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Accurate, simple, and reliable techniques are needed to perform sensitive and specific in vivo determinations of abnormalities within a given tissue. Successful, noninvasive "optical biopsies" might replace invasive, destructive biopsies, providing advantages of smaller sampling errors, reduction in cost and time for diagnosis, and resulting in easier integration of diagnosis and therapy by following progression of disease or its regression in response to therapy.
Clinically practical fluorescence imaging techniques must meet several requirements. First, the pathology under investigation must not lie at such a depth that the attenuation of the signal gives poor signal-to-noise ratio and resolvability. Second, the specificity of the marker must be high enough so that one can clearly distinguish between normal states and abnormal lesions. Finally, one must have a robust image reconstruction algorithm, which enables one to quantify the fluorophore concentration at a given depth. We have chosen Sjøgren's Syndrome (SS), as an appropriate test case for developing this noninvasive optical biopsy. SS is an autoimmune disease affecting minor salivary glands that are near (0.5 to 3.0 mm below) the oral mucosal surface. We are attempting to use exogenous fluorescent ligands (e.g., antibodies to CD4+ T cell-activated lymphocytes infiltrating the salivary glands) to provide high optical contrast in a quantitative relationship that will allow assessment of the stage of the disease process. We have developed a theoretical description of diffuse fluorescence photon migration for such a system, and have tested it with data acquired from a simulation experiment. Small (1mm3) rhodamine targets, which mimic diseased minor salivary glands labeled with fluorescent antibodies to infiltrating lymphocytes in SS, were embedded in a highly scattering tissue-like phantom at various depths. Excellent agreement between the experimentally measured surface profiles and those predicted by our theory was obtained.
We have also been investigating whether fluorescent lifetime imaging may be used to obtain functional information regarding local concentrations of specific substances such as O2, or information about environmental conditions such as temperature and pH. Quantification requires a fluorophore with a known dependence on the environmental factor. For deeply embedded sites in a turbid medium such as tissue, measuring the photon arrival delay caused by a specific fluorophore lifetime is made difficult by the photon arrival delays caused by multiple scattering of the photon on its transit through the tissue. However, RWT is well suited to solving this type of problem. The delay of photons that results from the excitation and later emission by a fluorophore can be modeled in RWT in the same way as transit delays due to multiple scattering. Hence, we have derived a RWT closed-form solution for time-resolved fluorescent lifetime imaging. This is now being tested with fluorophore-impregnated agarose phantoms. We have entered into two collaborative projects that will make use of the results of these studies. In collaboration with the Radiation Biology Program of the NCI, we have begun a mouse imaging study that will utilize malignant cells transfected with the gene(s) for synthesis of fluorescent proteins. In a second project we are investigating the use of IR-dependent fluorescent detection methods as alternatives to radionucleotide detection to determine the locations of sentinel lymph nodes for surgical treatment of cancer.
Hassan, M., J. Riley, V. Chernomordik, P. Smith, R. Pursley, S. B. Lee, J. Capala, A. H. Gandjbakhche. Fluorescence lifetime imaging system for in vivo studies. Molecular Imaging, in press.
S. Bloch, F. Lesage, L. McIntosh, A. Gandjbakhche, K. Liang, and S. Achilefu. Whole-body fluorescence lifetime imaging of a tumor-targeted near infrared molecular probe in mice. J. Biomed. Opt. 10(5):054003 (Sep-Oct 2005).
Morgan, N., S. English, W. Chen, V. Chernomordik, A. Russo, PD Smith, and A. Gandjbakhche. Real time in vivo non-invasive optical imaging using near-infrared fluorescent quantum dots. Acad Radiol. 12(3), 313-23 (Mar 2005).
Hassan, M., BA Klaunberg. Biomedical applications of fluorescence imaging in vivo. Comp Med. 54(6), 635-44 (Dec 2004).
Gannot, I., A. Garashi, V. Chernomordik and A. Gandjbachkhe. Quantitative optical imaging of the pharmacokinetics of fluorescent-specific antibodies to tumor markers through tissuelike turbid media. Optics Letters, 29(7), 742-4 (2004).
Gannot, I., R. Izhar, F. Hekmat, V. Chernomordik, and A. Gandjbakhche. Functional optical detection based on pH dependent fluorescence lifetime. Lasers in Surgery and Medicine 35, 342-348 (2004).
Gannot, I., A. Garashi, G. Gannot, V. Chernomordik, and A. Gandjbakhche. In vivo quantitative three-dimensional localization of tumor labeled with exogenous specific fluorescence markers. Applied Optics 42(16), 3073-3080 (June 2003).
Gannot, I., G. Gannot, A. Garashi, A. Gandjbakhche, A. Buchner and Y. Keisari.. Laser activated fluorescence measurements and morphological features: An in vivo study of clearance time of FITC tagged cell markers. Journal of Biomedical Optics 7,14-19 (2002).
Eidsath, A., V. Chernomordik, A. Gandjbakhche, P. Smith and A. Russo. Three-dimensional localization of flourescent masses deeply embedded in tissue. Phys. Med. Biol. 47, 4079-4092 (2002).
Chernomordik, V., D. Hattery, I. Gannot and A. Gandjbakhche. Inverse Method 3-D Reconstruction of Localized in vivo Fluorescence - Application to Sjogren Syndrome. IEEE Journal of Selected Topics in Quantum Electronics 5(4), 930-935 (July/Aug 1999).
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