Sound field detection of high intensity focused ultrasound HIFU piezo

First, the sound pressure of the HIFU ceramic transducer is very large. When the sound pressure exceeds the cavitation threshold, cavitation occurs in the measurement medium . At this time, the local offer extremely high pressure and high-speed jet generated by the cavitation bubble explosion. The shock wave will damage the sensor; secondly, the sound intensity is very high, the minimum sound intensity is not less than 1000W/cm2, which is easy to cause the temperature of the measuring device to rise. When the temperature of the sensitive piezo material rises to the curie temperature, the sensitivity of the sensor drops sharply. thirdly, if the vibration phase of each point of the radiating surface of the transducer is not uniform, the sound intensity at the focus is not necessarily proportional to the square of the sound pressure; fourth, if the radiated sound wave reaches the finite amplitude wave range, the propagation process is easy. 

Nonlinear harmonics are generated and sound saturation occurs. All makes the measurement difficult. There are three main aspects: First, the dynamic range of the HIFU sound field is very large, and the sound intensity at the focus is much higher than that at the non-focus point, and the value changes are three or four orders of magnitude; second, under the high sound intensity conditions, The nonlinear factors such as cavitation have a great influence on the measurement. Thirdly, the fine measurement of the acoustic focal region requires high spatial resolution of the measuring device and the sensor. Therefore, the design requirements of the measuring device are high.Accurately describing the parameters of the 4Mhz HIFU ceramic transducer sound field under high sound pressure has become an important issue in the development of HIFU technology. The main parameters describing the HIFU sound field are sound power, sound intensity, sound pressure, and sound focal length. The detection of the HIFU sound field has not yet been a completely ideal method. Each method uses one aspect of the HIFU sound field to detect one or several parameters of the sound field. At present, radiation pressure method, hydrophone measurement method, fiber inspection and optical detection are the main detection methods of HIFU sound field.

Common methods for HIFU sound field detection:

The radiation method for radiation force detection is a well-established measurement method that focuses on measuring the plane ultrasonic power in a liquid. In 1987, under the condition of geometrical acoustics,piezo ceramics for HIFU are no-diffraction high-frequency limitation and far-field directivity as a rectangular function, the formula for calculating the radiation force is in the focused ultrasonic sound field was derived, The deviation at 30°is less than 0.8%. In 1998, which applied the geometrical acoustic method to derive the general formula of the radiation force of focused ultrasound on the test target,it is discussed the radiation force on the total reflection target and the total absorption target, and carried out experimental verification. The deviation between the 1.6MHz ultrasonic power measurement result and the calorimetric method is not more than 3%. In 2005, Shaw of the National Physical Laboratory of the United Kingdom is used integrals and experiments to verify the validity of the general formula. The results were quite consistent.Using the radiation based water column method (water measures the acoustic radiation pressure or acoustic energy density of the measuring point), the sound field intensity of the anode power voltage of the piezo ceramics for laser machine driving circuit from 200V to 2600V is measured. 

It is pointed out that the water column method can increase the sound field measurement range to 2000W/cm2, but when the voltage rises to a certain level (2600V), the sound power is too large, and a clear cavitation bubble group will be generated in the water, and the sound field is disturbed to make the measuring sound power obvious. Unstable showing a saturated trend. China has passed the national standards of acoustic high intensity focused ultrasound sound power and sound field characteristics measurement. high-intensity focused ultrasound (HIFU) treatment system industry standards is the basic ideas of focusing ultrasound characteristics and ultrasonic power measurement. The method of HIFU piezo crystal is using the three-dimensional scanning method of the hydrophone to find the position of the sound pressure , scanning and measuring the focused sound field, and calculating the geometric parameters of the focal spot, and then directly measuring the sound pressure at full power at the focus to derive the maximum sound intensity. Hydrophones and radiation methods are recommended for the standard.