Studies on microwave antennas for hyperthermia applicatons

Abstract

Hyperthermia is a therapeutic procedure used to raise the temperature of the cancerous region in the range 41 — 45 °C. Various techniques are employed to produce hyperthermia in the cancerous tumor region. These techniques include microwave, RF and ultrasound techniques. Microwave radiation has the potential to heat the cancerous tumor (near spherical- or irregular-shaped tumor) effectively due to the fact that it can easily be focussed within the tumor region. External microwave applicators are designed and optimized to non-invasively couple the electromagnetic energy through human skin to kill cancerous cells. Effectiveness of hyperthermia is dependent on the type of applicator and its size, specific absorption rate (SAR) (which is a function of microwave power, frequency, wave polarization, dielectric properties and density of tumor), duration of treatment and blood flow rate. Various forms of waveguide/horns and planar applicators are available in the literature for microwave hyperthermia. At microwave frequencies penetration depth (PD) is generally shallow. Researchers are continuing to refine the existing waveguide/horn/planar antenna applicators and devise better applicators so that these systems can provide enhanced heating depth in the medium to treat tumors at greater depth. Focussing of electromagnetic energy at the tumor site is easily possible through high resolution exposure at microwave frequencies. This thesis deals with simulation, experimental and/or theoretical studies of efficient, practical, non-invasive, direct-contact horn antennas/conformal antenna (designed at 2450 and/or 915 MHz) in direct contact/close proximity with bio- medium/bio-media which can provide almost uniform power-absorption at greater depth in bio-medium/bio-media, have the focusing ability and/or remain compatible with the curved portion of the human body. These antennas include water-loaded conventional metal diagonal horn, water-loaded improved metal diagonal horn, water-loaded metal-dielectric wall diagonal horn (MDWDH) terminated in a bio-medium/bio-media along with a novel conformal and modified microstrip slot antenna integrated with a novel and compact artificial magnetic 1 Abstract conductor (AMC) in close proximity with a bio-medium/bio-media. All the proposed diagonal horn applicators provide circularly symmetric SAR distributions in the transverse plane to effectively heat spherical or near spherical tumors. Furthermore, a new conformal hyperthermia applicator integrated with a novel AMC is proposed. The person/operator operating this kind of proposed conformal and compact hyperthermia applicator will not get exposed to hazardous level of microwave radiation due to highly suppressed back field response/high front-to-back ratio of the applicator. The simulation/experimental/theoretical investigations of the fields, SAR and temperature distributions in a bio-medium/bio-media owing to water-loaded diagonal horns/conformal antenna applicators carried out during the course of the research work are described in six chapters.