Lung Ultrasound: A Practical Review for Radiologists SA–CME REVIEW the border of the pneumothorax (Figure 13, online video). 33,38 This can also be observed on M-mode where, within the same intercostal space, the seashore and stratosphere signs are intermittently present, revealing the point at which the visceral and parietal surfaces diverge, confirm- ing a pneumothorax. 39 A false lung point may occur when evaluating peridiaphragmatic portions of the lung, as temporary visualization into abdominal structures may falsely cause similar findings. 40 As noted previously, lung slid- ing may not be present without a pneumothorax in certain situations, such as atelectasis, ARDS, right mainstem intubation, among others (false positives). If B-lines or a lung pulse are present, no pneumothorax exists at this level. A lung pulse is a vertical pulsation of the pleural line in concordance with the heartbeat that excludes pneumothorax at a giv- en location in the intercostal space; both findings require the pleura to be in contact with each other (on- line video). 20,31 Pleural Effusion Pleural effusion was one of the first pathologies studied for the utilization of LUS. 4 In one study of intensive care patients, LUS showed a sensitivity and specificity of 100% Figure 14. Pleural effusion with spine sign. What has been described as the posterior spine with acoustic shadowing (arrows) can be seen as the ultrasound waves projecting through the fluid in the pleu al space. The effusion shows anechoic fluid, sugg sting simple effusion or very acute hemothorax (within dotted lines). Figure 15. Hemothorax with plankton sign. Mixed echogenic fluid in the pleu al cavity suggestive of hemothorax (dotted arrows). The plankton sign refers to swirling internal echoes within an otherwise anechoic pleural effusion and should be highly suspicious for hemothorax as a potential etiology in the setting of trauma. This can also be seen in exudative pleural effusions. Atelectatic lung is present, fl ating in the hemothorax (solid arrow). Applied Radiology 17 May / June 2023