SRS microscopy captures blood cells at video rate
Researchers at Harvard University in Cambridge, Mass., have captured blood cells squeezing through capillaries at video rate using stimulated Raman scattering microscopy.
Previously, SRS microscopy captured only about one image per minute, which was too slow to use in live animals or humans. The Harvard researchers, who included Brian G. Saar, Christian W. Freudiger, and X. Sunney Xie, were able to speed the collection of data by more than three orders of magnitude, fast enough for video-rate imaging.
SRS microscopy works by detecting the intrinsic vibrations in chemical bonds between atoms. The researchers improved detection of laser light backscattered by tissue by rearranging photodetectors to surround a small aperture through which laser light is directed at the tissue. This allowed the researchers to collect and analyze almost 30% of the laser light directed at a biological sample, a more than 30-fold increase over previous SRS microscopy.
The improved SRS microscopy technique can capture label-free chemical movies at the subcellular level, meaning it can capture video of proteins, lipids, and water inside cells. The researchers used the microscopy technique to view individual blood cells moving through a capillary and to track migration of medications in skin.
Xie says that the technique could aid and speed surgery to remove tumors and other lesions. Instead of sending excised samples for histological analysis while a patient waits on the operating table, SRS microscopy could provide equivalent information in real time.
"When we started this project 11 years ago, we never imagined we'd have an amazing result like this," says Xie, professor of chemistry and chemical biology at Harvard. "It took MRI more than 30 years to reach patients, but we're already looking forward with great anticipation to applications of SRS microscopy in hospitals. It's now clear that stimulated Raman scattering will play an important role in the future of biological imaging and medical diagnostics."
Figure 1: The images show a sebaceous gland wrapping around a hair in the viable epidermis of mouse skin. Left: CH2-image in red shows the lipid-rich gland cells with sub-cellular resolution. Nuclei appear as dark circles due to the lack of lipids. Center: CH3-image in green shows residual lipid signal but also new protein-rich structures such as a hair in the center of the image, collagen fibers surrounding the gland and red blood cells in the top left. It can be seen that hair (green solid center) is surrounded by lipid-rich sebum (red circle). Right: Because of water (in blue), the OH-image, water shows inverse image contrast from the sebaceous gland because the lipid rich cells exclude water. Scale: 20 microns. Images courtesy of Dr. Brian Saar and Christian Freudiger.
Figure 2: Left: SRS images of the viable epidermis at the CH3-vibration mainly highlight proteins. A capillary with individual red blood cells (arrow) is visible. The cells are imaged without motion blur due to video rate acquisition speed. Right: An x-t plot acquired by line-scanning across a capillary at the position of the arrow. Individual red blood cells are captured on the fly. Scale: 25 microns.