High-contrast spectroscopic photoacoustic characterization of thermal tissue ablation in the visible spectrum

PURPOSE: High-contrast tissue characterization of thermal ablation has been desired to evaluate therapeutic outcomes accurately. This paper presents a photoacoustic (PA) characterization of thermal tissue ablation in the visible spectrum, in which higher light absorbance can produce spectral contrast starker than in the near-infrared range. METHODS: Ex vivo experiments were performed to measure visible PA spectra (480–700 nm) from fresh porcine liver tissues that received a thermal dose in a range of cumulative equivalent minutes at 43°C (CEM(43)). The local hemoglobin lobe area between 510–600 nm and whole-spectral area under the curve were evaluated to represent the transition of hemoglobin into methemoglobin (MetHb) in the target tissue. RESULTS: The thermal process below an estimated therapeutic CEM(43) threshold (80–340 minutes) presented a progressive elevation of the PA spectrum and an eventual loss of local hemoglobin peaks in the visible spectrum, closer to the MetHb spectrum. Interestingly, an excessive CEM(43) produced a substantial drop in the PA spectrum. In the spectral analysis, the visible spectrum yielded 13.9–34.1 times higher PA sensitivity and 1.42 times higher contrast change than at a near-infrared wavelength. CONCLUSION: This novel method of PA tissue characterization in the visible spectrum could be a potential modality to evaluate various thermal therapeutic modalities at high-contrast resolution.