Measurement of high intensity focused ultrasound sound field

Sonoluminescence refers to a weak emission of light that is accompanied by cavitation in a liquid. The distribution of the cavitation field can be understood by directly observing with the naked eye or by sensitizing the substrate. The sonoluminescence intensity of the cavitation field can also be measured by the method of photomultiplier tube. The principle of HIFU piezoceramics transducer enhancement is proposed. In the alkaline solution, the oxidation of the cavitation field generated by high-power ultrasound is used to obtain the chemical luminescence agent luminol enhancement spectrum, and its emission wavelength mainly falls in the visible light range of 400-700 nm. Between the light blue, the color film can be used for photographic imaging. During the compression phase of the acoustic cycle, the collapse and rupture of the cavitation bubble produces smaller bubbles that pulsate regularly over multiple sound cycles, converge into a spherical shape and move to the pressure valley. It is described by a mathematical model and can be directly observed by imaging. The above phenomenon was simulated and experimented with a 28 kHz acoustic reactor. The consistency between the imaging method and the theoretical simulation was proved, and the sound field shape and sound intensity were visually described.

The laser method uses a laser to accurately measure the vibration of an object. Since the vibration velocity of the aqueous medium is related to the sound pressure, the sound pressure of HIFU ceramic transducer in the water can be measured by the laser vibration measuring device. Since the laser beam can be focused to the order of micrometers, measurement using a laser can achieve a relatively high spatial resolution. By scanning the laser beam, a very fine spatial distribution of the ultrasonic field sound can be obtained, and the axial sound position distribution of the transmitting transducer can be adjusted by the measuring device, and the axial sound pressure distribution of the transmitting transducer can also be obtained. Make measurements of the sound field of the focused ultrasonic transducer was measured by this method, and satisfactory results were obtained. Due to the complexity of the HIFU sound field, there has not been a completely ideal method to fully detect the HIFU sound field. The improved radiation balance system and hydrophone have been successfully applied to the measurement of HIFU fields, with a sound intensity of 5 kW/cm2 and a sound power of 500 W. Acoustic high intensity focused ultrasound sound power and sound field characteristics are measured by the national standard. This standard has passed the IEC exchange and is widely spread abroad. It shows that China has reached the forefront of the world in HIFU sound field measurement and standardization research. 

High-intensity focused ultrasound (HIFU) sound field sound pressure is generally more than 20MPa, negative pressure may exceed 10MPa, accompanied by strong nonlinear effects, cavitation and sound flow, which greatly affect the accuracy of the measuring device and stability, whether it is radiation method, hydrophone measurement or fiber measurement,which must consider the bearing device's ability to withstand the HIFU sound field and the measurement .And the temperature rise caused by the HIFU sound field in an instant will also have a great influence on the sensitivity of the measuring device such as the sensor,which is  causing measurement deviation, and it is likely to have an interaction of temperature and sound pressure. If fiber optics is used, temperature and sound pressure will cause changes in the shape and refractive index of the fiber. Therefore, how to effectively separate the interaction between temperature and sound pressure is also a major issue in HIFU sound field detection. In HIFU sound field measurement, cavitation of piezo ceramics is also one of the factors limiting its measurement accuracy. When the sound pressure exceeds the cavitation threshold, the extremely high pressure, high-speed jet and shock wave generated by the cavitation bubble which will affect the measuring device.The degassed water alleviates the influence of cavitation on the measuring device to a certain extent, but when the power is increased to a certain extent, a large amount of cavitation bubbles will be formed in the measuring medium (degassed water), which will result in a larger measuring device and the result. In short, the detection of HIFU sound field is still one of the bottlenecks restricting the development of HIFU technology. Its safe and effective measurement depends on the research progress of cavitation and nonlinear propagation, and also depends on the optical fiber, sensor technology and its materials.