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Thermal ablation has been investigated as a treatment for a variety of cancers. Heat treatments have not gained large-scale clinical acceptance due to inconsistencies in controlling heat deposition in vivo and the lack of precise temperature measurement. Interstitial ultrasound provide a good method of controlling the radial depth of a thermal lesion and the applicator designs evaluated in this study allow for dynamic angular control of the shape of the lesion. A trisectored internally water-cooled applicator (TriAD) and a rotating catheter water-cooled applicator (RIUS) angularly controlled thermal dose to a target area. Both devices were small in diameter (1.8 mm-2.4 mm), making them clinically feasible for minimally invasive treatment in device size-sensitive tissues. A biothermal model accounting for changes in acoustic attenuation and perfusion as a function of thermal dose was used to evaluate and predict applicator performance. The MR susceptibility artifact of the applicators was examined with MR temperature imaging (MRTI) sequences at 1.5 T and 0.5 T. Ex vivo experiments in turkey and beef muscle with realtime MRTI correlated well with results from the biothermal model. These results display the feasibility of thermally treating tumors with controllable interstitial ultrasound applicators under real-time MRTI and bracket the applicators' predicted performance in vivo.