  Fluorescent imaging in the second near infrared window (NIR II, 1-1.4 {\mu}m)
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 post-injection, 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 post-injection.
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, ICG) 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.
