Ultra-Efficient Conversion of Microwave into Ultrasound Wave through a Split Ring Resonator

TL;DR

This paper introduces a split ring resonator that significantly enhances microwave-to-ultrasound conversion efficiency, enabling deep tissue imaging and wireless ultrasound emission with potential biomedical applications.

Contribution

The study presents a novel SRR design that vastly improves thermo-acoustic conversion efficiency and demonstrates its use as a wireless ultrasound emitter in tissue phantoms.

Findings

Over three orders of magnitude increase in conversion efficiency.

Effective deep tissue ultrasound emission without batteries.

Potential for precise deep tissue imaging and localization.

Abstract

Thermo-elastic conversion of electromagnetic wave into ultrasound wave has enabled diverse biomedical applications such as photoacoustic imaging. Microwave, which has ~10 cm long wavelength, can penetrate deeper into tissue than photons, heralding exciting applications such as deep tissue imaging via thermo-acoustic tomography. However, the thermo-acoustic conversion efficiency is very low even with an exogenous contrast agent such as carbon nanotube. Here, we break this low conversion limit through using a split ring resonator (SRR) to effectively collect and concentrate the microwave energy into a sub-millimeter hot spot and subsequently convert the energy into ultrasound wave. Our SRR achieves over three orders of magnitude higher thermo-acoustic conversion efficiency than commonly used thermo-acoustic contrast agents. We further harness the SRR as a wireless, battery-free ultrasound emitter placed under a breast phantom. These results promise exciting potential of SRR for precise thermo-acoustic localization and modulation of subjects in deep tissue.