Due to the lack of sensitivity of chest X-ray in the supine position, US has progressively been taken into account. In 2010, an evidence-based review of chest X-ray and US in post-traumatic pneumothorax, found a US sensitivity ranging from 86 to 98% in the enrolled studies, an US specificity of 97–100%, a supine chest X-ray sensitivity of 28–75%, and a chest X-ray specificity of 100%, suggesting that bedside thoracic US is a more sensitive screening test than supine chest X-ray in detecting pneumothorax in blunt chest trauma [16, 22]. In a recent study, compared to chest CT as gold standard, the sensitivity of bed US to detect pneumothorax was 88% and the specificity 99.5%; these results are very similar to those reported in the previous review [21].

To detect a pneumothorax, the area of interest corresponds to the anterior part of the chest on both sides of the thoracic wall, approximately the third to eighth intercostal space between the parasternal and the midclavicular lines. It is essential that the results obtained have to be compared with the contralateral site. The probe is moved longitudinally in order to study the intercostal tissues on the real-time image. The hyperechogenic pleural line between the shadows of two ribs must be detected. The necessary criteria to diagnose a pneumothorax sonographically have been assessed in the “International evidence-based recommendations for point-of-care lung ultrasound” [17] and consist in absence of respiratory lung movement during dynamic examination (the so-called lung sliding sign), absence of lung pulse at power color Doppler imaging, absence of B-lines (vertical narrow based lines arising from the pleural line to the edge of the ultrasound screen, the so-called comet-tail image), detection of a “lung point” that reflects the border between the movement of the lung during respiration, and the area of absence of the sliding sign due to the pneumothorax [17] (Fig. 8.2).

While the absence of lung sliding can be seen in conditions in which there is no lung movement against the thoracic wall, for example, after pneumonectomy, pleuroparenchymal adhesion, or subpleural bullae [23], the “lung point” sign has been shown to be 100% specific for pneumothorax and should routinely be sought in all the patients with loss of lung sliding [24].

In spite of its high sensibility and specificity, we must take into account US limitations. In addition to the above-mentioned referring to the patient constitution, the presence of anterior subcutaneous emphysema and the operator skill, we must not forget that small, apical, and dependent pneumothoraces are poorly detected using thoracic US. A perspective study in 2012 correlated the site of pneumothoraces detected with chest CT with the capability of the previously acquired chest US to detect them. They found that the frequency and the extent of pneumothoraces both increments moving towards the medial and inferior thoracic regions which should always, in the authors opinion, be the first to be scanned [25]. Another limitation of chest US examination is that quantification of the pneumothorax is not easy [10] even if Volpicelli et al. in 2014 demonstrated that in supine patients, the more posterior the lung point is located, the greater the pneumothorax is, as compared to CT volume measurement [26].

MDCT offers confident diagnosis of the pneumothorax itself plus a panoramic view of all thoracic structures allowing the detection of associated injuries and accompanying pathological processes. In particular, it allows to detect the exact number and site of rib fractures, the associated lung injuries and the definition of presence and extent of an eventual subcutaneous emphysema, the latter frequently associated with pneumothorax when rib fractures are involved [11, 19, 27] (Fig. 8.3).

Intrapleural air usually collects in the nondependent regions of the thorax and, in particular, as the patients during CT scan is in a supine position, in the anteromedial basal pleural recesses [27] (Fig. 8.4). Sometimes however, especially in little pneumothorax, air can be detected nearby the rib fracture, wherever the fracture is (also if in a dependent region). This fact may be due to preexisting or trauma-related pleural adhesions which can delimitate small air collections (Fig. 8.5).

A tension pneumothorax is a less common and more dangerous variant of the simple pneumothorax and occurs when air continues to enter the interpleural space with each breath, either via laceration in the chest wall or via an injured lung, but cannot escape [20]. The resultant progressive increase in volume of trapped air will compress the adjacent lung and diminish its expansion leading initially to respiratory distress and hypoxemia. As the interpleural volume of air continues to increase, the mediastinal structures, including the vena cava, are also displaced and compressed, producing an eventual drop in cardiac preload due to diminished venous cardiac filling pressures. The result is compromised cardiac output which usually rapidly progresses to cardiogenic shock and eventual cardiac arrest [20].

In this life-threatening situation, chest X-ray permits to depict in a few minutes the hyperlucency of a hemithorax plus the opacification of the contralateral (occupied by the mediastinal structures and by the contralateral collapsed lung) thus permitting immediate pleural drainage which is necessary as promptly as possible in order to save the patient’s life [7] (Fig. 8.6). We must remember that only a mediastinal shift from its median localization is not a definite sign of a tension pneumothorax as it can also occur in a massive lung athelectasia. So it is important to look for hyperlucency of a hemithorax associated with the ancillary signs of overinflation: flattened or inverted diaphragm profile and enlarged intercostal spaces [7]. Sudden evacuation of a large pneumothorax with tube drainage can be complicated by re-expansion pulmonary edema [28, 29] which is clearly visible in a chest radiogram, being characterized mainly by immediate appearance of subtle opacifications in the ipsilateral lung that can rapidly progress to the total opacification of the entire hemithorax. The complication is more common in younger patients (20–50 years of age), occurs more often than was previously believed, and it has a reported variable mortality rate reaching 20% [29]. So, a control chest X-ray should always be acquired after positioning a drainage tube in a hypertension pneumothorax as it exactly localizes the tube apex and helps us to rapidly diagnose acute complications of the invasive maneuver [7].

MDCT signs are much the same as in conventional radiography, namely contralateral mediastinal shift and collapsed ipsilateral and contralateral lung in a hyperexpanded ipsilateral hemithorax. Also in MDCT imagines, we can find a flattened or inverted ipsilateral diaphragm [19] (Fig. 8.7). Sometimes, with MDCT we are able to find the exact site of the laceration responsible for the intrapleural air accumulation.