Real-time optoacoustic monitoring of temperature in tissues

There is a need to monitor tissue temperature and optical properties during thermotherapy in real time in order to control the boundaries of hyperthermia or coagulation of diseased tissues and minimize the damage to surrounding normal tissues. We propose to use optoacoustic technique for monitoring of tissue temperature. Efficiency of thermoacoustic excitation in water is dependent on temperature. Therefore, the optoacoustic technique may be utilized to monitor tissue temperature if the efficiency of thermoacoustic excitation in tissue is temperature-dependent. We performed experiments on real-time optoacoustic monitoring of temperature in freshly excised canine tissues (liver and myocardium) and aqueous solution during conductive heating. Laser-induced optoacoustic pressure signals were recorded from tissues and the aqueous solution during heating with the use of sensitive wide-band acoustic transducers. Fundamental harmonic of Q-switched Nd:YAG lasers were used for pressure wave generation. Amplitude of the optoacoustic pressure induced in tissues increased linearly with temperature. Good agreement was obtained between the experimental data and theory for the aqueous solution. We demonstrated that laser optoacoustic technique is capable of measuring 1-2 degrees C temperature change in tissue and at the distance of up to several centimeters between the investigated tissue volume and the transducer.