title: Functional optical detection based on pH dependent fluorescence lifetime.
abstract: OBJECTIVE
  Detection of possible alterations of physiological parameters (e.g., pH and temperature), resulting from malignant transformation of initially healthy tissue, can be a powerful diagnostic tool for earlier cancer detection. Such variations can be observed by comparing these parameters with those of healthy tissue surrounding the abnormality. Time-resolved spectroscopy of specifically targeted fluorescent labeled antibodies can be sensitive to such variations and provide a high resolution functional image of the region of interest. The goal of this study was to establish a forward experimental setup for calibration of the lifetime dependencies of near-IR fluorescent dyes on physiological parameters, and to develop analytical solutions, taking into account the effects of light propagation in turbid media (e.g., tissue), that was able to extract an original lifetime fluorescence signal from time-of-flight intensity distributions, measured in vivo from a deeply embedded live organ for further analysis.
  
  
  METHODS
  Tissue-like phantoms with embedded fluorescent dyes and background optical properties simulating those of live tissues were designed and created. Fluorescence decay curves were measured for different fluorophore positions, and pH values. Those measurements were made with a system based on a time-correlated single photon counting (TCSPC) instrument and a tunable femtosecond Ti-Sapphire system built by our group.
  
  
  RESULTS
  Decay curves were recorded for fluorophore depths of up to 5 mm and source-detector separation of 7 mm. It was shown that a forward model, based on the random walk theory, adequately described the experimental data. Measured pH dependencies of the fluorescence lifetime were characterized for two different dyes.
  
  
  CONCLUSIONS
  Good correlation between experimental data and predictions of the theoretical model allows the use of close-form analytical solutions to separate the effects of photon time delays due to multiple scattering in tissues from the original intensity fluorescence time decay curve, determined by the fluorophore itself and its immediate surroundings. It is the latter dependence that can be diagnostically important. Experimentally obtained scaling between lifetime and a parameter of interest can be used in vivo to obtain a map of physiological parameter changes which can serve as a base for an in vivo specific diagnostic system.
authors.affiliation: Lasers and Optics in Medicine Laboratory, Department of Biomedical Engineering, Faculty of Engineering Tel-Aviv University, Tel-Aviv 69978, Israel. gannoti@mail.nih.gov
authors.forename: Israel
authors.initials: I
authors.lastname: Gannot
authors.forename: Izhar
authors.initials: I
authors.lastname: Ron
authors.forename: Farid
authors.initials: F
authors.lastname: Hekmat
authors.forename: Victor
authors.initials: V
authors.lastname: Chernomordik
authors.forename: Amir
authors.initials: A
authors.lastname: Gandjbakhche
mesh_terms: D000818:Animals; D002294:Carcinoma, Squamous Cell / Q000175:diagnosis* / Q000378:metabolism; D003952:Diagnostic Imaging / Q000379:methods*; D005456:Fluorescent Dyes / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D051379:Mice; D008954:Models, Biological; D015233:Models, Statistical; D055095:Optics and Photonics; D019047:Phantoms, Imaging; D012016:Reference Values; D012542:Scattering, Radiation; D012996:Solutions; D013050:Spectrometry, Fluorescence / Q000379:methods*
chemical_list: D005456:Fluorescent Dyes; D012996:Solutions