Long path reverberation artefact
When appearing deep to the pleural line these are known as A-lines.
The ultrasound appearance of this artefact is repetitive, horizontal, echogenic lines deep to the second of two large reflective surfaces, between which the ultrasound energy is reverberating. Most commonly this artefact appears deep to a flat surface behind which is an gas collection, typically a pneumothorax, free gas in the abdomen, or gas within a loop of bowel.
How long path reverberation artefact is formed (A-lines)
The ultrasound beam hits the highly reflective pleural surface and is reflected back to the transducer. The first return results in a true image of the pleural surface on the monitor. The beam however reflects back again off the transducer face and the cycle repeats.
Subsequent horizontal lines are weaker and are artefacts. Their clarity depends on how good a reflective surface is encountered by the ultrasound beam.