Femtosecond Lasers for Cutting and Imaging Brain Tissue

Femtosecond lasers for cutting and imaging brain tissue have been demonstrated in a research collaboration that includes physicists and pharmacologists. Speaking at this week's CLEO/QELS meeting in Baltimore, Jeff Squier of the Colorado School of Mines described an automated, all-optical technique for performing histology, the study of biological tissue at the microscopic level. Used widely in clinical settings (e.g., to examine biopsied tissue from a suspected breast tumor) and in biological research (e.g, to study the anatomy of muscle), histology is presently a manual process, requiring a skilled technician to slice frozen tissue samples into thin pieces, and then view them with an optical microscope. Now, Squier and colleagues have demonstrated a way to do histology by using femtosecond lasers, which deliver light pulses that last just quadrillionths of a second. Compared to present methods, the femtosecond-laser technique does not require the freezing of biological samples (which can damage the specimen) and it can even slice and image very soft tissue (which is a challenge with standard histological techniques). In the new technique, the researchers first stain a tissue specimen with a layer of fluorescent dye to label desired structures (such as nerve cells) in tissue. Then, they use the laser beam at relatively low power (about 100 gigawatts per square centimeter) to obtain a picture (through various optical techniques) of these structures in a tissue specimen's first layer. The resolution of the image can approach 30 microns. After taking this first picture, the researchers increase the laser power (to levels of about 7000 terawatts per cm²) so that the light ablates (wears away) a 100-micron-deep layer of the tissue. To this newly exposed layer of tissue, the researchers add more fluorescent dye, and they lower the laser intensity to take another image. This process is repeated until no tissue remains. Stacking up the successive images to create a 3D picture, Squier and colleagues have obtained high-quality images of animal brain tissue, for example as blood vessels in rat neocortex. Since the femtosecond technique completely destroys its tissue samples, it may not be appropriate for certain clinical applications such as biopsies of breast tissue, as physicians may wish to preserve the tissue for future reference. However, the technique may be especially suited for many other applications, including studies in the burgeoning field of transgenic animals, which include genetically altered mice. For example, researchers could inject a fluorescently labeled gene into a mouse, and then obtain high-quality images showing how the gene gets expressed in mouse tissue (Paper CMN3 at the meeting).