Dynamic photophysical processes in laser-irradiated human cortical skull bone measured by means of modulated diffuse luminescence

The technique of modulated luminescence of bones was developed experimentally and theoretically and was subsequently used to interpret measurements performed on the cortical layer of human skull bones. The photophysical theory is based on the optical excitation and decay rate equations of the fluorescent endogenous chromophore and on the molecular interaction parameter with the photon field density in the matrix of the bone. An effective mean relaxation lifetime, τ_M, of skull cortical bone was derived theoretically and was found to depend on the endogenous chromophore decay lifetime, τ₂, in the upper energy state, on the generated luminescence field density through its dependence on the incident photon field density and on the thickness of the bone. A linear dependence of τ_M on laser beam intensity, I₀, was found and sensitivity of the value of τ_M to bone thickness, L, was observed for L≤6.2 mm. Both experimental dependencies of τ_M on I₀ and L were in excellent agreement with the theoretical model. The unusually long relaxation luminescence lifetime was accounted for theoretically by means of an excited-state manifold invoking intersystem crossing to a forbidden state followed by decay to the ground state of the chromophore. Best fits to the data were able to yield measurements of the following chromophore and photon field parameters: τ₂=19.7 ms, optical scattering coefficient μ_s(659 nm)=44 340 m⁻¹, optical absorption coefficient μ_a(659 nm)=13 m⁻¹, and coupling coefficient B₂₁=1.6×10⁴ m³ J⁻¹ s⁻¹, the decay coupling coefficient of the endogenous chromophore participating in the optical interaction in the form of stimulated luminescence emission mediated by the luminescence photon field between the long-lived excited state E₂ and the lower (ground) state E₁. The method of modulated luminescence can be used to measure photophysical properties of the chromophore in cortical skull bones, being a sensitive marker of bone diseases, namely, osteoporosis and cancer.