A-lines vs B-lines

Aerated lung is invisible to ultrasound, so lung ultrasound reads the artifacts the lung surface produces. The two that matter most are A-lines and B-lines.

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A-lines

Horizontal, evenly spaced bright lines below the pleural line, at multiples of its depth. They are reverberation between the probe and the air-filled lung. With lung sliding, an A-line pattern means normal, dry, aerated lung.

B-lines

Vertical, laser-like bright lines that start at the pleural line, run to the bottom of the screen without fading, move with sliding and erase the A-lines. They arise when the interstitium contains fluid or is thickened.

How many is abnormal?

One or two B-lines in a space can be normal. Three or more in a single intercostal space is a positive zone. Bilateral, diffuse B-lines suggest pulmonary oedema; focal B-lines suggest pneumonia, contusion or fibrosis.

Real scans

B-lines — Bright vertical bands (B-lines) start at the pleural line and run to the bottom of the screen, wiping out the A-lines. This patient had lung fibrosis. — **B-lines.** Bright vertical bands (B-lines) start at the pleural line and run to the bottom of the screen, wiping out the A-lines. This patient had lung fibrosis. Tinss · CC BY-SA 4.0 · source

Normal lung: pleural line and A-lines (clip). Normal lung pattern: a bright pleural line that slides with breathing, and evenly spaced horizontal echoes beneath it (A-lines). Gargani L · CC BY 2.0 · source

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FAQ

Do B-lines rule out pneumothorax?

Yes, at that spot — they require the lung surface to be in contact with the chest wall.

Are A-lines normal?

With lung sliding, yes. Without sliding, A-lines are compatible with pneumothorax.

Related guides

Lung sliding on ultrasoundWhat lung sliding is, how to see it with a linear probe, the bat sign, seashore sign, and why absent sliding suggests pneumothorax. Try it in a live simulator.
Diagnosing pneumothorax with ultrasoundUltrasound signs of pneumothorax: absent lung sliding, absent B-lines, barcode sign and lung point. Where to scan and the common false positives.
Ultrasound artifacts, explainedAcoustic shadowing, posterior enhancement, reverberation (A-lines), B-lines, mirror image and edge artifacts — why they happen and how they help diagnosis.

Educational content for learners; follow local protocols and supervision. Simulated images are generated by Sonoduck’s physics model; real images are credited individually.