Fig. 1

a Experimental setup. The HIFU transducer was driven by the RF amplifier. The input voltage to the transducer was monitored by an oscilloscope. Two function generators were used to send a certain number of pulses or a certain duration of continuous wave RF signals to the amplifier. b The output pressure of the transducer as function of input voltage. For applied voltages of 1–21 V, the transducer was calibrated by a hydrophone. For higher input voltages, the pressures were calculated by a modified KZK equation [17]