The fundamental goal of biomedical imaging is the retrieval of structural and dynamical information from the sample, with the greatest contrast, spatial, and temporal resolution possible. The workhorse of biomedical imaging is the optical microscope, which readily provides the resolution sufficient to observe subcellular organelles. The introduction of lasers and confocal optics brought the resolution scale down to diffraction-limited, i.e., on the order of the laser wavelength. Contrast enhancement came primarily from the use of fluorescent labeling compounds.