Bilirubin, a yellow pigment, is formed by the breakdown of hemoglobin. Neonates are susceptible to high bilirubin levels in their blood which places them at risk of neuronal damage, and monitoring of the bilirubin levels in these neonates is clinically required. Transcutaneous optical monitoring of the bilirubin will provide a non-invasive, inexpensive measurement of bilirubin in the skin.
The optical properties of skin are important for interpretation of the reflected light from the skin. In this report, the optical properties of neonatal skin were measured in the visible range on twenty in vitro skin samples. The scattering of the skin is dominated by the collagen fiber bundles in the dermis. Scattering in the dermis increases linearly with gestational maturity due to the accompanying increase in the size and number of the collagen fiber bundles. Scattering in the dermis was modelled by Mie and Rayleigh scattering.
The collection efficiency of an optical patch used for reflectance measurements at the skin surface varies with the skin's optical properties. The collection efficiency of the optical patch as a function of optical properties was determined by measurements in phantoms, and by Monte Carlo computer models. Dermal absorbers and epidermal melanin affect the reflected signals differently, and have to be analyzed separately.
In addition to bilirubin content of neonatal skin, other sources of variation include skin maturity, skin thickness, melanin content, blood depth, and blood content. Each of these factors affects the reflected spectrum. Each source of variation was analyzed individually and an algorithm was developed to determine the absorbances of bilirubin and blood in the dermis from optical patch reflectance measurements.
The algorithm was applied to analyze reflectance measurements performed on a heterogeneous clinical population consisting of 47 neonates. The algorithm was then adjusted to minimize a score designed to ensure that the determined in vivo cutaneous bilirubin concentrations were invariable with skin melanin and blood content. Consideration of optical transport in the skin has enabled the determination of cutaneous bilirubin concentration in heterogeneous neonatal populations.