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It is easy to measure the absorption coefficient (μ_a [cm^-1]) and the reduced scattering coefficient (μ_s' [cm^-1]) in biological tissues or materials using a variety of optical measurements that involve multiple photon scattering, for example:

• integrating sphere measurements of reflectance (R) and transmission (T)
• reflectance (R) as a function of distance (r) from source, R(r)
• added absorber experiments

However, separation of μ_s' = μ_s(1-g) into the separate values of scattering coefficient (μ_s') and anisotropy (g) is often problematic.

In this report, we use a reflectance-mode confocal microscope to scan down into tissues. The exponential decay of the reflected confocal signal as a function of depth of focus (z_f) is determined, fitting for an amplitude (A) and an exponential attenuation (μ):

SIGNAL = ρ exp(-μ z_f)

The two parameters, ρ and μ, map into the two unknown optical properties, μ_s and g.

Since confocal microscopy depends on singly scattered photons, it is sensitive to the value of anisotropy (g). Hence, it becomes possible to characterize an in vivo tissue site for its apparent values of μ_s and g, which are related to the density and size distribution of the ultrastructure (nuclei, mitochondria, collagen fibers, etc.) of a tissue.

This report describes measurements on 5 types of mouse tissues (brain white matter, brain gray matter, skin, muscle, liver). The results indicate the potential for using anisotropy (g) as an endogenous contrast agent in imaging.

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