Nonlinear Thermoacoustic Imaging Based on Temperature-Dependent Thermoelastic Response

In this paper, a novel nonlinear thermoacoustic imaging (NTAI) system is developed based on the temperature-dependent thermoelastic response under microwave irradiation. Specifically, we consider the high-pulse repetition frequency (HPRF) microwave regime, where the tissue temperature increases after microwave irradiation. In this circumstance, the temperature-dependent thermodynamic parameters of the tissue change; thus, the influence of absorbed microwave energy on the thermoacoustic (TA) amplitude is reflected in the change of thermodynamic parameters. Hence, this temperature dependence yields a nonlinear relationship between the TA amplitude and the absorbed microwave energy in the HPRF regime. In this paper, we obtain the nonlinear coefficient as a new parameter to distinguish biological tissues by performing differential measurements in the NTAI system. We further experimentally extract the nonlinear coefficient in phantom and ex vivo mouse, revealing the feasibility of applications in biological tissues identification with high contrast.