Method for Studying Gas Composition in the Human Mastoid Cavity by Use of Laser Spectroscopy

Abstract

Objectives:

We evaluated a method for gas monitoring in the mastoid cavity using tunable diode laser spectroscopy by comparing it to simultaneously obtained computed tomographic (CT) scans.

Methods:

The presented optical technique measures free gases, oxygen (O₂), and water vapor (H₂O) within human tissue by use of low-power diode lasers. Laser light was sent into the tip of the mastoid process, and the emerging light at the level of the antrum was captured with a detector placed on the skin. The absorption of H₂O was used to monitor the probed gas volume of the mastoid cavity, and it was compared to the CT scan–measured volume. The ratio between O₂ absorption and H₂O absorption estimated the O₂ content in the mastoid cavity and thus the ventilation. The parameters were compared to the grading of mastoid cavities based on the CT scans (n = 31). The reproducibility of the technique was investigated by measuring each mastoid cavity 4 times.

Results:

Both O₂ and H₂O were detected with good reproducibility. The H₂O absorption and the CT volume correlated (r = 0.69). The average ratio between the normalized O₂ absorption and the H₂O absorption signals was 0.7, indicating a lower O₂ content than in surrounding air (expected ratio, 1.0), which is consistent with previous findings made by invasive techniques. All mastoid cavities with radiologic signs of disease were detected.

Conclusions:

Laser spectroscopy monitoring appears to be a usable tool for noninvasive investigations of gas composition in the mastoid cavity, providing important clinical information regarding size and ventilation.