Part B: Interpretation

More on lung ultrasound interpretation.

5) A-Lines

Image showing A Lines. Image from A/Prof Abhilash Koratala @NephroPOCUS with permission.

A Lines are long path reverberation artefacts arising from the strongly reflective surfaces of the ultrasound probe and the pleural surface (Watch the Vimeo Video below by James Rippey, Em Med). They are horizontal lines occurring at increasing depth separated by equal distance. The distance of separation is equal to that between the probe and pleura.

A lines occur because ultrasound waves are reflected by strong reflectors (probe surface and pleural-air interface), and bounce (reverberate) between the 2 surfaces. The probe must be perpendicular to the pleura for A lines to be generated. A non perpendicular position of the probe will not cause the bouncing effect of the ultrasound waves required for A Line generation. This could be a potential cause of missing A lines, leading to misdiagnosis.

A Lines are present in normally well aerated lungs. They are reduced or absent when there is increased lung density – eg atelectasis, lung consolidation or interstitial disease.

The image on the left shows Long Path Reverberation Artefact which occurs when the probe and pleural surface are perpendicular to each other, giving rise to A lines, a feature of well aerated lungs. The image on the right shows an ultrasound beam hitting the pleural surface at 45 degrees. The beam is reflected of the pleural surface, away from the ultrasound probe surface and bouncing of the ultrasound wave between the 2 reflectors does not occur. Figure – available via license: Creative Commons Attribution-NonCommercial 3.0 Unported

6) B-Lines

B Lines are vertical laser like discrete artefacts that begin at the pleural line and extend all the way to the bottom of the screen. They move synchronously with lung sliding.

When the lung is normally aerated, there should be fewer than 2 B Lines per intercostal space. In moderate loss of lung aeration, there are usually more than 3 B Lines per intercostal space. Severe loss of lung aeration results in a pattern of multiple coalescing B Lines per intercostal space.

B Lines arise due to short path reverberation artifact (Watch the Vimeo Video above by James Rippey, Em Med) (also known as ring down artifact). This artifact arises when an ultrasound pulse wave reverberates within a tiny soft tissue or fluid filled collection – eg fluid filled alveoli – where the reflective surfaces are close together

B Lines are present in conditions associated with increased lung density:

• Extravascular Lung Water – eg Cardiogenic pulmonary oedema, ARDS, pneumonitis, interstitial pneumonias, lung consolidation
• Pus – Consolidation
• Blood – Alveolar Haemorrhage
• Protein – Consolidation, idiopathic interstitial pneumonias, alveolar proteinosis, pulmonary infarct
• Cells – Tumour
• Lipid – Lipid Pneumonia

B Lines are absent when visceral and parietal pleura are separated as in pneumothorax.

7) Shred / Tissue or Fractal Sign

This sign is present at the boundary between consolidated lung and normally or partially aerated lung. Indicates lobar consolidation.

8) Air Bronchogram

Air bonchograms are seen as hyperechoic spots or branch like structures present within areas of lung consolidation. They can be can be static or dynamic air bronchograms. Dynamic air bronchograms are present within areas of consolidation. Static air bronchograms in areas of atelectasis (obstructed airway causing atelectasis).

9) Lung Hepatization

Tissue like image representing severely increased lung density (solid lung) which occurs with complete or near complete loss of lung aeration. 

10) Quad Sign / Sinusoidal Sign

The quad sign is seen in 2D imaging in the presence of a pleural effusion. The sinusoidal sign is seen on M mode in pleural effusion. It is caused by respiratory variation in the presence of fluid in the pleural space.

11) Spine Sign

Case contributed by David Carrol (Creative Commons). Carroll, D. Anechoic pleural effusion (ultrasound). Case study, Radiopaedia.org. 

In normally aerated lung, the spine is not visible above the diaphragm. This is because air prevents transmission of ultrasound waves. A positive spine sign is seen in areas of supra diaphragmatic consolidation or when there is a pleural effusion. When pleural fluid or hemothorax is present, the transmission of ultrasound waves is enhanced. This allows visualization of the thoracic vertebrae above and below the diaphragm.

12) Curtain Sign

Curtain sign is present in normally aerated supra diaphragmatic lung. It describes the disappearance of the spine with respiration caused by expansion of normally aerated lung. The curtain sign disappears when there is supra diaphragmatic consolidation or in the presence of pleural effusion.

Summary – Medical Decision Making

Lung ultrasound signs are not always specific to pathology. Each sign may arise due to different causes. For example B-lines may be due to pulmonary oedema, ARDS and may be seeen in consilidation. Always correlate ultrasound findings with the history and physical examination.

Further imaging or ultrasound interrogation of other organs may be required before a definitive diagnosis can be made.