Improved Spectral Inversion of Blood Oxygenation due to Reduced Tissue Scattering: Towards NIR-II Photoacoustic Imaging

Abstract

Significance: Conventional spectral photoacoustic imaging (sPAI) to assess tissue oxygenation (sO₂) uses optical wavelengths in the first near infrared window (NIR-I). This limits the maximum imaging depth (~1 cm) due to high spectral coloring of biological tissues. Aim: Second near infrared or short-wave infrared (NIR-II or SWIR) wavelengths (950-1400 nm) show potential for deep tissue sPAI due to the exponentially reduced tissue scattering and higher maximum exposure threshold (MPE) in this wavelength range. However, to date, a systematic assessment of NIR-II wavelengths for sPAI of tissue sO₂ has yet to be performed. Approach: The NIR-II PA spectra of oxygenated and deoxygenated hemoglobin was first characterized using a phantom. Optimal wavelengths to minimize spectral coloring were identified. The resulting NIR-II PA imaging methods were then validated in vivo by measuring renal sO₂ in adult female rats. Results: sPAI of whole blood under a phantom and of circulating renal blood in vivo, demonstrated PA spectra proportional to wavelength-dependent optical absorption. NIR-II wavelengths had a ~50% decrease in error of spectrally unmixed blood sO₂ compared to conventional NIR-I wavelengths. In vivo measurements of renal sO₂ validated these findings and demonstrated a ~30% decrease in error of estimated renal sO₂ when using NIR-II wavelengths for spectral unmixing in comparison to NIR-I wavelengths. Conclusions: sPAI using NIR-II wavelengths improved the accuracy of tissue sO₂ measurements. This is likely due to the overall reduced spectral coloring in this wavelength range. Combined with the increased safe skin exposure fluence limits in this wavelength range, demonstrate the potential to use NIR-II wavelengths for quantitative sPAI of sO₂ from deep heterogeneous tissues.