| Total acoustic output power is a key parameter for high intensity focused ultrasound (HIFU) systems. Typically, HIFU systems generate ultrasound fields that are very intense and strongly focused: these features introduce problems when measuring output power. This article begins with the summery of various methods of acoustic power measurement, including radiation force balance, the method of reciprocity, the calorimetric method, the acousto-optic diffraction method, the method using calibrated hydrophone to scan the acoustic field, the holography method. The principle, characteristics and applications of different ultrasound power measurement methods were compared. However it can be difficult or inaccurate to apply many of the measurement methods to HIFU fields, either due to physical principle limits or to practical measurement problems. HIFU transducer output power was measured using radiation force balance method with absorbing target based on national standard. The results show that acoustic power has good linearity with the drive voltage squared of focusing transducer. Measurement error and problems were analyzed: nonlinear effects of the focused ultrasound field on the target, such as streaming and cavitation; water heating even explosive vapour in focus of HIFU field; force dependence on field geometry and target distance to the transducer; shielding by bubbles; limited accurate of balance in practical operation. According the practices measurement, also give some reasonable suggestion on using radiation force balance method to calibrate HIFU output power.A novel design of power measuring system based on calorimetric method for HIFU transducer was introduced. The system using parallelled copper constantan thermocouple as temperature sensor. The principle and uncertainty of parallelled thermocouple for mean temperature measurement was analyzed. The basic principle, the practical application, and the error of the HIFU output power measurement system were discussed. According to the characteristic of focused ultrasound field, the system improved the traditional calorimetric ultrasound power measurement setup. Small container, small acoustic window,cone shaped bottom, oil filling/discharge pipe, spatial average thermocouple placement were used in the system. HIFU transducer output power was measured with the system, and the result was compared with radiation force balance. The power measured by the system has good linearity with the drive voltage squared of HIFU transducer, and similar to the result measured by the radiation force balance method, and the measurement uncertainty less than±6.5%in the power range of 87-399W. Finally proposes some improvements of the system. The method offers several important advantages, such as simplicity, economy, practicality and its good accuracy and steadiness. It is suitable for measuring HIFU output power.The volume of the lesion created by conventional single-frequency HIFU is small, which lead to long treatment duration. A confocal dual-frequency HIFU technique was introduced. The lesions induced by confocal dual-frequency HIFU in optically transparent tissue mimicking phantom were investigated and compared with the lesions induced by conventional single-frequency HIFU. The results have shown that using different exposure time resulted in lesions of different sizes in both dual-frequency and single-frequency HIFU modes at the same acoustic power, but the dimensions of lesions in dual-frequency mode were significantly larger than those in single-frequency mode. Difference frequency acoustic field exist in the confocal region of dual-frequency HIFU may be the reason for the lesions dimensions enlargement. The dual-frequency HIFU mode may represents a new technique of improving the ablation efficiency of HIFU. |
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