Resonance effects in applicator water boluses and their influence on SAR distribution patterns

Spurious electromagnetic oscillations in the water layer of applicator water boluses are investigated. Two oscillation types--volume oscillations (VO) and surface standing wave oscillations (SWO)--are considered. Conditions leading to their excitation in water boluses, commonly used in clinical practice for local hyperthermia, are examined. It was found that the thickness d of the water layer is the main critical parameter which determines the possibility of spurious oscillation modes excitation. Analytical equations are derived, allowing calculations of critical d(c) and resonance d(r) values for a given water bolus, and determine the approximate structure of electrical field strength distribution at its radiating plane, taking account of spurious oscillations. Calculated d(c) and d(r) values are in satisfactory quantitative agreement with experimental data, whereas experimentally observed complicated SAR distribution perturbations correlate qualitatively with the calculations in terms of volume and/or surface standing wave oscillations excited within the water bolus.