Introduction The light distribution in a tissue can be characterized with the absorption coefficient µa, the scattering coefficient µs, and the anisotropy factor g. A method to calculate these from a measurement of total reflection Rt, collimated transmission Tc, and diffuse transmission Td, of a slab of tissue, the inverse adding-doubling method, has been developed earlier. We developed an experimental set-up to measure Rt, Tc and Td simultaneously. The accuracy of the system was tested using a known tissue phantom.
Materials and Methods A mathematical model of the set-up relating the radiance on the sphere wall to the reflection or transmission of the sample was developed. The model is calibrated using a set of standard reflection plates. Experiments are done using phantom solutions made of Evans Blue Intralipid-10%. By mixing these solutions in different proportions abledos from 0.001 to 0.999 can be obtained. The optical thickness, tau, of a phantom can be reduced by dilution. The range of tau thus obtained was from 50 to 0.1. The anisotropy factor is assumed to be constant.
Results & Conclusions The experiments show that results with an accuracy better than 5% are obtained if the optical depth tau of the sample in the direction of the laser beam is in the range 1‹tau‹10. Errors larger than 5% may occur in the µs and g if the albedo is smaller than 0.4 and in µa if the albedo is larger than 0.95. Within this range the combination of experimental set-up and method of data analysis are excellently suitable for measurements of µa, µs and g.
N. van Wieringen, S. A. Prahl, H. J. C. M. Sterenborg, and M. J. C. van Gemert. The limitations of the determination of the optical properties of tissue using a double integrating sphere set-up with collimated incident light. Lasers Med. Sci., 1990.