## 4 Discussion
Thus, the cutaneous landmark positioned would indicate pleural plane in a point very different from that evaluated by ultrasound, when the patient assumes a supine position for HRCT scans. It is known that changing posture, the position of the diaphragm, motility and lung volumes dramatically change as well [27; 28; 29; 30]. Also, the position of arms affects diaphragm height, and consequently the position of the lung [31]. Patient position must be necessarily the same during both LUS evaluation and HRCT scan. Changing position would be an unacceptable bias in a work in which the match between the lung surface assessed by ultrasound and HRCT scan is crucial. Methodological issues constitute the second point that needs to be discussed. Echographic assessment is a real time imaging technique. While recording videos, the patient is normally breathing. This implies that the imaging plane intercepts over time different areas of the pleura plane. During breathing, in fact, LUS pattern can change. In this study, the LUS pattern reported was the worst (the greater) observed during the evaluation of the ultrasound data. Thus, inter-observer variability can be an important limitation in this scoring system. Echographic acquisitions during breath holding at the end of a deep inspiration would make the condition closest to that of HRCT scans. Two blind observers would allow a better definition of the pattern by computing inter-observer agreement. Finally, in our work apical landmarks seem not to have a corresponding lung surface right below their location, by observation of the HRCT scans in the axial plane in three patients out of 14. Free hand echographic evaluation allows to scan the patient also in unconventional planes moving from coronal to axial. For these patients, the HRCT grade was provided guessing the ultrasound scan plane, but a better evaluation would be possible in future studies by confining the ultrasound evaluation to the axial plane as much as possible. It is extremely important to discuss the limitation and the methodological issues also with respect to previous studies, which already tried to report on the correspondence between LUS images and HRCT findings for fibrotic lung diseases [32; 33; 34]. When performing a comparison between LUS artifactual images and chest CT scans it is crucial to respect the same position and the same conditions in both techniques. Otherwise the obvious risk is that the comparison can be misleading. This is especially true for IPF, which is characterized by an extreme heterogeneity of peripheral changes. In this case series, this is an important methodological aspect which has been addressed for the first time. Moreover, the origins of vertical artifacts are only now beginning to be understood. As showed here, honeycomb and GGOs can result in misleading findings for LUS. On the other hand, weaknesses of this paper compared with previous works [18; 23; 32; 33; 34] are the lack of information about basal posterior areas, usually the more involved and more informative in the case of IPF patients, the lack of data about respiratory function tests [32], and the lack of ultrasound evaluations of the pleural line performed with linear probes. Finally, we do not agree with the possibility to distinguish lung interstitial patterns (probable UIP, definite UIP, NSIP etc.) with LUS, as reported in previous work [33]. The definition of these patterns is based on HRCT scan, which is able to provide morphological and panoramic views of these pathologies. In these contexts, ultrasound interact only with the pleural surface and peripheral airspace geometry. To differentiate radiological interstitial patterns, which is even a difficult task for a morphologic HRCT scan, is not the aim of artifactual sonography, especially when the underlying physical phenomena have not yet been completely understood. Counting vertical artifacts is likewise not the better approach to quantify these phenomena and explain their significance [34; 35].