Optical attenuation characteristics of breast tissues at visible and near-infrared wavelengths.

Optical experiments are described for measuring the attenuation characteristics of breast tissues at visible and near-infrared wavelengths. Total attenuation coefficients post mortem were measured directly in thin tissue sections. They are usually within the range from 10 to 30 mm-1, are rather higher in fat than in fibroglandular specimens and decrease with increasing wavelength. The scattering phase function is strongly forward-peaked with the mean cosine of scattering in the range from 0.85 to 0.97 and appearing more forward-peaked in fat than in fibroglandular tissue. The reduced scattering coefficient is of the order of 1 mm-1 in all tissues. Absorption coefficients were measured indirectly in optically thick sections. They are typically between 0.1 and 0.5 mm-1 at wavelengths around 580 nm and an order of magnitude lower at 850 nm. At 580 nm and shorter wavelengths the absorption in carcinoma is significantly higher than in adjacent uninvolved tissue. Significant differences were observed in the first-order derivatives of the transmission spectra of carcinoma and surrounding tissues at certain infrared wavelengths. Transmission spectra measured in vivo across the wavelength range from 500 to 860 nm have a similar form to the spectra of excised samples. Linear absorption coefficients are generally of the same order of magnitude as those found in vitro although they are lower at green wavelengths.