Design of power monitor for power measurement in HIFU systems

High-intensity focused ultrasound (HIFU) has recently won wide recognition as a noninvasive treatment for many clinical applications. HIFU can ablate tissue at predetermined spot through accurate and stable output of acoustic power. Real-time power measurement is required for output characterization of driving electronics in most clinical HIFU systems. The measured power is also used to obtain voltage standing wave ratio (VSWR), which can reflect the change of transducer impedance induced by aging or temperature drift. However power measurement is still lacked in numerous pre-clinical or lab-level HIFU systems due to the cost and/or the complexity of integrated power meter. Additionally few work about power measurement have been reported. In this study, a power monitor is designed and constructed to be integrated into HIFU systems for power measurement. The developed power monitor is composed of a dual directional coupler, a power detector and two analog-to-digital converters. It is calibrated by the power meter at four frequencies (0.1, 0.5, 1 and 2 MHz) using linear least-squares fitting, and the measurement error is assessed for different powers at the previous four frequencies after calibration. Both functions of power measurement and VSWR detection are test through two transducers with and without impedance matching networks (IMNs). The post-calibration error is at most 10% with the power lower than 15 W whereas it is reduced to 2% after increasing power. The test results demonstrate that the developed power monitor is able to measure the power delivered to the transducer during sonications. Besides, it is an independent but cost-effective module for VSWR detection in the design of IMNs.