The Audible Human Project: Modeling Sound Transmission in the Lungs and Torso
Auscultation has been used qualitatively by physicians for hundreds of years to aid in the monitoring and diagnosis of pulmonary diseases. Alterations in the structure and function of the pulmonary system that occur in disease or injury often give rise to measurable changes in lung sound production and transmission. A better understanding of sound transmission and how it is altered by injury and disease might improve interpretation of lung sound measurements. A long-term goal of the Audible Human Project (AHP) is to develop a computational acoustic model that would accurately simulate generation, transmission and noninvasive measurement of sound and vibration within the pulmonary system and torso. The goals of this dissertation research, fitting within the scope of the AHP, are to develop specific improved theoretical understandings, computational algorithms and experimental methods aimed at transmission and measurement. In this dissertation, tissue shear viscoelasticity was studied by applying two experimental identification approaches. Utilizing the frequency response function between the excitation location and points at known distances generally provides a more accurate estimation of viscoelastic parameters than utilizing the surface wave speed. A poroviscoelastic model based on Biot theory of wave propagation in porous media was used for compression waves in the lungs. The measured fast compression wave speed and attenuation on an excised pig lung had good agreement with theoretical predictions. Lung excitation through airway insonification was simulated in a real human lung geometry with integration of the simplified computer generated airway tree. Similar lung surface velocity distribution patterns were observed in pig lung experiments. Finally sound transmission was measured on human subjects by the scanning laser vibrometry and piezodisk sensors. The advantages and disadvantages of each technique were discussed.