May 14, 2007 - Harvard University’s Office of Technology Development (OTD) and Leica Microsystems, a developer in the optical and opto-electronic industries, today announced that Harvard has licensed its CARS microscopy technology to Leica for use in the company’s confocal microscopes to expand the optical microscopes imaging capabilities, enhancing scientists’ ability to image molecules in living cells and organisms.
“CARS microscopy has matured as a powerful imaging tool for biomedicine. It is complementary to magnetic resonance imaging (MRI). Although we do no have the penetration depth of MRI, we have much better spatial and time resolutions at a much lower cost,” stated Xiaoliang Sunney Xie, Ph.D., professor of Chemistry and Chemical Biology, Harvard University, one the developers of this technology.
Coherent anti-Stokes Raman scattering (CARS) microscopy allows rapid and non-perturbative imaging of biological specimens with chemical selectivity. The contrast in CARS microscopy arises from the intrinsic vibrations of molecules. Every molecule has one or more chemical bonds, the bending or stretching of which have characteristic vibrational frequencies that depend on the bond length and strength. For example, lipids, a primary component of fat, contain carbon-hydrogen bonds, which vibrate at certain distinct frequencies. CARS microscopy “tunes” into these characteristic frequencies to build chemically-selective images with extremely high sensitivity in living cells or organisms.
To image a specimen, such as tissues or cells, CARS microscopy utilizes two highly focused laser beams at different frequencies. By setting the difference between the two laser frequencies equal to the frequency of vibration of a particular chemical bond, molecules with that bond are made to vibrate coherently. This causes the sample to emit at a new frequency, called the “anti-Stokes” frequency, from the laser focus. An image is created by scanning the beams over the sample and detecting the intensity of the emitted anti-Stokes light at each position. In this way, one can map the concentration of the molecule of interest (e.g. lipid) throughout the tissue, or within a cell with 300nm lateral resolution. The method offers much higher time resolution than other vibrational imaging techniques, allowing movies of biological activity and chemical processes to be taken within a living cell or organisms.
By using excitation lasers at near-infrared wavelengths, which can penetrate deep into tissue, CARS microscopy can reach a depth of nearly 0.3 mm below the surface. Efforts are underway to extend CARS microscopy for not only cell biology applications, but also disease diagnostics and real-time surgical guidance.
The key uses of CARS Microscopy is as follows:
· Detecting the intrinsic vibrational signatures of molecules circumvents the need for fluorescent and other extrinsic labels, and permits "chemical mapping” – visualization of the distribution of specific molecules.
· The high sensitivity of CARS allows for data collection rates orders of magnitude faster than previous vibrational imaging techniques.
· The nonlinear nature of CARS process assures that the signal is generated only at the laser focal point, leading to high resolution 3-D imaging of tissue and cellular structures.
· Near-infrared excitation beams allows deeper penetration in tissues than visible light, allowing for noninvasive measurements with minimal photodamage.