Recently, additional near-infrared (NIR) optical windows beyond the conventional first therapeutic window have
been utilized for deep tissue imaging through scattering media. Biomedical applications using a second optical
window (1100 to 1300 nm) and a third (1600 to 1870 nm) are emerging. A fourth window (2100 to 2300 nm) has
been largely ignored due to high water absorption and a lack of high sensitivity imaging detectors and ultrafast laser
sources. In this study, optical properties of bone in this fourth NIR optical window, were investigated. Results were
compared to those seen at the first, second and third windows, and are consistent with our previous work on
malignant and benign breast and prostate tissues. Bone and malignant tissues showed highest uptake in the third and
fourth windows. As collagen is a major chromophore with prominent spectral peaks between 2100 and 2300 nm, it
may be that the fourth optical window is particularly useful for studying tissues with a higher collagen content, such
as bone or malignant tumors.
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