Sound Transmission in Human Lung by Percussion
Measuring sound transmission through the torso and lungs may be of value if altered transmission patterns correlate with pathology in ways that can be detected and used to provide a reliable and quantitative diagnosis of disease or injury. An accurate computer simulation of the sound transmission may aid in interpreting experimental measurements.
In the present study, vibratory excitation in the frequency range of 50-400 Hz is implemented on the chest surface of two normal human subjects. The technique used introduces mechanical waves of broad frequency content that is more controllable than the low frequency content obtained by tapping in conventional auscultatory percussion. The underlying hypothesis of the technique is that the high-frequency mechanical waves generated by the vibratory excitation on the anterior chest propagate to the posterior chest through internal organs between the location of measurement and the point of excitation. The change in intrathoracic organs would partially alter the wave propagation and the resulting motion amplitude at the measurement location. By applying the vibratory excitation to the sternum, the posterior torso surface motion excitation was experimentally measured and used to validate the results obtained from a computer simulation of the vibratory response of the torso to the external excitation source. This computer simulation model is then used to predict changes in acoustic transmission caused by a pneumothorax (PTX). The developed computational simulation models may be of use in assessing the performance of other acoustic approaches and the diagnosis of other injuries and diseases.
- Experimental studies were carried out on two healthy human subjects after receiving appropriate Institutional Review Board (IRB) approval. The subject was seated on a chair.
- A plexiglass disk of radius 15 mm was gently pressed against the chest surface on the top of the sternum and driven by an electromagnetic shaker (ET-132, Lab-Works Inc., Mesa Costa, CA) that was connected to a power amplifier (P 3500S, Yamaha, Buena Park, CA). The vibratory excitation has a spectral content from 50 – 400 Hz.
- The velocities on the back of the human subject were measured by a scanning laser Doppler vibrometer (SLDV) (PSV-400, Polytec, Irvine, CA).
- Retro reflective glass beads (nominal diameter = 45 – 63 µm) were applied to the skin surface of subjects to enhance the laser reflectivity to enable an improved signal-to-noise ratio. An impedance head (288D01, PCB Piezotronics, Depew, NY) was mounted on the disk to measure its acceleration, serving as a reference for the SLDV measurement.
- An array of points on the skin surface of the back of the subject were measured by the SLDV and processed to determine the frequency response function (FRF) of the acceleration on human back over the acceleration measured by the impedance head. There were 54 scan points on each side of the back (total of 108 scan points).
- For human subject simulations, a 3-dimensional (3D) computational model for simulating the vibratory response of the human upper torso is developed by using the CT image sets from the Visible Human Male (VHM) of the National Library of Medicine. The dimension of the human torso model was rescaled to the approximate size of the human subjects (HS) in our experiment.
- The soft tissue, ribcage, sternum, scapula and lungs were separated to create an accurate geometric model. Material property values used for the different tissue regions were based on previous studies.
- Harmonic vibratory excitation with displacement amplitude of 1 mm was applied on the sternum. The simulations were performed in a finite element (FE) software COMSOL® 4.3 using the harmonic analysis acoustic-solid interaction module.
- To simulate the pathological condition, the original right lung region of the 3D model was separated into two parts, the collapsed lung region and air region. Two different levels of collapse were considered, which resulted in air volume fraction ratio in lung & phi =28% and & phi =58% . Those two values, respectively, correspond to PTX by volume percentages of 89% and 53%. The compression wave speed and attenuation were used as the material properties, respectively, calculated by Biot theory for lungs at & phi = 75%, & phi = 58%, and & phi = 28%.
Results of Porcine Experimental and Simulation Studies
FRF of simulation and experiment at 60 Hz (color bar in dB scale)
FRF of simulation and experiment at 120 Hz (color bar in dB scale)
FRF results at points #15(a), #24(b), and #41(c) on posterior chest (left/right sides correspond to left/right columns).
Comparison of normal and PTX state on right side of the posterior chest by simulation (at point #41 ). FRF amplitude drops at PTX state.
Simulation for PTX on right lung at 60 and 120 Hz. FRF amplitude drops at PTX state.
Cross section of torso showing displacement in the anterior posterior direction. Color bar in µm. Vibratory wave in anterior chest and lung can be seen.