The invention provides for a medical instrument (100) comprising: a high intensity focused ultrasound system (122) for sonicating a target region (139) within a subject (118)； a light source (158) for exciting a temperature sensitive fluorescent dye, wherein the light source is configured for coupling to an optical fiber cable (148) of a urinary catheter (140)； a light source (160) for measuring fluorescence (190) emitted by the temperature sensitive fluorescent dye, wherein the light source is configured for coupling to the optical fiber cable； a memory (178) for storing machine executable instructions (180)； and a processor (174) for controlling the medical instrument. Execution of the machine executable instructions cause the processor to: receive (200) a sonication plan (188) descriptive of a sonication of the target region； measure (202) the fluorescence using the light sensor； calculate (204) a bladder temperature (192) using the fluorescence； and generate (206) sonication commands (194) using the sonication plan and the bladder temperature, wherein the sonication commands are adapted for controlling the high intensity focused ultrasound system to sonicate the target region.
