How I Do It: Lung Ultrasound
How I Do It: Lung Ultrasound
Assessment of the lung has always been considered off-limits for ultrasound, since it is standard textbook knowledge that «because ultrasound energy is rapidly dissipated by air, ultrasound imaging is not useful for the evaluation of the pulmonary parenchyma». The concept that ultrasound cannot be employed for evaluating the lung is linked to the presence of air, which determines a high acoustic mismatch with the surrounding tissues, causing a complete reflection of the ultrasound beam, preventing the creation of direct imaging of the pulmonary parenchyma. In a normally aerated lung, the only detectable structure is the pleura, visualized as a hyperechoic horizontal line. It is debated whether this line represents an artefact due to a reflection phenomenon at the interface between alveolar air and the soft tissues of the thoracic wall, or it images the real pleura. The pleural line moves synchronously with respiration: this dynamic horizontal movement is called lung sliding. In addition, there are some hyperechoic, horizontal lines arising at regular intervals from the pleural line: the A-lines. When combined with lung sliding, these reverberation artefacts represent a sign of normal or excessive content of air in the alveolar spaces (Figure 1, Additional file 1 http://www.cardiovascularultrasound.com/content/12/1/25/additional). When the air content decreases and lung density increases due to the presence in the lung of exudate, transudate, collagen, blood, etc. the acoustic mismatch between the lung and the surrounding tissues is lowered, and the ultrasound beam can be partly reflected at deeper zones and repeatedly. This phenomenon creates some vertical reverberation artefacts known as B-lines (Figure 2, Additional file 2 http://www.cardiovascularultrasound.com/content/12/1/25/additional). B-lines belong to the family of the comet-tail artifacts, well known in the setting of abdominal ultrasound. B-lines have also been addressed as comet-tail artifacts or ultrasound lung comets before an expert agreement on nomenclature was obtained. B-lines are defined as discrete laser-like vertical hyperechoic reverberation artifacts that arise from the pleural line, extend to the bottom of the screen without fading, and move synchronously with lung sliding. Multiple B-lines are considered the sonographic sign of lung interstitial syndrome, and their number increases along with decreasing air content and increase in lung density. When the air content further decreases, such as in lung consolidations, the acoustic window on the lung becomes completely open, and the lung may be directly visualized as a solid parenchyma, like the liver or the spleen (Figure 3). Consolidation of the lung may be the result of an infectious process, an infarction due to pulmonary embolism, a localization of cancer and metastasis, a compression or obstructive atelectasis, or a contusion in thoracic trauma. Additional sonographic signs may help determine the aetiology of the consolidation, such as the quality of the deep margins, the presence of air or fluid bronchogram, or the vascular pattern within the consolidation.
(Enlarge Image)
Figure 1.
Sonographic appearance of an aerated lung scan. Arrows indicate A-lines. Above A-lines the pleural line is visible with its horizontal movement, the lung sliding.
(Enlarge Image)
Figure 2.
Sonographic appearance of multiple B-lines (indicated by the white arrows).
(Enlarge Image)
Figure 3.
Sonographic appearance of a consolidated lung. The echo-texture of the lung becomes similar to the liver.
The acoustic limitations of ultrasound in the assessment of an air-rich organ such as the lung can paradoxically become a diagnostic advantage. In some conditions the presence of air between the chest wall and the lung parenchyma causes a decisive change of the dynamic characteristics of the sonographic artefact image of the lung described so far. In pneumothorax (PNX) lung sliding is always absent, since it can be observed if the lung and the parietal pleura are in direct apposition, but not when the physical acoustic enemy – the air – is between the two pleural layers. For similar reasons, no B-lines can be seen in the context of a PNX, since B-lines can be visualized only at an air-tissue acoustic interface, when the visceral pleura is opposing the parietal pleura. Another sign helps rule out PNX, the lung pulse, which refers to the subtle rhythmic movement of the lung upon the parietal pleura, synchronous with cardiac beats. Like the respiratory movement, this cardiac movement of the lung cannot be detected by ultrasound if air is present between the visceral and parietal pleura. An easy step-by-step sonographic algorithm has been proposed to diagnose/exclude PNX by LUS.
In summary, LUS may be defined as a powerful diagnostic imaging technique for anomalies of the pleural space and a reliable densitometer of the lung parenchyma. This definition of LUS includes both its virtues, which should be included in clinical practice, as it is often time-, cost- and potentially life-saving; as well as its limitations, which should never be forgotten for a correct use of this technique.
What is Lung Ultrasound?
Assessment of the lung has always been considered off-limits for ultrasound, since it is standard textbook knowledge that «because ultrasound energy is rapidly dissipated by air, ultrasound imaging is not useful for the evaluation of the pulmonary parenchyma». The concept that ultrasound cannot be employed for evaluating the lung is linked to the presence of air, which determines a high acoustic mismatch with the surrounding tissues, causing a complete reflection of the ultrasound beam, preventing the creation of direct imaging of the pulmonary parenchyma. In a normally aerated lung, the only detectable structure is the pleura, visualized as a hyperechoic horizontal line. It is debated whether this line represents an artefact due to a reflection phenomenon at the interface between alveolar air and the soft tissues of the thoracic wall, or it images the real pleura. The pleural line moves synchronously with respiration: this dynamic horizontal movement is called lung sliding. In addition, there are some hyperechoic, horizontal lines arising at regular intervals from the pleural line: the A-lines. When combined with lung sliding, these reverberation artefacts represent a sign of normal or excessive content of air in the alveolar spaces (Figure 1, Additional file 1 http://www.cardiovascularultrasound.com/content/12/1/25/additional). When the air content decreases and lung density increases due to the presence in the lung of exudate, transudate, collagen, blood, etc. the acoustic mismatch between the lung and the surrounding tissues is lowered, and the ultrasound beam can be partly reflected at deeper zones and repeatedly. This phenomenon creates some vertical reverberation artefacts known as B-lines (Figure 2, Additional file 2 http://www.cardiovascularultrasound.com/content/12/1/25/additional). B-lines belong to the family of the comet-tail artifacts, well known in the setting of abdominal ultrasound. B-lines have also been addressed as comet-tail artifacts or ultrasound lung comets before an expert agreement on nomenclature was obtained. B-lines are defined as discrete laser-like vertical hyperechoic reverberation artifacts that arise from the pleural line, extend to the bottom of the screen without fading, and move synchronously with lung sliding. Multiple B-lines are considered the sonographic sign of lung interstitial syndrome, and their number increases along with decreasing air content and increase in lung density. When the air content further decreases, such as in lung consolidations, the acoustic window on the lung becomes completely open, and the lung may be directly visualized as a solid parenchyma, like the liver or the spleen (Figure 3). Consolidation of the lung may be the result of an infectious process, an infarction due to pulmonary embolism, a localization of cancer and metastasis, a compression or obstructive atelectasis, or a contusion in thoracic trauma. Additional sonographic signs may help determine the aetiology of the consolidation, such as the quality of the deep margins, the presence of air or fluid bronchogram, or the vascular pattern within the consolidation.
(Enlarge Image)
Figure 1.
Sonographic appearance of an aerated lung scan. Arrows indicate A-lines. Above A-lines the pleural line is visible with its horizontal movement, the lung sliding.
(Enlarge Image)
Figure 2.
Sonographic appearance of multiple B-lines (indicated by the white arrows).
(Enlarge Image)
Figure 3.
Sonographic appearance of a consolidated lung. The echo-texture of the lung becomes similar to the liver.
The acoustic limitations of ultrasound in the assessment of an air-rich organ such as the lung can paradoxically become a diagnostic advantage. In some conditions the presence of air between the chest wall and the lung parenchyma causes a decisive change of the dynamic characteristics of the sonographic artefact image of the lung described so far. In pneumothorax (PNX) lung sliding is always absent, since it can be observed if the lung and the parietal pleura are in direct apposition, but not when the physical acoustic enemy – the air – is between the two pleural layers. For similar reasons, no B-lines can be seen in the context of a PNX, since B-lines can be visualized only at an air-tissue acoustic interface, when the visceral pleura is opposing the parietal pleura. Another sign helps rule out PNX, the lung pulse, which refers to the subtle rhythmic movement of the lung upon the parietal pleura, synchronous with cardiac beats. Like the respiratory movement, this cardiac movement of the lung cannot be detected by ultrasound if air is present between the visceral and parietal pleura. An easy step-by-step sonographic algorithm has been proposed to diagnose/exclude PNX by LUS.
In summary, LUS may be defined as a powerful diagnostic imaging technique for anomalies of the pleural space and a reliable densitometer of the lung parenchyma. This definition of LUS includes both its virtues, which should be included in clinical practice, as it is often time-, cost- and potentially life-saving; as well as its limitations, which should never be forgotten for a correct use of this technique.
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