Optical pathlength measurements on adult head, calf and forearm and the head of the newborn infant using phase resolved optical spectroscopy

TLDR

The differential pathlength factor (DPF) is defined as the absolute optical pathlength divided by the source‑detector spacing and is reported to be roughly constant for spacings over 2.5 cm. The study aims to quantify the statistical variation of DPF across 100 adults and 35 newborn infants. A portable intensity‑modulated near‑infrared spectrometer measuring phase shift at 200 MHz across four wavelengths (690, 744, 807, 832 nm) was used to determine absolute optical pathlengths at source‑detector spacings >4 cm. DPF at 807 nm ranged from 4.16 ± 18.8 % (adult arm) to 6.26 ± 14.1 % (adult head), with a 4.99 ± 9 % value for newborn heads; DPF also varies with wavelength and shows sex differences in adult arm and leg, enabling improved chromophore quantitation.

Abstract

The authors have used an intensity modulated optical spectrometer, which measures the phase shift across tissue experienced by intensity modulated near-infrared light, to determine the absolute optical pathlength through tissue. The instrument is portable and takes only 5 s to record pathlength at four wavelengths (690 nm, 744 nm, 807 nm and 832 nm). The absolute pathlength divided by the known spacing between the light source and detector on the skin is the differential pathlength factor (DPF) which previous studies have shown is approximately constant for spacings greater than 2.5 cm. DPF results are presented for measurements on 100 adults and 35 newborn infants to determine the statistical variation on the DPF. All measurements were made at a frequency of 200 MHz with source-detector spacings of >4 cm. Results at 807 nm show a DPF of 4.16(+or-18.8%) for adult arm, 5.51(+or-18%) for adult leg, 6.26(+or-14.1%) for adult head and 4.99(+or-9%) for the head of a newborn infant. A wavelength dependence was obtained for DPF on all tissues and a difference in DPF between male and female was observed for both the adult arm and leg. The results can be used to improve the quantitation of chromophore concentration changes in adults and newborn infants.