Deep-tissue anatomical imaging of mice using carbon nanotube fluorophores in the second near-infrared window

TLDR

Fluorescent imaging in the second near‑infrared window (NIR II, 1–1.4 µm) promises deep‑tissue visualization because of minimal autofluorescence and tissue scattering. Using well‑functionalized biocompatible single‑walled carbon nanotubes (SWNTs) as NIR II fluorophores, the authors performed high‑frame‑rate video imaging of mice during intravenous SWNT injection to trace the agents’ anatomical distribution. The study combined dynamic contrast‑enhanced imaging via principal component analysis, tissue‑phantom comparisons, and Monte Carlo simulations to show that NIR II SWNTs provide superior anatomical resolution and reduced scattering relative to NIR I dyes. Real‑time imaging revealed rapid SWNT circulation through lungs, kidneys, spleen, and liver; PCA enabled discrimination of deeper organs such as the pancreas and demonstrated that NIR II SWNTs maintain higher feature contrast than NIR I dyes, indicating that NIR II fluorescence with PCA offers powerful high‑resolution deep‑tissue imaging for biomedical research and diagnostics.

Abstract

Fluorescent imaging in the second near-infrared window (NIR II, 1–1.4 μm) holds much promise due to minimal autofluorescence and tissue scattering. Here, using well-functionalized biocompatible single-walled carbon nanotubes (SWNTs) as NIR II fluorescent imaging agents, we performed high-frame-rate video imaging of mice during intravenous injection of SWNTs and investigated the path of SWNTs through the mouse anatomy. We observed in real-time SWNT circulation through the lungs and kidneys several seconds postinjection, and spleen and liver at slightly later time points. Dynamic contrast-enhanced imaging through principal component analysis (PCA) was performed and found to greatly increase the anatomical resolution of organs as a function of time postinjection. Importantly, PCA was able to discriminate organs such as the pancreas, which could not be resolved from real-time raw images. Tissue phantom studies were performed to compare imaging in the NIR II region to the traditional NIR I biological transparency window (700–900 nm). Examination of the feature sizes of a common NIR I dye (indocyanine green) showed a more rapid loss of feature contrast and integrity with increasing feature depth as compared to SWNTs in the NIR II region. The effects of increased scattering in the NIR I versus NIR II region were confirmed by Monte Carlo simulation. In vivo fluorescence imaging in the NIR II region combined with PCA analysis may represent a powerful approach to high-resolution optical imaging through deep tissues, useful for a wide range of applications from biomedical research to disease diagnostics.