Phase measurement of light absorption and scatter in human tissue

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 ( a ) and scattering ( s Ј) 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 nm͒. Significant absorption is mainly due to 5%-10% of the tissue volume occupied by blood. Measurement of a and s Ј 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. © 1998 American Institute of Physics. ͓S0034-6748͑98͒01110-1͔ I. BACKGROUND OF THE PROBLEM SET TIME: INTRODUCTION TO THE NATURE OF THE MEDICAL DEVICESThe burgeoning interest in medical devices that are safe, economical, and efficacious, combined with recent observations of the feasibility of time domain optical measurements in human and animal brain, has led to significant interest in, and many publications on, the optical characteristics of human tissues, 1 particularly breast tissue, 2 brain tissue, 3,4 and skeletal muscle. 5 These novel techniques not only allow the measurement of the tissue absorption but also allow the independent measurement of tissue scatter which can also be of great clinical significance. 6,7 Most recently, flow, particularly of blood cells through tissue, can be detected using autocorrelation techniques in the time domain. 8 These reduced transport scattering ( s Ј) and absorption ( a ) parameters are measured in what is termed the near infrared ͑NIR͒ window lying between the declining absorption of blood and the increasing absorption of tissue water in the wavelength region 700-900 nm ͑see Fig. 1͒. In this region, scattering dominates absorption by 2 orders of magnitude and the average distance between isotropic scattering events is approximately 1 mm, while photon propagation pathways in the human head may readily reach 1 m enabling the sampling of large tissue volumes. 9 Thus, measurements at various wavelengths can separate the contributions of the parameters to the total absorption and scattering. For example, for oxy Hb, deoxy Hb, and water three wavelengths will suffice, and since rapidly modulated light is advantageous, laser diodes are most frequently employed as monochromatic light sources at...

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