A Versatile and Shelf-Stable Dielectric Coupling Medium for Microwave Imaging

A new compressed sensing (CS) image reconstruction method is proposed for high‐resolution wideband three‐dimensional synthetic aperture radar (SAR) imaging systems. In contrast to existing CS SAR methods that employ only a forward SAR transform in pre‐ or post‐processing, the proposed method employs both forward SAR and reverse SAR (R‐SAR) transforms in each CS iteration to improve the quality of reconstructed images. This study proposes a simple and elegant truncation repair method to combat the truncation error and utilises non‐uniform fast Fourier transform to reduce the SAR and R‐SAR transform errors, thereby ensuring the convergence of the CS algorithm and improving the quality of the reconstructed images. The proposed CS SAR method is applied to microwave and millimeter wave imaging systems for non‐destructive evaluation of materials embedded in stratified media. Three different specimens under test are measured by conventional uniform sampling and by random under‐sampling with 20% or 30% spatial points of the uniform sampling. The reconstructed images show that, albeit having much less measurement points, the proposed CS method achieves better image quality and lower background artefacts than the images reconstructed from the fully‐sampled uniform measurements.

ABSTRACTThis article presents the image reconstruction results of measurement data from a preliminary test using the first‐developed precision microwave tomography (MT) system operating at 3–6 GHz for medical diagnosis. A simple phantom is used in the MT system to verify the data validation. The measurement dataset has very different patterns owing to the significant contrast of dielectric properties according to the existence of a phantom in the sensing area of the illumination tank at microwave frequencies. Thus, it is possible to obtain the image reconstruction from this measurement data. The detailed results are presented and discussed. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:2445–2448, 2015

In this paper, we are going to touch some design essentials in terms of system considerations by taking a practical indoor millimeter wave (mmW) imaging system as a benchmark. Nowadays, system designers follow the guideline from classical imaging radars, which have a long history and aim at far-field operations. However, we are going to emphasize the difference when we focus on indoor mmW systems, which do not work in the far-field region. Basically, we think current indoor mmW imaging systems can be improved by two steps: first, proper sensor selection; second, optimum optical system design. We will illustrate how the resolution of imaging systems can be improved and discuss some design tradeoffs. The frequency of interest is around 100 GHz. However, the discussions can be extended to either microwave or THz imaging systems.

In this work, a conformal antenna designed using polydimethylsiloxane (PDMS) is presented, with copper used as its conducting elements. Such material combination enabled the realization of a highly flexible antenna structure for potential implementation on the human body in microwave imaging systems. This antenna is designed based on a planar monopole aimed at operation in the lower ultra-wideband (UWB) frequencies. The choice of this band is to enable a deeper penetration of radiation into the human body. It operates from 2.92 to 4.75 GHz, with a bandwidth of 1826 MHz and a gain of 2.41 dB in its planar condition. When evaluated in a bent configuration with a radius of 20 mm, 40 mm and 60 mm, a bandwidth of 47.59% (1826 MHz), 45.72% (1741 MHz) and 42.68% (1770 MHz) with a gain of 2.41 dB, 2.73 dB and 3 dB is observed respectively. These results indicate that the antenna can be potentially be used on a human body for microwave imaging purposes.

In the past decade, microwave imaging (MWI), employing advanced antenna-based systems, has emerged as a cutting-edge, non-invasive approach for pathological diagnosis. Multiple experimental studies suggest that MWI proposes promising results for early-stage tumor diagnosis. Recent studies on MWI show the importance of antenna types and configuration. However, it is observed that a very limited research has focused on exploring the large classes of antennas used in detail. In this review, a systematic study and firsthand classification of the antennas used, as the most important aspect of an MWI system, is proposed. The classification includes fundamental antennas, microstrip antennas, metamaterial/metastructure antennas, specialized antennas, and 3D antennas. A cross-sectional study is performed to depict the potential relationship between antenna type and efficiency of the MWI systems. This goal is achieved by quantitatively analyzing the gain and operating bandwidth, antenna configuration, and application models. Further, a critical analysis is presented on different signal processing and imaging algorithms for synthetic image generation. This review presents a modular analysis of MWI systems and draws several conclusions on the effectiveness of each essential module for a reliable MWI system. It also addresses the challenges, scope of improvement in the MWI systems, and prospects for the technology's integration into clinical practice.

Medical imaging refers to several different technologies that are used to view the human body in order to diagnose, monitor, or treat medical conditions. Each type of technology gives different information about the area of the body being studied depending on the radiation used to illuminate de body. Nowadays there are still several lesions that cannot be detected with the current methods in a curable stage of the disease. Moreover they present some drawbacks that limit its use, such as health risk, high price, patient discomfort, etc. In the last decades, active microwave imaging systems are being considered for the internal inspection of light-opaque materials thanks to its capacity to penetrate and differentiate their constituents based on the contrast in dielectric properties with a sub-centimeter resolution. Moreover, they are safe, relatively low-cost and portable. Driven by the promising precedents of microwaves in other fields, an active electromagnetic research branch was focused to medical microwave imaging. The potential in breast cancer detection, or even in the more challenging brain stroke detection application, were recently identified. Both applications will be treated in this Thesis. Intensive research in tomographic methods is now devoted to develop quantitative iterative algorithms based on optimizing schemes. These algorithms face a number of problems when dealing with experimental data due to noise, multi-path or modeling inaccuracies. Primarily focused in robustness, the tomographic algorithm developed and assessed in this thesis proposes a non-iterative and non-quantitative implementation based on a modified Born method. Taking as a reference the efficient, real-time and robust 2D circular tomographic method developed in our department in the late 80s, this thesis proposes a novel implementation providing an update to the current state-of-the-art. The two main contributions of this work are the 3D formulation and the multi-frequency extension, leading to the so-called Magnitude Combined (MC) Tomographic algorithm. First of all, 2D algorithms were only applicable to the reconstruction of objects that can be assumed uniform in the third dimension, such as forearms. For the rest of the cases, a 3D algorithm was required. Secondly, multi-frequency information tends to stabilize the reconstruction removing the frequency selective artifacts while maintaining the resolution of the higher frequency of the band. This thesis covers the formulation of the MC tomographic algorithm and its assessment with medically relevant scenarios in the framework of breast cancer and brain stroke detection. In the numerical validation, realistic models from magnetic resonances performed to real patients have been used. These models are currently the most realistic ones available to the scientific community. Special attention is devoted to the experimental validation, which constitutes the main challenge of the microwave imaging systems. For this reason, breast phantoms using mixtures of chemicals to mimic the dielectric properties of real tissues have been manufactured and an acquisition system to measure these phantoms has been created. The results show that the proposed algorithm is able to provide robust images of medically realistic scenarios and detect a malignant breast lesion and a brain hemorrhage, both at an initial stage.

In this work, we investigated dependence of the spatial resolution in a microwave tomography system on significant parameters such as an operating frequency, dielectric properties of testing material, number of testing antennas, diameter of the circular array, and total amount of measured data. The spatial resolution was calculated in 3 to 6 GHz, based on original estimation method in microwave imaging.

Microwave imaging (MWI) techniques demonstrate significant improvements in medical imaging relative to traditional imaging methods. For example, MWI systems are more cost-effective and portable than most magnetic resonance imaging (MRI) equipment. Due to its non-ionizing nature, MWI does not suffer from the safety issues associated with X-ray and positron emission tomography (PET). Consequently, several MWI prototype systems have been effectively used for medical imaging applications, encompassing medical diagnosis and therapy assessment.

Microwave imaging (MWI) based breast cancer detection has attracted widespread attentions because it has great advantages of non-ionization and low price. However, the current MWI systems require huge and expensive hardware, and complex signal calibration and present substantial challenges to the application of MWI system. For this reason, the embedded MW imaging (E-MWI) method is proposed for breast cancer detection. Firstly, the Software Define Radio (SDR) software is implemented on an embedded platform to reduce cost and miniaturize system. Secondly, an amplitude-based spatial domain confocal algorithm is used in E-MWI system to simplify the system calibration by avoiding signal phase acquisition. To verify the proposed method, experiments platform is set up using the E-MWI prototype system and a liquid breast phantom. The results show that the system successfully detects the target with different sizes.

Utilization of the pulses with ultra wide frequency band (UWB) in microwave imaging systems enables obtaining great resolution and simultaneously taking benefits from the microwaves' penetrating ability [4], [16]. Short duration of the pulses — which is what causes their wide frequency band — has also this advantage that it gives opportunity to perform time analysis and eliminate many unwanted signals, simply on a base of their time of arrival (when significantly different from the time of arrival of the signal of concern [13]). Known of UWB pulses is the fact that their shape is easily changed due to the convolution with pulse responses of the transmitter and receiver components [3], [21]. Antenna is here a recognized bottle neck [1]. However, this problem is possible to be mitigated by using better antenna or one of deconvolution techniques [6]. This paper identifies a problem with definition of the resolution of such imaging system. It is demonstrated that the pulse response of the object itself is also convoluted with measurement UWB pulse. Therefore, the resolution of the image of the object depends on the shape and dimensions of the object, what makes its definition rather ambiguous.

Measurement of whole-brain atrophy progression using microwave signal analysis

Microwave imaging (MWI) systems are being investigated for breast cancer diagnostics as an alternative to conventional x-ray mammography and breast ultrasound. This work aims at a next generation of tissue-mimicking phantoms modelling the temperature-dependent dielectric properties of breast tissue over a large frequency bandwidth. Such phantoms can be used to develop a novel kind of MWI systems that exploit the temperature-dependent permittivity of tissue as a natural contrast agent. Due to the higher water content in tumor tissue, a temperature increase leads to a different change in the complex permittivity compared to surrounding tissue. This will generate a tumor dominated scattering response when the overall tissue temperature increases by a few degrees, e.g. through the use of microwave hyperthermia systems. In that case a differential diagnostic image can be calculated between microwave measurements at reference (around 37 °C) and elevated temperature conditions. This work proposes the design and characterization of agar-oil-glycerin phantoms for fatty, glandular, skin and tumor tissue. The characterization includes measurements with an open-ended coaxial probe and a network analyzer for the frequency range from 50 MHz to 20 GHz in a temperature-controlled environment covering the temperature range from 25 °C to 46 °C. The phantoms show an unique temperature response over the considered frequency bandwidth leading to significant changes in the real and imaginary part of the complex permittivity. Comparative studies with porcine skin and fat tissue show a qualitative agreement.

Liver cancer is one of the most deadly diseases worldwide with an increasing yearly fatality rate. Thermal ablation treatments are considered to be an effective alternative to conventional surgery, but the lack of an effective imaging modality to monitor the treatment prevents from a full exploitation of their therapeutic potential. As such, there is an increasing interest in developing alternative imaging modalities. In this framework, due to the fact that thermally treated tissue exhibits different dielectric properties as compared to untreated tissue, microwave imaging is a potential candidate, offering the possibility of performing the treatment monitoring task in a truly non-invasive way and by means of a portable and low cost apparatus. In this communication the prototype of a microwave imaging system to monitor thermal ablation of liver is presented together with its initial experimental validation. The observed results, although still preliminary, confirm the anticipated treatment monitoring capabilities of microwave imaging.

Shortening the time between the onset of stroke and its detection/classification is extremely important for increasing the success of its treatment. Microwave imaging (MWI) represents one of the possible means that could allow such a shortening in near future. At the ELEDIA@CTU laboratory in Prague, a prototype of MW imaging system was developed, which was tested on a homogeneous human head phantom with a stroke phantom. Currently a new generation of the MWI system with more realistic human head phantom is being developed. Magnitudes of the transmission coefficient between antennas of the future system determine the electrical properties of the suitable microwave switching matrix. In this paper, numerical simulations in COMSOL Multiphysics are conducted to obtain information on the levels of amplitudes of transmission coefficients. Numerical transmission coefficient simulations were performed firstly for the 1D model of the human head, which from our point of view represents the highest theoretical limit of the transmission coefficient magnitudes. These simulations are followed by 2D and finally 3D models. The higher the number of the dimensions the lower magnitudes of the transmission coefficients were observed.

The microwave imaging system based on varifocal reconfigurable transmitarray (RTA) is presented and analyzed in this paper. Structure of the imaging system is presented, containing a focal-plane array and a vari-focal RTA. This imaging system can achieve flexible imaging distance, compared with conventional microwave front-view imaging method. Furthermore, by using cascaded RTAs, the image distance can be fixed, which is desirable for practical imaging system construction. This paper shows that the RTAs have promising potential in imaging systems.