OPTICAL-SCATTERING PROPERTIES OF SOFT-TISSUE - A DISCRETE PARTICLE MODEL

We introduce a micro-optical model of soft biological tissue that perm its numerical computation of the absolute magnitudes of its scattering coefficients. A key assumption of the model is that the refractive-in dex variations caused by microscopic tissue elements can be treated as particles with sizes distributed according to a skewed log-normal dis tribution function. In the limit of an infinitely large variance in th e particle size, this function has the same power-law dependence as th e volume fractions of the subunits of an ideal fractal object. To comp ute a complete set of optical coefficients of a prototypical soft tiss ue (single-scattering coefficient, transport scattering coefficient, b ackscattering coefficient, phase function, and asymmetry parameter), w e apply Mie theory to a volume of spheres with sizes distributed accor ding to the theoretical distribution. A packing factor is included in the calculation of the optical cross sections to account for correlate d 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 diame ters of the scatterers that contribute most to backscattering were fou nd to be significantly smaller (lambda/4-lambda/2) than the diameters of the scatterers that cause the greatest extinction of forward-scatte red Light(3-4 lambda). (C) 1998 Optical Society of America.