INSTITUTE OF LASER MEDICINE

Prof. Dr. P. Hering   

Investigation of the propagation of photon density waves in turbid media 

Frequency-resolved optical spectroscopy of tissues in vivo has a significant potential for clinical diagnostics. However, an accurate quantitative interpretation of the phase/modulation data still represents a problem for certain types of tissues and/or optical probe geometries. The standard diffusion model has limitations with regard to source-detector separation, boundary conditions, modulation frequency, and the optical properties of the medium. To evaluate the applicability of the diffusion model and other analytical approaches for particular experimental configurations, we developed a dedicated Monte Carlo technique for simulating the propagation of the photon density waves directly in the frequency domain (Fig. 1), thus eliminating the need to use a Fourier transform in time. Another limitation of the standard diffusion model is its inability to account for a finite scattering delay time, which generally leads to erroneous interpretation of the phase/modulation data. We suggest a modification of the standard diffusion model, which allows for a non-zero duration of the scattering process. The results of this study are currently being used to design novel tissue spectroscopy apparatus for in vivo applications. 
[References] 

Fig. 1. Comparison of the frequency-resolved reflected signals calculated with the diffusion approximation and with a dedicated Monte Carlo simulation, respectively.