Optoacoustic signal amplitude and frequency spectrum analysis laser heated bovine liver ex vivo

Optoacoustic imaging is being investigated as a potential tool for monitoring the onset and progression of laser thermal therapy. In this study OA images were acquired from ex vivo bovine liver using a reverse-mode OA imaging system consisting of a pulsed laser operating at 775 nm, and an 8 element annular array ultrasound transducer. LTT was performed with an 810 nm laser at 4 W for five minutes. OA signals were acquired for two minutes prior to, five minutes during, and seven minutes post treatment at a rate of 2 Hz. Treatment induced effects were identified based on the OA signal amplitude in combination with spectral analysis of the OA radio frequency (RF) data. The OA signal amplitude was compared with the measured tissue temperatures. Spectrum analysis commonly performed on ultrasound backscatter RF data, which calculate the spectral midband fit, slope, and intercept of the data was used to quantify the changes in the photoacoustic RF signal. The spectral midband fit and intercept increased on average 11 dB and 10 dB respectively. The amplitude of the OA signals increased during treatment on average 350%. However, posttreatment, the response varied. The results of this study support our hypothesis that LTT causes detectable changes in the amplitude and frequency components of OA signals. Both of these parameters may provide independent information about tissue state. These results demonstrate the potential of OA detection for monitoring LTT.