Up Conversion Imaging
Fluorescence labeling is an incredibly powerful technique used throughout biology and increasingly in medicine. Fluorophores can be genetically engineered into animals, injected with targeting antibodies, or activated by cancer-associated enzymes. The increasing use of fluorescence in whole-animal imaging has highlighted some of the limitations of existing fluorophores: broad excitation and emission spectra limit the number of independent fluorescence channels, endogenous fluorophores limit the signal-to-background ratio, and the shorter wavelength excitation and emission light used by many fluorophores is highly absorbed by tissue.
Up-Converting Nanoparticles (UCNs) are an alternative to fluorophores that solve many of these limitations. Under standard fluorescence excitation conditions, fluorophores are excited by a single photon. The fluorophore then emits a lower energy photon. Under extremely high peak excitation levels from femtosecond lasers, it is possible for fluorophores to be excited by multiple low-energy photons. Under multiphoton excitation, the fluorophore emits light at a higher energy than the excitation light. UCNs are unique because they can be excited by multiple photons at low power densities. This makes it possible to image UCNs at high contrast without interference from fluorophores or tissue background.
In collaboration with Scott Hildebrand at Massachusetts General Hospital, we have developed two up-conversion imaging systems. The In-Vivo Up-Conversion Microscope (IVUCM), captures cellular resolution images and movies of UCNs. It was used to image the vasculature in a live mouse. The Small-Animal Up-Conversion Imager (SAUCI) captures images over a wider field of view. It was used for intraoperative imaging of liver accumulation of UCN in a mouse.
S. Hildebrand, F. Shao, C. Salthouse, U. Mahmood, and R. Weissleder, "Upconverting Luminescent Nanomaterials: Application to in vivo Bioimaging" Chemical Communications preprint online
C. Salthouse, S. Hildebrand, R. Weissleder, and U. Mahmood, "Design and Demonstration of a Small-Animal Up-Conversion Imager" Optics Express, vol. 16, no. 26, pp. 21731-21737, December 2008.