A comparison of blood vessels imaged with short-wave fluorescence imaging (right) and near-infrared fluorescence imaging (left). Both images rely on a fluorescent dye called ICG, but the vessels can be seen more clearly with short-wave fluorescence imaging. [Image from MIT]
Fluorescence imaging is often used to visualize biological tissues or blood vessels during reconstructive surgery to see if vessels are connected properly. Currently, researchers use a dye that runs at the near-infrared (NIR) level of the light spectrum that gets imaged through a specialized camera that can pick up that light that runs at 700 to 900 nanometers.
Researchers have recently found that light running at more than 1,000 nanometers, known as short-wave infrared (SWIR), gives clearer images than NIR. However, there are no FDA-approved fluorescence dyes with peak emission that can run at the SWIR range.
Massachusetts Institute of Technology and Massachusetts General Hospital researchers discovered that a dye that is used for NIR imaging has also been effective for SWIR imaging, making SWIR imaging more widely available.
“What we donut is that this de, which has been approved since 1959, is really the best, the brightest fluorophore that we know of at this point for imaging in the short-wave infrared,” Moungi Bawendi, a professor of chemistry at MIT, said in a press release. “Now clinicians can start to try short-wave imaging for their applications because they already have fluorophore, which is approved for human use.”
The dye, known as indocyanine green (ICG), strongly fluoresces around 800 nanometers, which is still in the near-infrared range. It travels through the bloodstream when injected into the body to provide better visualization of blood flowing through vessels. Some robot-assisted surgical systems already use NIR fluorescence imaging to see blood vessels and other anatomical features during procedures.
ICG’s use in SWIR was discovered mistakenly as part of another experiment the researchers were doing. They were testing the fluorescence output of ICG against the fluorescence output of quantum dots in short-wave infrared. The researchers anticipated that ICG would have no output. Instead, ICG showed a strong output.
The researchers wanted to create better fluorophores for SWIR because it offers better clarity and contrast than NIR.
“In the near-infrared, a lot fo the features you see in tissue can look foggy, and once you move into the short-wave infrared, the image clears up and everything becomes sharp,” said Oliver Bruns, a senior author on the study and an MIT research scientist.
SWIR can go deeper into tissue than NIR. Although the exact depth at which the light can go is hard to pinpoint, the researchers could see several hundred micrometers into the tissue with a regular fluorescence microscope in this study. The researchers suggest that this depth can normally only be reached using two-photon microscopy, which tends to be a more expensive type of imaging.
“We found that short-wave infrared is particularly useful for imaging small objects that are on top of a large background, so when you want to do angiography of small vessels, or capillaries, that’s significantly easier in the short-wave infrared than in the near-infrared,” Daniel Franke, one of the study’s lead authors, said.
The researchers also found that ICG gave stronger signals than other SWIR dyes that are currently available. The dye fluoresces brighter at 800 nanometers, but the researchers saw that it creates a strong signal at longer wavelengths as well. Even though it doesn’t fluoresce efficiently in the SWIR range, the ICG dye does absorb enough light that a small percentage can be emitted as fluorescent light, making a brighter signal than other SWIR dyes. The brighter the light, the quicker it can produce an image.
“If you don’t have a strong enough signal, it slows down how long it takes to take the image, so you can’t use it for imaging motion such as blood flowing or the heart beating,” said Jessica Carr, one of the study’s lead authors.
Another dye that fluoresced brightly in SWIR was IRDye 800CW, which is similar to ICG. IRDye 800CW attaches to antibodies that can target proteins like the ones found in tumors and could work in near-infrared. It doesn’t shine as bright as ICG, but it has proven to be beneficial to image cancerous tumors in the brain.
The researchers suggest that in order for SWIR imaging to be more accessible, research labs and hospitals would have to switch from silicon cameras to indium gallium arsenide cameras, which have been expensive in the past.
The team is now working on figuring out why ICG works well for SWIR imaging while also trying to figure out what wavelength it is best used at to create better applications for the imaging type.
The research was published in the journal Proceedings of the National Academy of Sciences and was funded by the National Institutes of Health through the Laser Biomedical Research Center, MIT through the Institute for Soldier Nanotechnologies, the National Science Foundation and the Department fo Energy Office of Science.
