## 4 Discussion  
Lung ultrasound is able to intercept peripheral subversion of airspace geometry (PAS) which can occur in fibrotic interstitial lung diseases. Some of them, especially idiopathic pulmonary fibrosis (IPF), can be a good target for ultrasound because of its peculiar propensity to involve peripheral lung parenchyma. Irregular, unpredictable, disordered acoustic traps at the pleural plane are able to generate pneumogenic sonographic interstitial syndrome (SIS). The more advanced and distributed is SIS, the greater will be the number and the distribution of acoustic traps. Consequently, vertical artifacts tend to prevail on horizontal reverberations accordingly with number and distribution of acoustic traps [10; 11; 18; 19]. PAS geometry is typically subverted for structural matters. The architecture of the lung is deeply altered, especially on its periphery. On the contrary, in the case of early cardiogenic pulmonary edema, the structure of the lung remains unchanged, and acoustic traps are generated by thickened interlobular and intralobular septa. These kinds of acoustic traps are more regular, ordered, and predictable. The size, shape, regularity, as well as the nature of the content of acoustic traps, are able to influence the generation of different vertical artifacts. A fibrotic pulmonary disease, as IPF, alters the PAS geometry by producing heterogeneous and irregular shaped acoustic traps with fibrotic content. The corresponding pneumogenic sonographic interstitial syndrome can thus become a specific pattern with a thickened pleural line and more attenuated vertical artifacts, often without a clear periodic structure [20; 21]. In this study four LUS patterns were described. The choice of this classification deserves to be motivated. LUS pattern 1 corresponds to a normal or near normal lung surface, which can be seen as formed by closely packed air-filled alveoli. Consequently, due to the high acoustic impedance mismatch between air and the intercostal tissue, the lung surface behaves essentially as a specular reflector to ultrasound waves. Ultrasound penetration is thus hindered beyond the pleura line, and the \begin{table}
\begin{tabular}{c c c} \hline  & \multicolumn{3}{c}{**HRCT Grade 2**} \\ \hline
**LUS Pattern 2** & **Positive** & **Negative** \\ \hline
**Positive** & 24 & 6 \\
**Negative** & 4 & 50 \\ \hline  & Cohen’s Kappa: 0.737. Agreement: 88.1\%. & \\ \end{tabular}
\end{table} 
Table 3: Agreement between LUS pattern 2 and HRCT grade 2. \begin{table}
\begin{tabular}{c c c} \hline  & \multicolumn{3}{c}{**HRCT Grade 3**} \\ \hline
**LUS Pattern 3** & **Positive** & **Negative** \\ \hline
**Positive** & 19 & 4 \\
**Negative** & 3 & 58 \\ \hline  & Cohen’s Kappa: 0.788. Agreement: 91.7\%. & \\ \end{tabular}
\end{table} 
Table 4: Agreement between LUS pattern 3 and HRCT grade 3. \begin{table}
\begin{tabular}{c c c} \hline  & \multicolumn{3}{c}{**HRCT Grade 4**} \\ \hline
**LUS Pattern 4** & **Positive** & **Negative** \\ \hline
**Positive** & 2 & 4 \\
**Negative** & 8 & 70 \\ \hline  & Cohen’s Kappa: 0.176. Agreement: 85.7\%. & \\ \end{tabular}
\end{table} 
Table 5: Agreement between LUS pattern 4 and HRCT grade 4.  
typical reverberant A-line artifacts are displayed by standard imaging systems. With the progression of a pathology such as fibrosis, for which there is a replacement or the lung volume originally occupied by air in favor of tissue, the lung surface becomes progressively permeable to ultrasound, resulting in the increasing appearance of vertical B-line artifacts associated to irregularities of pleural line, and ultimately leading to the visualization of images displaying White Lung (LUS pattern 4) [21]. This preliminary work was able to demonstrate some level of agreement between LUS patterns, as previously described, and HRCT grades of fibrotic alterations. First of all, LUS pattern 1 in 100% of cases was indicative of mild to moderate fibrotic alterations.