Abstract

We have designed and constructed a near-infrared spectrometer for the non-invasive optical study of biological tissue. This instrument works in the frequency-domain and employs multiple source-detector distances to recover the absorption coefficient ((mu) <SUB>(alpha</SUB> )) and the reduced scattering coefficient ((mu) <SUB>s</SUB>') of tissue. The light sources are eight light emitting diodes (LEDs) whose intensities are modulated at a frequency of 120 MHz. Four LEDs emit light at a peak wavelength of 715 nm ((lambda) <SUB>1</SUB>), while the other four LEDs emit at a peak wavelength of 850 nm ((lambda) <SUB>2</SUB>). From the frequency-domain raw data of phase, dc intensity, and ac amplitude obtained from each one of the eight light sources, which are located at different distances from the detector fiber, we calculate (mu) <SUB>(alpha</SUB> ) and (mu) <SUB>s</SUB>' at the two wavelengths (lambda) <SUB>1</SUB> and (lambda) <SUB>2</SUB>. The concentrations of oxy- and deoxy-hemoglobin, and hence hemoglobin saturation, are then derived from the known extinction coefficients of oxy- and deoxy-hemoglobin at (lambda) <SUB>1</SUB> and (lambda) <SUB>2</SUB>. The statistical error in the measurement of the optical coefficients due to instrument noise is about 1 - 2%. The accuracy in the determination of the absolute value of the optical coefficients is within 10 - 20%. Preliminary results obtained in vivo on the forearm of a volunteer during an ischemia measurement protocol are presented.

Highlights

  • AND BACKGROUNDIn the study ofbiological tissue for medical diagnostic, monitoring, and therapeutical purposes, optical methods can achieve relevance if they provide a quantitative determination of the tissue optical properties

  • To model the non-invasive configuration where light sources and detector fiber are placed on the surface of the tissue, we apply semi-infinite medium boundary conditions.2'10 Our instrument implements a multiple source-detector distances protocol, which is effective in independently and simultaneously measuring the absolute values of Iia(absorption coefficient) and p'

  • We employed a tissue-like phantom consisting of an aqueous solution of Liposyn 20% and black India ink

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Summary

INTRODUCTION

In the study ofbiological tissue for medical diagnostic, monitoring, and therapeutical purposes, optical methods can achieve relevance if they provide a quantitative determination of the tissue optical properties. The optical study of tissue in the near-infrared is complicated by the strong scattering experienced by photons in tissue In these conditions, light intensity attenuation is caused by both absorption and scattering, and the average optical path length is unknown. 100 MHz frequency region, and we have pointed out the properties that render LEDs suitable devices for medical applications.[8] The theoretical background for light propagation in turbid media is provided by diffusion theory.[9] To model the non-invasive configuration where light sources and detector fiber are placed on the surface of the tissue, we apply semi-infinite medium boundary conditions.2'10 Our instrument implements a multiple source-detector distances protocol, which is effective in independently and simultaneously measuring the absolute values of Iia(absorption coefficient) and p'

DESCRIPTION OF THE SPECTROMETER
In vitro
In vivo
DISCUSSION
CONCLUSION
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