Diode laser spectroscopy for oxygen detection in the lungs of infants

Abstract: This thesis work is part of a larger project with final goal to be able to measure the oxygen concentration in the lungs of infants, using non-invasive laser spectroscopy. This thesis is specifically dealing with simulations of light transport through the upper body of infants, as well as constructing a tissue-like model (phantom) that can be used in experiments. The goal is to use these simulations, along with measurements on the phantoms to obtain a better knowledge about how the final measurement system should be designed. The phantoms are constructed to have similar structures to an infant and correct optical properties. The gas detection in the lung of the phantom is implemented by GASMAS, which stands for GAs in Scattering Media Absorption Spectroscopy. It utilises a laser at 760 nm for oxygen detection and a laser at 935 nm for water vapour detection. The software employed to do numerical simulations is named Nirfast. It includes two parts, one is to do segmentation of different tissues from a series of CT images, and based on that, light propagation simulations are done; the other one is to do measurement geometry evaluation, in which we define a parameter of sensitivity to indicate how a measurement geometry is sensitive to the gas in the lung. The measurement geometries that have a high sensitivity value are selected and are used for GASMAS measurements. An extensive literature research on optical properties at 760 nm of skin, fat, muscle, lung, heart and bone (the segmented tissues) is done and the values are applied to the simulation. There are two kinds of phantoms, one is the 3D printed models of only the boundary of different tissues. The other one is the liquid phantom of fat, muscle and heart with correct absorption and scatter coefficients, made of Indian ink and intralipid. GASMAS measurements on both water vapour and oxygen are done at atmospheric conditions (21% oxygen) and the results are compared to the simulation results.