A-Lines

Air below the pleural line reflects most US back to the transducer. This is itself a reflector, meaning some of the US waves will bounce back and forth between the pleura and transducer generating artifacts called A lines. They are horizontal lines below the pleura with the same spacing as the distance between the probe and the pleural line.

Because they demonstrate the presence of air below the pleura, they are present both in normal lungs and in pneumothorax.