title: Method for studying gas composition in the human mastoid cavity by use of laser spectroscopy.
abstract: OBJECTIVE
  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.
authors.affiliation: Department of Otorhinolaryngology, University Hospital Lund, Lund, Sweden.
authors.forename: Sven
authors.initials: S
authors.lastname: Lindberg
authors.forename: Märta
authors.initials: M
authors.lastname: Lewander
authors.forename: Tomas
authors.initials: T
authors.lastname: Svensson
authors.forename: Roger
authors.initials: R
authors.lastname: Siemund
authors.forename: Katarina
authors.initials: K
authors.lastname: Svanberg
authors.forename: Sune
authors.initials: S
authors.lastname: Svanberg
mesh_terms: D006801:Humans; D054023:Lasers, Semiconductor; D008416:Mastoid / Q000378:metabolism*; D010100:Oxygen / Q000032:analysis; D015203:Reproducibility of Results; D013057:Spectrum Analysis; D014057:Tomography, X-Ray Computed; D014867:Water / Q000032:analysis
chemical_list: D014867:Water; D010100:Oxygen