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When a pneumothorax exists, free air should rise to the most nondependent region within the chest. Current ultrasound (US) examination methodologies may exclude visualization of these areas that may limit the sensitivity of the examination. This retrospective study uses computed tomography (CT) scans to precisely evaluate where free air within the thorax occurs and correlates this location with a presumably optimal US interrogation window.A total of 94 CT scans of patients with a pneumothorax in a single institution from December 2006 to January 2010 were examined. The borders and volumes of each pneumothorax were precisely measured by a radiologist. Logistic regression was used to determine the relationship between volume and location of intrapleural air at specified areas of the hemithorax. Sensitivities relating location of intrapleural air at a specific landmark and side of thorax were calculated.All but 3 of the pneumothoraces extended to the sternum. In all, 83 patients demonstrated a pneumothorax between rib interspaces 3 and 6 (mean pneumothorax volume 300.4 mL, 95% confidence interval [CI] 217.4-383.3), and 11 patients did not (mean pneumothorax volume 4.5 mL, 95% CI 1.7-7.3; P < .0001). The cumulative sensitivity for the presence of intrapleural air at rib interspaces 3 to 6 along the sternal border was 88%. This was consistent regardless of the side of hemithorax (right 91% and left 86%).The CT scans demonstrate that intrapleural air most often collects along the mediastinum between ribs 3 and 6 on either side of the chest. Although no USs were performed in this retrospective study, one may infer that a parasternal approach along rib interspaces 3 to 6 is an easy and sensitive window to diagnose pneumothorax with US.