Optical path length of 754nm and 816nm light emitted into the head of infants

Abstract

The emerging importance of light as a diagnostic and therapeutic tool in medicine has created a need to determine the optical properties of living tissues, such as the path length (L) of light through tissue. We investigated path length using modulated 754nm and 816nm light emitted into and scattered back from heads of 34 human infants, aged 1 day to 3 years, by measuring the phase shift in the returning signal. Measurements taken at 3 emitterldetector separations (1.8, 2.5, and 3.0cm) yielded L= 3.4 to 55cm. Path significantly increased with larger head circumference and emitterldetector separation. This relationship may be a reflection of the geometry between the emitter and detector, but only a fraction of the variability can be accounted for by these factors. We propose that in the assessment of cerebral optical properties, L is an important variable to measure if equations utilizing path length are to be quantitatively solved, such as when determining blood oxygen saturation via Beer’s Law. KEY WORDS: time of flight, scattering, infant, spectrophotometry, reflectance, path length.