Acoustic characteristics of air cavities at low audible frequencies with application to pneumoperitoneum detection.

Air accumulations within living organisms are sometimes pathologic. An example is free air within the abdomen from perforation of the intestines (a condition called pneumoperitoneum). The objectives of the described research were to define the acoustic signatures of abdominal air cavities at low frequencies and to investigate the feasibility of using these signatures for pneumoperitoneum diagnosis. The central hypothesis was that low-frequency vibro-acoustic property changes are detectable using broad-band acoustic excitation applied at the abdominal surface. Band-limited white noise (0-3200 Hz) was introduced at the abdominal surface of sedated dogs and response was measured by a surface vibro-acoustic sensor. The transfer function and coherence were estimated from these measurements. The presence of pneumoperitoneum caused increased resonances and anti-resonances (p < 0.01). Measures of the latter parameters were proposed and evaluated to quantitatively measure their magnitude. Resonant spectral peaks of more than 3 dB were consistent with pneumoperitoneum (p < 0.01), and both resonance and anti-resonance increased with condition severity (p < 0.03). The data also suggest a possible reduction in the resonant and anti-resonant frequencies with decreasing air cavity volumes (p = 0.14) as supported by theoretical predictions. Finally, anti-resonance was also found to be associated with a drop in coherence. These findings suggest that the proposed technique may be useful in the diagnosis of pneumoperitoneum.