This work presents a design and experimental validation of a compact ultrawideband (UWB) bowtie antenna and balanced-to-unbalanced (balun) circuit for medical microwave imaging (MWI) applications working in the frequency band of 1–6 GHz. The UWB balun is perpendicularly attached to the planar bowtie arms, whose dimensions were miniaturized by rounding the bowtie edges. The antenna reflection coefficient was lower than −12 dB for tissues with a higher water content. The antenna possesses a high radiation efficiency (over 80%) and very low backward radiation. The predicted and measured specific absorption rate (SAR) distributions together with the electric field $\vert E\vert $ distribution proved a symmetrical radiation pattern. The potential of the presented antenna element was demonstrated by successful reconstruction of the images from the measured data by MWI methods. For this purpose, an MWI system equipped with eight antennas was implemented. The radar approach was used to determine the region of interest (ROI) where the area for microwave tomography reconstruction was identified. The dielectric properties within the ROI were reconstructed by using the Born approximation method at 1 GHz. This confirmed that the antenna can be used as the basis for more accurate multifrequency MWI systems and can be implemented in UWB MWI hybrid systems.

This work presents the design and manufacturing of an anatomically and dielectrically realistic layered phantom of the human head that allows the insertion of ischemic and hemorrhagic stroke phantom models. A 2.5D physical phantom was designed using a representative anatomical image of the human head, which was simplified into 5 different layers that mimic the scalp, skull, cerebrospinal fluid, brain, and stroke regions in terms of anatomy and dielectric properties. Apart from the brain phantom, all other layers consist of a mixture of polyurethane rubber, graphite powder, and carbon black powder. The brain phantom is in the liquid form to facilitate the insertion of different stroke models (ischemic or hemorrhagic) with different positions and shapes. Phantoms were designed with dielectric properties valid within the frequency range 0.5–3.0 GHz, which is relevant for microwave stroke detection and classification. Molds for casting individual parts of the phantom were printed in 3D. The presented phantom is suitable for the development and testing of microwave systems and algorithms used in the detection and classification of vascular events relevant to stroke diagnosis.

The design of proper antenna element (AE) for microwave-based head imaging or brain stroke detection is a crucial challenge in the development process of microwave imaging (MWI) systems. The main purpose of this paper was to design, fabricate, and experimentally verify the compact and dimensions-reduced H-slot antenna suitable for the new generation of multichannel MWI system for brain stroke detection. The slot antenna type was chosen based on the numerical study of three AEs available in the literature, i.e. bow tie, slot, and waveguide-based. The study was focused on the sensitivity of the antennae (change of magnitude and phase of S21) due to dielectric parameters change or type and diameter of inclusion in a head phantom representing a hemorrhagic (HEM) or ischemic (ISCH) stroke phantom, respectively. Further, the analysis of antenna radiation to lossy medium/air and its immunity against plane wave exposure was carried out. The H-slot antenna was fabricated and experimentally verified (measurements of reflection as well as transmission coefficients) using a liquid head phantom with inserted HEM stroke phantom (both prepared as a mixture of propylene glycol, water, and salt). The phantoms were filled inside the designed two-port test system. Numerical models were validated by comparing calculated and measured S-parameters. The sensitivity of the H-slot antenna to the presence of the HEM stroke phenomenon within the phantom of the head was also demonstrated. The main advantage of the proposed H-slot antenna is its small dimensions, easy, inexpensive, and repeatable fabrication as well as mechanical stability.