The light absorption is due to oxy- and deoxyhemoglobin, melanin, lipid, and other compounds found in living tissue2, 3, 4. These compounds cause tissue a u t o f l u o rescence throughout the visible spectral range up to approximately 700 nm5, 6. Because the absorption coefficient of tissue is considerably smaller in the near infrared region (700 nm-900 nm), light can penetrate more deeply into the tissues to depths of several centimeters3, 7, 8.
A key to enabling optical imaging has been the development of suitable NIR fluorochromes with high molar extinction coefficients, good quantum yields and low non-specific tissue binding. There are several commercially available candidate fluorochromes which can be used for optical imaging, including I R D ye 800CW, IRDye 680, IRDye 700DX, Cy5.5, and Alexa Fluor 750. Quantum dots have been used; however, their size often precludes efficient clearance from the circulatory and renal systems and there are questions about their long-term toxicity4.
A number of studies have been published using NIR
dyes. Two of the dyes commonly used for optical
imaging are Cy5.5 and IRDye 800CW. Cy5.5 has
been used in the past primarily due to the lack of
other candidate dyes suitable for imaging. Cy5.5 has
excitation/emission maxima at 675 nm/694 nm,
making it a borderline candidate labeling agent1, 3.
In contrast, a recently developed fluoro c h rome,
IRDye 800CW, has its excitation/emission maxima
at 785 nm/810 nm precisely centered in the region
known to give optimal signal to background in optical
imaging (see Figure 1)