NIR-II Photoacoustic Imaging

Overview

Photoacoustic imaging in the second near-infrared window (NIR-II, 1000–1700 nm) offers a compelling combination: optical absorption contrast at depths and resolutions typically associated with ultrasound. By operating in the NIR-II window, we access a regime where tissue scattering is reduced and water absorption windows allow deeper penetration than conventional NIR-I systems.

Technical Approach

Wavelength selection. We identified optimal excitation wavelengths within the NIR-II window by modeling the competition between reduced scattering (favoring longer wavelengths) and water absorption peaks (creating transmission windows around 1064 nm, 1300 nm, and 1550–1620 nm). Our system uses a tunable OPO source covering 1000–1700 nm.

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.

Contrast agent development. In collaboration with the chemistry group, we are developing NIR-II absorbing nanoparticles with engineered absorption spectra, enabling multi-wavelength photoacoustic spectroscopy for molecular imaging.

Current Status

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.