Permittivity Maps Research Articles

Detection of Anti-Counterfeiting Markers through Permittivity Maps Using a Micrometer Scale near Field Scanning Microwave Microscope.

This paper describes the use of microwave technology to identify anti-counterfeiting markers on banknotes. The proposed method is based on a robust near-field scanning microwave microscope specially developed to measure permittivity maps of heterogeneous paper specimens at the micrometer scale. The equipment has a built-in vector network analyzer to measure the reflection response of a near-field coaxial probe, which makes it a standalone and portable device. A new approach employing the information of a displacement laser and the cavity perturbation technique was used to determine the relationship between the dielectric properties of the specimens and the resonance response of the probe, avoiding the use of distance-following techniques. The accuracy of the dielectric measurements was evaluated through a comparative study with other well-established cavity methods, revealing uncertainties lower than 5%, very similar to the accuracy reported by other more sophisticated setups. The device was employed to determine the dielectric map of a watermark on a 20 EUR banknote. In addition, the penetration capabilities of microwave energy allowed for the detection of the watermark when concealed behind dielectric or metallic layers. This work demonstrates the benefits of this microwave technique as a novel method for identifying anti-counterfeiting features, which opens new perspectives with which to develop optically opaque markers only traceable through this microwave technique.

GPRInvNet: Deep Learning-Based Ground-Penetrating Radar Data Inversion for Tunnel Linings

A DNN architecture referred to as GPRInvNet was proposed to tackle the challenges of mapping the ground-penetrating radar (GPR) B-Scan data to complex permittivity maps of subsurface structures. The GPRInvNet consisted of a trace-to-trace encoder and a decoder. It was specially designed to take into account the characteristics of GPR inversion when faced with complex GPR B-Scan data, as well as addressing the spatial alignment issues between time-series B-Scan data and spatial permittivity maps. It displayed the ability to fuse features from several adjacent traces on the B-Scan data to enhance each trace, and then further condense the features of each trace separately. As a result, the sensitive zones on the permittivity maps spatially aligned to the enhanced trace could be reconstructed accurately. The GPRInvNet has been utilized to reconstruct the permittivity map of tunnel linings. A diverse range of dielectric models of tunnel linings containing complex defects has been reconstructed using GPRInvNet. The results have demonstrated that the GPRInvNet is capable of effectively reconstructing complex tunnel lining defects with clear boundaries. Comparative results with existing baseline methods also demonstrated the superiority of the GPRInvNet. For the purpose of generalizing the GPRInvNet to real GPR data, some background noise patches recorded from practical model testing were integrated into the synthetic GPR data to retrain the GPRInvNet. The model testing has been conducted for validation, and experimental results revealed that the GPRInvNet had also achieved satisfactory results with regard to the real data.

CNN for Compressibility to Permittivity Mapping for Combined Ultrasound-Microwave Breast Imaging

Combined ultrasound-microwave breast imaging requires a mechanism to guide one imaging modality using the other. To this end, a convolutional neural network (CNN) is proposed for the mapping of ultrasound property images to dielectric property images for combined ultrasound-microwave breast imaging applications. In this approach, higher resolution ultrasound images are obtained by inverting the ultrasound scattered pressure data using a linearized inverse scattering algorithm. The reconstructed Born-based ultrasound compressibility images are then used to predict dielectric images for the same breast phantoms. These predicted dielectric images can then be used to guide microwave imaging reconstruction to achieve higher accuracy images. To this end, a CNN is trained based on the input of the reconstructed quantitative ultrasonic compressibility and the output of the true quantitative dielectric properties corresponding to the same numerical phantom. Several numerical MRI-derived breast phantoms are used to train and test this CNN. The predicted dielectric properties are tested using different numerical MRI-derived breast phantoms and the predicted profiles show promising results. The predicted dielectric properties are also used as the initial guess prior for the microwave inversion algorithm which leads to the enhancement of the reconstruction of dielectric properties as compared to the blind microwave inversion.

Microwave Microscopy and Its Applications

Understanding the nanoscale electrodynamic properties of a material at microwave frequencies is of great interest for materials science, condensed matter physics, device engineering, and biology. With specialized probes, sensitive detection electronics, and improved scanning platforms, microwave microscopy has become an important tool for cutting-edge materials research in the past decade. In this article, we review the basic components and data interpretation of microwave imaging and its broad range of applications. In addition to the general-purpose mapping of permittivity and conductivity, microwave microscopy is now exploited to perform quantitative measurements on semiconductor devices, photosensitive materials, ferroelectric domains and domain walls, and acoustic-wave systems. Implementation of the technique in low-temperature and high-magnetic-field chambers has also led to major discoveries in quantum materials with strong correlation and topological order. We conclude the review with an outlook of the ultimate resolution, operation frequency, and future industrial and academic applications of near-field microwave microscopy.

Magnetic resonance electrical property mapping at 21.1 T: a study of conductivity and permittivity in phantoms, ex vivo tissue and in vivo ischemia

Electrical properties (EP), namely conductivity and permittivity, can provide endogenous contrast for tissue characterization. Using electrical property tomography (EPT), maps of EP can be generated from conventional MRI data. This report investigates the feasibility and accuracy of EPT at 21.1 T for multiple RF coils and modes of operation using phantoms. Additionally, it demonstrates the EP of the in vivo rat brain with and without ischemia.Helmholtz-based EPT was implemented in its Full-form, which demands the complex field, and a simplified form requiring either just the field phase for conductivity or the field magnitude for permittivity. Experiments were conducted at 21.1 T using birdcage and saddle coils operated in linear or quadrature transceive mode, respectively. EPT approaches were evaluated using a phantom, ex and in vivo Sprague-Dawley rats under naïve conditions and ischemic stroke via transient middle cerebral artery occlusion.Different conductivity reconstruction approaches applied to the phantom displayed average errors of 12%–73% to the target acquired from dielectric probe measurements. Permittivity reconstructions showed higher agreement and an average 3%–8% error to the target depending on reconstruction approach. Conductivity and permittivity of ex and in vivo rodent brain were measured. Elevated EP in the ischemia region correlated with the increased sodium content and the influx of water intracellularly following ischemia in the lesion were detected.The Full-form technique generated from the linear birdcage provided the best accuracy for EP of the phantom. Phase-based conductivity and magnitude-based permittivity mapping provided reasonable estimates but also demonstrated the limitations of Helmholtz-based EPT at 21.1 T. Permittivity reconstruction was improved significantly over lower fields, suggesting a novel metric for in vivo brain studies. EPT applied to ischemic rat brain proved sensitivity to physiological changes, motivating the future application of more advanced reconstruction approaches.

Computed Tomography-Based Modeling of Water Vapor-Induced Changes in Permittivity During Microwave Ablation.

Measurements of tissue permittivity with small open-ended coaxial probes during microwave tissue heating have been plagued by high variability as tissue water becomes vaporized. Analysis of such variability has been hampered by a lack of direct visualization of the measurement volume. The objective of this study was to determine if X-ray computed tomography (CT) could be used to visualize the measurement volume and then predict dielectric permittivity based on the visualized tissue composition. CT attenuation at 120kVp was measured at the end of an open-ended coaxial probe during microwave ablation of ex vivo liver (2.45GHz at 50 W delivered for 5minutes). Tissue composition was estimated from attenuation maps and used to predict dielectric properties based on established mixture equations. Predicted permittivity was then compared to measured values using error metrics and linear regression. There was a good agreement between measured and modeled permittivity during 5-minute ablations at 2.45GHz (r = 0.94, p < .001). Normalized root mean squared errors were below 17% in permittivity modeling at 2.45GHz. CT-based model predictions of dielectric properties are feasible in ex vivo liver. The models may facilitate real-time imaging-based permittivity mapping.

Accuracy and precision of electrical permittivity mapping at 3T: the impact of three mapping techniques.

PurposeTo investigate the sequence‐specific impact of B1+ amplitude mapping on the accuracy and precision of permittivity reconstruction at 3T in the pelvic region.Methods B1+ maps obtained with actual flip angle imaging (AFI), Bloch–Siegert (BS), and dual refocusing echo acquisition mode (DREAM) sequences, set to a clinically feasible scan time of 5 minutes, were compared in terms of accuracy and precision with electromagnetic and Bloch simulations and MR measurements. Permittivity maps were reconstructed based on these B1+ maps with Helmholtz‐based electrical properties tomography. Accuracy and precision in permittivity were assessed. A 2‐compartment phantom with properties and size similar to the human pelvis was used for both simulations and measurements. Measurements were also performed on a female volunteer’s pelvis.ResultsAccuracy was evaluated with noiseless simulations on the phantom. The maximum B1+ bias relative to the true B1+ distribution was 1% for AFI and BS and 6% to 15% for DREAM. This caused an average permittivity bias relative to the true permittivity of 7% to 20% for AFI and BS and 12% to 35% for DREAM. Precision was assessed in MR experiments. The lowest standard deviation in permittivity, found in the phantom for BS, measured 22.4 relative units and corresponded to a standard deviation in B1+ of 0.2% of the B1+ average value. As regards B1+ precision, in vivo and phantom measurements were comparable.ConclusionsOur simulation framework quantitatively predicts the different impact of B1+ mapping techniques on permittivity reconstruction and shows high sensitivity of permittivity reconstructions to sequence‐specific bias and noise perturbation in the B1+ map. These findings are supported by the experimental results.

Influence of the description of the scattering matrix on permittivity reconstruction with a quantitative imaging procedure: polarization effects.

This paper focuses on the role of polarization-and more specifically, the effect of its selection-in 3D quantitative imaging obtained from scattered field measurements. Although polarization is now commonly used in linear imaging procedures (when unknowns are linked by a linear relationship to the measured signal), the influence of polarization choice is generally ignored in nonlinear imaging problems. In this paper, we propose a formulation to obtain the 3D permittivity map, by a nonlinear imaging procedure, from the scattering matrix. This allows one to select, from the same data set, the desired polarization case as input data for the imaging algorithm. We present a study of the influence of the input data polarization choice on the reconstructed permittivity map. This work shows that a suitable basis choice for the description of the scattering matrix and an appropriate selection of the element of this scattering matrix can greatly improve imaging results.

Permittivity mapping in the VLF–LF range using a multi‐frequency EMI device: first tests in archaeological prospection

ABSTRACTIn the early years of near‐surface geophysical applications, measurements obtained using electromagnetic induction (Slingram or dipole–dipole) instruments were first interpreted in terms of electrical conductivity. Afterwards, studies began considering the importance of magnetic susceptibility. Until now, however, no attention has been paid to possible responses in dielectric permittivity. After reviewing the current state of knowledge regarding the expected theoretical responses in the very low frequency to low frequency (VLF‐LF) range (3–300 kHz) and the value range of this property for soils, we propose the use of multi‐frequency instruments to define a process allowing for the determination of the apparent conductivity and permittivity in the higher part of the frequency range above 20 kHz. We test this process through a series of experiments at archaeological sites in Greece using the GEM‐2 (Geophex Ltd) instrument. In our experiments, the soil permittivity values fall between several hundredths and several thousandths. The results indicate lateral variations different from the other properties with a significant influence of the stone content and of the ionic strength, while a decrease with frequency may also provide further information on soil dielectric behaviours.

Effect of Physical and Chemical Hair Removal Methods on Skin Barrier Function in vitro: Consequences for a Hydrophilic Model Permeant

Background: Although very common in our society, the effect of hair removal on physiological skin parameters and on the ingress of applied chemicals has not been systematically investigated. Thus, as a first step, the aim of the present study was to elucidate the effect of hair removal through epilation (electric epilation, waxing) and depilation (dry and wet shaving, depilatory cream) on skin properties in vitro using the porcine ear model. Methods: Attenuated total reflection Fourier transform infrared spectroscopy, measurement of the transepidermal water loss (TEWL), visualization by capacitance-based contact imaging, confocal Raman spectroscopy (CRS), diffusion cell studies and tape stripping experiments were employed. Results: Increased TEWL and altered skin permittivity maps were observed. Decreased stratum corneum thickness was observed after waxing. Diffusion cell studies showed increased skin permeation especially in case of dry shaving, electric epilation and waxing. Conclusion: Considering CRS and diffusion cell data, a moderate if significant decrease in skin barrier function was found after hair removal by dry shaving (physical skin/material interaction) and epilation methods (plucking out the entire hair, for example, by electrical epilation and waxing). Subsequent experiments will include testing of different permeants covering a broad range of physicochemical properties in vitro and confirming our findings in vivo.

Monitoring osseointegrated prosthesis loosening and fracture using electrical capacitance tomography.

A noncontact, noninvasive, electrical permittivity imaging technique is proposed for monitoring loosening of osseointegrated prostheses and bone fracture. The proposed method utilizes electrical capacitance tomography (ECT), which employs a set of noncontact electrodes, arranged in a circular fashion around the imaging area, for electrical excitations and measurements. An inverse reconstruction algorithm was developed and implemented to reconstruct the electrical permittivity distribution of the interrogated region from boundary capacitance measurements. In this study, osseointegrated prosthesis phantoms were prepared using plastic rods and Sawbone femur specimens, which were subjected to prosthesis loosening and fracture monitoring tests. The results demonstrated that the spatial location and extent of prosthesis loosening and bone fracture could be estimated from the ECT reconstructed permittivity maps. The resolution of the reconstructed images was further enhanced by a limited region tomography algorithm, and its accuracy in terms of identifying the severity, location, and shape of bone fracture was also investigated and compared with conventional full region tomography.

Modeling Error and Calibration Techniques for a Faceted Metallic Chamber for Magnetic Field Microwave Imaging

This paper introduces and analyzes a novel 3-D microwave imaging chamber composed of a number of intersecting flat metallic facets that form a hemiellipsoid. Synthetic studies demonstrate that the faceted chamber geometry provides advantages in modeling error reduction and reconstruction quality at a greater number of frequencies than a comparable cylindrical chamber. The experimental chamber is introduced along with small magnetic field half-loop probes mounted on the chamber walls. An examination of incident and scattered field modeling errors in the faceted chamber is carried out for simple cylindrical targets in an air background. A variety of calibration and measurement techniques are compared, and the reconstructed permittivity maps are compared very well with an equivalent synthetic problem. Finally, the system is used to image a simple two-region breast phantom in air, and with the incorporation of prior information, tumor detection is shown to be possible.

Noncontact Electrical Permittivity Mapping and pH-Sensitive Films for Osseointegrated Prosthesis and Infection Monitoring.

The objective of this paper is to develop a noncontact, noninvasive system for detecting and monitoring subcutaneous infection occurring at the tissue and osseointegrated prosthesis interface. It is known that the local pH of tissue can change due to infection. Therefore, the sensing system integrates two parts, namely, pH-sensitive thin films that can be coated onto prosthesis surfaces prior to them being implanted and an electrical capacitance tomography (ECT) algorithm that can reconstruct the spatial permittivity distribution of a region of space in a noncontact fashion. First, a thin film pH sensor was fabricated by spray coating, and tests confirmed that the film exhibited changes in its permittivity due to pH. Second, the ECT forward and inverse problems were implemented. Third, an aluminum rod was employed as a representative phantom of an osseointegrated prosthesis and then spray coated with the pH sensor. Finally, the film-coated phantom was immersed in different pH buffers, dried, and subjected to ECT interrogation and spatial permittivity reconstruction. The results validated that ECT was able to detect and localize permittivity variations correlated to pH changes.

A Split Ring Resonator Dielectric Probe for Near-Field Dielectric Imaging

A single split-ring resonator (SRR) probe for 2D surface mapping and imaging of relative dielectric permittivity for the characterisation of composite materials has been developed. The imaging principle, the analysis and the sensitivity of the SRR surface dielectric probe data is described. The surface dielectric properties of composite materials in the frequency range 1–3 GHz have been measured based on the magnetic resonance frequency of the transmission loss of the SRR dielectric probe when in contact with the surface. The SRR probe performance was analysed analytically and using full-wave simulation, and predictions showed close agreement with experiment for composite materials with spatially varying dielectric permittivity manufactured by 3D printing. The spatial and permittivity resolution of the SRR dielectric probe were controlled by the geometrical parameters of the SRR which provided flexibility to tune the SRR probe. The best accuracy of the dielectric permittivity measurements was within 5%.

Global permittivity mapping of the Martian surface from SHARAD

SHARAD is a subsurface sounding radar aboard NASA's Mars Reconnaissance Orbiter, capable of detecting dielectric discontinuities in the subsurface caused by compositional and/or structural changes. Echoes coming from the surface contain information on geometric properties at metre scale and on the permittivity of the upper layers of the Martian crust. A model has been developed to estimate the effect of surface roughness on echo power, depending on statistical parameters such as RMS height and topothesy. Such model is based on the assumption that topography can be characterized as a self-affine fractal, and its use allows the estimation of the dielectric properties of the first few metres of the Martian soil. A permittivity map of the surface of Mars is obtained, covering several large regions across the planet surface. The most significant correspondence with geology is observed at the dichotomy boundary, with high dielectric constant on the highlands side (7 to over 10) and lower on the lowlands side (3 to 7). Other geological correlations are discussed.

Direct mapping of the electric permittivity of heterogeneous non-planar thin films at gigahertz frequencies by scanning microwave microscopy.

We obtained maps of electric permittivity at ∼19 GHz frequencies on non-planar thin film heterogeneous samples by means of combined atomic force-scanning microwave microscopy (AFM-SMM). We show that the electric permittivity maps can be obtained directly from the capacitance images acquired in contact mode, after removing the topographic cross-talk effects. This result demonstrates the possibility of identifying the electric permittivity of different materials in a thin film sample irrespectively of their thickness by just direct imaging and processing. We show, in addition, that quantitative maps of the electric permittivity can be obtained with no need for any theoretical calculation or complex quantification procedures when the electric permittivity of one of the materials is known. To achieve these results the use of contact mode imaging is a key factor. For non-contact imaging modes the effects of local sample thickness and of the imaging distance make the interpretation of the capacitance images in terms of the electric permittivity properties of the materials much more complex. The present results represent a substantial contribution to the field of nanoscale microwave dielectric characterization of thin film materials with important implications for the characterization of novel 3D electronic devices and 3D nanomaterials.

Non-contact Tomographic Imaging and Nanocomposite Thin Films for Monitoring Human-prosthesis Interfaces

The objective of this work is to develop a non-contact, non-invasive technique for monitoring human-prosthesis interfaces, such as the development of infection. The monitoring scheme proposed works by coupling electrical capacitance tomography (ECT) with a pH-sensitive nanocomposite thin film applied at the human-prosthesis interface. In short, ECT uses applied electrical excitations and measurements at the boundaries to reconstruct the spatial permittivity distribution inside the sensing region. The pH-sensitive nanocomposite pronounces permittivity changes due to issues occurring at the human-prosthesis interface, thereby enhancing detection sensitivity and resolution. First, numerical simulations were performed by altering the permittivity at selected locations in a mathematical model to simulate the effect of infections near a prosthesis. The extent and shape of the perturbed permittivity was captured by numerical simulations of ECT. Second, experiments were conducted in the laboratory to validate the monitoring system and sensing scheme. Here, a pH-sensitive thin film was fabricated using carbon nanotubes and polyaniline. The thin film's pH sensitivity was characterized by exposing it to different pH buffer solutions and measuring its corresponding dielectric property changes. A circular plastic rod was then used as a phantom for a human prosthesis, for example, to simulate the portion of an artificial limb that would be embedded in and bonded to the human body. The thin film was then deposited onto the surface of the prosthetic phantom and exposed to different pH buffer solutions. For each case, the ECT monitoring system was used to interrogate the prosthetic phantom placed in the sensing area. Not only did the system detect changes in permittivity due to different pH buffers, but the reconstructed permittivity maps also identified the location of the permittivity changes, as well as the prosthesis itself.

Study of variability of permittivity and its mapping over lunar surface and subsurface using multisensors datasets

In the present paper, permittivity maps of equatorial lunar surface are generated using brightness temperature (TB) data obtained from Microwave Radiometer (MRM) of Chang'e-1 and physical temperature (TP) data obtained from Diviner of Lunar Reconnaissance Orbiter (LRO). Here, permittivity mapping is not carried out above 60° latitudes towards the lunar poles due to large anomaly in the physical temperature obtained from the Diviner. Microwave frequencies, which are used to generate these maps are 3GHz, 7.8GHz, 19.35GHz and 37GHz. Permittivity values are simulated using TB values at these four frequencies. Here, weighted average of physical temperature obtained from Diviner are used to compute permittivity at each microwave frequencies. Longer wavelengths of microwave signals give information of more deeper layers of the lunar surface as compared to smaller wavelength. Initially, microwave emissivity is estimated using TB values from MRM and physical temperature (TP) from Diviner. From estimated emissivity the real part of permittivity (ε), is calculated using Fresnel equations. The permittivity maps of equatorial lunar surface is generated. The simulated permittivity values are normalized with respect to density for easy comparison of simulated permittivity values with the permittivity values of Apollo samples as well as with the permittivity values of Terrestrial Analogue of Lunar Soil (TALS) JSC-1A. Lower value of dielectric constant (ε′) indicates that the corresponding lunar surface is smooth and doesn't have rough rocky terrain. Thus a future lunar astronaut can use these data to decide proper landing site for future lunar missions. The results of this paper will serve as input to future exploration of lunar surface.

Electrical conductivity and permittivity maps of brain tissues derived from water content based on T1 -weighted acquisition.

To develop an electrical properties tomography (EPT) technique that can provide in vivo electrical conductivity and permittivity images of biological tissue without performing complex-valued radiofrequency field measurements. Electrical conductivity and permittivity images are modeled as a monotonic function of tissues' water content (W) under the principle of Maxwell's mixture theory. Water content maps are estimated from two spin-echo images having different repetition times (TRs). For the modeling functions, physically measured parameters (electrical properties, water content, and T1 ) of brain cerebrospinal fluid (CSF), gray matter, and white matter are used as landmark literature references. The formulations are validated by a developed electrolyte-protein phantom and by human brain studies at 3 Tesla (T). The electrical properties (EPs) of the phantom estimated by the proposed method match well with the values measured on the bench. The conductivity and permittivity maps from all experiments show uncompromised spatial resolution without boundary artifacts and higher contrast when compared with water content maps. Human brain and phantom EP images suggest that water content is a dominating factor in determining the electrical properties of tissues. Despite possible literature inaccuracies, the proposed method offers EP maps that can provide complementary information to current approaches, to facilitate EPT scans in clinical applications. Magn Reson Med 77:1094-1103, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

MRI-Based Electrical Property Retrieval by Applying the Finite-Element Method (FEM)

In this study, the finite-element method is applied to reconstruct the electrical properties (conductivity and permittivity) maps of media with 1-D, 2-D, and 3-D inhomogeneities based on the RF magnetic field distributions in a magnetic resonance imaging (MRI) system. The proposed method handles the discontinuity of electrical properties well. It therefore shows high accuracy at the boundary compared to retrieval methods in the literature. The method has successfully been validated using both analytical and numerical data. Moreover, the noise sensitivity of the proposed method is studied. Good accuracy of retrievals can be obtained when proper coils are chosen (e.g., transverse electromagnetic coils). It is an effective approach showing one step further towards accurate electrical property mapping based on MRI data. It helps to pave the way to accurate calculations of specific absorption rate distributions and temperature distributions of human body under MRI scans, which are crucial for the evaluation of the safety of MRI systems. Moreover, the increased accuracy of the electrical property maps of human body show their potential to be a diagnostic tool for tumors and cancers, especially at their early stages. Following-up research is expected to make the proposed method robust and practical for clinical applications.