Feasibility of Adult Lung Monitoring Using GASMAS: Development and Evaluation of Advanced Large-Geometry Phantoms

Abstract

Pulmonary complications such as pneumothorax and atelectasis are common in intensive care patients. Current diagnostic techniques, including computed tomography (CT) and X-ray imaging, are not suitable for continuous monitoring and expose patients to ionizing radiation. This motivates the development of continuous and non-invasive monitoring methods. Gas in Scattering Media Absorption Spectroscopy (GASMAS) is an optical technique that has shown promising results for neonatal lung monitoring. However, extending GASMAS to adult lung monitoring remains challenging because the thicker chest wall causes significant attenuation of the optical signal. This thesis investigates the feasibility of adult lung monitoring using Gas in Scattering Media Absorption Spectroscopy (GASMAS) through numerical simulations, phantom development, and experimental measurements. Monte Carlo simulations of photon transport were performed in voxelized thoracic geometries using the pmcx framework for both external and internal (esophageal) illumination configurations over a range of chest wall thicknesses and source-detector separations. A durable tissue-mimicking phantom was developed using a gelatin matrix containing titanium dioxide and India ink, and its optical properties were characterized by time-of-flight spectroscopy. In addition, the water-vapor-based GASMAS calibration method was evaluated numerically by comparing photon pathlength distributions at the oxygen (764 nm) and water vapor (935 nm) absorption wavelengths. The results show that a detectable GASMAS oxygen signature can be obtained through an adult-equivalent chest wall thickness of 3 cm. For realistic adult geometries, internal esophageal illumination produced consistently stronger relative signals than external illumination. In both simulation and experimental measurements, increasing the source-detector separation enhanced the GASMAS signal, although it also increased the noise level. The simulated and experimental signal trends were in good agreement. In addition, the equal-pathlength assumption underlying the water-vapor-based calibration method was validated, with photon pathlength distribution overlaps of 82 - 94 % across the simulated conditions. These findings support the feasibility of GASMAS for adult lung monitoring and identify internal illumination as a promising approach for clinical translation. At the same time, the results demonstrate that external illumination can also provide measurable GASMAS signals, highlighting its potential for non-invasive lung monitoring in adult thoracic geometries.

Popular Abstract

Every breath fills our lungs with millions of tiny air sacs. This research investigates whether laser light can pass through the chest and reveal whether these air sacs are healthy or whether problems have developed within the lungs.

The technique is called GASMAS (Gas in Scattering Media Absorption Spectroscopy). It works by sending near-infrared laser light through the body. As the light scatters through tissue, oxygen gas in the lungs absorbs specific wavelengths, leaving a unique fingerprint on the detected light. By measuring this fingerprint, GASMAS can determine whether air is present inside the lungs.

In intensive care units (ICUs), patients on mechanical ventilators can develop serious lung problems, such as a collapsed lung or air leaking into the chest cavity. Doctors currently rely on CT scans and chest X-rays to detect these complications. However, these methods expose patients to radiation and only provide occasional snapshots of the lungs. Continuous monitoring is not possible.

GASMAS offers a potential solution. The technique has already shown promising results in premature newborns, whose thin chest walls allow light to reach the lungs more easily. Applying the same method to adults is much more difficult because adult chest walls are thicker and absorb much more light.

This thesis explored whether GASMAS could work for adults. First, computer simulations were used to track how millions of light particles travel through the chest and lungs. Second, a reusable laboratory gelatin-based model of the adult chest wall was carefully formulated to mimic the way human tissue scatters and absorbs light. Laser measurements were then performed using this artificial tissue model.

The results were encouraging. Both simulations and experiments showed that an oxygen signal can still be detected through a chest wall up to three centimetres thick, which is representative of many adults.

The study also investigated a new approach in which the light source is placed inside the oesophagus, the tube that carries food from the mouth to the stomach. Because the oesophagus lies very close to the lungs, this method allows more light to reach the lung tissue and significantly improves the oxygen signal. Since many ICU patients already have feeding tubes inserted through the oesophagus, this approach could be practical in clinical settings.

Counter-intuitively, moving the detector further away actually strengthens the signal, as light taking a longer path is more likely to have travelled through the lungs. However, if the detector is moved too far away, the signal becomes too weak to measure accurately. Finding the best distance is therefore important.

Finally, the study tested a calibration method that uses water vapour naturally present in the lungs as a reference signal. This method helps determine oxygen levels without needing to know the exact path travelled by the light. Computer simulations confirmed that the approach works well under realistic conditions.

Overall, the results demonstrate the potential of GASMAS as a safe, radiation-free method for continuous monitoring of adult lungs. Delivering light through the oesophagus emerged as the most promising approach, bringing us closer to a future where laser light can continuously monitor lung health at the patient's bedside.

• author: Mawella Withanawasam, Isuru Chanilka
• supervisor: Anna-Lena Sahlberg; Emilie Krite Svanberg
• organization: Combustion Physics; Department of Physics
• course: PHYM03 20261
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: GASMAS, Near-infrared spectroscopy, Lung monitoring, Optical gas detection, Intensive care, Tissue-mimicking phantoms, Monte Carlo simulation
• language: English
• id: 9236328
• date added to LUP: 2026-06-15 07:07:38
• date last changed: 2026-06-15 07:07:38