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

Air accumulations within living organisms are sometimes pathologic. An exam ple is free air within the abdomen from perforation of the intestines (a co ndition called pneumoperitoneum). The objectives of the described research were to define the acoustic signatures of abdominal air cavities at low fre quencies and to investigate the feasibility of using these signatures for p neumoperitoneum diagnosis. The central hypothesis was that low-frequency vi bro-acoustic property changes are detectable using broad-band acoustic exci tation applied at the abdominal surface. Band-limited white noise (0-3200 H z) was introduced at the abdominal surface of sedated dogs and response was measured by a surface vibro-acoustic sensor. The transfer function and coh erence were estimated from these measurements. The presence of pneumoperito neum caused increased resonances and anti-resonances (p <0.01). Measures of the latter parameters were proposed and evaluated to quantitatively measur e their magnitude. Resonant spectral peaks of more than 3dB were consistent with pneumoperitoneum (p <0.01), and both resonance and anti-resonance inc reased 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. Th ese findings suggest that the proposed technique may be useful in the diagn osis of pneumoperitoneum.