This is a longitudinal view of trachea, the air-mucosa interface just beneath the tracheal wall. What are the structures “A” and “B”?
- A â€“ Reverberation artifact from air-mucosal interface
- B â€“ Mirror image of cricoid cartilage.
Reflection at the tissue interfacesÂ 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 â€œAâ€ 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.
Whatâ€™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:
- They occur when there is a bright reflective surface
- They are always deeper than the real image
- They are always less distinct than the real image
Next time, look out forÂ the mirror in the tracheal wall.