Open-ended Coaxial Probe Method Research Articles

A software tool for fabricating phantoms mimicking human tissues with designated dielectric properties and frequency.

Dielectric materials play a crucial role in assessing and refining the measurement performance of dielectric properties for specific tasks. The availability ofviable and standardized dielectric materials could greatly enhance medical applications related to dielectric properties. However, obtaining reliable phantoms with designated dielectric properties across a specified frequency range remains challenging. In this study, we propose software to easily determine the components of dielectric materials in the frequency range of 16 MHz to 3 GHz. A total of 184 phantoms were fabricated and measured using open-ended coaxial probe method. The relationship among dielectric properties, frequency, and thecomponents of dielectric materials was fitted through feedforward neural networks. Software was developed to quickly calculate the composition of dielectric materials. We performed validation experiments including blood, muscle, skin, and lung tissue phantoms at 128 MHz, 298 MHz, 915 MHz, and 2.45 GHz. Compared with literature values, the relative errors of dielectric properties are less than 15 %. This study establishes a reliable method for fabricating dielectric materials with designated dielectric properties and frequency through the development of the software. This research holds significant importance in enhancing medical research and applications that rely on tissue simulation using dielectric phantoms.

Using the Probability Density Function-Based Channel-Combination Bloch-Siegert Method Realizes Permittivity Imaging at 3T.

Magnetic resonance electrical properties tomography (MR EPT) can retrieve permittivity from the B1+ magnitude. However, the accuracy of the permittivity measurement using MR EPT is still not ideal due to the low signal-to-noise ratio (SNR) of B1+ magnitude. In this study, the probability density function (PDF)-based channel-combination Bloch-Siegert (BSS) method was firstly introduced to MR EPT for improving the accuracy of the permittivity measurement. MRI experiments were performed using a 3T scanner with an eight-channel receiver coil. The homogeneous water phantom was scanned for assessing the spatial distribution of B1+ magnitude obtained from the PDF-based channel-combination BSS method. Gadolinium (Gd) phantom and rats were scanned for assessing the feasibility of the PDF-based channel-combination BSS method in MR EPT. The Helmholtz-based EPT reconstruction algorithm was selected. For quantitative comparison, the permittivity measured by the open-ended coaxial probe method was considered as the ground-truth value. The accuracy of the permittivity measurement was estimated by the relative error between the reconstructed value and the ground-truth value. The reconstructed relative permittivity of Gd phantom was 52.413, while that of rat leg muscle was 54.053. The ground-truth values of relative permittivity of Gd phantom and rat leg muscle were 78.86 and 49.04, respectively. The relative error of average permittivity was 33.53% for Gd and 10.22% for rat leg muscle. The results indicated the high accuracy of the permittivity measurement using the PDF-based channel-combination BSS method in MR EPT. This improvement may promote the clinical application of MR EPT technology, such as in the early diagnosis of cancers.

Microwave dielectric characterization studies of PVDF-TrFE-Nafion blended films

The solution-based casting method fabricates the free-standing pure PVDF-TrFE, PVDF-TrFE-Nafion blended films and tests them for structural, morphological, and microwave studies. The X-ray diffraction pattern and FTIR spectrum confirmed the formation of the ferroelectric β-phase of PVDF-TrFE. The surface morphology of blended films is studied by scanning electron microscope (SEM), and noticeable changes in the morphological features are observed in the blended films resulting from the differences in the nucleation process in these films. Then using the open-ended coaxial probe method, the complex permittivity measurements were carried out over a large frequency range, from 0.5 GHz to 50 GHz. These results were verified with the Transmission-Reflection method using a rectangular waveguide in the X-band and Ku-band frequency range. The microwave absorption properties of the blends are also studied in the same frequency range. The stability of the dielectric properties and low microwave absorption indicates that the blending of PVDF-TrFE with Nafion results in a new potential material for high frequency flexible capacitor applications.

Dielectric Spectroscopy Shows a Permittivity Contrast between Meningioma Tissue and Brain White and Gray Matter-A Potential Physical Biomarker for Meningioma Discrimination.

The effectiveness of surgical resection of meningioma, the most common primary CNS tumor, depends on the capability to intraoperatively discriminate between the meningioma tissue and the surrounding brain white and gray matter tissues. Aiming to find a potential biomarker based on tissue permittivity, dielectric spectroscopy of meningioma, white matter, and gray matter ex vivo tissues was performed using the open-ended coaxial probe method in the microwave frequency range from 0.5 to 18 GHz. The averages and the 95% confidence intervals of the measured permittivity for each tissue were compared. The results showed the absence of overlap between the 95% confidence intervals for meningioma tissue and for brain white and gray matter, indicating a significant difference in average permittivity (p ≤ 0.05) throughout almost the entire measured frequency range, with the most pronounced contrast found between 2 GHz and 5 GHz. The discovered contrast is relevant as a potential physical biomarker to discriminate meningioma tissue from the surrounding brain tissues by means of permittivity measurement, e.g., for intraoperative meningioma margin assessment. The permittivity models for each tissue, developed in this study as its byproducts, will allow more accurate electromagnetic modeling of brain tumor and healthy tissues, facilitating the development of new microwave-based medical devices and tools.

Utilization of Poisonous Marine invader in Development of Low Losses Microwave Devices

This research paper presents constitutive parameters of poisonous marine invaders (Diadema setosum) for microwave application. Two different powder form of these invaders were prepared and calcined at different temperatures. These biomaterials have been obtained for grinding shell and spines of sea urchin were turned into powder. Constitutive parameters of these classified biomaterials have been investigated in 3.2-20GHz frequency range with open-ended coaxial probe method. Measured results show that these biomaterials have a potential usage area in microwave range are mentioned firstly in literature. Measured complex permittivity properties of samples of shell and spines of invasive long-spined sea urchin have demonstrated that samples can be utilized in development of high-quality microwave radomes with high impedance matching to air and low loss microwave substrates.

Testing of chlorine dosage in drinking water using microstrip patch sensor

A new method using microstrip patch sensor is designed, simulated, and measured to detect the dosage of added chlorine to the drinking water using the relation between permittivity and matching impedance. The simulation is done using computer simulation technology-microwave studio software. The proposed sensor is designed on a single FR-4 layer with a low profile, 0.499 GHz bandwidth, center frequency of 2.94 GHz, gain of 6.55 dBi, and a good front to back ratio of 17.84 dB. In this work, the antenna design is started from a conventional patch antenna equation. Then, an optimization process is performed to achieve good parameter values for obtaining the desired objective. The objective is to utilize the microwave frequency to measure the chlorine dosage in water. The measurement results proved that the matching impedance can be used to find out the dosage of chlorine in drinkable water. The proposed method overcomes the open-ended coaxial probe method in terms of experimental time and contactless testing and shows high matching with the latter actual data. Moreover, the proposed method overcomes the conventional methods in terms of continuity and large scale of testing, non-contacting with the samples of water, and no additive chemical materials, which make the water samples not drinkable.

Sensitivity investigation of open-ended coaxial probe in skin cancer detection

Open-ended coaxial probe method is one of the most common modalities in measuring dielectric properties (DPs) of biological tissues. Due to the significant differences between the tumors and normal tissues in DPs, the technique can be used to detect skin cancer in the early stage. Although various studies have been reported, systematic assessment is in urgent need to advance it to clinical applications, for its parameters interactions and detecting limitations remained unclear. In this study, we aim to provide a comprehensive examination of this method, including the minimum detectable tumor size by using a three-layer skin model via simulation and demonstrated that open-ended coaxial probe method can be used for detection of early-stage skin cancer. The smallest detecting size are subject to different subtypes: for BCC, inside the skin is 0.5mm radius × 0.1mm height; for SCC, inside the skin is 1.4mm × 1.3mm in radius and height; the smallest distinguishing size of BCC is 0.6mm × 0.7mm in radius and height; for SCC is 1.0mm × 1.0mm in radius and height; for MM is 0.7mm × 0.4mm in radius and height. The experiment results showed that sensitivity was affected by tumor dimension, probe size, skin height, and cancer subtype. The probe is more sensitive to cylinder tumor radius than height growing on the surface of the skin while the smallest size probe is the most sensitive among the working probes. We provide a detailed systematic evaluation of the parameters employed in the method for further applications.

Design of a simple radio frequency circuit for implementing the open-ended coaxial probe method for permittivity measurement.

The development of electromagnetic (EM)-based therapeutic and diagnostic tools, as well as safety assessment of EM interactions with the human body, requires adequate measurement of the complex permittivity of different biological tissues. Such measurement techniques must be low-cost, readily available, and easy to implement. In this study, a simple circuit with basic radio frequency electronics was used to implement the open-ended coaxial probe method for permittivity measurement, as opposed to the widely used vector network analyzers. The non-ideal behavior of the circuit due to spurious reflections and ohmic losses was accounted for by a scattering matrix (SM) that relates the measured reflection coefficient to the true reflection coefficient at the probe tip. Parameters of SM were obtained using three calibration standards, and the circuit was used to measure the complex permittivity of a standard, tissue-equivalent, American Society of Testing Materials (ASTM) polymer gel. A more intuitive approach to circuit analysis is also introduced. For both methods, the dielectric constant and electrical conductivity of the gel were found to agree with the recommended uncertainties of the ASTM standard and validate the utility of the circuit at the test frequency.

Wideband frequency-dependent dielectric properties of rat tissues exposed to low-intensity focused ultrasound in the microwave frequency range

Tissue dielectric properties can vary upon the incident of an acoustic wave. The goal of this study is to quantify this change due to the acoustoelectric effect (AE), and to obtain the frequency-dependent dielectric properties of tissues exposed to low-intensity focused ultrasound (LIFU). The dielectric properties of the blood, brain, chest muscle, heart, kidney, leg muscle, liver, lung, pancreas, and spleen of rats were measured by an open-ended coaxial probe method. The acoustic intensity of LIFU focus was 2.97 MPa (67.6 W/cm2), 3.95 MPa (120 W/cm2), and 5.17 MPa (204 W/cm2), respectively, and the measurement frequency band was 0.1–7.08 GHz. The measurement results show that with the LIFU modulation, the conductivity and dielectric constant decreased in the high-frequency band, and on the contrary, they increased in the low-frequency band, and the larger the acoustic intensity was, the more obvious the phenomenon was. This work contributes to a better understanding of the mechanisms by which ultrasound acts on the dielectric properties of biological tissues. It is expected that the findings from this study will provide a basis that the response of tissue to LIFU modulation can be monitored by noninvasive techniques such as microwave-induced thermoacoustic imaging (MTI) and microwave imaging, present a new idea for improving the endogenous contrast between different biological tissues in MTI and acoustoelectric imaging, and possibly lead to the development of a new imaging method based on the relaxation time of tissue after LIFU modulation.

Characterization of Open-Ended Coaxial Probe Sensing Depth with Respect to Aperture Size for Dielectric Property Measurement of Heterogeneous Tissues.

The open-ended coaxial probe (OECP) method is frequently used for the microwave dielectric property (DP) characterization of high permittivity and conductivity materials due to inherent advantages including minimal sample preparation requirements and broadband measurement capabilities. However, the OECP method is known to suffer from high measurement error. One well-known contributor to the high error rates is tissue heterogeneity, which can potentially be managed through the selection of a probe with a proper sensing depth (SD). The SD of the OECP is dependent on many factors including sample DPs and probe aperture diameter. Although the effects of sample DPs on SD have been investigated to some extent in the literature, the probe aperture diameters, particularly small diameters, have not been fully explored. To this end, the SDs of probes with three different apertures (0.5, 0.9 and 2.2 mm-diameters) were analyzed in this study. Probes’ SDs were first investigated with simulations using a double-layered sample configuration (skin tissue and olive oil). Next, experiments were performed using a commercial OECP with a 2.2 mm aperture diameter. The SD was categorized based on 5%, 20% and 80% DP change. Among these threshold values, a 5% DP change was selected as the benchmark for SD categorization. The findings suggest that probes with a smaller aperture size and correspondingly smaller SD should be utilized when measuring the DPs of thin and multilayered samples, such as healthy and diseased skin tissues, to increase the measurement accuracy.

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.

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.

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.

Enhancement of microwave ring resonator based on poly-lactic acid thermoplastic substrate

In this paper, the development of a single-port microwave ring resonator (MRR) sensor based on a thermoplastic material, which is poly-lactic acid (PLA), is presented. The open-ended coaxial probe method was applied to identify the dielectric properties of PLA in terms of dielectric constant (2.25) and loss tangent (0.0001). The PLA substrate was fabricated using a hot press machine and with the same thickness (1.6 mm) as FR4. Hence, to consider the PLA as microwave substrate, microwave ring resonator (MRR) operating at 1.1 GHz resonance frequency was designed, simulated, and measured. Based on the observation, the return loss of MRR for the simulation and the measurement of the conventional design are -5.37 dB and -5.02 dB, respectively. The quality factor (Q-factor) for both are 122.22 and 183.33, respectively. Then, the enhanced coupling gap method was applied to improve the performance of MRR sensitivity in terms of return loss and Q-factor. It is observed that the return loss of the enhanced design for the simulation and the measurement are -26.67 dB and -20.23 dB, respectively, and the Q-factor are 122.22 and 200, respectively. Thus, the performance of the MRR based on different designs were compared in order to validate the sensor’s sensitivity and PLA can be recognized as a substrate material for RF and microwave applications.

Microwave absorption performance enhancement of corn husk-based microwave absorber

This paper presents corn husk-based microwave absorber along with the enhancement of microwave absorption performance by adding a different percentage of charcoal in the frequency range of 1–20 GHz. The open-ended coaxial probe method is used to measure the S11 parameter, and also dielectric properties of the samples. The computer simulation technology microwave studio (CST MWS) is used to design the proposed microwave absorber. The result shows that the sample with the utmost charcoal content (4.44 wt%) poses the best return loss value of − 50.2696 dB at 19.221 GHz frequency.

Dielectric profile of blood clots to inform ischemic stroke treatments.

Platelet and fibrin-rich blood clots can respond differently to red blood cell rich clots during ischemic stroke treatment, which includes thrombolysis and mechanical thrombectomy. Currently, there is no accurate way to identify the type of clot in advance of treatment. If the type of blood clot can be identified, the optimum clot removal process can be chosen and patient outcomes can be improved. In this paper we fabricate physiologically relevant blood clot analogues from human blood, that cover a range of red blood cell, fibrin, and platelet concentrations. We characterize the dielectric profile of these formed clots using an open-ended coaxial probe method across a wide frequency range. After the dielectric measurements are completed, histology on each blood clot is performed to determine the concentration of red blood cells present. In total, 32 unique blood clots were measured.With this completed analysis, we investigate the correlation between the dielectric properties across this frequency range and the red blood cell count of the formed blood clots. Furthermore, we develop a model to predict whether an unknown blood clot can be categorized as red blood cell rich or platelet and fibrin-rich based solely on the measured dielectric properties.Clinical Relevance-Using the dielectric profile of a clot we can predict whether a clot is platelet and fibrin-rich or red blood cell rich allowing clinicians to more easily determine treatment methods during an intervention for ischemic stroke.

A comparison between the open-ended coaxial probe method and the parallel plate method for measuring the dielectric properties of low-moisture foods

The dielectric properties of reference materials (deionized water and heptane) and selected low-moisture foods (ground black pepper, all-purpose bleached wheat flour, non-fat milk powder, and whole milk powder) were measured with the open-ended coaxial probe (OECP) and parallel plate (PP) methods from 20 to 60 °C and in the frequency ranges of 5 MHz to 3 GHz for OECP and 5–30 MHz for PP. Overall, the PP method was more accurate at measuring the dielectric properties of the reference materials than the OECP method. Notably, the OECP deviated from expected values below 200 MHz or when the magnitudes of the dielectric properties were small. As for the food products, the OECP and PP data sets were interpolated well with a modified power law model at lower temperatures when the OECP data below 200 MHz were excluded. However, the OECP data largely deviated from PP data at higher temperatures. The results suggest that the PP method may be preferable if the frequencies of interest are below 200 MHz or if the dielectric properties are expected to be small in magnitude.

Broadband Dielectric Properties of Ex Vivo Bovine Liver Tissue Characterized at Ablative Temperatures

To investigate the thermal and frequency dependence of dielectric properties of ex vivo liver tissue - relative permittivity and effective conductivity - over the frequency range 500 MHz to 6GHz and temperatures ranging from 20 to 130°C. We measured the dielectric properties of fresh ex vivo bovine liver tissue using the open-ended coaxial probe method (n = 15 samples). Numerical optimization techniques were utilized to obtain parametric models for characterizing changes in broadband dielectric properties as a function of temperature and thermal isoeffective dose. The effect of heating tissue at rates over the range 6.4-16.9°C/min was studied. The measured dielectric properties were used in simulations of microwave ablation to assess changes in simulated antenna return loss compared to experimental measurements. Across all frequencies, both relative permittivity and effective conductivity dropped sharply over the temperature range 89 - 107°C. Below 91°C, the slope of the effective conductivity changes from positive values at lower frequencies (0.5-1.64GHz) to negative values at higher frequencies (1.64-6GHz). The maximum achieved correlation values between transient reflection coefficients from measurements and simulations ranged between 0.83 - 0.89 and 0.68 - 0.91, respectively, when using temperature-dependent and thermal-dose dependent dielectric property parameterizations. We have presented experimental measurements and parametric models for characterizing changes in dielectric properties of bovine liver tissue at ablative temperatures. The presented dielectric property models will contribute to the development of ablation systems operating at frequencies other than 2.45GHz, as well as broadband techniques for monitoring growth of microwave ablation zones.

Dielectric properties of litchi fruit (Litchi chinensis Sonn) at microwave frequencies.

The dielectric properties of litchi fruit were determined using the open-ended coaxial probe method. The measurements were performed in the frequency range from 0.5 to 20GHz during 3days of storage at room temperature (~ 24°C). The dielectric properties increased with storage time. Additionally, measurements at different temperatures (24, 30, 40 and 50°C) were determined. The dielectric constant (ε') decreased with increasing temperature in a frequency range of 0.5-5GHz; at higher frequencies, ε' increased with increasing temperature. The loss factor (ε″) value increased at frequencies higher than 2GHz and decreased with increasing temperature. The results will be useful for further applications using microwaves, such as microwave-assisted drying, sensing of quality parameters, modeling, and heating to protect against molds or insects, among other applications.