Control of interstitial thermal coagulation: Comparative evaluation of microwave and ultrasound applicators

This study presents a comparative evaluation of the control of heating and thermal coagulation with microwave (MW) and ultrasound (US) interstitial ap plicators. Helical coil MW antennas (17 mm and 25 mm length radiating anten nae) were tested using an external implant catheter (2.2 mm o.d.) with wate r-cooling. US applicators with tubular transducers (2.2 and 2.5 mm o.d., 10 mm length, single-element and 3-element) were utilized with a direct-coupl ed configuration and internal water-cooling. Measurements of E-field distri butions (for MW) and acoustic beam distributions (for US) were used to char acterize the applicator energy output. Thermal performance was evaluated th rough multiple heating trials in vitro (bovine liver) and in vivo (porcine thigh muscle and liver) at varied levels of applied power (20-40 W for micr owave, 15-35 W for ultrasound) and heating times (0.5-5 min). Axial tempera ture distributions in the tissue were recorded during heating, and dimensio ns of the resulting lesions of thermal coagulation were measured. Both MW a nd US applicators produced large volumes of tissue coagulation ranging from 8 to 20 cm(3) with singular heating times of 5 min. Radial depth of lesion s for both MW and US applicators increased with heating duration and power levels, though US produced notably larger lesion diameters (30-42 mm for US vs 18-26 mm for MW. 5 min heating). Characteristic differences between the applicators were observed in axial energy distribution, tissue temperature s, and thermal lesion shapes. MW lesions increased significantly in axial d imensions (beyond the active applicator length) as applied power level and/ or heating duration was increased, and lesion shapes were generally not uni form. US provided greater control and uniformity of heating, with energy de position and axial extent of thermal lesions corresponding to the length of the active transducer(s). The improved ability to control the extent of th ermal coagulation demonstrated by the US applicators provides greater poten tial to target a specific region of tissue. (C) 2001 American Association o f Physicists in Medicine.