Simulation of Photon Propagation in Multi-layered Tissue for Non-invasive Fetal Pulse Oximetry

Abstract

Monitoring the state of health from fetus and mother non-invasively is still a big challenge in prenatal diagnostics. In this contribution the photon propagation in the maternal abdomen of pregnant women was simulated in the near-infrared range. To determine the fetal oxygenation it is necessary to ensure a sufficient penetration depth. This means that the photons have to pass through the maternal layers, the amniotic fluid and finally have to reach the fetus. It is essential that the reflected signal, recorded by a photo sensor, contains information about both mother and fetus.

The penetration depth of the injected photons and their propagation inside of the tissue were analyzed by using a model of steady-state light transport which is based on the Monte Carlo method. A simplified multi-layer tissue model with maternal tissue and blood vessel, amniotic fluid, fetal tissue and blood vessel layer was implemented. The dominant reflection layer is a fetal skull layer and the fetal brain is the last considered layer of the tissue model. The dilatation of the maternal and fetal blood vessels was realized by varying the layer thickness for each of the simulation scenarios. To analyze the penetration depth inside of the tissue, a photon tracking was implemented to mark whether the photon reaches the fetus.

The simulation results show the physical photon interaction in tissue by using the Monte Carlo method. Furthermore the analysis illustrates the fundamental feasibility of the noninvasive fetal pulse oximetry. The investigations demonstrate an inversely proportional dependence between the dilatation of blood vessels and the number of reflected photons which passed the maternal and fetal tissue. This relation allows the simulation of abdominal pulse curves of pregnant women. These synthetic signals might provide basic data for the evaluation of a separation algorithm which is able to estimate the fetal oxygenation.