In this study, we investigated the feasibility of using 3.5-Fr circular phased array IVUS catheters for minimally invasive, image-guided hyperthermia treatment of tumors in the brain. Feasibility was demonstrated in two ways: 1) by fitting a 3.5-Fr IVUS catheter through burr holes, for 20 MHz imaging, and 2) by testing a modified circular array for therapy potential with 18.5-MHz and 9.0 MHz CW excitation. The imaging transducer's performance was compared to our previous 9 MHz mechanical IVUS prototype. The therapy transducer was driven by CW electrical power, achieving temperature changes reaching +8°C at a depth of 2 mm in human glioblastoma tissue grown on the flank of a mouse. Further hyperthermia trials showed that resistive heating of the transducer was minimal producing a +2°C increase with CW excitation.