Frequency dependence of the acoustic field generated from a spherical cavity transducer with open ends

Faqi Li,Min He,Dan Song,Zhibiao Wang,Junru Wu,Guangrong Lei,Deping Zeng,Dong Zhang,Zhou Lin

doi:10.1063/1.4938523

Abstract

Resolution of high intensity focused ultrasound (HIFU) focusing is limited by the wave diffraction. We have developed a spherical cavity transducer with two open ends to improve the focusing precision without sacrificing the acoustic intensity (App Phys Lett 2013; 102: 204102). This work aims to theoretically and experimentally investigate the frequency dependence of the acoustic field generated from the spherical cavity transducer with two open ends. The device emits high intensity ultrasound at the frequency ranging from 420 to 470 kHz, and the acoustic field is measured by a fiber optic probe hydrophone. The measured results shows that the spherical cavity transducer provides high acoustic intensity for HIFU treatment only in its resonant modes, and a series of resonant frequencies can be choosen. Furthermore, a finite element model is developed to discuss the frequency dependence of the acoustic field. The numerical simulations coincide well with the measured results.

Highlights

High intensity focused ultrasound (HIFU) generated by an extracorporeal focusing ultrasound (US) transducer has been used clinically to treat various solid tumors seated inside a patient’s body for years.[1,2,3,4] The principal physical mechanism is that acoustic energy generated by the transducer can be strongly focused to a small volume of ellipsoidal shape inside a human body and absorbed by the tissue within the volume and turned to be thermal energy
This work aims to theoretically and experimentally investigate the frequency dependence of the acoustic field generated from the spherical cavity transducer with two open ends
The measured results shows that the spherical cavity transducer provides high acoustic intensity for high intensity focused ultrasound (HIFU) treatment only in its resonant modes, and a series of resonant frequencies can be choosen

Summary

Introduction

High intensity focused ultrasound (HIFU) generated by an extracorporeal focusing ultrasound (US) transducer has been used clinically to treat various solid tumors seated inside a patient’s body for years.[1,2,3,4] The principal physical mechanism is that acoustic energy generated by the transducer can be strongly focused to a small volume of ellipsoidal shape inside a human body and absorbed by the tissue within the volume and turned to be thermal energy. The focusing resolution is limited by the smallest axial and lateral dimensions of HIFU focusing region, which is about the order of wavelength according to the wave theory, i.e., the higher the frequency (the smaller the wavelength), the better the resolution. The acoustic absorption coefficient of soft-tissue increases with frequency, the higher the frequency, the shorter depth the ultrasound can penetrate inside the body. As compromise between the US penetration depth and resolution of the focusing, the frequency of HIFU in current use is between 0.5-3.5 MHz, the

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