Research And Development Of On-line Power Monitor For Driving Circuit Of High-intensity Focused Ultrasound

As a noninvasive treatment,High-intensity focused ultrasound(HIFU)has been widely recognized in the field of tumor thermal ablation.Ultrasound can be focused deep in the body,inducing local temperature elevation and ablating target tissue through accurate control of output acoustical power.Acoustical power is determined by the electrical power and electro-to-acoustic efficiency of transducer,so the on-line monitor of electrical power can indirectly monitor the acoustical power and provide output closed-loop feedback to the HIFU driving system to ensure the safety and effectiveness of HIFU thermal ablation.On the other hand,the change of transducer impedance,inducing by the temperature drift or aging,leads impedance mismatching between the output impedance and transducer,the electrical power can’t be fully loaded to transducer,thereby influence the defined acoustical power and finally affect the target ablation.Therefore,on-line power monitor of driving circuit is an indispensable part of HIFU system.At present,the power of driving circuit is monitored by the voltage and current of transducer,commercial power meter or the current of driving circuit power,etc.Among these methods,the driving power can be directly obtained by the first two methods,but the measuring circuit is difficult to be integrated into exiting HIFU system and limited to be used in the preclinical and laboratory HIFU system,the power of driving source can’t truly reflect the power of transducer.This paper presents an on-line power monitoring scheme based on directional and power detector,which can be integrated in to exiting HIFU system.The directivity of directional coupler can separate the forward and reflected signal in the driving circuit,and the power detector can measure the forward and reflected power in real time.At the same time,the standing wave ratio(VSWR)of the driving circuit can be calculated by the forward and reflected power,to evaluate the impedance change of transducer.This paper firstly analyzes the feasibility of the on-line power monitoring scheme based on the directional coupler and power detector,then designs and develops the power monitor of driving circuit,including power coupling unit,power detection units,power data acquisition unit,power data transmission unit and power data display unit.According to the design requirements of each unit,the electronic devices were selected,the circuit was schematic design and the software of power monitor was developed.Further more,the function of the power monitor was tested,the Bird4421 power meter was used to calibrate the power monitor at two frequencies,1MHz and 1.4MHz,and power range of 3W to 30 W.After the calibration,the power monitor was used to measure the forward power and reflected power of several transducers compared with the results of power meter,and the VSWR of the transducers before and after impedance matching between 1.32 MHz and 1.42 MHz.The relative error of measured result is less than 10% compared to the power meter,the VSWR decreases obviously after impedance matching and reaches minimum at 1.38 MHz,which is consistent with the actual impedance.In summary,the power monitor meets the on-line power monitoring demands of driving circuit of HIFU system,and can reflect the change of transducer impedance.