Phase measurement of light absorption and scatter in human tissue

TLDR

Human tissues are highly scattering (mean inter‑scatterer distance < 1 mm) and absorb strongly (5–10 % blood volume), prompting the development of analog and digital photon‑delay measurement technologies. The apparatus is designed to quantify tissue absorption (μa) and reduced scattering (μs′) using frequency‑domain photon‑delay measurements. Frequency‑domain techniques are employed, comparing homodyne and heterodyne, single‑ and multi‑sideband, and multiplexed optical/radio‑frequency encoding methods. These phase‑modulation systems enable quantitative brain and muscle blood oximetry, forebrain functional imaging, and other medical diagnostics.

Abstract

Analog and digital technologies are presented for precise measurement of propagation delay of photons from source and detector placed on portions of the human body. The goal of the apparatus design is to quantify absorption (μa) and scattering (μs′) induced by biological pigments and biological structures, respectively. Body tissues are highly scattering with a mean distance between scatterers of less than a mm (at 700–850 nm). Significant absorption is mainly due to 5%–10% of the tissue volume occupied by blood. Measurement of μa and μs′ is done by both time and frequency domain equipment. This article focuses upon frequency domain equipment because of its simplicity, reduced noise bandwidth, versatility, and the strong analogy to very high frequency/ultrahigh frequency communication devices, particularly those using phase modulation. Comparisons are made of homodyne and heterodyne systems together with evaluation of single and multiple side band systems, with particular emphasis on methods for multiplexed optical and radio frequencies by frequency encoding or time-sharing technologies. The applications of these phase modulation systems to quantitative brain and muscle blood oximetry, functional activity of the forebrain, and other important problems of medical science, are presented.