QLF: Red Fluorescence Monte-Carlo Simulation of Photons in Tooth Tissue

Objectives: To investigate the ability of the Quantitative Light-Induced Fluorescence technique to detect a subsurface cavity filled with plaque, emitting so-called red fluorescence, in dry human tooth enamel by simulating excitation and fluorescence photon paths in tooth tissue and cavity.

Methods: The simulation model was based on a 2x2 mm² block of dry sound human tooth enamel with a thickness of 1 mm. Within this block a cavity with plaque was constructed with a size of 0.7 x 0.7 mm² at a variable depth with respect to the enamel surface extending to the bottom of the enamel. The Henyey-Greenstein phase function was used to derive the path of each photon traveling through the enamel. It was assumed all photons had an average cosine scattering angle g = 0.68, a linear scattering coefficient s = 10mm^-1, and a linear absorption coefficient a = 0.1 mm^-1. In this test blue excitation photons entered the tooth tissue perpendicularly at a random position on the outer enamel surface and were scattered around until they left the enamel block or were absorbed, and emitted isotropically as a green photon, or hit the cavity block, and were consequently emitted isotropically back in to the enamel as a red photon. The number of red (N_r) and green (N_g) fluoresced photons eventually emitting out of the tooth surface was registered and parameter ΔR=N_r/N_g was calculated.

Results: At cavity depths 0, 0.25, 0.50, 0.75 and 0.95 mm ΔR was found to be respectively 570, 160, 55, 15 and 2 % indicating an exponential-like decrease with cavity depth.

Conclusions: It is postulated that that a subsurface cavity filled with plaque in human dry enamel up to a depth 1 mm into the enamel is detectable by the QLF technique.