title: Propagation of fluorescent light.
abstract: OBJECTIVE
  In general, the remitted fluorescence spectrum is affected by the scattering and absorption properties of tissue. Other important factors are boundary conditions, geometry of the tissue sample, and the quantum yield of tissue fluorophores. Each of these factors is examined through a series of Monte Carlo simulations.
  
  
  METHODS
  Monte Carlo modeling is used to simulate the propagation of excitation light and the resulting fluorescence. Remitted fluorescence is determined for semi-infinite single and multiple layer geometries and for cubic geometries representing small tissue samples. Monte Carlo results are compared to approximations obtained with a heuristic model.
  
  
  RESULTS
  Remitted fluorescence as a function of (1) the depth of fluorescence generation and (2) radial escape position is presented for semi-infinite single and multiple layer geometries. Fluorescence from a small tissue sample is simulated in terms of a cubic geometry, and losses from the sides and bottom are presented as a function of cube dimensions in terms of optical depth of the excitation wavelength. Monte Carlo results for a homogeneous semi-infinite layer are compared to a simple, fast heuristic model.
  
  
  CONCLUSIONS
  Both Monte Carlo simulations and the heuristic model clearly detail the volume of tissue interrogated by fluorescence. Since approximately 35-40% of the remitted fluorescence is due to photons originally directed away from the surface, distal layers affect the remitted fluorescence. Fluorescence spectra from small biopsy samples may not produce the correct line shape owing to wavelength dependent losses.
authors.affiliation: Biomedical Engineering Program, University of Texas, Austin 78712, USA.
authors.forename: A J
authors.initials: AJ
authors.lastname: Welch
authors.forename: C
authors.initials: C
authors.lastname: Gardner
authors.forename: R
authors.initials: R
authors.lastname: Richards-Kortum
authors.forename: E
authors.initials: E
authors.lastname: Chan
authors.forename: G
authors.initials: G
authors.lastname: Criswell
authors.forename: J
authors.initials: J
authors.lastname: Pfefer
authors.forename: S
authors.initials: S
authors.lastname: Warren
mesh_terms: D001011:Aorta; D001161:Arteriosclerosis; D003198:Computer Simulation; D005453:Fluorescence; D006801:Humans; D066298:In Vitro Techniques; D008962:Models, Theoretical; D009010:Monte Carlo Method; D055095:Optics and Photonics