Abstract
Pulmonary diseases and injury lead to structural and functional changes in the lung parenchyma and airways, often resulting in measurable sound transmission changes on the chest wall surface. Additionally, noninvasive imaging of externally driven mechanical wave motion in the chest (e.g., using magnetic resonance elastography) can provide information about lung stiffness and other structural property changes which may be of diagnostic value. In the present study, a comprehensive computational simulation (in silico) model was developed to simulate sound wave propagation in the airways, parenchyma, and chest wall under normal and pathological conditions that create distributed structural (e.g., pneumothoraces) and diffuse material (e.g., fibrosis) changes, as well as a localized structural and material changes as may be seen with a neoplasm. Experiments were carried out in normal subjects to validate the baseline model. Sound waves with frequency content from 50 to 600 Hz were introduced into the airways of three healthy human subjects through the mouth, and transthoracic transmitted waves were measured by scanning laser Doppler vibrometry at the chest wall surface. The computational model predictions of a frequency-dependent decreased sound transmission due to pneumothorax were consistent with experimental measurements reported in previous work. Predictions for the case of fibrosis show that while shear wave motion is altered, changes to compression wave propagation are negligible, and thus insonification, which primarily drives compression waves, is not ideal to detect the presence of fibrosis. Results from the numerical simulation of a tumor show an increase in the wavelength of propagating waves in the immediate vicinity of the tumor region.
Highlights
1.1 Motivation The lungs are comprised of soft tissue, airways, alveoli and vasculature structures, acoustically behaving as a poroviscoelastic material over a broad frequency range [1]
From fig. 6, it is observed that the frequency response function (FRF) obtained from computational simulation follows a similar trend as that of the average plus one standard deviation FRF with respect to the spectral magnitudes and features
It is to be noted that this trend is captured more so in the measurements performed on the left side of the torso (fig. 6 (a), (c) and (e)) than in the FRF values computed for points on the right side of the torso (fig. 6 (b), ((d) and ((f))
Summary
1.1 Motivation The lungs are comprised of soft tissue, airways, alveoli and vasculature structures, acoustically behaving as a poroviscoelastic material over a broad frequency range [1] Due to this fine-structure heterogeneity, conventional imaging modalities such as magnetic resonance imaging (MRI), ultrasound (US) and X-ray computed tomography (CT) have limited resolution to detect and quantify changes in the anatomy and properties in vivo that may be indicative of many pulmonary pathologies, including subtle fibrosis, inflammation, tumors, pneumonia and pneumothorax [2]. A better understanding of propagation of mechanical waves inside the pulmonary structures may help improve the utility of MRE for diagnosis and monitoring of disease states This understanding can be achieved through the development of an experimentally validated comprehensive computational model of sound propagation inside the human torso, as described in this article. The techniques of insonification and percussion (defined and explained in section 1.2) used in the present study, focus not on impedance but rather on how sound is transmitted throughout the torso region, and measured on the torso surface
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