Transcranial optical path length in infants by near-infrared phase-shift spectroscopy

Near-infrared spectroscopy (NIRS) is an emerging technique for noninvasive, bedside monitoring of cerebral oxygenation and blood flow. Traditionally, it has relied on the Beer's Law relationship in which the concentration of light-absorbing oxygen-carrying pigments is proportional to their light absorbance, and inversely proportional to an optical path length (a measure of the distance traveled by photons passing through the tissue). In practice, NIRS has been based upon assumptions that mean transcranial optical path length, the average optical path length for a given patient, is constant among patients and independent of the wavelength of light used.

The objective of our study was to measure mean optical transcranial path length in infants as a step in allowing quantitation of cerebral oxygenation.

We measured mean transcranial optical path length in 34 infants, aged 1 day to 3 years, using amplitude-modulated phase-shift spectroscopy at 754 nm and 816 nm. Optical transcranial path lengths (mean +/- SEM) were 8.6 +/- 0.9 cm, 11.1 +/- 0.9 cm, and 11.3 +/- 0.9 cm at 754 nm, and 8.8 +/- 0.9 cm, 11.2 +/- 0.8 cm, and 11.1 +/- 0.9 cm at 816 nm, using emitter-detector separations of 1.8, 2.5, and 3.0 cm, respectively. Optical path length increased as emitter-detector separation, head circumference, or age increased. Variance in the ratio of mean optical path lengths at the two different wavelengths exceeded that accounted for by variation in repeated measures alone (p < 0.001), suggesting that optical path length is also not independent of wavelength.

NIRS instrument emitter-detector geometry, subject age, head size, and wavelength used each influence optical path length. Quantitative NIRS measurements in clinical use may require concurrent measurement of both absorbance and optical path length at each wavelength, or use of newer measures that are not based upon Beer's Law assumptions.