Numerical study on lung ultrasound B-line formation as a function of imaging frequency and alveolar geometries

Abstract

Lung ultrasound (LUS) has become a widely adopted diagnostic method for several lung diseases. However, the presence of air inside the lung does not allow the anatomical investigation of the organ. Therefore, LUS is mainly based on the interpretation of vertical imaging artifacts, called B-lines. These artifacts correlate with several pathologies, but their genesis is still partly unknown. Within this framework, this study focuses on the factors affecting the artifacts' formation by numerically simulating the ultrasound propagation within the lungs through the toolbox k-Wave. Since the main hypothesis behind the generation of B-lines relies on multiple scattering phenomena occurring once acoustic channels open at the lung surface, the impact of changing alveolar size and spacing is of interest. The tested domain is of size 4 cm × 1.6 cm, the investigated frequencies vary from 1 to 5 MHz, and the explored alveolar diameters and spacing range from 100 to 400 μm and from 20 to 395 μm, respectively. Results show the strong and entangled relation among the wavelength, the domain geometries, and the artifact visualization, allowing for better understanding of propagation in such a complex medium and opening several possibilities for future studies.

State of the Art in Lung Ultrasound, Shifting from Qualitative to Quantitative Analyses

“…Only recently researchers have started to perform studies on LUS in silico. The first study investigated the dependence of vertical artifacts on alveolar diameter and spacing (i.e., distance between alveoli) (Peschiera et al 2021). The numerical simulations were performed using the k-Wave MATLAB toolbox (Treeby and Cox 2010), which was used to replicate a simplified lung structure.…”

“…The numerical simulations were performed using the k-Wave MATLAB toolbox (Treeby and Cox 2010), which was used to replicate a simplified lung structure. Specifically, the first 2 cm of depth was composed of muscle tissue, whereas air inclusions (alveoli) were introduced beyond 2 cm (Peschiera et al 2021). These alveoli were periodically arranged to maintain the same distance between each other, in both the axial and lateral directions.…”

Diagnostic Value of Color Doppler Ultrasonography in Subacute Thyroiditis

“…The ratio of male to female patients was 47 : 43. The highest age of the patients was 69 years old, and the lowest was 21 years old, with an average of (40.37 ± 4.36) years old; the longest course of disease was 8 months and the shortest was 3 days, with an average of (45.38 ± 7.25) days [ 11 ]. In the same period, 90 healthy persons were selected as the control group, of which the male to female ratio was 55 : 45.…”

All B-lines are equal, but some B-lines are more equal than others

“…The frequency response of these bubbly structures is more likely associated to specific resonance phenomena. A subsequent work [ 18 ] by the same group tested the genesis of the artifacts on a laboratory model by reproducing the alveoli shape with the alveolar interstitium level, with known dimensions; the authors showed that low imaging frequencies (i.e., f 2 MHz) generally do not allow the generation of consistent artifacts, regardless of the alveolar diameter choice. Only when the alveolar spacing assumes the largest investigated value (i.e., s 395 lm) do the lowest frequencies start to enable the B-lines’ formation.…”

“…Therefore, the final representation of the vertical artifact on B-mode ultrasound seems to strongly depend on the frequency of the ultrasounds employed by the probe (therefore on the setting of the machine), and on the three-dimensional structure of the interstitium-alveolar spaces. The authors argued that those factors can affect the representation of the artifacts on the ultrasound image, and in the near future it will be possible to differentiate different pictures of different diseases (with alveolar interstitium involvement) on the basis of the iconographic representation of the vertical artifacts [ 17 , 18 ]. The frequency characterization of vertical artifacts can be used as an indirect measure of the state of the lung, i.e., the lower the frequency at which B lines are visualized, the larger the channels formed between the alveoli and the more severe the lung condition [ 17 , 18 ].…”