B-line (ultrasound)

The B-line is an artifact relevant in lung ultrasonography. As originally described, it has seven defining features :

  • a hydroaeric comet-tail artifact
  • arising from the pleural line
  • hyperechoic
  • well-defined
  • extending indefinitely
  • erasing A-lines 
  • moving in concert with lung sliding, if lung sliding is present

B-line distribution corresponds with sub-pleural thickened interlobular septa, as demonstrated by CT, and are absent under normal conditions and present in alveolar-interstitial syndromes . Subpleural septal edema is postulated to provide a bubble-tetrahedron interface, generating a series of very closely spaced reverberations at a distance below the resolution of ultrasound which is interpreted as a confluent vertical echo which does not fade with increasing depth .

A few B-lines (<3/field of view) can often be found under normal circumstances, especially in the elderly, and around the base of the lungs .