A radiation force balance calibrator for calibrating a high intensity focused ultrasound transducer, the radiation force balance calibrator comprising: - a balance frame; - a force sensor for measuring force, wherein the force sensor is attached to the balance frame; - a balance with first and second ends and comprising a pivot, wherein the first end is attached to the pivot for enabling the balance to pivot relative to the balance frame, wherein the second end is adapted for exerting force onto the force sensor; - a calibration weight holder attached to the second end; - an oil target phantom adapted to absorb the ultrasound radiation (112, 114) radiated from the high intensity focused ultrasound transducer, wherein the oil target phantom is connected to the balance such that the absorption of ultrasonic radiation by the oil target phantom causes a reduction in the force exerted by the balance on the force sensor; and wherein the radiation force balance calibrator is constructed of non-magnetic materials, wherein the radiation force balance calibrator further comprises a processor and a memory (426, 428), wherein the memory contains instructions (446, 448, 450, 452, 454) for execution by the processor, wherein execution of the instructions causes the processor to perform the steps of: i. computing an estimated force on the force sensor due to the absorbing by the oil target phantom, of the ultrasound radiation (112, 114) radiated from the high intensity focused ultrasound transducer, on the basis of activation signals used to activate the high intensity focused ultrasound transducer to emit ultrasound radiation and of model parameters that are descriptive for the relationship between the activation signals and the output power of the transducer and the transfer of the output power to the force exerted by the balance on the force sensor, ii. activating the high intensity focused ultrasound transducer and measure the force exerted by the balance on the force sensor, iii. computing the difference between the measured force and the estimated force iv. updating the model parameters on the basis of the computed difference and iterating steps i. through iv. until the difference is less than a pre-determined threshold. - a balance frame; - a force sensor for measuring force, wherein the force sensor is attached to the balance frame; - a balance with first and second ends and comprising a pivot, wherein the first end is attached to the pivot for enabling the balance to pivot relative to the balance frame, wherein the second end is adapted for exerting force onto the force sensor; - a calibration weight holder attached to the second end; - an oil target phantom adapted to absorb the ultrasound radiation (112, 114) radiated from the high intensity focused ultrasound transducer, wherein the oil target phantom is connected to the balance such that the absorption of ultrasonic radiation by the oil target phantom causes a reduction in the force exerted by the balance on the force sensor; and i. computing an estimated force on the force sensor due to the absorbing by the oil target phantom, of the ultrasound radiation (112, 114) radiated from the high intensity focused ultrasound transducer, on the basis of activation signals used to activate the high intensity focused ultrasound transducer to emit ultrasound radiation and of model parameters that are descriptive for the relationship between the activation signals and the output power of the transducer and the transfer of the output power to the force exerted by the balance on the force sensor, ii. activating the high intensity focused ultrasound transducer and measure the force exerted by the balance on the force sensor, iii. computing the difference between the measured force and the estimated force iv. updating the model parameters on the basis of the computed difference and iterating steps i. through iv. until the difference is less than a pre-determined threshold.