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GASMAS offers nonivasive gas sensing for lung conditions

04 Nov 2021

Tests using Tyndall National Institute lung tissue phantom indicates value of technique in clinics.

Gas in Scattering Media Absorption Spectroscopy (GASMAS) is a laser-based technique for measuring gas concentrations enclosed by diffuse media.

Although first demonstrated back in 2001, the technique has not yet made a substantial transition into clinical practice for the diagnosis and monitoring of respiratory conditions.

A project at Ireland's Tyndall National Institute (TNI) has now demonstrated the potential value of technique by applying it to a custom lung tissue phantom structure, one designed to mimic the alveolar anatomy of the lung often left out of existing models due to its inherent complexity.

As published in Journal of Biomedical Optics, the Tyndall research also demonstrates the investment in time and resources required to take promising research technologies such as GASMAS and make them realistic clinical techniques.

"The clinical translation of existent technologies is tremendously challenging," commented Andrea Pacheco of the Biophotonics@Tyndall Group. "The work related to this article started two years ago."

GASMAS combines narrow-band diode-laser spectroscopy with diffuse media optical propagation, exploiting the fact that the absorption signal of gases is unique against the enclosing tissue background.

In the case of air in the lungs, absorption bands of oxygen and water are typically 0.001 nanometer in width, which can be distinguished from the absorption imprint of the organs around the lungs which is generally in the 10 nanometer range, according to the Tyndall project.

Application of this principle involves illuminating the thorax with tunable diode lasers operating at 760 nanometers to match the absorption bands of oxygen and 820 or 935 nanometers to match absorption of water as a reference gas. The water absorption spectra is used to calculate the gas absorption path length in the lung, via the Beer-Lambert law, and this parameter can then be used alongside the measured oxygen signal to work out the oxygen concentration in the lung.

The Tyndall project set out to determine whether GASMAS could therefore effectively be used to monitor changes in gas volume in the lungs, as an alternative to conventional spirometry techniques which can struggle to indicate localized volume changes in the lungs' complex alveolar structures.

Respiratory treatment and diagnosis

This in turn required a suitably realistic lung phantom model to use as a target for GASMAS analysis, one that could recreate inflated alveoli surrounded by lung tissue.

The novel lung phantom developed by the TNI Biophotonics team features a system of capillaries and air pockets that can be variably and progressively filled with liquid that matches the optical properties of lung tissue, allowing incrementally variable pockets of air that mimic air-filled alveolar sacs.

In trials the lung model successfully enabled study of the changes in the GASMAS signals generated by the dual oxygen/water system when the transmittance geometry changed. This proved the feasibility of sensing volume changes with a GASMAS technique, and potentially opens up a new application of GASMAS in respiratory treatment and diagnostics.

The project team commented that future work could see the implementation of GASMAS via pulmonary endoscopy, in a similar way as OCT has become a complementary tool in bronchoscopy procedures.

"The implementation of GASMAS technology in the clinic creates the capability to locally assess the inflation of lung tissue, which is not possible with the existing diagnostic procedures such as spirometry and plethysmography," noted TNI in its paper.

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