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ARTEFACTS
In all lung views, examine the pleura, the parenchyma and any effusion in detail. In order to do this you need to be familiar with the artefacts seen in lung US and some of the physics behind their generation.
The most common artefacts/signs are:
1. A lines
2. B lines
3. Consolidation
4. Quad sign
5. Spine sign
A LINES
A lines are the horizontal lines which are seen deep to the pleura and which fade down the screen.
A lines are reverberation artefacts created when the US beam meets the highly reflective air tissue interface at the visceral pleura. They are horizontal lines which are mirror images of the subcutaneous and pleural line and they become fainter towards the inferior screen.
In the normal lung, the air beneath the visceral pleura reflects a large proportion of the US waves back to the transducer. Having travelled through subcutaneous tissue and pleura, the reflected US waves carry information regarding these structures back to the machine. Thus, the machine displays an image on the screen of the subcutaneous tissues and a bright white pleura. The pleura is hyperechoic because a bulk of the US beam is reflected here.
A lines: reverberation artefact due to reflection at the visceral pleura and the transducer face
The US beam travels through subcutaneous tissue well. But at the pleura, most of it is reflected back to the transducer. Thus the machine receives information on the subcutaneous tissue and pleura only. At the transducer face, some of the reflected US waves are reflected back into the tissues. They go through the same path as described above, however, this time, the reflection takes longer and the returning waves are weaker due to attenuation. Due to this, the machine displays an image of the subcutaneous tissues and the pleura deeper and more hypoechoic than the previous set. This is how A lines are created.
This keeps going down the screen until the US beam is too attenuated to create an image, similar to ripples expanding from a disturbance on the water's surface.
A lines indicate the presence of air at the pleural line preventing US from penetrating further.
A Lines = Air
B LINES
B lines are vertical laser like lines, originating at the pleura and extending to the bottom of the screen without fading. The area with fluid moves with respiration, thus B lines also move back and forth on the screen with respiration (1). B lines erase A lines
Vertical laser like B lines erase the horizontal A lines
B lines are ring down artefacts (2) and indicate the presence of fluid in the lung parenchyma. There are different theories about the generation of B lines and there is no consensus yet. The easiest way to understand the physics behind B lines is to visualise a tiny locule of fluid surrounded by air. The US wave penetrates this locule but becomes trapped within it bouncing back and forth causing the fluid to vibrate. This sends a continuous US signal back to the machine. The trapped wave doesn't become attenuated and so the artefact doesn't fade down the screen like A lines.
Up to 3 B lines is normal at R/L4 and R/L6 as these areas may have some dependent atelectasis. B lines increase and become confluent as the amount of lung water increases (3). Machine generated quantification of B lines is more accurate. However, if you have a machine without this software: eye ball quantification of B lines is simple. Where the B lines are separated, simply freeze the frame and count the number of B lines. Where the B lines are confluent, estimate the percentage of the pleura which generates B lines and then divide this by 10 (4). Add any solitary B lines to this number for a final score for that zone.
But mostly, I do a visual estimate of mild (3-4 seperated) moderate (5-8 seperated) and severe (confluent - like a curtain or waterfall).
Studies have shown that B lines are dynamic and increase or decrease with therapy. Changes to the number of B lines have been seen 1-2hours following diuretic therapy (5). B lines in COVID19 have been shown to decrease with disease resolution (6). Thus, they may be use to monitor fluid/diuretic therapy or resolution of acute interstitial disease.
Quantification of B lines: image 1: one B line; image 2: 4 B lines; image 3: 90% of the pleura line generates B lines: 9 B lines
B lines are in themselves non specific and simply indicate an increase in lung water. Other features of the examination such as pleural pathology, localised vs diffuse B lines, upper vs lower zones, associated effusions all help narrow the differential diagnosis.
There is still debate about the best way to visualise and quantify B lines. Horizontal transducer orientation leads to a wider field of view than the vertical orientation we have described and may allow a more uniform assessment of the B lines in that area (7). However, most guidelines still recommend vertical orientation of the transducer.
For more detailed information on B lines, click on the tab below.
B Lines = Extravascular Lung water
CONSOLIDATION
As seen in pathological dissections, consolidation of the lungs leads to a process of hepatisation in which the normal aerated alveoli become filled with debris and the lung parenchyma begins to resemble liver like tissue. This process is visualised with US with a high accuracy (8).
Small consolidations are typically hypoechoic, bordered by aerated lung with confluent B lines (shred sign).
hypo echoic consolidation with punctuate hyper echoic areas (air-bronchograms) and shred sign
Larger, lobar consolidations have easily visualised air-bronchograms and vessels.
Blood vessels seen within a lobar consolidation with colour doppler
Hyperechoic branching structures within the consolidation are bronchioles. With respiration, hyperechoic dots within these branches are seen to move (dynamic air-bronchogram)
Consolidation on US is non specific and may be due to pulmonary infarction, malignancy, contusion or infection.
Consolidation = tissue-like lung
Anechoic/hypoechoic areas anterior to the visceral pleura may be due to localised pleural effusions, debris or thickened pleura. The space created resembles a quadrangle: hence the name quad sign. A localised effusion will increase and decrease in size with expiration and inspiration respectively. On the other hand, debris or thickened pleura will not change size with respiration.
Quad sign: Anechoic space above the pleural line and below the ribs
The appearance of spine above the diaphragm indicates lung pathology which is allowing US to penetrate to the spine. Normally, aerated lung prevents US waves from penetrating beyond the visceral pleura. Thus when there is aerated lung, US cannot visualise the thoracic spine.
At R/L4 in the supine patient, in the normal lung, vertebra are seen caudal to but not cranial to the diaphragm. In the presence of consolidated lung or pleural effusions, the thoracic spine is seen to extend above the diaphragm. This is called the spine sign. Using the spine sign, as little as 15-30ml of pleural effusion may be detected with US (9).
Spine sign: top two images show normal lung with the visualised spine ending at the diaphragm. The lower two images show a large pleural effusion (anechoic space to the left of screen) and the thoracic spine extending cranial to the diaphragm.
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REFERENCES
1. Volpicelli G, Elbarbary M, Blaivas M, et al. International evidence-based recommendations for point-of-care lung ultrasound. Intensive Care Med. 2012;38(4):577-591
2. Garrido S, Duran J, Melendez A. Ring down versus commit tail: two artefacts uncovered. Eur Soc Radiol 2013: 1-17.
3. Brusasco C, Santori G, Bruzzo E, et al. Quantitative lung ultrasonography: a putative new algorithm for automatic detection and quantification of B-lines. Crit Care. 2019;23(1):288. Published 2019 Aug 28. doi:10.1186/s13054-019-2569-4
4. Gargani, L Ultrasound of the Lungs. Heart Fail Clin. 2019. 15 (2): 297-303
5.. Martindale JL, Secko M, Kilpatrick JF, et al. Serial sonographic assessment of pulmonary edema in patients with hypertensive acute heart failure. J Ultrasound Med 2018;37(2):337-45.
6. Xing C, Li Q, Du H, Kang W, Lian J, Yuan L. Lung ultrasound findings in patients with COVID-19 pneumonia. Crit Care. 2020;24(1):174.
7. Mongodi S, Bouhemad B, Orlando A, et al. Modified Lung Ultrasound Score for Assessing and Monitoring Pulmonary Aeration. Modifizierter Lungen-US-Score zur Bewertung und Überwachung der Belüftung der Lunge. Ultraschall Med
8.Lichtenstein, D.A., Lascols, N., Mezière, G. et al. Ultrasound diagnosis of alveolar consolidation in the critically ill. Intensive Care Med 2004. 30: 276-281.
9. Dickman, E., Terentiev, V., Likourezos, A., Derman, A. and Haines, L., 2015. Extension of the Thoracic Spine Sign: A New Sonographic Marker of Pleural Effusion. Journal of Ultrasound in Medicine, 34(9), pp.1555-1561.