ were observed with both a decrease in frequency and an increase in frequency. The σ_ac is found to increase with both increasing frequency and voltage. In addition, the experimental dielectrical data have been analyzed considering electric modulus formalism. It can be concluded that the interface charges and interfacial polarization have strong influence on the dielectric properties of metal-insulator-semiconductor (MIS) structures especially at low frequencies and both in depletion and accumulation regions.     

Microwave Dielectric Behavior of Wet Soil-Part 1: Empirical Models and Experimental Observations

This is the first paper in a two-part sequence that evaluates the microwave dielectric behavior of soil-water mixtures as a function of water content, temperature, and soil textural composition. Part I presents the results of dielectric constant measurements conducted for five different soil types at frequencies between 1.4 and 18 GHz. Soil texture is shown to have an effect on dielectric behavior over the entire frequency range and is most pronounced at frequencies below 5 GHz. In addition, the dielectric properties of frozen soils suggest that a fraction of the soil water component remains liquid even at temperatures of -24° C. The dielectric data as measured at room temperature are summarized at each frequency by polynomial expressions dependent upon both the volumetric moisture content m and the percentage of sand and clay contained in the soil; separate polynomial expressions are given for the real and imaginary parts of the dielectric constant. In Part II, two dielectric mixing models will be presented to account for the observed behavior: 1) a semiempirical refractive mixing model that accurately describes the data and requires only volumetric moisture and soil texture as inputs, and 2) a theoretical four-component mixing model that explicitly accounts for the presence of bound water.     

Hot electron microwave conductivity of wide bandgap semiconductors

Hot electron microwave conductivity of the wide bandgap semiconductors GaN, SiC and Diamond has been calculated using displaced Maxwellian approximation for the electron distribution function. The effects of both the energy and momentum relaxation times due to scattering by acoustical, optical intervalley phonons and by ionized impurities are included in the derivations. Numerical results for the microwave conductivity and the change in dielectric constant as a function of frequency and bias electric field are presented. It is found that significant change in the conductivity and dielectric constant contribution for a fixed bias field occurs at very high frequencies on the order of 10¹² Hz, which is well beyond the range of current microwave device interest.     

多层介质膜光栅和介质膜反射镜抗激光损伤阈值研究

对多层介质膜光栅以及介质膜反射镜的激光损伤阈值进行了系统的研究。测试方法采用国际测试标准。测试结果表明,介质光栅的损伤阈值远低于未刻蚀的多层介质膜。对样品损伤形貌的扫描电镜照片分析发现,相比于未刻蚀的多层介质膜,介质膜光栅的初始损伤主要发生在光栅槽形的侧壁,且损伤主要是由驻波场的空间分布引起的本征吸收、制备过程中引入的杂质污染以及刻蚀过程中HfO2的化学计量机失衡引起的。