DECEMBER 11, 2008-- In the latest issue of the journal Proceedings of the National Academy of SciencesProfessor Vasilis Ntziachristos and his team at the Helmholtz Zentrum München and the Technische Universität München (both in Munich, Germany), together with colleagues from the Harvard Medical School and the Massachusetts General Hospital (Massachusetts, USA), report on the use of "Early Photon Tomography" (EPT) to image lung tumors in living mice.
"Early arriving 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.
The researchers injected into to the animals a substance that 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 further development of the equipment will likely enable niche applications of the technique with larger organisms including humans.
Paper describing the technique