Will pleural fluid affect surface wave speed measurements of the lung using lung ultrasound surface wave elastography: experimental and numerical studies on sponge phantom?

Pleural effusion manifested as compression of pleural fluid on the lung parenchyma, contributing to hypoxemia. Medical procedure such as drainage of plural fluid releases this compression and increase the oxygenation. However, the effect of pleural effusion on the elasticity of lung parenchyma is unknown. By using the lung ultrasound surface wave elastography (LUSWE) and finite element method (FEM), the effect of pleural effusion on the elasticity of superficial lung parenchyma in terms of surface wave speed measurement was evaluated in a sponge phantom study. Different thickness of ultrasound transmission gel simulated as pleural fluid was inserted into a condom which was placed between the sponge and standoff pad. A mechanical shaker was used to generate vibration on the sponge phantom at different frequencies ranging from 100 to 300 Hz while ultrasound transducer was used to capture the motion for measurement of surface wave speed of the sponge. FEM was conducted based on the experimental setup and numerically assess the influence of pleural effusion on the surface wave speed of the sponge. We found the influence of thickness of ultrasound transmission gel was statistically insignificant on the surface wave speed of the sponge at 100 and 150 Hz both from experiments the FEM.

💡 Research Summary

The paper investigates whether the presence and thickness of pleural fluid influence the surface wave speed measured by lung ultrasound surface wave elastography (LUSWE). Because pleural effusion compresses the lung parenchyma and contributes to hypoxemia, clinicians often drain the fluid to improve oxygenation, yet the mechanical effect of the fluid on lung elasticity has not been quantified. To address this gap, the authors designed an experimental phantom consisting of a household sponge (as a surrogate for lung tissue), an acoustic standoff pad (simulating thoracic muscle), and ultrasound transmission gel placed inside a condom to mimic pleural fluid. The gel thickness was varied at four levels: 0 mm (baseline), 2 mm, 7 mm, and 12 mm, representing increasing amounts of pleural fluid.

A mechanical shaker generated harmonic vibrations of 0.1 s duration at five frequencies (100, 150, 200, 250, and 300 Hz). A linear array transducer (6.4 MHz) captured the resulting motion on the sponge surface. For each frequency‑gel‑thickness combination, measurements were repeated three times, and surface wave speed was calculated from phase‑delay versus distance using eight equally spaced points along an ~8 mm segment of the sponge. Statistical analysis employed an unpaired two‑tailed t‑test with significance set at p < 0.05.

Experimental results showed that at the two lowest frequencies (100 Hz and 150 Hz) the surface wave speed was essentially unchanged across all gel thicknesses (e.g., 3.28 ± 0.08 m/s at 100 Hz). In contrast, at higher frequencies (200 Hz, 250 Hz, 300 Hz) the speed increased with gel thickness: approximately 10 % higher at 2 mm, 15 % higher at 7 mm, and up to 35 % higher at 12 mm relative to the baseline. This suggests that at higher frequencies the additional fluid layer contributes appreciable mass and shear stiffness, thereby accelerating wave propagation.

To complement the experiments, a finite‑element model (FEM) was built in ABAQUS 6.14. The model represented the sponge as a linear poro‑viscoelastic material (elastic modulus = 36.7 kPa, shear viscosity = 24 Pa·s), the standoff pad as a linear elastic solid (modulus = 6.83 kPa), and the transmission gel as a homogeneous medium with density 1000 kg/m³. The geometry matched the physical setup: a 12 cm × 2 cm sponge, a 9 cm × 1.5 cm pad, and gel layers of the experimentally measured thicknesses. A harmonic point load was applied to the top surface of the pad to simulate the shaker, and the bottom of the sponge was attached to an infinite element boundary to suppress reflections. Meshes used 1 mm × 1 mm quadrilateral plane stress elements, and the implicit dynamic solver with automatic time stepping ensured numerical stability. The simulated surface wave speeds closely reproduced the experimental trends: negligible variation at 100 Hz and 150 Hz, and progressive increases at higher frequencies proportional to gel thickness.

The authors draw several key conclusions. First, for LUSWE applications that employ low frequencies (≤150 Hz), pleural fluid thickness does not significantly bias surface wave speed measurements, implying that clinicians can reliably assess lung elasticity even in patients with moderate effusions without needing to drain the fluid first. Second, at frequencies ≥200 Hz, the presence of fluid does affect the measured speed, so either lower frequencies should be preferred or correction factors must be introduced when interpreting high‑frequency data. Third, the FEM framework proved valuable for predicting the influence of intervening fluid layers and can be extended to more realistic 3‑D lung models incorporating non‑linear, anisotropic tissue behavior.

Future work suggested includes (i) constructing a 3‑D, patient‑specific lung‑pleura‑fluid model that captures the heterogeneous, viscoelastic nature of real lung tissue; (ii) validating the phantom findings in vivo by comparing LUSWE measurements before and after therapeutic thoracentesis; and (iii) developing multi‑frequency inversion algorithms that automatically compensate for fluid‑induced speed changes, thereby enhancing the diagnostic utility of LUSWE for a broad range of pulmonary pathologies.