Visualization of internal structures of opaque biologic objects is essential in many biomedical studies. Limited by the penetration depth of the probing sources (photons and electrons) and the lack of endogenous contrast, conventional forms of microscopy such as optical microscopy and electron microscopy require tissues to be sectioned into thin slices and stained with organic chemicals or heavy-metal compounds prior to examination. These invasive and destructive procedures, as well as the harmful radiation in the case of electron microscopy, make it difficult to obtain three-dimensional information and virtually impossible to study biologic tissues in vivo.