Laser photoablation: a new biomedical tool

Abstract

Most diseases and biological processes can only be studied in a living animal in which all the complex regulatory processes are occurring. New techniques using ultrashort laser pulses are increasingly allowing researchers to manipulate cells and tissue within living organisms to study normal and diseased functionality. The study of microstrokes may, in particular, benefit from these advances. These small strokes are thought to be caused by clots or bleeding in small blood vessels within the brain that lead to the death of neurons. Clinical studies have shown that such microstrokes are relatively common in the elderly and may play a major role in dementia and diseases such as Alzheimer’s. However, no treatment is presently available, partly because reproducing microstroke lesions in animals has proved elusive. Ultrashort pulsed laser ablation provides a novel tool to produce such lesions, finally enabling the study of this disease. The technique has recently emerged as an excellent tool for the precision machining of solid state materials and has been used for the fabrication of waveguides inside transparent materials without affecting the surface. The extremely high light intensities achieved when focusing ultrashort laser pulses enable the nonlinear optical absorption of the laser light. The resulting ionized volume can be as small as the size of a diffraction-limited focus, typically ∼1μm3 , and can be confined deep below the surface. This same technique can be applied to manipulating biological structures deep within a living organism. To study the effect of microstrokes in the living brain, windows are made in the skulls of anesthetized rats and the blood vessels are labeled with an intravenous injection of fluorescent dye. We use another nonlinear optical technique, two-photon laser scanning microscopy, to image blood vessels and measure blood flow in the brain with microscopic resolution. To perturb microvessels located as deep as 500μm below the brain surface, Figure 1. Diagram of the femtosecond laser-induced rupture of a microvessel followed by hemorrhage (top, left). The other panels show the time series of five images showing the rupture of a vessel 140μm below the surface of a rat cortex. Each image is labelled with the time relative to irradiation onset. The images were recorded during two-photon excited fluorescence microscopy. The targeted vessel first explodes, leaving a dark central volume filled with red blood cells and fluorescentlylabeled blood plasma penetrating the brain tissue. The illustrations are adapted from Nishimura et al. and used with kind permission of the Nature Publishing Group.