common method of HIFU for sound field testing

The radiation method is a well-established measurement method that focuses on measuring the plane piezoelectric ceramic shear plates in a liquid. In 1987, Beissner derived the calculation formula of the radiation force on the fully absorbed target in the focused ultrasonic sound field under the assumption that the geometrical acoustics, the non-diffractive high-frequency limitation and the far-field directivity are rectangular functions, When the angle≤ 30 ° deviation is less than 0. 8%. In 1998, he applied the geometrical acoustic method to derive the general formula of the radiation force of focused ultrasound on the test target, discussed the radiation force on the total reflection target and the total absorption target, and carried out experiment verified that the deviation of the sound power measured by the ultrasonic power measurement of 1.6 MHz and the calorimetry method was not more than 3%. In 2005, Shaw of the National Physical Laboratory of the United Kingdom used Rayleigh integrals and experiments to verify the validity of the general formula. The results were quite consistent. In 2003,he measured the anode power amplifier voltage of the HIFU transducer drive circuit by using the radiation-based water column method (water measuring the acoustic radiation pressure or acoustic energy density of the measurement point by measuring the height of the liquid column in the thin tube).

The sound field intensity at 200 V to 2600 V indicates that the water column method can increase the sound field measurement range to 2000 W / cm 2 , but when the voltage rises to a certain level (2600 V), the sound power is too large, and significant cavitation will occur in the water. The bubble group interferes with the sound field, making the measuring sound power significantly unstable and exhibits a saturation trend. In 2004 and 2005, China passed the national standard of “acoustic high-intensity focused ultrasound sound power and sound field characteristics measurement” and the “high-intensity focused ultrasound (H IFU) treatment system” industry standards, which focus on ultrasound characteristics and ultrasound. The basic idea and method of power measurement is to find the position of the sound pressure focus by the hydrophone three-dimensional scanning method, scan and measure the focused sound field, calculate the geometric parameters of the focal spot, and then piezoelectric ceramics plates transducer directly measure the sound pressure at full power at the focusing. Hydrophones and radiation methods are recommended for the standard.

2 hydrophone detection

A hydrophone is a transducer that converts an underwater sound pressure signal into an electrical signal. When the pressure (acoustic disturbance) on the piezoelectric material changes, the charge distribution inside the piezoelectric material changes proportionally and is reflected in the form of a voltage signal, so it can be extracted by the electrode on the surface of the piezoelectric ceramics element. These charges are amplified by a voltage amplifier or a charge amplifier, and the signal processing displays an image that reflects the waveform of the sound wave. Thus, the sound pressure measurement in the ultrasonic sound field is completed in a very straightforward manner. Traditional materials used for ultrasonic sound field detection are piezoelectric ceramics and PVDF (polyvinylidene fluoride).

Piezoelectric ceramics have high hardness and sensitivity, and can withstand a certain range of sound pressure in the HIFU field at low power, but the sound intensity When increased, the piezoelectric ceramic is easily broken, the linear dynamic range is small, and the acoustic impedance is high, so that the hydrophone has a certain interference to the measured sound field. The PVDF acoustic impedance is close to the acoustic impedance of water, and it has good acoustic impedance matching with water, soft texture, easy processing, stable chemical properties, wide frequency response and excellent linearity. The dynamic range is larger than that of piezoelectric ceramic hydrophones. Therefore, PVDF is currently generally used for measurement. It can improve the uneven frequency response produced by piezoelectric ceramics and reduce the interference to the measured sound field as long as the film is thin enough. PVDF is available in both film and needle types. The diameter of the film type is greater than 5 cm, while the diameter of the needle is less than 1 mm, which is easily damaged in the HIFU sound field.

The size of the HIFU focal region is about 1. 1 mm × 2. 1 mm × 3. 2 mm. Therefore, PVDF has the disadvantage of low spatial resolution, and it has edge effect, and the volume cannot be made very small. Temperature limitation, depolarization occurs when the temperature reaches 60 °C, the re-use rate is low, and the hydrophone measurement requires a mechanical method for point-by-point scanning, even if scanning 10 × 10 cm plane, the fastest It also takes a few hours, so it is using a few simple lines to describe the sound field distribution becomes inevitable. In 2002, he use high-frequency piezoelectric ceramic hollow spheres as hydrophones, and it has unique advantages in terms of geometry, size and sensitivity. The diameter of the ball is 0.7 to 1 mm, the resonance frequency is 1. 8 to 2. 7 MHz, the sensitivity is twice that of the hydrophone, and it has good stability ultrasonic transducer piezoceramic plate, and the pressure is needle-type hydrophone. the device is considered to be the ideal sensor for measuring high-intensity sound fields. 

In 2004, it is reported a new type of membrane water heater for HIFU sound field measurement, indicating that the sensor can measure sound power immediately in HIFU treatment, thus ensuring accurate delivery of energy during treatment, and radiation force measurement and water .Compared to the measurement of the receiver, the components are durable and have a small influence on temperature. In 2006, Zanelli and Howard designed a hydrophone that effectively avoids cavitation damage. The piezoelectric ceramic is placed in a metal shield to provide a smooth outer surface that minimizes the possibility of cavitation nucleus on the surface. Small, in the degassed deionized water, the sound field measurement of the transducer with a frequency of 1.50 MHz, a diameter of 100 mm and a focal length of 150 mm has achieved good results. However, the linear dynamic range of piezoelectric ceramics is insufficient, affecting its upper limit for use in HIFU measurements.