Sonographic Manifestations of Normal Lungs
Pleural line [1, 2]: The echo reflection of the pleural is formed by interface of the pleural lung surface. The linear hyperechoic structure that manifests as smooth and regular under the ultrasound is called the pleural line (Fig. 2.1). Under normal circumstances, the disappearance, roughness, significant thickening, or irregularity of the pleural line represents an abnormality. There are two kinds of artifacts that stem from the pleural line. One is the horizontal A-line, and the other is perpendicular B-line. These two artifacts are perpendicular to each other and lay a foundation for lung ultrasound.
A-line : The A-line is one kind of reverberation artifact located below the pleural line. It presents as a series of horizontal linear hyperechoic structures parallel to the pleural line; these are equidistant from each other and become weakened in echo strength with increasing depth, and the spacing is the distance from the skin to the pleural line. The number of A-lines is associated with the depth of the ultrasound machine; when the exploration position is relatively deeper, more A-lines are visibly displayed on screen.
B-line, comet-tail artifacts, and compact B-line [4, 5]: The B-line is one kind of laser-like vertical hyperechoic artifact that arises from the pleural line; it spreads to the edge of the screen without fading and synchronously moves with lung sliding and respiratory movements (Figs. 2.2, 2.3, and 2.4). We think whether the B-line reaches the edge of the screen or not depends on the setting of the depth. The B-line indicates filling of intralobular or interlobular septa, and it is invisible per intercostal in 30% of the lungs of normal children or adults under the ultrasound. However, as the lungs of newborns are rich in liquid, a small quantity of B-lines are visible on the ultrasound. There are two points to note here: (1) Previous findings in the literature indicate that B-lines and comet-tail signs can completely disappear within 48–72 h after birth, but in our experience, a few lines or comet-tail signs can be present during the neonate period in small premature infants. (2) In the past, it has been considered that the A-line would disappear in the presence of B-lines; however, we find that after long-term clinical practice, B-lines and the A-line can coexist. This is not only seen early in disease and during disease recovery but is also found to a lesser extent of disease in children and part of the normal newborn. Compact B-lines refer to coalescent (dense) B-lines without rib shadows as a result of B-lines fused together (Fig. 2.5). Comet tails are short-path reverberation artifacts that weaken with each reverberation, resulting in vertical echogenic artifacts that rapidly fade as they continue further into the ultrasound image, resembling the shape of a comet tail.
Lung sliding : During real-time ultrasound, it can be seen that the pleural line can move with the movement of respiration and presents the relative movement of the chest wall of upper and lower round trip, which is called “lung sliding” (Fig. 2.6).
Lung consolidation [3, 6]: An ultrasonic image presents the lung tissues with “hepatization,” probably associated with air bronchograms or fluid bronchograms. Lung consolidation is one of the most important signs in lung ultrasound imaging and is of vital value to the diagnosis of respiratory distress syndrome, pneumonia, and atelectasis (Figs. 2.7, 2.8, and 2.9).
Spared areas : The normal lung tissues are surrounded by at least one intercostal area size and alveolar-interstitial syndrome (AIS) region (Fig. 2.10).
Alveolar-interstitial syndrome (AIS) : When more than two B-lines are present in each the lung field, it is called alveolar-interstitial syndrome (Fig. 2.11).
Diffuse white lung : The six regions of the lung field are manifested as dense B-lines, disappearance of A-lines, and no presence of “spared area.” “White lung” is a manifestation of serious AIS and is induced by the presence of a large amount of liquid in the pulmonary interstitium and alveoli.
Lung pulse : The pulmonary tissues, with disappearance of lung sliding and visible consolidation at the pleural lines, beat with the heart pulsation. Lung pulsation is an important ultrasonic imaging characteristic of serious lung consolidation (such as atelectasis). Lung pulse can be observed and measured through the real-time two-dimensional ultrasound or M-mode (Figs. 2.12 and 2.13).
Lung point : Under real-time ultrasound, the existence of a transition point with respiratory movement between the intermittent presence and absence of a lung sliding is called the lung point. The lung point is a specific sign of pneumothorax and can accurately locate the position of the gas boundary when mild-moderate pneumothorax is present.
Double lung point : Due to differences in severity or the nature of pathological changes in different areas of the lung, a longitudinal scan shows a clear difference between upper and lower lung fields; this sharp cutoff point between the upper and lower lung field is known as a “double lung point” (Fig. 2.14).
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