Researchers used Fluorescence Imaging is widely used for visualizing biological tissues such as the back of the eye, where signs of macular degeneration can be detected. It is also commonly used to image blood vessels during reconstructive surgery, allowing surgeons to make sure the vessels are properly connected. For these procedures, such as imaging tumors, researchers use a portion of the light spectrum known as the near-infrared (NIR), 700 to 900 nanometers, just beyond what the human eye can detect.

A dye that fluoresces at this wavelength is administered to the body or tissue and then imaged using a specialized camera. Researchers have shown that light with wavelengths greater than 1,000 nanometers, known as short-wave infrared (SWIR), offers much clearer images than NIR, but there are no FDA-approved fluorescence dyes with peak emission in the SWIR range.

Researchers have taken a major step toward making SWIR imaging widely available. They have shown that an FDA-approved, commercially available dye now used for near-infrared imaging also works very well for short-wave infrared imaging. Imaging this dye with a camera that detects short-wave infrared light could allow doctors and researchers to obtain much better images of blood vessels and other body tissues for diagnosis and research.

The dye that the researchers used in this study, known as indocyanine green (ICG), fluoresces most strongly around 800 nanometers, which falls within the near-infrared range. When injected into the body, it travels through the bloodstream, making it ideal for angiography (the visualization of blood flowing through vessels). Some robot-assisted surgical systems have incorporated NIR fluorescence imaging to help visualize blood vessels and other anatomical features.

In the new study, the researchers were able to see several hundred micrometers into tissue using a regular fluorescence microscope. Normally, this depth can be reached only by two-photon microscopy, a much more complicated and expensive type of imaging.

A strong signal

In their study, the researchers further explored ICG and showed that it gives a stronger signal than other SWIR dyes now in development. Previous studies of ICG had focused on its emission around 800 nanometers, where it fluoresces the brightest, so no one had observed that the dye also produced a strong signal at longer wavelengths. 

The researchers also tested another dye that works in the near-infrared. This dye, called IRDye 800CW, is similar to ICG and can be attached to antibodies that target proteins such as those found in tumors. They found that IRDye 800CW also fluoresces brightly in the shortwave-infrared light, though not as brightly as ICG, and showed that they could use it to image a cancerous tumor in the brains of mice.

The research team is now further investigating why ICG works so well for shortwave-infrared imaging and is trying to identify the optimal wavelength for its use, which they hope will help them determine the best applications for this kind of imaging. They are also working with other labs to develop dyes that are similar to ICG and might work even better.

To do shortwave-infrared imaging, research labs and hospitals would need to switch from the silicon cameras now used for NIR imaging to an indium gallium arsenide (InGaAs) camera. Until recently, these cameras have been prohibitively expensive, but the prices have been coming down in the past several years.