Optical scattering properties of soft tissue: a discrete particle model

We introduce a micro-optical model of soft biological tissue that permits numerical computation of the absolute magnitudes of its scattering coefficients. A key assumption of the model is that the refractive-index variations caused by microscopic tissue elements can be treated as particles with sizes distributed according to a skewed log-normal distribution function. In the limit of an infinitely large variance in the particle size, this function has the same power-law dependence as the volume fractions of the subunits of an ideal fractal object. To compute a complete set of optical coefficients of a prototypical soft tissue (single-scattering coefficient, transport scattering coefficient, backscattering coefficient, phase function, and asymmetry parameter), we apply Mie theory to a volume of spheres with sizes distributed according to the theoretical distribution. A packing factor is included in the calculation of the optical cross sections to account for correlated scattering among tightly packed particles. The results suggest that the skewed log-normal distribution function, with a shape specified by a limiting fractal dimension of 3.7, is a valid approximation of the size distribution of scatterers in tissue. In the wavelength range 600 less than or equal to lambda less than or equal to 1400 nm, the diameters of the scatterers that contribute most to backscattering were found to be significantly smaller (lambda/4-lambda/2) than the diameters of the scatterers that cause the greatest extinction of forward-scattered Light(3-4 lambda). (C) 1998 Optical Society of America.