Modeling photon transport in transabdominal fetal oximetry

The possibility of optical oximetry of the blood in the fetal brain measured across the maternal abdomen just prior to birth is under investigation. Such measurements could detect fetal distress prior to birth and aid in the clinical decision regarding Cesarean section. This paper uses a perturbation method to model photon transport through an 8-cm-diam fetal brain located at a constant 2.5 cm below a curved maternal abdominal surface with an air/tissue boundary. In the simulation, a near-infrared light source delivers light to the abdomen and a detector is positioned up to 10 cm from the source along the arc of the abdominal surface. The light transport [W/cm² fluence rate per W incident power] collected at the 10 cm position is T_m = 2.2×10^-6 cm^-2 if the fetal brain has the same optical properties as the mother and T_f = 1.0×10^-6 cm^-2 for an optically perturbing fetal brain with typical brain optical properties. The perturbation P = (T_f-T_m)/T_m is -53% due to the fetal brain. The model illustrates the challenge and feasibility of transabdominal oximetry of the fetal brain.