INSTITUTE OF LASER MEDICINE

Prof. Dr. P. Hering   

Knowledge of the optical properties (absorption coefficient, scattering coefficient, and scattering phase function) of biological tissues is a requirement for laser applications in medicine. An established method for measuring the optical parameters of turbid materials is the integrating-sphere technique. This technique is widely used for the determination of the optical properties of biological tissues in vitro and for the verification of the optical techniques intended for use in vivo. A mathematical model of light propagation in the sample is required to deduce the optical properties from the measured quantities. In earlier studies, various simplifications of the radiative transport theory were used for this purpose, including Kubelka-Munk theory, diffusion approximation, and adding-doubling method (the latter being a rigorous, but essentially 1D model). To overcome the limitations of these methods and avoid simplifying assumptions which do not hold in general, we have developed an inverse Monte Carlo technique. It enabled us to take into account: 

• the real 2D geometry of the measurements, 
• contribution of the scattered light into the measured collimated signal, 
• side losses of light at the edges of the sample, 
• arbitrary phase function of the scattering material, and 
• realistic boundary conditions. 

The Monte Carlo method was combined with an effective quasi-Newton algorithm to solve the inverse problem. Currently the method is routinely used for the determination of the optical properties of various phantom materials and tissues including human vessel walls, blood, and brain tissues (Fig. 1). 
[References] 

Feel FREE to download our inverse Monte Carlo software 
(for Win 95/NT) SetupML3.exe (740k) 

Fig. 1. Optical properties of human gray matter determined in vitro with an integrating-sphere set up and an inverse Monte Carlo technique