Microwave Imaging System Research Articles

The impact of the inverse chirp z-transform on breast microwave radar image reconstruction

This work examines the impact of the inverse chirp z-transform (ICZT) for frequency-to-time-domain conversion during image reconstruction of a pre-clinical radar-based breast microwave imaging system operating over 1–8 GHz. Two anthropomorphic breast phantoms were scanned with this system, and the delay-multiply-and-sum beamformer was used to reconstruct images of the phantoms, after using either the ICZT or the inverse discrete Fourier transform (IDFT) for frequency-to-time domain conversion. The contrast, localization error, and presence of artifacts in the reconstructions were compared. The use of the IDFT resulted in prominent ring artifacts that were not present when using the ICZT, and the use of the ICZT resulted in higher contrast between the tumor and clutter responses. In one of the phantoms, the tumor response was only visible in reconstructions that used the ICZT. The use of the ICZT evaluated with a time-step size of 11 ps resulted in the reduction of prominent artifacts present when using the IDFT and the successful identification of the tumor response in the reconstructed images.

Microwave imaging of breast cancer with factorization method: SPIONs as contrast agent.

We demonstrate an imaging technique that can locate cancerous tumors inside the breast due to electric field variations caused by SPION tracers under different magnetic field intensities. Furthermore, we assess the feasibility of SPION-enhanced microwave imaging for breast cancer with simulations performed on a multi-static imaging configuration. The imaging procedure is realized as the factorization method of qualitative inverse scattering theory, which is essentially a shape retrieval algorithm for inaccessible objects. The formulation is heuristically modified to accommodate the scattering parameters instead of the electric field to comply with the requirements of experimental microwave imaging systems. With full-wave electromagnetic simulations performed on an anthropomorphically realistic breast phantom, which is excited with a cylindrical imaging prototype of 18 dipole antenna arranged as a single row, the technique is able to locate cancerous tumors for a experimentally achievable doses. The technique generates nonanatomic microwave images, which map the cancerous tumors depending on the concentration of SPION tracers, to aid the diagnosis of the breast cancer.

Classification of breast tumor models with a prototype microwave imaging system.

The assessment of the size and shape of breast tumors is of utter importance to the correct diagnosis and staging of breast cancer. In this paper, we classify breast tumor models of varying sizes and shapes using signals collected with a monostatic ultra-wideband radar microwave imaging prototype system with machine learning algorithms specifically tailored to the collected data. A database comprising 13 benign and 13 malignant tumor models with sizes between 13 and 40mm was created using dielectrically representative tissue mimicking materials. These tumor models were placed inside two breast phantoms: a homogeneous breast phantom and a breast phantom with clusters of fibroglandular mimicking tissue, accounting for breast heterogeneity. The breast phantoms with tumors were imaged with a monostatic microwave imaging prototype system, over a 1-6GHz frequency range. The classification of benign and malignant tumors embedded in the two breast phantoms was completed, and tumor classification was evaluated with Principal Component Analysis as a feature extraction method, and tuned Naïve Bayes (NB), decision trees (DT), and k-nearest neighbours (kNN) as classifiers. We further study which antenna positions are better placed to classify tumors, discuss the feature extraction method and optimize classification algorithms, by tuning their hyperparameters, to improve sensitivity, specificity and the receiver operating characteristic curve, while ensuring maximum generalization and avoiding overfitting and data contamination. We also added a realistic synthetic skin response to the collected signals and examined its global effect on classification of benign vs malignant tumors. In terms of global classification performance, kNN outperformed DT and NB machine learning classifiers, achieving a classification accuracy of 96.2% when classifying between benign and malignant tumor phantoms in a homogeneous breast phantom (both when the skin artifact is and is not considered). We experimentally classified tumor models as benign or malignant with a microwave imaging system, and we showed a methodology that can potentially assess the shape of breast tumors, which will give further insight into the correct diagnosis and staging of breast cancer.

Development of Microwave Brain Stroke Imaging System using Multiple Antipodal Vivaldi Antennas Based on Raspberry Pi Technology

This paper proposes a Microwave Imaging System (MIS) for brain stroke detection. In the MIS, the primary challenge is to improve in terms of cost, size, and stroke image quality. Thus, the main contribution of this work is the economy and the compact rotation platform integrated with an array of nine antipodal Vivaldi antenna in circular arrangement and single computer board, Raspberry Pi Module (RPM) as microcontroller developed. The design and fabrication of wideband antenna based on Computer Simulation Technology (CST) software and Rogers RO4350B substrate, which operated from 2.06 GHz to 2.61 GHz. In the RPM, the Python programming language used for regulating the angle of rotation and antenna switching process. The process of receiving reflection signals from the head phantom for each antenna supervised by Single-Pole 8-Throw (SP8T) Radio Frequency (RF) switch. The fabricated head phantom based on the primary tissues of the brain, white matter using inexpensive materials, and located in the middle of the platform. Platform rotation is a combination of wood-based platform with the size 0.36m2 and material Perspex. Then, through an interfacing process between Python script and Vector Network Analyzer (VNA), the raw data in S-Parameters transferred to the MATLAB software for analysis. The fabricated antenna able to realize high directivity, 86.92% efficiency, and 2.45 dBi gain. Overall, the proposed system offers the cost-effective, compact, and able to collect the data effectively around the head phantom that consist of a target clot and without a target clot at 50 different positions. It successfully tracked the presence of stroke clots through color differences in color plots.

Food Security Sensing System Using a Waveguide Antenna Microwave Imaging through an Example of an Egg.

In this paper, we present a form of food security sensing using a waveguide antenna microwave imaging system through an example of an egg. A waveguide antenna system with a frequency range of 7–13 GHz and a maximum gain of 17.37 dBi was proposed. The maximum scanning area of the waveguide antenna microwave imaging sensing system is 30 × 30 cm2. In order to study the resolution and sensitivity of the waveguide antenna microwave imaging sensing system, the circular and triangular high-k materials (with the same thickness but with different dielectric constants of the materials) were used as the testing sample for observing the microwave images. By using the proposed waveguide antenna microwave imaging sensing system, the high-k materials with different dielectric constants and shapes could be easily sensed. Therefore, the waveguide antenna microwave imaging sensing system could be potentially used for applications in rapid, non-destructive food security sensing. Regarding the example of an egg, the proposed waveguide antenna microwave imaging sensing system could effectively identify the health status of many eggs very quickly. The proposed waveguide antenna microwave imaging sensing system provides a simple, non-destructive, effective, and rapid method for food security applications.

Effects of and Compensation for Translational Position Error in Microwave Synthetic Aperture Radar Imaging Systems

Translational position error in microwave and millimeter-wave synthetic aperture radar (SAR) imaging systems can cause significant image quality degradation, particularly in nondestructive testing and evaluation (NDT&E) applications where the distance to the imaging object is relatively short. In this paper, this translational position error problem is fully studied through electromagnetic simulation. The results show that among possible geometrical causes of error, a translational position error, in the height direction, is the dominant factor in image quality degradation. Subsequently, a corresponding height position error compensation method is proposed and analyzed. The extensive simulations and measurement are performed in the X-band (8.2–12.4 GHz) frequency range. Then, by defining several evaluation metrics, the relationship between image quality and height position error is discussed quantitatively. The measured results show good agreement with the simulated results, which validates the effectiveness of the proposed analysis approach and the compensation method. The methodology proposed in this paper can be used to evaluate the feasibility or help define the required specifications of a microwave SAR imaging system for a specific application.

Feature-Supervised Compressed Sensing for Microwave Imaging Systems

Traditional compressed sensing (CS) applications use sparse information for downsampling but ignore overall system objectives such as feature extraction. This article jointly designs the sensing and feature-extraction process to improve the efficiency of the microwave imaging systems in time, storage, and feature extraction. A feature-supervised CS (FsCS) is proposed for the cases where not all data contribute to feature extraction. Compared with the traditional spatial–spectral sweep and CS solutions, a feature constraint is added in designing the CS measurement matrix. More efficient sensing and feature extraction are achieved, because only the data contributing to feature extraction are sampled and reconstructed. To improve the time efficiency of CS reconstruction, an aligned spatial–spectral sensing (ASSS) is involved in FsCS to enable joint reconstruction. The proposed scheme is validated in an open-ended waveguide imaging system for low-energy impact damage feature detection. The experimental results demonstrate one order of magnitude improvement in time and two orders of improvement in the data compression ratio compared with the state-of-the-art method, while preserving the interested feature. This article can inspire the joint sensing-processing designs for more intelligent industrial processes.

Accurate Near-Field Millimeter-Wave Imaging of Concave Objects—A Case Study of Dihedral Structures Under Monostatic Array Configurations

With the fast progress of high-resolution microwave and millimeter-wave (MMW) near-field imaging systems, image artifacts caused by complex high-contrast target become a nonnegligible issue for accurate surface reconstruction and recognition. Among different types of artifacts, those caused by high-order scattering within concave structures are particularly difficult to interpret and to correct. This article focuses on analyzing these unconsidered effects in conventional imaging or beamforming algorithms and chooses a typical imaging scenario with dihedral structures under near-field monostatic cylindrical imaging geometry for discussion. The mechanism of multiple reflections and the formation of artifacts are firstly investigated, and the artifacts are classified into two categories based on the number of reflection times. Then shooting and bouncing rays (SBRs) method is utilized to establish an analytical forward model including the mechanism of multiple reflections. Furthermore, a new imaging algorithm based on this forward model is proposed to obtain more accurate reconstructions. By compensating phase shifts of different multiple reflection signals with the corresponding range equations, dihedral-type objects with different opening angles can be accurately recovered. Numerical results are presented to verify the applicability of the forward model and to validate the effectiveness of the proposed imaging algorithm. In the end, experimental results confirm the practicability of the proposed method.

Miniaturized on‐body antenna for small and wearable brain microwave imaging systems

Miniaturized on‐body antenna for small and wearable brain microwave imaging systems

Microwave Imaging for Breast Cancer Detection: The Discrimination of Breast Lesion Morphology

Radar based microwave imaging system is proposed and used for breast cancer detection. Tumour location can be detected based on high dielectric differences between tumour and surrounding tissues. In clinical medicine, malignant tumour usually has more irregular morphology than benign tumour. However, previous work has ignored the discrimination between benign and malignant tumours, and this issue is solved in our work. In this paper, two major novel contributions are presented. Firstly, a complex natural responses (CNRs) method is proposed for discrimination between the benign and malignant tumours. The CNRs of the tumour show a unique feature that the CNRs only depend on the morphological appearance of the tumour. Simulation results show that the use of the CNRs can successfully discriminate between a 5 mm radius spherical benign tumour and a 5 mm average radius malignant tumour with a spiculated periphery. Secondly, microwave images of the benign and malignant tumours are created by a proposed mono-static imaging system which uses a single Vivaldi antenna to scan the breast. Both benign and malignant tumour positions can be correctly indicated from the images.

Metamaterial-Inspired Antenna Array for Application in Microwave Breast Imaging Systems for Tumor Detection

This paper presents a study of a planar antenna-array inspired by the metamaterial concept where the resonant elements have sub-wavelength dimensions for application in microwave medical imaging systems for detecting tumors in biological tissues. The proposed antenna consists of square-shaped concentric-rings which are connected to a central patch through a common feedline. The array structure comprises several antennas that are arranged to surround the sample breast model. One antenna at a time in the array is used in transmission-mode while others are in receive-mode. The antenna array operates over 2–12 GHz amply covering the frequency range of existing microwave imaging systems. Measured results show that compared to a standard patch antenna array the proposed array with identical dimensions exhibits an average radiation gain and efficiency improvement of 4.8 dBi and 18%, respectively. The average reflection-coefficient of the array over its operating range is better than S11 ≤ −20 dB making it highly receptive to weak signals and minimizing the distortion encountered with the transmission of short duration pulse-trains. Moreover, the proposed antenna-array exhibits high-isolation on average of 30dB between radiators. This means that antennas in the array (i) can be closely spaced to accommodate more radiators to achieve higher-resolution imaging scans, and (ii) the imagining scans can be done over a wider frequency range to ascertain better contrast in electrical parameters between malignant tumor-tissue and the surrounding normal breast-tissue to facilitate the detection of breast-tumor. It is found that short wavelength gives better resolution. In this experimental study a standard biomedical breast model that mimics a real-human breast in terms of dielectric and optical properties was used to demonstrate the viability of the proposed antenna over a standard patch antenna in the detection and the localization of tumor. These results are encouraging for clinical trials and further refinement of the antenna-array.

A novel UWB compact elliptical-patch antenna for early detection of breast cancer in women with high mammographic density

Microwave imaging is one of the emerging technologies for early detection of breast cancer among women having dense mammographic densities. One of the critical and valuable components of an accurate, effective and compact, involving minimum risk - microwave imaging system for early breast cancer detection is an ultra wideband (UWB) antenna. A novel, compact elliptical UWB microwave antenna is presented in this research article that might be suitable for early breast cancer detection. The simulation of antenna structure is carried out using HFSS13 FEM-based EM software. The simulation results yield better UWB response. The antenna structure provides a wide practical fractional bandwidth of more than 156%. A significant performance factor of the proposed antenna is its ability to provide sufficient gain level for short distance communication. Thus, the proposed antenna is a strong candidate for design and development of microwave imaging system for early detection of breast cancer among women with dense mammographic densities.

Multi-modality strain estimation using a rapid near-field microwave imaging system for dielectric materials

Strain distribution is an important indicator of stress concentration, damage initiation and evolution. Many dielectric materials sustain very large strain before failure. In this paper, a near-field microwave high-resolution imaging (NMHI) system is presented to estimate very large deformation. The sensitivity of the microwave imaging system to the dielectric property and geometric changes have been utilized in the present work. A multi-modality data fusion technique is applied to experimentally evaluate the strain in ASTM-D638 standard dog bone structure made of Polyamide 11 (PA-11) material. Plastic strain in the range of 0.3–0.7 has been successfully correlated to the microwave probe response in PA-11 materials. The comparison of strain distribution obtained from NMHI and digital image correlation (DIC) indicate the potential of NMHI as a fast, non-contact method to estimate large mechanical strain.

A Low Cost and Portable Microwave Imaging System for Breast Tumor Detection Using UWB Directional Antenna array

Globally, breast cancer is a major reason for female mortality. Due to the limitations of current clinical imaging, the researchers are encouraged to explore alternative and complementary tools to available techniques to detect the breast tumor in an earlier stage. This article outlines a new, portable, and low-cost microwave imaging (MWI) system using an iterative enhancing technique for breast imaging. A compact side slotted tapered slot antenna is designed for microwave imaging. The radiating fins of tapered slot antenna are modified by etching nine rectangular side slots. The irregular slots on the radiating fins enhance the electrical length as well as produce strong directive radiation due to the suppression of induced surface currents that radiate vertically at the outer edges of the radiating arms with end-fire direction. It has remarkable effects on efficiency and gain. With the addition of slots, the side-lobe levels are reduced, the gain of the main-lobe is increased and corrects the squint effects simultaneously, thus improving the characteristics of the radiation. For experimental validation, a heterogeneous breast phantom was developed that contains dielectric properties identical to real breast tissues with the inclusion of tumors. An alternative PC controlled and microcontroller-based mechanical MWI system is designed and developed to collect the antenna scattering signal. The radiated backscattered signals from the targeted area of the human body are analyzed to reveal the changes in dielectric properties in tissues. The dielectric constants of tumorous cells are higher than that of normal tissues due to their higher water content. The remarkable deviation of the scattered field is processed by using newly proposed Iteratively Corrected Delay and Sum (IC-DAS) algorithm and the reconstruction of the image of the phantom interior is done. The developed UWB (Ultra-Wideband) antenna based MWI has been able to perform the detection of tumorous cells in breast phantom that can pave the way to saving lives.

One microwave imaging system based on automotive radar chip

The interest in microwave imaging system is increasing because it can serve as a useful tool for concealed threat detecting. The current imaging systems have two major challenges: (i) multi-static arrays suffer from complex hardware design and ineffective backprojection-imaging algorithm, and (ii) monostatic linear arrays suffer from large scanning time due to the electronically scanning one by one of the co-located receive–transmit antennas. To address these challenges, this study introduces a novel linear imaging array based on automotive radar chips and sub-array time-division multiple-input multiple-output working principles, which is hardware efficient and compatible fast Fourier transform (FFT) imaging technique. Through simulation, it is shown that the scanning time is only one-fourth of the traditional monostatic linear array and the FFT image reconstruction technique can be used.

A Low-Cost and Compact Three-Dimensional Microwave Holographic Imaging System

With the significant growth in the use of non-metallic composite materials, the demands for new and robust non-destructive testing methodologies is high. Microwave imaging has attracted a lot of attention recently for such applications. This is in addition to the biomedical imaging applications of microwave that are also being pursued actively. Among these efforts, in this paper, we propose a compact and cost-effective three-dimensional microwave imaging system based on a fast and robust holographic technique. For this purpose, we employ narrow-band microwave data, instead of wideband data used in previous three-dimensional cylindrical holographic imaging systems. Three-dimensional imaging is accomplished by using an array of receiver antennas surrounding the object and scanning that along with a transmitter antenna over a cylindrical aperture. To achieve low cost and compact size, we employ off-the-shelf components to build a data acquisition system replacing the costly and bulky vector network analyzers. The simulation and experimental results demonstrate the satisfactory performance of the proposed imaging system. We also show the effect of number of frequencies and size of the objects on the quality of reconstructed images.

Accelerating microwave tomography based on generalized analytical singular value decomposition

In this paper, a 2-D microwave tomography system with circular observation domain is proposed. By means of Analytical Singular Value Decomposition (ASVD) object reconstruction process significantly accelerated. The spectral expansion of the Green’s function is employed to derive generalized SVD of scattering integral operator. The analytical formulation of singular values and vectors is straightforward without considerable computation cost. Influence of medium properties and configuration on singular values are studied numerically. Also, the performance of the proposed method is validated against experimental data. Thanks to the spectral representation, truncation parameter for singular values can be determined in a closed form. As a result, small singular values can be filtered out to ensure stability.

A printed compact UWB Vivaldi antenna with hemi cylindrical slots and directors for microwave imaging applications

A new compact UWB (Ultra Wide Band) hemi cylindrical slotted Vivaldi antenna is proposed for microwave imaging system to detect the breast cancer. The hemi cylindrical slots are added on the edges of the Vivaldi Antenna to increase the electrical length and focus the energy towards the aperture centre. The novel combination of hemi cylindrical slots and directors are introduced to improve the performance characteristics of the antenna such as UWB impedance matching, high gain, constant E plane beam tilt, better FBR (Front to Back Ration) and Cross Polarization Discrimination (XPD). To exemplify the effectiveness of this design, a prototype of the VAHCSD (Vivaldi antenna with Hemi Cylindrical Slots and Directors) is fabricated and tested. The antenna is designed on 0.8 mm thick low cost FR4 substrate (49 × 48.5 × 0.8 mm3) and operates from 2.9 GHz to more than 9 GHz with S11 < −10 dB. The gain of the antenna is more than 3 dBi for frequency (3–5 GHz) and greater than 6 dBi from 5 GHz. The FBR value greater than 15 dBi, the XPD value more than −20 dB and the stable radiation pattern are achieved. The calculated fidelity factor is 0.9557 which proves the directionality and low distortion of the transmitted UWB signal. Furthermore, the time domain study of the antenna is also carried out which shows good pulse handling ability with nearly flat group delay having a small variation of ±0.5 ns over operating UWB band.

Microwave tomography sensing for potential agarwood trees imaging

Agarwood or its scientific name, Aquilaria is an expensive and precious non-timber forest product. The problems faced by the agarwood industry are the indiscriminate harvesting of agarwood and ineffective identification and inspection process of Aquilaria tree by agarwood prospectors and researchers. Hence, microwave tomography (MWT) is proposed to evaluate agarwood in order to encounter these problems. This technique operates in a safe, non-invasive and insensitive to environmental impacts. A simulation-based study had been performed in which 16 antennas microwave tomography system was modelled using the finite element analysis. In this study, six profiles were being tested and simulated at 1 GHz. An electric field was simulated based on transverse electric mode to be emitted into the modelled wood with agarwood. Besides, electric field at the receiving antennas was also simulated when the simulated incident field was emitted into the modelled wood. The readings of the electric field for both incidents and receiving antennas were recorded to find the difference of electric field which will be used for image reconstruction. The images for the six profiles were reconstructed using Linear Back Projection (LBP), Filtered Back Projection (FBP), Newton’s One-Step Error Reconstruction (NOSER) and Tikhonov Regularization (TR) image reconstruction algorithms. The reconstructed images were then analysed using the Mean Structural Similarity (MSSIM) index. The phantom and reconstructed images were analysed qualitatively and quantitatively. The findings showed promising results as the proposed MWT was able to distinguish and identify agarwood in Aquilaria tree.

Towards real‐time through‐obstacle imaging based on compressed sensing for sparse objects

Although significant progress has been made in microwave imaging, real-time imaging, especially for objects behind walls or closed obstacles, remains a technical challenge. In this work, highly efficient imaging for complex-structured objects surrounded by a closed obstacle was experimentally demonstrated. The imaging equations are derived based on a combination of the inverse-scattering problem and the concept of compressed sensing. Making use of the spatial sparsity of objects and obstacles, the compressed imaging can be implemented using a time-division multi-antenna setup with reduced transmitting antennas. Owing to the spatial compressed sensing applied to the sparse imaging region and objects, the imaging time can be reduced by two orders of magnitude compared with the conventional twofold subspace-based optimisation method with a comparable imaging quality. Taking advantage of the sparsity of the entire imaging area, objects with larger relative permittivity can also be reconstructed. The proposed method can be potentially used in applications such as security examination through boxes. It also provides a new clue for solving the practicability difficulty faced by existing microwave imaging systems.