B lines, also known as comet tail artifacts, are caused by echo reverberations of the air-filled lung and appear as narrow hyperechoic ray-like opacities extending from the pleural line to the edge of the ultrasonography screen that move with lung sliding without fading. Because pleural air would block the visualization of the underlying lung, the presence of B lines and lung sliding rules out a pneumothorax with a negative predictive value of 100% in the location of the chest probe.26,27 It is important to examine several locations on the thorax, especially the superior anterior and lateral chest wall, where air would normally accumulate. Lung sliding can be limited by pleural-parenchymal adhesions, endobronchial obstruction, or diaphragmatic paralysis, and, therefore, the main use of ultrasonography for assessment of pneumothorax lies in its ability to rule out a pneumothorax. Lung ultrasonography can also be used to determine pneumothorax by identifying the point where the lung separates from the chest wall. This is seen as an area where normal lung sliding meets an area where no lung sliding is seen, and it has been termed the “lung point.” The lung point can be visualized with both B-mode and M-mode ultrasonography, and, when seen, has a 100% specificity for pneumothorax.28 The sensitivity of the lung point for pneumothorax, however, is inversely proportional to the size of the pneumothorax, because a large pneumothorax would prevent the parenchyma from opposing the chest wall (Fig 2).