Ultrasound interstitial syndrome is defined by the visualization of three or more B-lines simultaneously visible between two ribs, in a longitudinal scan (Fig. 12.1). Three or more B-lines were coined lung rockets, since the pattern is reminiscent of a rocket at liftoff. Said differently, lung rockets include eight features: they are comet-tail artifacts, arising from the pleural line, moving with lung sliding, usually long, usually well-defined, usually erasing A-lines, usually hyperechoic, and multiple in one longitudinal scan.

By definition, lung rockets are plural. Less than three B-lines are not consistent with interstitial syndrome. The b-line (lower case): It is defined by a single B-line between two ribs. It cannot be assimilated to interstitial syndrome nor any disease. Can be the sign of a minor fissura (see Fig. 11.​2).

Isolated lung rockets (i.e., visible at only one focal area) define focalized interstitial syndrome, of minor importance in the BLUE-protocol. Lung rockets disseminated to the whole lung define diffuse interstitial syndrome, i.e., a characteristic of most disorders seen in the emergency.

Our princeps study, assessing 121 cases of patients with diffuse alveolar-interstitial syndrome on radiography, and comparing them with 129 patients without any alveolar-interstitial pattern, showed a sensitivity of 93 % and a specificity of 93 % for the disseminated lung rockets [1]. When CT was used as a reference, the concordance was complete with interstitial syndrome.

These data mean that no disorder can yield lung rockets, if not interstitial syndrome. This was published in 1997, and we wanted to see how these data would age. With time, we never saw diffuse lung rockets in the countless healthy models we have insonated during workshops, which shows that time passing, ultrasound sensitivity would be 100 %. With time, a case of interstitial syndrome without lung rockets will maybe be described, but this will be an extreme rarity. Extremely rare cases of lung rockets not related to interstitial syndrome should now be described (read Peculiar Note 1).

The average distance between two B-lines in the septal variant is roughly 6–7 mm. This corresponds to the average size of a lobule. The polyedric shape of these lobules explains that the 6-/7-mm distance is an average; it can be less, depending on the section. Between two ribs in an adult, 2 cm of pleural line is visible, i.e., space for three or four subpleural interlobular septa, let us keep in mind the minimal value of three.

The question whether one may miss deep interstitial syndrome (without superficial extension) is solved by looking any CT: the subpleural interlobular septal thickening is a representative sample of deeper changes.

There is a subtle gradation of the number of B-lines, with a dichotomy inside (Fig. 12.2). All data must be understood “between two ribs in longitudinal scans.” To the left, all patterns which are not lung rockets are first the O-line (a variant of A-line), then the A-lines, then the isolated B-line, dubbed b-line (lower case), then two B-lines between two ribs, and dubbed bb-lines (lower case). To the right, there are three B-lines or more, i.e., lung rockets, i.e., interstitial syndrome. The label B-lines does not infer a specific number. Some B-lines can be large with a fusiform pattern; this does not mean, without mistake, a severe form of edema (Fig. 12.3).