Simulation and experimental investigation of tooth tissue in photothermal radiometry dynamic response induced by modluated laser

Abstract

The photon-density wave is generated in tooth tissue due to the scattering induced by modulated laser beams, and furthermore, thermal-wave will form because of photothermal effect. A one-dimensional thermal-wave model for three-layer tooth tissue using modulated laser stimulation is developed based on 1D diffusion approximation of the radiative transfer theory in combination with 1D heat conduction equation. Effects of photothermal properties (i.e. light absorption coefficient, scattering coefficient and thermal diffusivity coefficient), enamel depth and caries depth on the photothermal radiometry (PTR) dynamic responses are investigated based on the 1D thermal wave model coupling with photon-density wave. The PTR amplitude and phase delay (the phase difference between the PTR signal and reference signal) are strongly dependent on the photothermal parameters of the dental enamel caries layers (DECLs). PTR amplitude and phase delay increase with increasing DECL absorption coefficient, scattering coefficient and thermal diffusivity. Additionally, PTR amplitude may also increase due to the larger thickness of caries layer, and the PTR phase peak value is generated at low frequencies. The inhomogeneous photothermal properties of dental enamel healthy layer (DEHL) also have obviously influenced PTR amplitude and phase. Increasing DEHL scattering coefficient leads to the increase of PTR amplitude, but has no apparent effect on the PTR phase. While the PTR phase delay increases with increasing DEHL absorption coefficient. The delay of PTR amplitude and phase is enlarged at the high value of DEHL thermal diffusivity. However, the DEHL layer thickness has no apparent effect on the PTR amplitude and phase.The PTR signal of tooth tissue induced by the 808 nm diode laser is monitored using an infrared detector (HgCdTe, spectral width 2.012.0 m), and the PTR amplitude and phase response are obtained using lock-in amplifier (SR830). Through frequency-scanning experiments of dental tissue, PTR dynamic responses can be measured and employed to characterize the inhomogeneity and caries of the tooth tissue. The photothermal parameters and caries characteristic of the tooth issue can be simultaneously obtained by multi-parameters statistic best-fit.Simulation and experimental results show that the PTR dynamic response has the advantages of high sensitivity and high contrast for inhomogeneity and caries of the tooth tissue.