ultrasound of tracheaThis is a longitudinal view of trachea, the air-mucosa interface just beneath the tracheal wall. What are the structures “A” and “B”?


  • A – Reverberation artifact from air-mucosal interface
  • B – Mirror image of cricoid cartilage.

Reflection at the tissue interfaces occur when there is a difference in acoustic impedance between two tissues. The greater the difference, the stronger the reflection, the brighter the image.

A . A transmitted ultrasound beam hits the air-mucosa interface and is reflected back to the transducer (1st reflection). Based on the time taken for the reflected beam to return (assuming a constant speed of 1540 m/s), the machine calculated the distance this 1st image is away from the transducer (at around 1.15cm) and registers it. The skin-transducer interface itself also results in the 1st reflected beam being partially reflected back into the air-mucosa interface, which again gets reflected back to the transducer as a 2nd reflection. This 2nd reflection takes twice the time compared to the first; therefore the machine (assuming all beams travel only once to and from an object) registers a 2nd image, the reverberation artifact, at twice the depth (around 2.3cm in this case). Lichtenstein called these artifacts “A” lines when they arise from the pleura.

B. A similar explanation accounts for the mirror image of the cricoid cartilage below the air-mucosa interface, only that the 2nd reflection occurs at the cartilage-soft tissue interface.

What’s the difference between the two? The reproduction of tissue interfaces is called reverberation artifact; whereas the reproduction of objects is termed mirror image. Both artifacts follow the same principles:

  1. They occur when there is a bright reflective surface
  2. They are always deeper than the real image
  3. They are always less distinct than the real image

Next time, look out for the mirror in the tracheal wall.