Photoacoustic imaging of rat kidney tissue oxygenation using second near-infrared wavelengths.

Significance: Conventionally, spectral photoacoustic imaging (sPAI) to assess tissue oxygenation ( ${\mathrm{sO}}_2$ ) uses optical wavelengths in the first near-infrared (NIR-I) window. This limits the maximum photoacoustic imaging depth due to the high spectral coloring of biological tissues and has been a major barrier to the clinical translation of the technique.

Aim: We demonstrate the second near-infrared (NIR-II) tissue optical window (950 to 1400 nm) for the assessment of blood and tissue ${\mathrm{sO}}_2$ .

Approach: The NIR-II PA spectra of oxygenated and deoxygenated hemoglobin were 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 kidney ${\mathrm{sO}}_2$ in adult female rats.

Results: sPAI of whole blood, in a phantom, and of blood in kidneys in vivo produced PA spectra proportional to wavelength-dependent optical absorption. Using the NIR-II wavelengths for spectral unmixing resulted in a $\sim 50\%$ decrease in the error of the estimated blood ${\mathrm{sO}}_2$ , compared with conventional NIR-I wavelengths. In vivo measurements of kidney ${\mathrm{sO}}_2$ validated these findings, with a similar 50% reduction in error when using NIR-II wavelengths versus NIR-I wavelengths at larger illumination depths.

Conclusions: sPAI using NIR-II wavelengths improved the accuracy of tissue ${\mathrm{sO}}_2$ measurements. This is likely due to reduced scattering, which reduces the attenuation and, therefore, the impact of spectral coloring in this wavelength range. Combined with the increased safe skin exposure fluence limits in this wavelength range, these results demonstrate the potential to use NIR-II wavelengths for quantitative sPAI of ${\mathrm{sO}}_2$ from deep heterogeneous tissues.