Photoacoustic and ultrasound imaging to guide photothermal therapy: Ex vivo study

In photothermal therapy, a localized temperature increase is achieved by using a continuous wave laser and optically tuned metal nanoparticles. However, the successful outcome of therapy depends on identifying the presence of nanoparticles in the tumor before therapy and monitoring temperature rise during the photothermal procedure. In this paper, we investigate the utility of photoacoustic and ultrasound imaging to guide photothermal therapy. Differences in the optical properties of tissue, enhanced by the presence of nanoparticles, provide a contrast for photoacoustic imaging. Thus, an uptake of nanoparticles in the tumor can be detected by monitoring a photoacoustic image over time. A temperature rise causes the photoacoustic signal amplitude to increase. In addition, a temperature change also leads to time shifts in an ultrasound signal, primarily due to the change in speed of sound. Therefore, by measuring the change in the photoacoustic signal, and differential motion of ultrasound speckle, the temperature rise during photothermal therapy can be computed. Combined imaging was performed with a tunable pulsed laser and an array-based ultrasound transducer. Experiments were carried out on ex-vivo animal tissue injected with composite and broadly absorbing gold nanoparticles. The photoacoustic imaging identified the presence of nanoparticles in tissue. In addition, a localized temperature increase, obtained during therapy, was monitored using photoacoustic and ultrasound imaging. The temperature profiles, obtained by both imaging techniques, were spatially and temporally co-registered. Therefore, the experimental results suggest that photoacoustic and ultrasound imaging can be used to guide and monitor photothermal therapy.