Fig. 4. Ex vivo spectroscopic PA imaging for needle guidance with nanoparticle injection into chicken breast.

a Laser sequence and timing diagram for the following three operations: needle insertion into the chicken breast, GNR injection, and needle pullout. For each operation, the laser pulse sequence was designed to scan with a fixed wavelength of 775 nm, followed by 10 cycles sweeping over 10 wavelengths. b PAUS imaging results including needle insertion into chicken breast (top row), GNR injection (center row), and needle pullout (bottom row). Four simultaneous but different imaging modalities are compared, including: B-mode (left column), wavelength-compounded Σλ-PA (second column), GNR-weighted PA (third column), and needle-weighted PA (fourth column). GNR and needle-weighted PA images are produced by the product of Σλ-PA and measured spectra correlated with individual reference spectra. The reference spectrum of GNR was measured by UV–VIS whereas that of the needle was determined by PA measurements in de-ionized water (Supplementary Note 5). The horizontal and vertical axes represent lateral and axial dimensions in units of mm, respectively.