基于自易校准方法的中心开孔球面换能器电声特性研究

Introduction:

In clinical practice, the monitoring of ultrasound imaging is often required during HIFU treatment. The spherical transducer with a central aperture (spherical annular transducer) is a key component in ultrasound ablation therapy that accommodates imaging probes. Therefore, its radiation acoustic power is closely related to treatment efficacy and human safety, and is a parameter that requires strict control.

In acoustic power research, the recommended method by IEC is the Radiation Force Balance (RFB) method, which was originally used for measuring the average acoustic power of flat sound fields. However, this method is not sensitive enough to sudden burst signals that may be used in HIFU treatment. Additionally, the absorption of the target can convert high-intensity acoustic energy into thermal energy, affecting the accuracy of the measurement and even causing permanent thermal and mechanical damage in long-term, high-power measurements. The Water Calorimetry method can obtain the acoustic pressure and intensity distribution information while scanning the sound field, but it is not widely used in the HIFU acoustic power measurement field due to the high energy at the focal point, which is likely to cause damage to the water calorimeter. Furthermore, both methods require expensive specialized measurement equipment and specific measurement environments.

In order to achieve more accurate and simpler measurement devices with minimal measurement errors, it is necessary to study the electroacoustic characteristics of the transducer. The self-calibration method was first proposed by Cartensen in 1947 and has long been used as an absolute calibration method for transducer calibration in ocean acoustics. With the improvement of acoustic field theory, the self-calibration technique has been applied to both planar piston and spherical transducers, and can be used for calibration of the water calorimeter, as well as for measuring electroacoustic parameters such as acoustic power. Based on this, the international standard for self-calibration of spherical transducers was established, which further extended the application of this method to the measurement of electroacoustic parameters such as acoustic power of cylindrical focusing transducers, proving the scientific validity of this method.

In this paper, based on the reciprocity principle of the spherical wave sound field, we systematically studied the electroacoustic characteristics of the central aperture spherical transducer. We provide the calculation methods and data tables for diffraction correction coefficients and average reflection coefficients, thus realizing the application of the reciprocity principle in the central aperture spherical transducer and expanding the self-calibration theory of the transducer. Based on this, we established a method for self-calibration of the central aperture spherical transducer using reflection signals from a planar reflector. The results show that the self-calibration method has better stability and anti-interference ability than the RFB method, and is more suitable for measuring mW-level acoustic power. This method can be used to measure five electroacoustic parameters, including output acoustic power, radiation conductance, electroacoustic conversion efficiency, free-field current response, and free-field voltage sensitivity. The feasibility and effectiveness of the method were verified using a transducer under experimental conditions.