Early photon tomography provides clearer picture of tumors in living animals

Fluorescent molecules - i.e. substances which can be stimulated to emit light - are extremely valuable tools in biological research and medical diagnosis.

Fluorescence can be used for instance to analyze the regulation and expression of genes, to locate proteins in cells and tissues, to follow metabolic pathways and to study the location and migration of cells. Of particular importance is the combination of fluorescence imaging with novel techniques that allow tomographic three-dimensional visualization of objects in living organisms. At the Helmholtz Zentrum München - German Research Center for Environmental Health together with the Technische Universität München an own institute is concerned with the development and refinement of such new technologies: the Institute for Biological and Medical Imaging headed by Professor Vasilis Ntziachristos.

The quality of optical imaging in tissues is naturally limited, since beyond a penetration depth of a few hundred micrometers the photons are massively scattered due to interactions with cell membranes and organelles which results in blurred images. In the latest issue of the journal Proceedings of the National Academy of Sciences Prof. Ntziachristos and his team, together with colleagues from the Harvard Medical School and the Massachusetts General Hospital in Boston, USA, report on the use of the so-called early arriving photons together with tomographic principles. Early photons are the first photons that arrive onto a photon detector after illumination of tissue by an ultra-short photon pulse and undergo less scattering in comparison to photons arriving at later times. Compared to continuous illumination measurements a combination of these less scattered photons with 360-degree illumination-detection resulted in sharper and more accurate images of mice under investigation.

With this technique, called 'Early Photon Tomography' (EPT), the scientists imaged lung tumors in living mice. For this purpose they injected a substance into to the animals, which normally does not fluoresce, but becomes fluorescent after contact with certain cysteine proteases such as cathepsins. The amount of these proteases is enriched in lung tumors which allows fluorescence imaging of the tumor tissue. Comparison with conventional x-ray tomography showed, that EPT is not only a very sensitive technique for imaging of lung tumors in living organisms, but also has the potential to reveal biochemical changes that reflect the progression of the disease, which could not be detected by conventional X-ray imaging.

While early-photons are typically associated with reduced signal available for image formation, the authors demonstrated that due to the wide-field implementation, EPT operates with very small reduction in average signal strength as in conventional tomographic methods operating using continuous light illumination. In this respect EPT is a practical method for significantly improving the performance of fluorescence tomography in animals over existing implementations. At present EPT is practicable only with small animals, but - as stated by the authors of the paper - further development of the equipment can allow niche applications of the technique also with larger organisms including humans.

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