The temperature dependence of ultrasound-stimulated acoustic emission

Given the high variability of tissue properties during sonication, temperature monitoring is one of the most crucial components for accurate thermal treatment of tissues with focused ultrasound and other thermotherapy devices. Recently, the method of ultrasound-stimulated acoustic emission (USAE) has been introduced as a potential method for measurements of mechanical properties of tissues. In this paper, the dependence of USAE on tissue temperature is determined. Because USAE depends on the acoustic and mechanical properties, both of which vary with temperature, it is hypothesized that the USAE signal is also temperature-dependent and in such a way that it can be used to guide thermal therapy. In a series of experiments, ex vivo porcine muscle and fat samples were exposed to ultrasound at power levels that induce temperature elevation. In both tissue types, below the coagulation threshold, the USAE amplitude was found to vary linearly with temperature. However, at higher powers, the correlation with temperature was lost due mainly to the irreversible nature of the changes in the tissue properties. Theoretical simulations were used to interpret the USAE response change with temperature involving both reversible and irreversible changes and during both heating and cooling. These results indicate that USAE may have important promise as a potential method for localizing temperature elevation and, thus, thermal surgery monitoring, as well as detection of irreversible changes in tissues.