Localizing Molecules in Cellular CT Scans

Abstract

Much like medical CT scans reveal anatomical structures in the body, soft x-ray tomography (SXT) visualizes and quantifies the organization of sub-cellular structures within a cell. In SXT, the specimen is illuminated with x-ray photons from within a region of the spectrum known as the ‘water window’ (284 - 543eV). ‘Water window’ x-ray photons are absorbed an order of magnitude more strongly by carbon- and nitrogen-containing organic material than by water. Consequently, variation in biomolecule composition and concentration gives rise to quantitative, high-contrast images of intact, fully hydrated cells without the need to use contrast-enhancing agents. Cells imaged by SXT are, therefore, highly representative of the cell in its native, functional state. Attenuation of soft x-rays, as they pass through the specimen, adheres to the Beer-Lambert Law. Attenuation is, therefore, a function of chemical composition and concentration of organic material, yielding unique quantitative Linear Absorption Coefficient (LAC) measurements for cellular components. LAC values are enormously powerful in terms of quantifying alterations in cell structures during events such as cell differentiation, progression or etiology of disease states, genetic manipulation, and application of exogenous agents. To localize molecules without perturbing cell structures, we use correlated high numerical aperture cryogenic fluorescence tomography (CFT). This multi-modal approach - imaging the same cell using both CFT and SXT - allows localization of labeled molecules directly in the context of a high-resolution 3-D tomographic reconstruction of the cell. I will show examples of data collected using these imaging technologies developed at the National Center for X-ray Tomography, an NIGMS-NIH supported Biomedical Technology Research Resource.