NIR-II Photoacoustic Imaging

Why NIR-II

The second near-infrared window (NIR-II, 1000–1700 nm) gives you optical absorption contrast at depths and resolutions usually reserved for ultrasound. Tissue scattering falls with wavelength and there are water-absorption windows that let light reach deeper than a conventional NIR-I system can, which is the whole reason to work out here rather than at shorter wavelengths.

The amplifier and the reconstruction

Wavelength selection. I picked excitation wavelengths by modeling the competition between reduced scattering (favoring longer wavelengths) and water-absorption peaks (which open transmission windows around 1064 nm, 1300 nm, and 1550–1620 nm). The source is a custom all-fiber stimulated Raman scattering amplifier seeded at 1064 nm: cascaded Raman shifting yields a set of discrete excitation lines across the NIR-II window rather than a single fixed wavelength.

Reconstruction algorithms. Standard delay-and-sum beamforming is supplemented with a model-based iterative reconstruction that accounts for acoustic heterogeneity and frequency-dependent attenuation. We incorporate the optical fluence distribution as a spatially varying sensitivity map.

Where it stands

The benchtop imaging system is operational and has demonstrated photoacoustic imaging at depths exceeding 4 cm in tissue-mimicking phantoms. We are currently characterizing the system’s spectroscopic capabilities using multi-wavelength excitation.