Researchers want to image individual cells inside living subjects because it will give them insight into how cellular behavior gives rise to the properties of organisms as a whole. For instance, the nerve cells of the hippocampus region of the brain give rise to important mental processes such as learning and memory.
Imaging living cells below the surface has been difficult to accomplish using conventional techniques. Electron microscopy can't be used on living tissue, and optical (light) microscopy can't penetrate very deeply into tissue because light scatters as it travels through tissue near the surface. Thus traditionally microscopic images of the living brain have only been made near the surface. Yet researchers would like to know more about certain deep-tissue areas of the brain, which are critical to understanding Alzheimer's and Parkinson's disease, for example.
Scientists often use some form of fluorescence microscopy to image tissue. In conventional "one-photon" fluorescence imaging, the scientist injects a dye into tissue and then shines a bright light. The tissue fluoresces, or radiates, light of a different color in response. However, a problem with one-photon fluorescence is that the deep tissue causes the photons to ricochet, or scatter, as they return to the detector. The result is a background haze in the images, almost like viewing the sample through a cloud.
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Source:American Institute of Physics