What is normal?

Our thorax is lined by a continuous serous membrane called the pleura. The pleura is further divided into the visceral pleura which is attached to the lung and the parieta pleural which is attached to the chest wall. The pleural space is a potential space between the visceral and parietal pleura. It is normally filled with small amounts of physiologic fluid. On ultrasound, this interface is seen as a hyperechoic line running deep to the ribs. As the patient breathes the fluid moves between the pleura and is called lung sliding or “shimmering” (video 2).

Video 2: Normal Lung Sliding

In addition to shimmering, normal lung sliding leads to a reverberation artefact called a comet tail. Comet tails are formed by ultrasound pulses bouncing between the reflective surfaces of the visceral and parietal pleura and appear as vertical echogenic lines originating from the pleura which move with respiration (figure 2). Because the surfaces are not parallel many of the bouncing beams are scattered so the artefact fades as it gets further from the pleural line.

Figure 2: Comet tail artifact

A normal pleural interface at the level of the probe is indicated by the PRESENCE OF EITHER:

• Lung sliding
• Comet tail artifacts