Electromagnetic Tomography using Microwave Signals and Numerically Realistic Human Head Model

Abstract

Imaging modalities that are widely utilized in the world include Computed Tomography (CT), Magnetic Resonance Imaging (MRI) and the Positron Emission Tomography (PET). The underlying physical properties of the object being imaged, the design of radiation/ detection system and the imaging performance of each modality are quite similar; both in absolute terms as well as relative to an ideal observer. However, each technique has some associated drawbacks e.g. CT possesses ionizing effects, MRI is not appropriate for patients with metallic medical implants and PET involves a radioactive material injection. Moreover, they all are time-consuming, costly and immovable imaging modalities. Therefore, to overcome these deficiencies, there is a need of an alternate imaging technique that can provide a safe, low-cost, fast and portable imaging solution for brain anomalies diagnostics. This forms the basis of research question considered in this study. Our study looks closely into brain stroke incidences which are not only life-threatening but also bring with them a very poor prognosis. There is a need to investigate the onset of stroke symptoms in a matter of few hours by the doctor. The dawn of the 21st century has brought exciting vision for innovative Microwave Imaging (MWI) systems to address the diagnostic needs in medicine and industry. The ultimate objective of MWI technique is to exploit a dielectric properties contrast which is sensitive to any physiological or pathological feature of clinical interest. Through this study, we highlight our four major scientific contributions. Primarily, this work investigates the feasibility of Electromagnetic Tomography (EMT) for brain stroke diagnostics. We achieved this by evaluating the interaction between MW signals and the stroke-affected head models. The maximum electric field differences are observed at an approximate location of stroke that vary with type and location of stroke inside the head model. It is inferred that MW scattering from a head model changes considerably, once its complexity is increased by making it anatomically more realistic. We also evaluated the MW scattering behavior of a complete human head for two types of stroke at various locations inside the brain. A preliminary Finite Element Method (FEM) based analysis is presented using a hemorrhagic-affected three-dimensional (3D) ellipsoid head model. The simulation results are validated through an analytical solution involving a two-dimensional (2D) multilayer head model. Later on, an anatomically more realistic and structurally detailed 3D head model is generated by implementing a novel tissue-mapping scheme along with a mixed-model approach. We also developed an improved 2D image reconstruction algorithm for EMT of a human head named as Adaptive Additive-Regularized Contrast Source Inversion (CSI) method. It is based on basic CSI method and an adaptively-regularized total-variation minimization additive constraint function. The processing of MW scattering data, generated through FE simulations Electromagnetic Tomography using Microwave Signals and Numerically Realistic Human Head Model vi of 2D/ 3D realistic head model EMT setup, was done during its validation. The algorithm successfully estimated the dielectric properties of head tissues and produced better-quality images by spatially mapping these properties. It precisely highlighted the locations of clinical importance to perform an accurate stroke diagnosis. The algorithm also took into account real-life noise conditions. Later on, we modified our 2D imaging algorithm for EMT of a 3D realistic head model, following a scalar approximation approach. We were able to obtain meaningful head images with an acceptable stroke diagnostics results. A simulation-driven antenna array design, an appropriate matching medium and the optimal frequency range were utilized. In addition, a safety analysis was also conducted to ensure the safe exposure of MW signals to a human head. It is concluded that EMT using MW signals may potentially substitute the existing brain imaging modalities; especially at rural areas and in emergency situations like brain stroke and traumatic injuries.