A contrast agent that has long been used for near-infrared (NIR) fluorescence has been found to be also suitable for short-wave infrared (SWIR) fluorescence. The fluorescent contrast agent has already been approved by regulators and can therefore be used directly for SWIR fluorescence, which can provide much clearer images than NIR fluorescence.
Fluorescent imaging is used to visualise tissue in the human body that cannot be seen with the naked eye - such as the back of the eye. The technology is also used to image blood vessels, including during reconstructive surgery. This allows surgeons to check that blood vessels have been sutured correctly.
Looking deeper into tissue
Typically, NIR is used in fluorescent imaging. Previously, researchers showed that using SWIR in fluorescent imaging can produce clearer images than NIR fluorescence. SWIR can also be used to look deeper into tissue. However, there was previously no FDA-approved fluorescent contrast agent available that performed well in the SWIR spectrum.
This contrast agent is what researchers from the Massachusetts Institute of Technology (MIT) and Massachusetts General Hospital have now found. It is a liquid called indocyanine green (ICG), which was approved decades ago by the US FDA for use on humans. ICG is injected and travels through the human body via the bloodstream. This makes it very suitable for imaging blood vessels, which is also known as angiography.
"We found that this contrast agent, which was approved in 1959, is the best and brightest fluorophore for short-wave infrared that we know of," says Moungi Bawendi, professor of chemistry at MIT. ICG is a liquid that fluoresces most strongly around 800 nanometres, which falls in the spectrum of NIR. "Doctors can now try out short-wave imaging for their applications, since they have a fluorophore at their disposal that is already approved for use on humans."
Discovered by chance
That ICG is also suitable for SWIR fluorescence was discovered by the researchers by accident. They were working on a control experiment for another paper, investigating the fluorescence yield of quantum dots compared to the fluorescence yield of ICG in shortwave infrared. The researchers expected ICG to have no fluorescence yield. To their surprise, however, the contrast agent actually turned out to yield a very strong signal.
Further experiments show that ICG produces a stronger signal than many other fluorescent contrast agents currently being developed specifically for SWIR fluorescence. The researchers indicate that although ICG does not fluoresce efficiently in the short-wave infrared spectrum, it absorbs a very large amount of light. Even if only a small percentage is emitted as fluorescent light, this makes the signal stronger than that of other SWIR contrast agents. The researchers hope their discovery will make SWIR fluorescence widely available.
IRDye 800CW
In addition to ICG, a contrast agent called IRDye 800CW also appears to be suitable for SWIR fluorescence. This liquid is similar to ICG and can be coupled to antibodies that target proteins such as those found in tumours. IRDye 800CW also appears to fluoresce brightly in the short-wave infrared spectrum, but not as brightly as ICG.
Author: Wouter Hoeffnagel
Source: MIT
Source photo: Pixabay / whitesession
