Sudden onset right chest pain in a young man
The pleural line is identified as a bright, echogenic line approximately 0.5-1cm deep to the anterior surface of the ribs or costal cartilage (depending on how near the sternum you are looking). Normally there is movement seen as the visceral pleura slides back and forth under the parietal pleura ("ants running back and forth" = sliding sign). In a pneumothorax this is replaced by a static, unmoving line. The lung point sign is where the abnormal, unmoving line is replaced by the normal movement which appears to "slide in" (it actually is the shifting contact point as the expanding lung makes contact with the parietal pleura). This is the most specific sign for a pneumothorax.
Ultrasound is much more sensitive than a supine CXR for pneumothorax. Generally, the left and right parasternal regions around the 3-4th rib are examined first (the most anterior space in the supine patient). Absence of lung sliding can also occur in apnoea, single lung ventilation, atelectasis, adhesions or large bullae. The point sign, however, only occurs in a pneumothorax, and can also be used to estimate the size of the pneumothorax by demonstrating where the lung is no longer in contact with the chest wall.