Microwave Imaging System Research Articles

Design and Experimental Validation of a Multiple-Frequency Microwave Tomography System Employing the DBIM-TwIST Algorithm.

We present a first prototype of a wideband microwave tomography system with potential application to medical imaging. The system relies on a compact and robust printed monopole antenna which can operate in the 1.0–3.0 GHz range when fully immersed in commonly used coupling liquids, such as glycerine–water solutions. By simulating the proposed imaging setup in CST Microwave Studio, we study the signal transmission levels and array sensitivity for different target and coupling liquid media. We then present the experimental prototype design and data acquisition process, and show good agreement between experimentally measured data and results from the CST simulations. We assess imaging performance by applying our previously proposed two-dimensional (2-D) DBIM TwIST-algorithm to both simulated and experimental datasets, and demonstrate that the system can reconstruct simple cylindrical targets at multiple frequencies.

Imaging of Near-Surface Defects using Microwaves and Ultrasonic Phased Array Techniques

Detecting and imaging near-surface defects is of significant importance in many applications. Detecting corrosion/defects under thin paint layer applied on metallic substrates, evaluating debond under a Teflon coat, as well as evaluating the integrity of the dielectric coats remains a challenge in many industries settings. Periodic testing is imperative for a wide range of industries to isolate material defects that could lead to catastrophic failures, expensive repairs, or a complete shutdown. Out of the many non-destructive testing (NDT) techniques available, phased array ultrasonic testing (PAUT) is widely used in various industries for material inspection. It has shown exceptional capability in detection of defects, including material loss and corrosion in metals. On the other hand, near-field microwave NDT is emerging as a powerful inspection modality for similar applications. In this paper, imaging capabilities of PAUT (5 MHz) as well as near-field microwave imaging system (33.5 GHz) are discussed and evaluated for a number of specimens with man-made surface defects. The evaluated defects include flat bottom slots, holes and corrosion-under-paint. Images produced by both techniques are presented. The images produced by the microwave imaging system are benchmarked with PAUT system.

A Homogeneous Breast Phantom Measurement System with an Improved Modified Microwave Imaging Antenna Sensor.

Microwave breast imaging has been reported as having the most potential to become an alternative or additional tool to the existing X-ray mammography technique for detecting breast tumors. Microwave antenna sensor performance plays a significant role in microwave imaging system applications because the image quality is mostly affected by the microwave antenna sensor array properties like the number of antenna sensors in the array and the size of the antenna sensors. In this paper, a new system for successful early detection of a breast tumor using a balanced slotted antipodal Vivaldi Antenna (BSAVA) sensor is presented. The designed antenna sensor has an overall dimension of 0.401λ × 0.401λ × 0.016λ at the first resonant frequency and operates between 3.01 to 11 GHz under 10 dB. The radiating fins are modified by etching three slots on both fins which increases the operating bandwidth, directionality of radiation pattern, gain and efficiency. The antenna sensor performance of both the frequency domain and time domain scenarios and high-fidelity factor with NFD is also investigated. The antenna sensor can send and receive short electromagnetic pulses in the near field with low loss, little distortion and highly directionality. A realistic homogenous breast phantom is fabricated, and a breast phantom measurement system is developed where a two antennas sensor is placed on the breast model rotated by a mechanical scanner. The tumor response was investigated by analyzing the backscattering signals and successful image construction proves that the proposed microwave antenna sensor can be a suitable candidate for a high-resolution microwave breast imaging system.

Impact of Information Loss on Reconstruction Quality in Microwave Tomography for Medical Imaging.

This paper studies how limited information in data acquired by a wideband microwave tomography (MWT) system can affect the quality of reconstructed images. Limitations can arise from experimental errors, mismatch between the system and its model in the imaging algorithm, or losses in the immersion and coupling medium which are required to moderate this mismatch. We also present a strategy for improving reconstruction performance by discarding data that is dominated by experimental errors. The approach relies on recording transmitted signals in a wide frequency range, and then correlating the data in different frequencies. We apply this method to our wideband MWT prototype, which has been developed in our previous work. Using this system, we present results from simulated and experimental data which demonstrate the practical value of the frequency selection approach. We also propose a K-neighbour method to identify low quality data in a robust manner. The resulting enhancement in imaging quality suggests that this approach can be useful for various medical imaging scenarios, provided that data from multiple frequencies can be acquired and used in the reconstruction process.

Ultra-broadband wide-angle linear polarization converter based on H-shaped metasurface

We report the design, fabrication, and measurement of an ultra-broadband wide-angle reflective cross-polarization convertor using the compact H-shaped metasurface. The significant bandwidth expansion is attributed to the four electromagnetic resonances generated in an H-shaped unit. The simulation results show that the polarization conversion ratio (PCR) of the proposed metasurface is above 90% in the frequency range from 7 to 19.5 GHz and the relative bandwidth reaches 94%. The proposed metasurface is valid for a wide range of incident angles, and the mean polarization conversion ratio remains 80% even though the incident angle reaches 41.5°. The experimental results are in good agreement with the simulation results. Compared with the previous designs, the proposed linear polarization converter has a simple geometry but an excellent performance and hence has potential applications in microwave communications, remote sensors, and imaging systems.

Preliminary Assessment of a Microwave System to Detect Contrast Enhancing Agents.

This paper presents a preliminary study of the impact of potential contrast enhancing agents on a 2-port microwave imaging system. To this end, we have conducted microwave measurements inside a dual cylindrical tank, comprised of an outer and inner cylinder filled with high and low loss liquids, respectively. A third smaller cylinder inside the low loss filled tank represents a target. The target materials consisted of safflower oil, water, PEGylated zinc oxide nanoparticles dispersed in water, and saline with varying concentrations of salt. To benchmark our experimental results, we have also performed accurate numerical simulations of the system under three target scenarios: safflower oil, water, and PEGylated zinc oxide suspension. Our results show that the difference in received signal strength in scenarios with and without PEGylated zinc oxide nanoparticles in our 2-port system was comparable to measurement error. However, with the use of varying concentrations of saline solutions as a target, we have observed a significant difference in received signal strength.

Dual-Laser Integrated Microwave Imaging System for Nondestructive Testing of Construction Materials and Structures

Microwave near-field imaging techniques have been successfully used for the nondestructive testing of construction materials in civil infrastructures. In this technique, the distance between the microwave antenna and the material, which is referred to as standoff distance is an important parameter. The standoff distance should be kept constant during the scanning process or there should be a proper compensation method to overcome any change in the distance. The undesired change in the standoff distance may mask the indication of flaws. However, civil infrastructures are composed of construction materials with different geometrical shapes and sharp edges, and maintaining the constant standoff distance during the scanning process is a challenging task. For this purpose, a novel dual-laser integrated microwave imaging system is proposed. The proposed system consists of a novel integrated sensing unit with two laser displacement sensors and a single microwave antenna that automatically follows the contour of the material under test at a constant standoff distance and generates microwave images. The proposed system performs contour following at hardware level and does not require any calibration or complex compensation algorithm. The comparison of two microwave antennas is performed to find the suitable antenna for different materials. The proof of concept of the developed system for the nondestructive testing of construction materials with different geometrical shapes is provided.

Single-frequency 3D synthetic aperture imaging with dynamic metasurface antennas.

Through aperture synthesis, an electrically small antenna can be used to form a high-resolution imaging system capable of reconstructing three-dimensional (3D) scenes. However, the large spectral bandwidth typically required in synthetic aperture radar systems to resolve objects in range often requires costly and complex RF components. We present here an alternative approach based on a hybrid imaging system that combines a dynamically reconfigurable aperture with synthetic aperture techniques, demonstrating the capability to resolve objects in three dimensions (3D), with measurements taken at a single frequency. At the core of our imaging system are two metasurface apertures, both of which consist of a linear array of metamaterial irises that couple to a common waveguide feed. Each metamaterial iris has integrated within it a diode that can be biased so as to switch the element on (radiating) or off (non-radiating), such that the metasurface antenna can produce distinct radiation profiles corresponding to different on/off patterns of the metamaterial element array. The electrically large size of the metasurface apertures enables resolution in range and one cross-range dimension, while aperture synthesis provides resolution in the other cross-range dimension. The demonstrated imaging capabilities of this system represent a step forward in the development of low-cost, high-performance 3D microwave imaging systems.

Fast and Precise Soft-Field Electromagnetic Tomography Systems for Multiphase Flow Imaging

In the process industry, measurement systems are required for process development and optimization, as well as for monitoring and control. The processes often involve multiphase mixtures or flows that can be analyzed using tomography systems, which visualize the spatial material distribution within a certain measurement domain, e.g., a process pipe. In recent years, we studied the applicability of soft-field electromagnetic tomography methods for multiphase flow imaging, focusing on concepts for high-speed data acquisition and image reconstruction. Different non-intrusive electrical impedance and microwave tomography systems were developed at our institute, which are sensitive to the local contrasts of the electrical properties of the materials. These systems offer a very high measurement and image reconstruction rate of up to 1000 frames per second in conjunction with a dynamic range of up to 120 dB. This paper provides an overview of the underlying concepts and recent improvements in terms of sensor design, data acquisition and signal processing. We introduce a generalized description for modeling the electromagnetic behavior of the different sensors based on the finite element method (FEM) and for the reconstruction of the electrical property distribution using the Gauss–Newton method and Newton’s one-step error reconstructor (NOSER) algorithm. Finally, we exemplify the applicability of the systems for different measurement scenarios. They are suitable for the analysis of rapidly-changing inhomogeneous scenarios, where a relatively low spatial resolution is sufficient.

Development of an ultra-wideband, stable and high-directive monopole disc antenna for radar-based microwave imaging of breast cancer

ABSTRACTRadar-based ultra-wideband (UWB) microwave imaging (MI) system has recently attracted considerable attention for detection of breast cancer at early stage. A key component of the system is the sensor (antenna) that is used to radiate and receive the UWB pulses. In this study, a compact-size, UWB, directive, and microstrip-fed planar circular disc monopole antenna (PCDMA) with modified L-shaped ground plane for the radar-based UWB MI system is developed. Analysis and the performance optimization of the proposed antenna are performed by HFSS software. In the development process of the antenna design, some modifications such as adjusting feed point, adding parasitic element, slot, notches are made on the conventional PCDMA to improve the bandwidth, directivity and half-power beamwidth (HPBW) of the antenna. After obtaining the optimum parameters, the proposed antenna is fabricated and then tested by using Vector Network Analyser. Impedance bandwidth of the designed antenna covers the broad frequency range of 3–10 GHz (108%) for Return Loss < –10 dB. The good stability of the directional radiation patterns is obtained in the 3–10 GHz. The direction of the maximum radiation moves from 32° to 54° and HPBW decreases from 56° to 30° through the frequency band. The directivity changes between 6.2 and 9.4 dB in the 3–10 GHz. Results confirm that the proposed antenna is suitable for using in radar-based UWB MI of the breast cancer.

Interview

Dr Saeid Karamzadeh at Istanbul Aydin University, Turkey, talks to Electronics Letters about the work behind his Letter ‘Millimeter-Wave High-Gain Circularly polarised SIW End-Fire Bow-tie Antenna by utilising Semi Planar Helix Unit Cell’, page 411. Saeid Karamzadeh Years ago, during the design of a system to rescue living people left under earthquake rubble, we found that the antenna is one of the most important parts of the system and should be studied in more detail. The next step was designing an antenna for satellite communication. Size reduction, designing a compact antenna, bandwidth improvement, gain increment and getting circularly polarisation were our goals. In the meantime, I finished my PHD degree and I won the most successful PHD thesis award of Istanbul Technical University. Recently, as mm-wave circularly polarised antennas have many benefits, we focused on designing novel works on this subject. One of the important issues is generation of circular polarisation from a linearly polarised antenna. Identifying linearly polarised antennas that have the ability to change to antennas with circularly polarised features and realising the methods for this is a challenge for antenna designers. Circularly polarised antennas have several important advantages compared to antennas using linear polarisation and are becoming a key technology for various wireless systems. Circularly polarised antennas are very effective in combating multi-path interferences or fading. The reflected radio signal from the ground or other objects will result in a reversal of polarisation, that is, right-hand circular polarisation (RHCP) reflections show left-hand circular polarisation (LHCP). An RHCP antenna will have a rejection of a reflected signal which is LHCP, thus reducing the multi-path interferences from the reflected signals. The second advantage is that a circularly polarised antenna is able to reduce the ‘Faraday rotation’ effect due to the ionosphere. The Faraday rotation effect causes a significant signal loss (about 3 dB or more) if linearly polarised signals are employed. Another advantage of using CP antennas is that no strict orientation between transmitting and receiving antennas is required. This is different from linearly polarised antennas, which are subject to polarisation mismatch losses if arbitrary polarisation misalignment occurs between transmitting and receiving antennas. We managed to create a circularly polarised antenna on a mm-wave linear polarisation antenna by employing several spiral unit cells. This is helpful because linear polarisation antennas have few limitation parameters, therefore designing them is relatively simple compared with circularly polarised antennas and by this method we can convert linearly polarised to circularly polarised antennas. The importance of CP antennas and consequently the advantage of this antenna is shown in this Letter. Undoubtedly, to report a work with a novel technique you must conquer many issues. In this work, finding a co-planer cell which enables a linearly polarised antenna to circularly polarise was a big challenge; in addition to issues such as selecting an antenna to radiate directionally. We are working on increasing the bandwidth and 3-dB axial ratio of this antenna and also concentrating on mm-wave antennas to improve their characteristics. In further steps, we would like to work on applying this to mm-wave CP array antennas. Over those years, many works about mm-wave antennas have been reported. We are hoping that as this continues over the next ten years, mm-wave antennas can be released in most countries, in order to use high data transfer in this frequency band. This could be so helpful for real-time communication as well as helping to save many lives through use in automotive radar. Additionally, this frequency band, which can achieve high accuracy in microwave imaging systems, can be used in biomedical systems for so many applications like tumour detection and prediction. By increasing the data transfer rate to allow 3-D vision, it will have an impact in the entertainment sector too.

Experimental Demonstration of In‐Place Calibration for Time Domain Microwave Imaging System

AbstractIn this study, the experimental demonstration of in‐place calibration was conducted using the developed time domain measurement system. Experiments were conducted using three calibration methods—in‐place calibration and two existing calibrations, that is, array rotation and differential calibration. The in‐place calibration uses dual receivers located at an equal distance from the transmitter. The received signals at the dual receivers contain similar unwanted signals, that is, the directly received signal and antenna coupling. In contrast to the simulations, the antennas are not perfectly matched and there might be unexpected environmental errors. Thus, we experimented with the developed experimental system to demonstrate the proposed method. The possible problems with low signal‐to‐noise ratio and clock jitter, which may exist in time domain systems, were rectified by averaging repeatedly measured signals. The tumor was successfully detected using the three calibration methods according to the experimental results. The cross correlation was calculated using the reconstructed image of the ideal differential calibration for a quantitative comparison between the existing rotation calibration and the proposed in‐place calibration. The mean value of cross correlation between the in‐place calibration and ideal differential calibration was 0.80, and the mean value of cross correlation of the rotation calibration was 0.55. Furthermore, the results of simulation were compared with the experimental results to verify the in‐place calibration method. A quantitative analysis was also performed, and the experimental results show a tendency similar to the simulation.

Dielectric resonator antenna array for X‐band and microwave imaging applications

AbstractIn this article, simulation and experimental studies of compact dielectric resonant antenna array have been performed for microwave imaging systems and X‐band applications. A novel feed structure has been proposed to excite the four cylindrical dielectric resonators. The proposed antenna array shows dual band characteristics i.e., 4.15–7.2 GHz and 9.35–17.2 GHz with the bandwidth of 3.05 GHz and 7.85 GHz respectively. The proposed antenna has been fabricated and tested. Measurement results have been extracted and found in good agreement with simulated results. The proposed antenna array affords appropriate gain, high radiation efficiency, wide bandwidth, small size, and stable radiation patterns over the entire bands which are essential performance characteristics to be utilized for C band, X band, and Ku band applications.

Microwave Breast Imaging: Clinical Advances and Remaining Challenges.

Microwave breast imaging has seen significant academic and commercial development in recent years, with four new operational microwave imaging systems used with patients since 2016. In this paper, a comprehensive review of these recent clinical advances is presented, comparing patient populations and study outcomes. For the first time, the designs of operational microwave imaging systems are compared in detail. First, the current understanding of dielectric properties of human breast tissues is reviewed, considering evidence from operational microwave imaging systems and from dielectric properties measurement studies. Second, design features of operational microwave imaging systems are discussed in terms of advantages and disadvantages during clinical operation. Reported results from patient imaging trials are compared, contrasting the principal results from each trial. Additionally, clinical experience from each trial is highlighted, identifying desirable system design features for clinical use. Increasingly, evidence from patient imaging studies indicate that a contrast in dielectric properties between healthy and cancerous breast tissues exists. However, despite the significant and encouraging results from patient trials, variation still exists in the microwave imaging system design. This study seeks to define the current state of the art in microwave breast imaging, and identify suitable design characteristics for ease of clinical use.

Monostatic imaging of an embedded object using a confocal algorithm

AbstractIn this paper, we present simulation results of monostatic configuration for microwave imaging system by using confocal algorithm to detect and localize metallic bars inserted in a concrete pillar. An ultra‐wideband antenna has been used. The dielectric property values of the model were chosen near those of the realities. However, we have changed the permittivity of the concrete pillar to study the effectiveness of the algorithm despite the introduction of different kind of concrete. Successful detection and localization of different shapes and positions of metallic bars of diameter as small as 4 mm are presented. The efficacy of the proposed algorithm was also demonstrated by adding noise to the backscattered signals before creating the image.

A Time-Reversal Imaging System for Breast Screening: Theory and Initial Phantom Results.

In this paper, a working ultrawideband (UWB) microwave imaging (MI) system using basic techniques is proposed to detect and localize a tumor model. The system harnesses the time reversal-decomposition of time reversal operator (DORT) signal processing method. We suggest a modified background Green's function and suitable spiral antennas for time reversal imaging of a breast cancer tumor. In this method, scatterings from a breast phantom are measured. Then, reflected signals are processed using the decomposition of time reversal (TR), known under French acronym DORT, algorithm. In a conventional TR imaging method, free-space Green's function is used for detection. However, in the present method, we employ cylindrical Green's functions of a two-layer medium. This makes a better model of the background medium for TR-DORT imaging. Also, the performance of TR-DORT detector against signal to clutter ratio, and its ability to detect and localize different shapes, sizes, and permittivity values of breast tumor are discussed. Furthermore, a fatty tissue phantom model is used to verify the results experimentally. The MI system is tested to localize a tumor within the breast Debye phantom at the frequency range of 3.1-10.6GHz. The experimental results show the validity of our proposed system.

Application of microwave tomography to investigation the wet gas-solids flow hydrodynamic characteristics in a fluidized bed

Fluidized beds are widely used in pharmaceutical industry for particles coating and granulation. Due to the complex gas solids flow process, it is extremely desirable that a real time monitoring and control system is used in the process in order to enable a bubbling stable fluidization and reduce the chance of defluidization due to agglomeration with high moisture contents particles. The microwave tomography (MWT) technology has been developed for more than twenty years and was mainly applied in medical sectors. MWT has a wide range of frequency (1–2.5 GHz) and can be used to measure materials with high permittivity and conductivity. Therefore, it is a good choice for the monitoring of fluidized bed coating and granulation processes. In this paper, a MWT system with 16 antennas was designed and applied for the monitoring of wetting and coating processes in a lab-scale fluidized bed with top-spray and bottom-spray system. The hydrodynamics characteristics of wet gas-solids flow with different moisture content were investigated by MWT technology. This is the first time that MWT was applied in a real dynamics gas-solids fluidized bed. Experiment result shows that the MWT is available for the online monitoring of the gas solids flow with different moisture content. Different flow regimes were revealed based on the MWT images combined with pressure signals. The results indicate that the MWT provides valuable information to identify the defluidization in the fluidized bed process with high moisture contents materials. It is evident that microwave tomography has clear advantages over single point based measurement technology for process fault diagnose.

Boundary Estimation of Imaged Object in Microwave Medical Imaging Using Antenna Resonant Frequency Shift

The performance of microwave imaging systems can be significantly improved by incorporating boundaries of the imaged object as a priori information. While it might be possible to manually measure that boundary in a controlled laboratory environment, this cannot be achieved in the clinical environment due to the impracticality of such a measurement in addition to the effect of the natural subject’s movement. In this paper, a method for boundary identification at the same time of imaging and using the same data captured for imaging is presented. The method is based on the relation between the imaging antennas’ resonant frequency and the location of the imaged object. The imaging antennas’ resonant frequency shifts when that antenna faces an imaging object, which is effectively a lossy dielectric in biomedical application, and that shift depends on the distance between the antenna and the object. The proposed technique is quite fast in scanning, computation, and image creation as it does not need additional devices for accurate boundary estimation. The method is tested via simulations and human trials using a torso imaging system. The collected data across the band 0.75–1.75 GHz using a 12-element antenna array, which is extended to 24 virtual elements, enclosing the human torso are processed to successfully estimate the torso boundary in a more accurate way than other methods. The included results in imaging a lung cancer case indicate that the accurate detection of the torso boundary improves microwave images.

Optimized matching liquid with wide-slot antenna for microwave breast imaging

In this paper, we introduce an optimized matching liquid together with a wide-slot ultra-wideband antenna to be used in a breast microwave imaging system. The slot antenna is small with 13 mm × 14 mm × 0.625 mm dimensions so that many of them can be placed around a breast. The matching liquid is needed to effectively couple the microwave signal into breast tissues. Exhibiting a suitable permittivity and moderate loss, the matching liquid is a mixture of 83% sunflower oil with 17% distilled water. We build a phantom in order to investigate the penetration of microwave signal into the breast tissues. Experimental measurements and simulation results indicate that this matching liquid provides, nearly, the maximum scattering power from inside of breast tissues in interval [2.5, 7 GHz] that is an appropriate frequency range for breast imaging.

Novel Microwave Tomography System Using a Phased-Array Antenna

In this paper, a novel microwave tomography (MWT) system that utilizes a phased-array transmitter antenna is presented. In contrast to contemporary MWT systems, the system presented herein consists of a single transmitter and multiple receivers. The single transmitter is a four-element array antenna; by varying the phase for each element in the array, the field distribution inside the imaging chamber varies, which produces a different set of measurements per phase configuration. Furthermore, the individual elements of the transmitting array antenna and the receiving antennas are of different types; the receiving antenna type is selected to maximize the coupling between the transmitter and the receiver. Due to the system’s complexity, the system is modeled using a neural network; the trained network is used as the forward solver for an inversion algorithm based on a Bayesian regularized Levenberg–Marquardt algorithm. The system along with the inversion algorithm is tested using several targets. Furthermore, the performance of the algorithm against various levels of experimental noise is analyzed and evaluated.