Open-ended Coaxial Probe Research Articles

Study of Freezing and Defrosting Effects on Complex Permittivity of Biological Tissues

In this letter we study the effect of freezing and defrosting on the dielectric properties of biological tissues. The electromagnetic characterization of tissues at microwave frequencies is crucial for the development of microwave-based biomedical devices. These measurements are often not practical, as tissue degradation restricts the time available between tissue excision and dielectric measurements. For this reason, measurement of tissues that underwent freezing and defrosting may provide researchers with more flexibility in setting measurement campaigns, thus speeding up the development of microwave-based biomedical devices. To this end, this letter presents dielectric measurement on frozen and defrosted tissue, which translates into the following objectives: first, investigate if the dielectric properties of defrosted tissues depend on frozen storage time; second, determine if defrosted tissue dielectric properties differ from those of fresh tissues. As a result, we measure the dielectric properties of ten samples of chicken muscle, bovine liver, and bovine fat, each before and after freezing (up to 14 days) and defrosting. The measurements are performed with the open-ended coaxial probe method at the frequency band of 0.5–8.5 GHz. We observe a slight increase—less than 10%—in complex permittivity of high-water-content tissues (muscle and liver) after defrosting, and negligible effect on fat tissues.

Comparison of dielectric properties of normal human esophagus and esophageal cancer using an open-ended coaxial probe

To study the difference in dielectric properties (relative permittivity and electrical conductivity) between normal human esophageal tissues and esophageal cancer tissues resected in esophageal cancer surgery for rapid identification of esophageal cancer during surgery. An open- end coaxial probe in the frequency range of 50 MHz to 4 GHz was used for dielectric measurement of the normal tissues and tumor tissues immediately after resection in 51 esophageal cancer cases. The difference in dielectric characteristics of the tissues was analyzed in the full frequency range, and the measurement data at 6 specific frequencies (64, 128, 298, 433, 915, and 2450 MHz) were analyzed statistically. The Cole-Cole model was used for data fitting in the entire frequency band and the measured values were compared with reported values. In the frequency range of 50 MHz- 4 GHz, the dielectric measurement values of esophageal cancer tissues were significantly higher than those of normal esophageal tissues. At the 6 specific frequencies, the dielectric properties also differed significantly between normal esophageal tissues and esophageal cancer tissues (P < 0.01). The Cole-Cole model achieved a good fitting result of the measured data. The measured values in this study were lower than the reported values. There are significant differences in the dielectric properties between normal esophageal tissue and tumor tissue, which may provide a new theoretical basis for rapid identification of esophageal cancer during surgery.

Functionalized-CNT Polymer Composite for Microwave and Electromagnetic Shielding.

In this research work, we studied the microwave properties of multi-wall carbon nanotube (MWCNT) surface functionalized with metallic oxides composites. Three different concentrations (5%, 10%, and 20%) of metallic oxides were used, namely cobalt, iron, and cobalt ferrite. The surface-decorated CNTS were impregnated into polyurethane (PU) matrix. The surface-decorated MWCNTs and the MWCNTs-PU composites were characterized using electron microscopy. The dielectric properties of the samples are studied using an open-ended coaxial probe technique in a wide frequency range of (5–50 GHz). The metallic oxide-decorated surface MWCNTs-PU composites demonstrated different microwave-frequency absorption characteristics depending on the concentration of the metallic oxides.

Experimental tissue mimicking human head phantom for estimation of stroke using IC-CF-DMAS algorithm in microwave based imaging system

This paper presents the preparation and measurement of tissue-mimicking head phantom and its validation with the iteratively corrected coherence factor delay-multiply-and-sum (IC-CF-DMAS) algorithm for brain stroke detection. The phantom elements are fabricated by using different chemical mixtures that imitate the electrical properties of real head tissues (CSF, dura, gray matter, white matter, and blood/stroke) over the frequency band of 1–4 GHz. The electrical properties are measured using the open-ended dielectric coaxial probe connected to a vector network analyzer. Individual phantom elements are placed step by step in a three-dimensional skull. The IC-CF-DMAS image reconstruction algorithm is later applied to the phantom to evaluate the effectiveness of detecting stroke. The phantom elements are preserved and measured multiple times in a week to validate the overall performance over time. The electrical properties of the developed phantom emulate the similar properties of real head tissue. Moreover, the system can also effectively detect the stroke from the developed phantom. The experimental results demonstrate that the developed tissue-mimicking head phantom is time-stable, and it shows a good agreement with the theoretical results in detecting and reconstructing the stroke images that could be used in investigating as a supplement to the real head tissue.

Effect of moisture content on dielectric properties of banana leaves and peels in frequency range of 1–20 GHz

Abstract The quality of agricultural products can be remotely sensed and enhanced by determining the dielectric properties. This paper studies the dielectric properties of banana leaf and banana peel over the frequency range 1–20 GHz using the open-ended coaxial probe (OCP) method. A new curve fitting model is proposed to characterize the dielectric properties of banana leaf and banana peel. The different moisture content (MC) levels are considered for both banana leaf and banana peel samples and, their dielectric properties are characterized. Further, the banana leaf and banana peel’s measurement data are compared with the data obtained using the proposed model. In addition, Root Mean Square Error (RMSE) and R-squared (R 2) are calculated to validate the performance of the proposed model. In case of banana leaf at 68.26% MC, the dielectric constant achieves the value of R 2 and RMSE of 0.98 and 0.0648, respectively. Similarly, dielectric loss achieves the value of R 2 and RMSE of 0.88 and 0.0795, respectively. Further, for banana peel at 80.89% MC, the dielectric constant achieves the value of R 2 and RMSE of 0.99 and 0.2989, respectively. Similarly, dielectric loss achieves the value of R 2 and RMSE of 0.96 and 0.6132, respectively.

Temperature-dependent dielectric properties of human uterine fibroids over microwave frequencies

Microwave ablation is under investigation as a minimally-invasive treatment for uterine fibroids. Computational models play a vital role in the development, evaluation and characterization of candidate ablation devices. The temperature-dependent dielectric properties of fibroid tissue are essential for accurate computational modeling. Objective: To measure the broadband temperature-dependent dielectric properties of uterine fibroids excised during hysterectomy procedures. Methods: The open-ended coaxial probe method was employed for measuring the broadband dielectric properties of freshly excised human uterine fibroid samples (n = 6) obtained from an IRB-approved tissue bank. The dielectric properties (relative permittivity, ε r, and effective electrical conductivity, σ eff) were evaluated at temperatures ranging from 23 °C–150 °C, over the frequency range of 0.5–6 GHz. Linear piecewise parametrization with respect to temperature and quadratic parametrization with respect to frequency was applied to characterize broadband temperature-dependent dielectric properties of fibroid tissue. Results: The baseline room temperature values of ε r vary from 57.5 ± 5.29 to 44.5 ± 5.77 units and σ eff changes from 0.91 ± 0.19 to 6.02 ± 0.7 S m−1 over the frequency range of 0.5–6 GHz. At temperatures close to the water vaporization point, ε r, drops considerably i.e. to 12%–14% of its baseline value for all measured frequencies. σ eff values initially rise till 98 °C and then fall to 11%–13% of their baseline values at 125 °C for frequencies ≤2.45 GHz. The σ eff follows a decreasing trend for frequencies >2.45 GHz and drops to ∼6 % of their baseline room temperature values. Conclusion: The temperature dependent dielectric properties of uterine fibroid tissues over microwave frequency range are reported for the first time in this study. Parametric models of uterine fibroid dielectric properties are also presented for incorporation within computational models of microwave ablation of fibroids.

Application of Machine Learning to Predict Dielectric Properties of In Vivo Biological Tissue.

In this paper we revisited a database with measurements of the dielectric properties of rat muscles. Measurements were performed both in vivo and ex vivo; the latter were performed in tissues with varying levels of hydration. Dielectric property measurements were performed with an open-ended coaxial probe between the frequencies of 500 MHz and 50 GHz at a room temperature of 25 °C. In vivo dielectric properties are more valuable for creating realistic electromagnetic models of biological tissue, but these are more difficult to measure and scarcer in the literature. In this paper, we used machine learning models to predict the in vivo dielectric properties of rat muscle from ex vivo dielectric property measurements for varying levels of hydration. We observed promising results that suggest that our model can make a fair estimation of in vivo properties from ex vivo properties.

High-Temperature and High-Sensitivity Pressure Sensors Based on Microwave Resonators

This paper reports a novel diaphragm-assisted open-ended hollow coaxial cable resonator (OE-HCCR) for highly sensitive pressure measurements at elevated temperatures. The coaxial line resonator is constructed using two highly reflective reflectors in a homemade hollow coaxial cable, including a metal post shorting the two concentric conductors (i.e., the inner conductor and the outer conductor) and the open end of the coaxial line. In the pressure sensor device design, a sealed metal diaphragm is mounted on the open end of the OE-HCCR and is employed as the pressure responsive element. The diaphragm’s deflection can be accurately captured by the coaxial line resonator in real-time, in response to a change in differential pressure. The sensor device’s responses to hydraulic pressures at ambient temperature and air pressures at elevated temperatures up to 1000 °C were investigated in the demonstration experiments, showing user-configurable measurement sensitivity and high-temperature survivability. The device’s pressure measurement resolution is also demonstrated as high as sub-Pascal by probing tiny hydrostatic pressure changes. A positive feedback loop is constructed as an alternative interrogator for the novel device, showing high signal quality and ease of demodulation. The pressure sensor described in this work is robust, easy-to-manufacture, user-configurable, low-cost, highly sensitive, and has great potential for industrial applications involving high-temperature and harsh environments. The presented work opens new avenues to develop a new generation of high-sensitivity and high-temperature microwave sensors based on well-known open-ended coaxial probes.

Effects of Particle Size on the Dielectric, Mechanical, and Thermal Properties of Recycled Borosilicate Glass-Filled PTFE Microwave Substrates.

Low dielectric loss and low-cost recycled borosilicate (BRS) glass-reinforced polytetrafluoroethylene (PTFE) composites were fabricated for microwave substrate applications. The composites were prepared through a dry powder processing technique by dispersing different micron sizes (25 µm, 45 µm, 63 µm, 90 µm, and 106 µm) of the recycled BRS filler in the PTFE matrix. The effect of the filler sizes on the composites’ thermal, mechanical, and dielectric properties was studied. The dielectric properties of the composites were characterised in the frequency range of 1–12 GHz using an open-ended coaxial probe (OCP) connected to a vector network analyser (VNA). XRD patterns confirmed the phase formation of PTFE and recycled BRS glass. The scanning electron microscope also showed good filler dispersion at larger filler particle sizes. In addition, the composites’ coefficient of thermal expansion and tensile strength decreased from 12.93 MPa and 64.86 ppm/°C to 7.12 MPa and 55.77 ppm/°C when the filler size is reduced from 106 μm to 25 μm. However, moisture absorption and density of the composites increased from 0.01% and 2.17 g/cm3 to 0.04% and 2.21 g/cm3. The decrement in filler size from 106 m to 25 m also increased the mean dielectric constant and loss tangent of the composites from 2.07 and 0.0010 to 2.18 and 0.0011, respectively, while it reduced the mean signal transmission speed from 2.088 × 108 m/s to 2.031 × 108 m/s. The presented results showed that PTFE/recycled BRS composite exhibited comparable characteristics with commercial high-frequency laminates.

Dielectric spectroscopy of Baijiu over 2-20GHz using an open-ended coaxial probe.

The complex electric permittivity of Baijiu (one of the most consumed beverages) is measured over a frequency range of 2 to 20GHz using a low-cost open-ended coaxial probe and a portable microwave analyzer. The Cole-Cole equation is used to fit the permittivity data, and the parameter values related are extracted using the particle swarm optimization. A linear relationship is obtained between these parameters and the alcohol by volume. A strong dependence of the permittivity on the alcohol content is also observed for other alcoholic drinks, including brandy and whisky. The study demonstrates that the microwave technique provides an efficient solution in the effort to determine the alcohol content and the principal component analysis is a powerful tool for the quick estimation of the dielectric properties. The sensor presented can contribute to efficient on-site quality control of alcoholic drinks. PRACTICAL APPLICATION: The microwave approach presented could offer an economical approach for the characterization of Baijiu and a general solution to the alcohol content determination for the alcoholic drinks industry.

Development of a Solid and Flexible Matching Medium for Microwave Medical Diagnostic Systems.

This paper reports the development of a new composite material as a matching medium for medical microwave diagnostic systems, where maximizing the microwave energy that penetrates the interrogated tissue is critical for improving the quality of the diagnostic images. The proposed material has several advantages over what is commonly used in microwave diagnostic systems: it is semi-flexible and rigid, and it can maximize microwave energy coupling by matching the tissue’s dielectric constant without introducing high loss. The developed matching medium is a mirocomposite of barium titanate filler in polydimethylsiloxane (PDMS) in different weight-based mixing ratios. Dielectric properties of the material are measured using a Keysight open-ended coaxial slim probe from 0.5 to 10 GHz. To avoid systematic errors, a full dielectric properties calibration is performed before measurements of sample materials. Furthermore, the repeatability of the measurements and the homogeneity of the sample of interest are considered. Finally, to evaluate the proposed matching medium, its impact on a printed monopole antenna is studied. We demonstrate that the permittivity of the investigated mixtures can be increased in a controlled manner to reach values that have been previously shown to be optimal for medical microwave imaging (MWI) such as stroke and breast cancer diagnostic applications. As a result, the material is a good candidate for supporting antenna arrays designed for portable MWI scanners in applications such as stroke detection.

Classification of Metastatic and Non-Metastatic Thoracic Lymph Nodes in Lung Cancer Patients Based on Dielectric Properties Using Adaptive Probabilistic Neural Networks.

ObjectiveDielectric properties can be used in normal and malignant tissue identification, which requires an effective classifier because of the high throughput nature of the data. With easy training and fast convergence, probabilistic neural networks (PNNs) are widely applied in pattern classification problems. This study aims to propose a classifier to identify metastatic and non-metastatic thoracic lymph nodes in lung cancer patients based on dielectric properties.MethodsThe dielectric properties (permittivity and conductivity) of lymph nodes were measured using an open-ended coaxial probe. The Synthetic Minority Oversampling Technique method was adopted to modify the dataset. Feature parameters were scored to select the appropriate feature vector using a Statistical Dependency algorithm. The dataset was classified using adaptive PNNs with an optimized smooth factor using the simulated annealing PNN (SA-PNN). The results were compared with traditional Probabilistic, Support Vector Machines, k-Nearest Neighbor and the Classify functions in MATLAB.ResultsThe conductivity frequencies of 3959, 3958, 3960, 3978, 3510, 3889, 3888, and 3976 MHz were selected as the feature vectors for 219 lymph nodes (178 non-metastatic and 41 metastatic). Compared with the other methods, SA-PNN achieved the highest classification accuracy (92.92%) and the corresponding specificity and sensitivity were 94.72% and 91.11%, respectively.ConclusionsCompared with the other methods, the SA-PNN proposed in the present study achieved a higher classification accuracy, which provides a new scheme for classification of metastatic and non-metastatic thoracic lymph nodes in lung cancer patients based on dielectric properties.

Soil dielectric characterization during freeze–thaw transitions using L-band coaxial and soil moisture probes

Abstract. Soil microwave permittivity is a crucial parameter in passive microwave retrieval algorithms but remains a challenging variable to measure. To validate and improve satellite microwave data products, precise and reliable estimations of the relative permittivity (εr=ε/ε0=ε′-jε′′; unitless) of soils are required, particularly for frozen soils. In this study, permittivity measurements were acquired using two different instruments: the newly designed open-ended coaxial probe (OECP) and the conventional Stevens HydraProbe. Both instruments were used to characterize the permittivity of soil samples undergoing several freeze–thaw cycles in a laboratory environment. The measurements were compared to soil permittivity models. The OECP measured frozen (εfrozen′=[3.5; 6.0], εfrozen′′=[0.46; 1.2]) and thawed (εthawed′=[6.5; 22.8], εthawed′′=[1.43; 5.7]) soil microwave permittivity. We also demonstrate that cheaper and widespread soil permittivity probes operating at lower frequencies (i.e., Stevens HydraProbe) can be used to estimate microwave permittivity given proper calibration relative to an L-band (1–2 GHz) probe. This study also highlighted the need to improve dielectric soil models, particularly during freeze–thaw transitions. There are still important discrepancies between in situ and modeled estimates and no current model accounts for the hysteresis effect shown between freezing and thawing processes, which could have a significant impact on freeze–thaw detection from satellites.

Towards Accurate Microwave Characterization of Tissues: Sensing Depth Analysis of Open-Ended Coaxial Probes with Ex Vivo Rat Breast and Skin Tissues.

Dielectric properties of biological materials are commonly characterized with open-ended coaxial probes due to the broadband and non-destructive measurement capabilities. Recently, potential diagnostics applications of the technique have been investigated. Although the technique can successfully classify the tissues with different dielectric properties, the classification accuracy can be improved for tissues with similar dielectric properties. Increase in classification accuracy can be achieved by addressing the error sources. One well-known error source contributing to low measurement accuracy is tissue heterogeneity. To mitigate this error source, there is a need define the probe sensing depth. Such knowledge can enable application-specific probe selection or design. The sensing depth can also be used as an input to the classification algorithms which can potentially improve the tissue classification accuracy. Towards this goal, this work investigates the sensing depth of a commercially available 2.2 mm aperture diameter probe with double-layered configurations using ex vivo rat breast and skin tissues. It was concluded that the dielectric property contrast between the heterogeneous tissue components has an effect on the sensing depth. Also, a membrane layer (between 0.4–0.8 mm thickness) on the rat wet skin tissue and breast tissue will potentially affect the dielectric property measurement results by 52% to 84%.

Dielectric properties of fresh rabbit meat in the microwave range.

Dielectric properties (DPs) of fresh rabbit meat in the microwave range (0.5 to 20GHz) were determined. Three different muscles (Biceps femoris, Tensor fasciae latae, and Longissimus thoracis) from California rabbits (male and female) were measured with the open-ended coaxial probe method at temperatures of 20, 40, and 60 °C. To assess the possible effect of age, females of 100 and 180 days old were analyzed. DPs were affected by frequency, muscle type, age, gender, and temperature (p<0.05). Dielectric constant decreased with increasing frequency. Loss factor decreased from 0.5 to 2.5GHz due to ionic conduction, followed by an increase up to 20GHz, dominated by dipolar relaxation. PRACTICAL APPLICATION: These results are key parameters for further quality sensing applications and for heating processes of meat rabbit using microwaves. Longer penetration depths were achieved at 915MHz; this frequency is recommended for further applications. Besides, dielectric properties have potential to be a tool for identification of gender and age for slaughtered rabbits.

Towards Non-Invasive Diagnosis of Skin Cancer: Sensing Depth Investigation of Open-Ended Coaxial Probes.

Simple SummaryThis work investigates the effect of skin tissue heterogeneity on the sensing depth of the open-ended coaxial probe to exploit the potential use of the probe for skin cancer detection and to establish a simple measurement protocol for skin depth characterization. Skin depth was calculated through simulations and measurements. Heterogeneity was obtained using double-layered materials composed of gel-like skin mimicking material and liquid olive oil, triton X-100. It was concluded that the sensing depth was not dependent on the frequency between 0.5 to 6 GHz, was affected by the material located at the aperture of the probe, and lastly the dielectric property contrast between layers. Namely the degree of heterogeneity affects the probe sensing depth.Dielectric properties of biological tissues are traditionally measured with open-ended coaxial probes. Despite being commercially available for laboratory use, the technique suffers from high measurement error. This prevents the practical applications of the open-ended coaxial probes. One such application is the utilization of the technique for skin cancer detection. To enable a diagnostic tool, there is a need to address the error sources. Among others, tissue heterogeneity is a major contributor to measurement error. The effect of tissue heterogeneity on measurement accuracy can be decreased by quantifying the probe sensing depth. To this end, this work (1) investigates the sensing depth of the 2.2 mm-diameter open-ended coaxial probe for skin mimicking material and (2) offers a simple experimental setup and protocol for sensing depth characterization of open-ended coaxial probes. The sensing depth characterized through simulation and experiments using two double-layered configurations composed to mimic the skin tissue heterogeneity. Three thresholds in percent increase of dielectric property measurements were chosen to determine the sensing depth. Based on the experiment results, it was concluded that the sensing depth was effected by the dielectric property contrast between the layers. That is, high contrast results in rapid change whereas low contrast results in a slower change in measured dielectric properties. It was also concluded that the sensing depth was independent of frequency between 0.5 to 6 GHz and was mostly determined by the material located immediately at the aperture of the probe.

Open-Ended Coaxial Probe for Dielectric Thickness Measurements

Open-ended coaxial probes have been used extensively in determining the dielectric properties of materials in the radio and microwave frequencies. The probes are used in conjunction with an automatic network analyzer to measure electrical and material properties. The technique is based on the principle that measured reflection coefficient of a loaded coaxial probe is related to the dielectric properties of the material in contact with the probe. The dielectric properties, in turn, provide information on the electrical energy storage, dissipation, hydration and quality of the test material.

Ultrawide band microwave system as a non-invasive technology to predict beef carcase fat depth

A portable ultra-wide band microwave system (MiS) coupled with an open-ended coaxial probe (OCP) or Antipodal Vivaldi Antenna (VPA) was tested as a non-invasive objective measurement to predict beef carcase single site fat depth at commercial abattoirs. Experiment one tested the effectiveness of MiS coupled with a VPA. The VPA was used to predict hot carcase P8 (fat depth on the rump) across 4 slaughter groups (n = 241). The VPA was also used to predict cold carcase rib fat (at the quartering site, 75% along the rib eye muscle) across 5 slaughter groups (n = 598). Experiment two tested the ability of MiS coupled with OCP to measure hot carcase P8 across two slaughter groups (n = 435). A machine learning stacking ensemble method was used to create the prediction equations. Datasets were grouped by prediction trait (P8 or ribfat) and probe/antenna then randomly divided into 5 groups based on tissue depth. Precision was greatest using OCP to predict P8 fat depth with a RMSEP of 2.47 mm and R2 of 0.70. The VPA precision was similar for the two tissue depths assessed, hot carcase P8 had an average RMSEP of 2.86 mm and R2 of 0.58 compared to cold carcase rib fat RMSEP of 2.60 mm and R2 of 0.55.

Application of microwave waveguide techniques for investigating the effect of concrete dielectric and reflection properties during curing

This study deals with the measurement of dielectric constant, the magnitude of reflection coefficient, and compressive strength for G30, G40, G50, and G60 concrete mix after 0, 3, 7, 14, 21, and 28 days curing. The open-ended coaxial probe of 4.8 cm, WR510, and WR340 waveguide models operating over a frequency range of 85 MHz to 1.8 GHz, 1.45–2.2 GHz, and 2.17–3.3 GHz, respectively, are applied. Fifteen concrete cube samples 150 × 150 × 150 mm size were casted for each mix. The experimental result shows that the dielectric constant and magnitude of the reflection coefficient decrease gradually until 21 days, then increase at 28 days age of concrete. Correlated the concrete dielectric constant with design compressive strength. The results corroborate that microwave techniques correlate well for evaluating the strength of concrete. The strength characterization is suitable with related material composition, necessary experimental setup, and environmental conditions applied in this study. Furthermore, curing conditions, effects must be accounted; it is valuable in establishing a correlation between conventional properties and microwave properties of concrete.

Dielectric Characterization of Ex Vivo Ovine and Human Adrenal Glands for Microwave Thermal Ablation Applications

Historically, adrenal glands diseases causing hypertension, such as Primary Aldosteronism (PA), have been treated through pharmacotherapy or surgical resection. Given the shortcomings of the available treatment options, the interest in alternative and less invasive treatment modalities such as microwave ablation (MWA), has increased. In order to develop and optimize this novel electromagnetic-based therapy, an accurate knowledge of the dielectric properties of human adrenal glands, as well as preclinical animal models, is crucial. In particular, ovine models represent a feasible animal model to test the safety and performances of MWA. In this study, the dielectric properties of ovine adrenal glands and of normal and diseased human adrenal glands are characterized <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex vivo</i> in the microwave frequency range. The dielectric properties of the two functional tissues (cortex and medulla) composing ovine adrenal glands are measured using the open-ended coaxial probe technique and represented with a two pole Cole-Cole model in the frequency range from 0.5 GHz to 8 GHz. This paper presents the first dielectric data of normal and diseased human adrenal tissues, including a functioning adenoma responsible for PA and it compares the human data with data from the animal model.