Experimental study of water/tissue conductivity variation under 1 MHz HIFU radiation

Abstract High Intensity Focused Ultrasound (HIFU) has long demonstrated its effectiveness and advantage on tumor treatment, drug delivery, and some other therapies. In order to efficiently and accurately control HIFU, HIFU monitoring technology became critical. B-type scan or MRI are the two kinds of effective ways to monitor HIFU. While B-type scan has to interleave with HIFU to avoid white noise on the B-scan images, the MRI is expensive and bulky, not an ideal way to monitor HIFU either. This paper presents research to explore the possibility to monitor HIFU via impedance/resistance property. The theoretical analysis, the experimental setup and results with water phantom and two kinds of tissue holders are presented. The experimental results show that the resistance value changes instantaneously at the moment the HIFU is turned on/off and the magnitude of the resistance change is proportional to the square root of HIFU power, which indicates that the resistance property could be an effective way to monitor the existence and the power level of HIFU. The resistance is sensitive to the temperature variation and the formation of lesions as well. More research with ex vivo and in vivo experiments at different HIFU frequencies and power levels are strongly suggested to investigate the detailed mechanisms of the resistance change during HIFU and hence further clearly specify the way to monitor HIFU via resistance measurement.