Photoacoustic thermometry for therapeutic hyperthermia

Local hyperthermia is widely studied as a treatment option for small tumors. This study investigates the feasibility of exploiting the photoacoustic (PA) effect to monitor the in situ temperature rise during high-intensity focused ultrasound (HIFU) exposures for therapeutic hyperthermia. Polyacrylamide phantoms with a cylindrical inclusion (3 × 20 mm) of graphite (0.01 g/ml) were heated using 30 s exposures from a 2 MHz HIFU transducer. The transducer focus was aligned to the tip of a wire thermocouple embedded in the inclusion. A 532 nm pulsed laser was used to illuminate the inclusion. A 15 MHz broadband transducer was employed as a passive receiver (PR) to detect the PA response, which was an ultrasonic pulse emanating from the inclusion due to thermoelastic expansion induced by optical absorption. The native temperature and PR signals were recorded before, during, and after HIFU exposure. Singular-value decomposition (SVD) was performed on the matrix consisting of the PR signals to extract temperature data. SVD-deduced, PA-based temperatures correlated well with the thermocouple measurements (RMS error<5°C). A temperature rise of 20°C corresponded to a 30% increase in PA amplitude. The PA temperature-measurement technique was able to track heating and cooling phases over a range of temperatures characteristic of HIFU-induced hyperthermia.