Abstract

The quantitation of μ_a and μ_s′ in suspensions of intracellular organelles identifies the principal scatterers and allows a comparison with whole cells. Small quantities of the specimens or small biological objects are available (a few grams), and the assumption of semi-infinite boundaries for photon diffusion in highly scattering media are violated. Immersing the sample in a semi-infinite scattering medium of matched μ_a/μ_s′ affords a solution to this problem. A match is assumed when no change of μ_a and μ_s′ in the external medium is observed when the sample is immersed into the optical field. Suspensions of mitochondria give μ_a ≤ 0.04 cm^-1 and μ_s′= 15 cm^-1. Ascites tumor cells give μ_a ≤ 0.003 cm^-1 and μ_s′ = 12 cm^-1. Localization of such small objects by interference of coherent waves of photon density¹ employs a four element phased array with a sensitivity at the null of>20°/mm at 200 MHz in a medium of μ_s′ = 10cm^-1. This localized beam is used to detect the small changes of μ_a and μ_s′ characteristic of small breast tumors or localized absorbers/scatterers in the brain as in Alzheimers disease.

© 1992 Optical Society of America