Tracking molecules in live tissue at video rate

New advances in microscopy using stimulated Raman scattering (SRS) have allowed researchers at Harvard University to capture blood cells being squeezed through capillaries.

Previously, SRS microscopy could capture only about one image per minute, which was too slow for use in live animals or humans. The team of scientists was able to speed the collection of data by more than three orders of magnitude to provide video-rate imaging.

SRS microscopy works by detecting intrinsic vibrations in chemical bonds between atoms. The optical technique complements MRI, which is suited to imaging organs, tumors and other large objects deep within the body.

At 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. At right is 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 μm. Courtesy of Dr. Brian Saar and Christian Freudiger.

The team improved detection of signals backscattered by tissues in the body by rearranging photodetectors to surround a small aperture through which a light beam is directed at the tissue. This allowed the researchers to collect and analyze almost 30 percent of the laser light directed at the biological sample, a more than 30-fold increase over previous SRS microscopy. For the first time, it is possible to see label-free, streaming footage of proteins, lipids and water at a subcellular level.

SRS microscopy could aid and speed surgery to remove tumors and other lesions. It is already used by the team to track migration of medication in skin, as well as to view three-dimensional sections of tissue, layer by layer, when used in conjunction with endoscopy.

The work was published in the journal Science, Vol. 330, No. 6009, pp. 1368-1370 (2010).