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Arrays of multielement ultrasound applicators for interstitial hyperthermia have been developed and tested both in vitro and in vivo. The system includes multielement applicators, a 64 channel RF driving unit, a power measuring unit, a 112 channel multisensor temperature measuring unit, and a water cooling unit. Ninety-five arrays of single-element and nine arrays of three-element ultrasound applicators were designed, built, and characterized by measuring transducer efficiency and ultrasound field distribution. Improved uniformity in the azimuthal direction was achieved by using multiple driving frequencies. In addition, production of ultrasound in a desired sector of the transducer was possible by selecting a suitable frequency. Both in vitro and in vivo experiments showed that 92% of monitored temperature points within the target volume of 30 mm×30 mm×35 mm achieved a therapeutic temperature rise (above 5°C) when an array of five three-element applicators were used. These results indicated that the arrays of multielement ultrasound applicators have distinct advantages over present interstitial hyperthermia modalities in terms of the capability to control the temperature distribution with a large catheter spacing. As a conclusion, the feasibility of a practical arrays of multielement ultrasound applicators for interstitial hyperthermia was demonstrated.