Ultrasound lung surface: Basic considerations of ultrasound physics.

Abstract

We read with interest the study by Zoneff et al.1 focused on the prevalence of lung surface abnormalities in young and older healthy volunteers. Agreeing with the authors that the presence of B-lines increases in various lung diseases, we will consider it necessary to make some observations. Ultrasound scanner machines are calibrated at a homogeneous and constant sound speed of approximately 1500 m/s. However, propagation speed of the ultrasound in lungs is approximately 440 m/s. So that, more than 96% of the ultrasound beam is reflected at tissue-chest wall/air-lung interface. This results in a hyperechoic pleural line, that does not have a real anatomic match, and in vertical artefacts (called B-lines or ‘ring down’ artefacts) and in horizontal artefacts (called A-line or simple reverberations).2 Indeed the real anatomic thickness of the pleura, resulting from the addition of the visceral, parietal, and virtual space, averages approximately 150 μm (0.15 mm).3 For instance, the thickness of the pleural line of the same subject would be 0.7–1.8 mm using a high-frequency probe, and 1.4–2.8 mm with a middle-low-frequency probe (3.5–5 MHz).2 The B-line and A-line artefacts generation mainly depends on the great difference in acoustic impedance encountered by the ultrasound beam when it crosses surfaces with a different density (i.e. chest wall/aerated lung or gas/fluid-film).2 Nevertheless, these artefacts can be seen in the residual cavity of the post-pneumonectomy space3 (Figure 1C), in the bowel loops4 (Figure 1A) and also in normal lungs, in particular at the bases, where the hydrostatic pressure gives a more fluid-rich interstitium.5 Furthermore, the number and intensity of the B-lines depend on the type and frequency (2–12 MHz) of the probe used (linear, sectorial or convex), the degree of total gain compensation (TGC), as well as the position of the electronic focus respect to the pleural line and the used tissue harmonics and compounding.6 Moreover, also the spatial relationship existing between the probe and the pulmonary surface may contribute to modify the perception, intensity and number of B-lines. In fact, these artefacts are not visible during intraoperative lung ultrasound7 (Figure 1D), since there is no longer the great acoustic impedance difference of the chest wall/lung. Understanding the nuances in lung ultrasound artefacts production is important to avoid errors in the results interpretation. An increase number of B-lines are found in patients with different lung and cardiac diseases: pleural effusion in congestive heart failure, fibrosis, emphysema, exacerbations of chronic obstructive pulmonary disease (COPD) and lymphangitis, without enable distinction between acute pulmonary oedema from other diffuse lung diseases.6, 8 Therefore, considering the multitude of conditions in which these artefacts appear to be increased, the use of B-lines as a pathognomonic marker of any one type of lung disease is questionable. In addition, in reason of the numerous and above-mentioned variables that affect the generation of the B-lines artefacts, it seems an useless challenge to ‘count’ B-line artefacts, being not useful for a specific diagnosis.9 Finally, because of the anatomic constraints of the thoracic cage, TUS can at best explore about 70% of the pleural surface, and even in zones amenable to TUS examination, only lesions adherent to the pleural surface may be visualized.10 For this reason, we believe that all scans are necessary to view all 70% of the surface that could be viewed ecographically. From the back, we can opt for longitudinal and transversal intercostal middle scapolar and paravertebral scans, exploring from the base up to the ipsilateral posterior pulmonary apex, passing, then, to the exploration of the lateral chest side along the posterior, middle and anterior axillary line. From the front, in addition to the longitudinal and transversal intercostal scans, parasternal, middle clavicular and supraclavicular are all useful views.10 The authors scanned the bilateral lung areas in only ‘four anterior sites (R1, R2, L1 and L2), four lateral sites (R3, R4, L3 and L4) and with the addition of four posterior sites (R5, R6, L5 and L6).’ Therefore our doubt is the following: would be this the reason why the authors have seen a few/absent B-lines in the subjects examined with ultrasound? Indeed in this study, the prevalence of B-lines seems to be not correlated to the increase in age, while in the opinion of other authors, who have performed a number of complete scans in a population of subjects with characteristics similar to those in this study, the increase of number of B-line artefacts is correlated to the increase in age according to the physiological process of lung ageing (which also seems to be accelerated in smokers).11-13