Quantitation and localization of scattering and absorption properties of small tissue volumes by time and frequency domain spectroscopy

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.