Design and development of triangular body coupled monopole antenna for head imaging application

Abstract

Current head imaging technologies, such as computed tomography (CT) and magnetic resonance imaging (MRI), can accurately diagnose brain injuries like stroke and brain tumors. However, these technologies have several limitations, notably their bulkiness, high cost, lengthy scanning times, and stationary nature. In addition, most of the antennas used for microwave imaging suffer high signal loss due to strong reflections from the body surface. This issue would lead to inaccurate diagnosis of diseases such as false negative results for cancer detection. Typically, standard antennas release electromagnetic (EM) energy into free space with a dielectric constant (or relative permittivity), 𝜀𝜀𝑟𝑟=1. However, the dielectric properties of the human body are approximately 20 to 30. Due to this impedance mismatch, almost ¾ of the energy transmitted into the body is reflected back at the body-air boundary. Considering the very low limit set upon the level of electromagnetic energy that the human body should be exposed to, simply increasing the transmitted power to probe inside the body is not an option. Moreover, the existing antennas are rigid to be used for wearable head imaging applications. This thesis proposes a flexible body-coupled (co-planar waveguide) CPW-fed triangular planar monopole antenna (PMA) for wide-band microwave sensing and imaging, particularly for head imaging applications. The proposed antenna has been made with advanced ceramic material barium titanate (BaTiO3) incorporated with silicone-based hyperplastic elastomer to synthesize a flexible body-matched impedance substrate with low loss. The proposed barium titanate silicon-elastomer composite layer is designed with the dielectric property of 20 which acts as an impedance-matching layer for the CPW-fed triangular PMA. The dimension of the proposed antenna is 70mm × 30mm × 6mm. The mixed ratio by weight of barium titanate powder and silicon used to synthesize the impedance matching layer is 1:1. The measured dielectric constant of the impedance matching layer is 6.9. It has been shown that the power radiated into an artificial head phantom improved by almost 160% as compared to an antenna without an impedance-matching layer. Moreover, the SAR level is 0.0286 W/kg when 1 mW of power is transmitted, which is well below the limit set by the FCC (<1.6 w/kg for mass) regulation for the public at microwave frequencies. The proposed antenna has a bandwidth of almost 900 MHz (0.6 GHz - 1.5GHz) when measured directly on the actual human head. This makes the antenna suitable for wearable head imaging applications due to its wideband characteristic and improved power penetration into the human head.