Relationship Between Tissue Microscopic Structure and Scattering Properties

We have studied the optical properties of mammalian cell suspensions to provide a mechanistic basis for interpreting the optical properties of tissues in vivo. Measurements of the wavelength dependence of the reduced scattering coefficient and measurements of the phase function demonstrated that there is a distribution of scatterer sizes. The volumes of the scatterers are equivalent to those of spheres with diameters in the range between ~0.4 and 2.0 mum. Measurements of isolated organelles indicate that mitochondria and other similarly sized organelles are responsible for scattering at large angles, whereas nuclei are responsible for small-angle scattering. Therefore optical diagnostics are expected to be sensitive to organelle morphology but not directly to the size and shape of the cells.

We introduce a micro-optical model of soft biological tissue thatpermits numerical computation of the absolute magnitudes of itsscattering coefficients. A key assumption of the model is that therefractive-index variations caused by microscopic tissue elements canbe treated as particles with sizes distributed according to a skewedlog-normal distribution function. In the limit of an infinitelylarge variance in the particle size, this function has the samepower-law dependence as the volume fractions of the subunits of anideal fractal object. To compute a complete set of opticalcoefficients of a prototypical soft tissue (single-scatteringcoefficient, transport scattering coefficient, backscatteringcoefficient, phase function, and asymmetry parameter), we apply Mietheory to a volume of spheres with sizes distributed according to thetheoretical distribution. A packing factor is included in thecalculation of the optical cross sections to account for correlatedscattering among tightly packed particles. The results suggest thatthe skewed log-normal distribution function, with a shape specified bya limiting fractal dimension of 3.7, is a valid approximation of thesize distribution of scatterers in tissue. In the wavelength range 600