Measuring oxygen in lungs with lasers (Finished Master Project)

Published 1 November 2022

Many hands working on measuring on a pair of boar lungs. Photo — Here, a wild boar lung is used as a model similar to the lungs of a human child. A diode laser is used to check if the lung is filled with air.

Throughout life, our lungs are vital organs. For newborns as well as for elderly patients, there is a need for safe and continuous lung monitoring in the healthcare. Some of today's common methods of examining the lungs, such as X-rays and computed tomography are limiting since they can not be applied continuously and carry ionizing radiation. A new technology for clinical lung monitoring is now on the way: GASMAS. GASMAS stands for GAs in Scattering Media Absorption Spectroscopy. In other words, a laser is used to to try and measure the gas concentration in for example lungs. This technique is free of ionizing radiation and has the potential to provide continuous lung monitoring. Today, GASMAS can successfully measure oxygen in the lungs of newborns with lung problems which is simpler since distances are small in the small body. This thesis takes the next step and studies how GASMAS can be scaled up for larger patients.

A GASMAS system for measuring oxygen in lungs was put together in this thesis with good results. The system consisted of a diode laser with a continuous red light at 760 nm. This wavelength is used to match oxygen absorption energy. With low laser powers of 26 mW through a boar lung (seen in the image above), good oxygen signals could be measured through about four cm of lung tissue. Although expected, this good result was promising for scaling up the system for larger patients. Here, the biggest challenge is that there is more tissue and the laser signal will have a harder time to get through since the tissue both absorbs and scatters the light. To counteract this, the laser power must be turned up. Therefore, the next step was to amplify the laser light, while keeping the existing and successful GASMAS system intact. For this purpose, an optical semiconductor amplifier was incorporated. The amplifier had a three-micrometer input surface towards which the laser beam was directed. Fortunately, the laser light that hit the input surface was amplified, but unfortunately it was only a very small part of the total laser light. The challenge was that three micrometers is a really tiny opening (a strand of hair is about 50 micrometers thick). In order to obtain a GASMAS system with better amplification, future studies should continue the work of incorporating the amplifier. Achieving higher laser powers is critical to the goal of safe GASMAS monitoring of larger lungs. To verify that higher laser powers indeed allow light penetration into thicker tissue, a high-power system was used. This system could not be used for GASMAS, but it was suitable for studying light penetration through tissue of different thicknesses. Sliced Kassler pieces were used as a tissue model. The results show that it really pays to increase the laser power. With two Watts the light penetrate a full 14 cm of tissue!

Based on existing research, this thesis has constructed a low-power GAMAS system to examine the oxygen content of lungs over short distances. To use the system for adult persons, the power needs to be turned up. The next important challenge is therefore to continue the attempts at using the optical ampilifer which has more potential compared to what is show in this work where we have taken a step towards safe monitoring of lungs for more patient groups.

This work was done by Anna Brandt.