A Mathematical Framework to Analyze the Achievable Resolution From Microwave Tomography

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

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.

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.

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.

We consider the use of matching metasurfaces for enhanced interrogation of the object of interest (OI) in microwave imaging. For an OI immersed in water, we perform synthetic data collection in air with the aid of matching metasurfaces, and then invert the scattered field data under the assumption of infinite water background medium.

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.

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.

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.

To develop a new class of emulsions using a protein-based emulsifier as the coupling fluid for microwave imaging systems. In this paper, we provide a theoretical basis for engineering shelf-stable dielectric fluids, a step-by-step formulation method, and measurements of complex dielectric properties in the frequency range of 0.5-3 GHz, which can be applicable for many of the recent microwave imaging systems. This medium was primarily designed for long-term stability while providing a controllable range of complex dielectric permittivities given different fractions of its constituents. Consequently, this emulsion shows dielectric stability in open air throughout a 7-day experiment and temperature insensitivity over the range of 0 ° C to 60 ° . This control over dielectric permittivity enables formulations that tune the background-to-target contrast to the linearizable regime of iterative inverse scattering algorithms. Accordingly, the emulsion conductivity can also be controlled and reduced to maintain the required signal-to-noise ratio within the dynamic range of the imaging system. The new formulation overcomes the practical challenges of engineering coupling fluids for microwave imaging systems, e.g., temporal stability, non-toxic, low sensitivity to temperature variation, and easy formulation from readily available and inexpensive materials. The achieved properties associated with this new fluid are of particular benefit to microwave imaging systems used in thermal therapy monitoring.

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.

Speed selectivity in visual short term memory for motion

This paper describes a fast-forward electromagnetic solver (FFS) for the image reconstruction algorithm of our microwave tomography system. Our apparatus is a preclinical prototype of a biomedical imaging system, designed for the purpose of early breast cancer detection. It operates in the 3-6-GHz frequency band using a circular array of probe antennas immersed in a matching liquid; it produces image reconstructions of the permittivity and conductivity profiles of the breast under examination. Our reconstruction algorithm solves the electromagnetic (EM) inverse problem and takes into account the real EM properties of the probe antenna array as well as the influence of the patient's body and that of the upper metal screen sheet. This FFS algorithm is much faster than conventional EM simulation solvers. In comparison, in the same PC, the CST solver takes ~45 min, while the FFS takes ~1 s of effective simulation time for the same EM model of a numerical breast phantom.

Accelerating microwave tomography based on generalized analytical singular value decomposition

Most microwave tomography system design uses only TM polarized wave for transmitting and receiving mode, but TE waves will also gets excited because of depolarization of incident waves from composite complex target that cannot be captured by using single polarized antenna. The aim of this project is to design a dual polarized DRA for use in microwave tomography system to collect coplanar and cross polar responses. The proposed antenna is reduced in size with a factor of 6.7 and the bandwidth of 72% (2.6-5.52 GHz).