Quantitative Thermal Imaging for Plasmonic Photothermal Therapy

Continuous-wave (CW) laser-induced thermotherapy (LIT) can heat tumors to above 42 C and lead to tumor destruction. During LIT, antibody-conjugated gold nanorods (AuNRs) can bind to tumor tissue and improve therapeutic efficacy when implementing plasmonic photothermal therapy (PPTT). This study uses a noninvasive photoacoustic (PA) technique to monitor temperature during PPTT. A pulsed laser is used to generate the PA signal. AuNRs are used to further increase the PA-signal contrast during PA imaging. Ultrasound (US) is combined with PA imaging for anatomical imaging. The bimodal system of US and PA imaging is superior to conventional systems at locating tumors and recognizing nanoparticles, and facilitates the selection of an optimal treatment plan. This technique allows temperature to be monitored in real-time during therapy, thereby ensuring both safety and efficacy. The technique is based on the fact that the Gruneisen parameter (which determines the acoustic pressure amplitude) is linearly related to the temperature of soft tissue. The results of a phantom study show that the temperature and the thermal mapping of the laser-irradiated area can be exactly measured and drawn quantitatively. Under the ANSI Z136.1 safety guidelines, inducing thermotherapy without AuNRs is difficult with low laser power density. However, by using specific antibody-conjugated AuNRs, targeted thermotherapy can be implemented selectively. An in vivo study is performed to confirm the feasibility of the proposed technique. A quantitative thermal image of the laser-irradiated tumor shows that the temperature is above 45 C, indicating the induction of thermal therapy. The tumor after thermotherapy is examined histologically. The tumor architecture is destroyed whereas the peripheral and intratumor noncarcinoma cells are preserved despite also being irradiated by the CW laser. This indicates that antibody-conjugated AuNRs can facilitate targeted thermotherapy to selectively kill tumor cells. Both phantom and in vivo imaging results demonstrate that real-time quantitative thermal imaging is feasible for AuNR-assisted PPTT.