Effect of complex bolus-tissue load configurations on SAR distributions from dual concentric conductor applicators

Power deposition [specific absorption rate (SAR)] distributions from a two element array configuration of 4-cm-square 915-MHz dual concentric conducto r (DCC) microwave antennas were characterized theoretically for several cli nically realistic complex bolus-tissue load models using the finite differe nce time domain (FDTD) numerical method. The purpose of this effort was to determine the perturbing effects on SAR of three often unavoidable heteroge neities in the bolus-tissue load, The three cases studied in this work cons ist of bone (two ribs spaced 1 cm apart) embedded 5-mm or I-cm deep in musc le or layered fat-muscle tissue, small air bubbles trapped between the coup ling bolus and tissue surface, and variable thickness water bolus layer due to sharply contoured anatomy. Results of the FDTD simulations demonstrate rather small effects on SBR distribution for both rib-sized bones greater t han or equal to 5-mm deep in muscle and small air pockets less than or equa l to 1-mm thick. Larger air bubbles >1-cm diameter by 3-mm depth showed a d istinct concentration of SAR near the lateral sides of the air bubbles, and a blocking effect under the bubbles when located directly under the center of a DCC aperture where there is a higher normal E-field component. Variat ion from 2.5- to 7.5-mm bolus thickness under the two aperture array produc ed only minor perturbation of the uniformity and penetration of SAR, along with minor reduction in SAR under the thicker bolus which should be accommo dated sufficiently by changes in applied power to the array elements.