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Imaging of Thoracic Trauma


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Introduction

Thoracic injury is a common sequela of acute trauma and is the third most common injury in trauma patients, after head and extremity injuries. The overall mortality rate approaches 25%, with acute aortic, tracheobronchial, and cardiac injuries often having an even worse prognosis. Given the high mortality rate and the impact on clinical management, rapid and accurate assessment of thoracic trauma imaging is imperative.

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Appropriateness of Imaging Modalities and Technique

When CT Should Be Performed

There are conflicting data in the literature on whether routine chest CT should be performed in all cases of thoracic trauma. Chest CT is much more accurate than chest radiography in evaluating thoracic trauma, with at least 50% of pneumothoraces, rib fractures, and pulmonary contusions missed on routine anteroposterior chest radiographs. Some studies have shown that CT changes management in a significant proportion of patients, and potentially fatal injuries such as aortic laceration could be missed. Other studies have shown that with appropriate clinical assessment and use of other radiographic and demographic data, patients can be selected for whom chest CT would be more likely to identify injuries, thus saving cost and radiation. The American College of Radiology Appropriateness Criteria for blunt thoracic trauma in 2014 have suggested that CT should be strongly considered in patients with “high-mechanism injury, abnormal chest radiographs, altered mental status, distracting injuries, or clinically suspected thoracic injuries. CT angiogra[phy] should be routinely used in patients with suspected acute aortic laceration.”

Appropriate Imaging Protocol

Unless contraindicated due to allergy or renal failure, contrast should be used in all CT scans of thoracic trauma. This allows for much more accurate assessment of vascular injury, which is a significant cause of morbidity and mortality in thoracic trauma. In most cases, precontrast images are not necessary. It is also important to choose a reconstruction algorithm with some degree of overlap. This allows for high-quality multiplanar reconstructions, which can be very helpful in the detection of certain types of thoracic injury.

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Abnormal Gas and Fluid Collections

In the setting of trauma, gas and fluid can accumulate in different thoracic spaces, with different causes and clinical significance. It is important to be aware of each of these to direct the appropriate management of the trauma patient.

Pneumothorax

Incidence of Pneumothorax in Trauma; Occult Pneumothorax

Pneumothorax or air within the pleural space occurs in up to 20% to 40% of blunt trauma patients presenting to the emergency department. Up to 50% of pneumothoraces may be missed with portable radiography ( Fig. 43.1A ). These are termed occult pneumothoraces and are rarely symptomatic. However, in the setting of positive end-expiratory pressure mechanical ventilation, they can significantly enlarge, and therefore are often drained with a chest tube.

Figure 43.1, (A) Supine frontal radiograph in a patient involved in a motor vehicle collision demonstrates a hyperlucent and deep right costophrenic angle (arrow). This finding, known as the deep sulcus sign, occurs when gas within a pneumothorax collects inferiorly in a supine patient. (B) Image of the same patient taken 1 hour later shows that the patient now has a large right pneumothorax, with collapse of the right lung toward the hilum (arrow). There is no tension effect, however, because the mediastinum remains centered, and the right hemidiaphragm is not displaced. (C) In a different patient, status post–gunshot wound to the left chest, the left chest is opacified secondary to the presence of a large tension hemopneumothorax. The tension hemopneumothorax displaces the mediastinum to the right and the left hemidiaphragm inferiorly.

Pitfalls in Diagnosis

On a chest radiograph, one should be careful not to mistake a skin fold for a pneumothorax. The pleural line in a pneumothorax is seen as a thin white line, with no lung markings beyond it, whereas a skin fold is often less dense, has lung markings beyond it, and can be seen projecting into the soft tissues. Mach bands, which are due to an optical illusion resulting in incorrect perception of density at the borders, of varying shades of gray, can also simulate pneumothorax on chest radiograph. On chest CT, use of a sharp kernel such as with a bone algorithm can result in an artifact mimicking pneumothorax.

Determining if There Is Tension Physiology

One of the most important findings to assess is whether there are signs of tension physiology because this can be lethal to the patient. Signs of tension in the pleural space from pneumothorax or hemothorax include contralateral mediastinal shift, flattening or inversion of the ipsilateral hemidiaphragm, and a hyperexpanded ipsilateral chest (see Fig. 43.1B and C ). If these signs are seen, emergent decompression is warranted because cardiovascular collapse can occur.

Associated Injuries

Rib fractures are commonly associated with pneumothorax, and careful attention should be given to the ribs when a pneumothorax is seen in the setting of trauma. Multiple contiguous segmental rib fractures are termed flail chest, which has a high (≈50%) association with injuries requiring surgical treatment and prolonged mechanical ventilation.

Pneumomediastinum

Cause of Pneumomediastinum in the Traumatic Setting

Most cases of pneumomediastinum in blunt thoracic trauma are due to the Macklin effect, which is alveolar rupture resulting in gas dissecting along the bronchovascular sheath into the mediastinum ( Fig. 43.2 ). This is typically due either to barotrauma or increased pressure against a closed glottis.

Figure 43.2, (A) Supine radiograph in a patient status post–motor vehicle collision shows vertical lucencies about the mediastinum (arrows), consistent with pneumomediastinum. Note that the left heart border is outlined with gas, which has tracked inferiorly and can be difficult to distinguish from pneumopericardium. (B) An image from the CT of this patient confirms the presence of pneumomediastinum in the anterior mediastinum. Note that the gas in pneumomediastinum is insinuating among the mediastinal fat, which can be seen as displaced islands of increased density (arrow). (C) Image in a different patient who was stabbed in the chest also shows gas outlining the heart (arrows), but note the absence of gas more superiorly in the mediastinum. (D) An image from the CT scan of this patient shows gas in the pericardium (arrow), consistent with pneumopericardium. Note that in contrast to the gas shown in (B), which was present in the potential space of the mediastinum, this gas is present in a true space (the pericardial space) and does not have islands of fat or strands of tissue within it. Given the presence of pneumopericardium, the patient underwent surgical exploration, which found a pericardial defect but no evidence of myocardial injury.

Associated Injuries

In the setting of trauma, one should always carefully assess the esophagus and tracheobronchial tree because direct injury to these structures can result in pneumomediastinum.

Pneumopericardium

Importance of Pneumopericardium and Associated Injuries

Pneumopericardium, or air within the pericardial space, implies a defect in the pericardium and is unusual after blunt thoracic trauma. Pneumopericardium in the setting of penetrating trauma, however, can be enough to warrant surgical exploration to assess for cardiac injury. When encountered, one should carefully assess for any pericardial defect and look for signs of cardiac injury. CT cannot exclude cardiac injury because the right ventricular free wall, the most anterior portion of the heart, is only 2 to 3 mm thick, and lacerations are often not seen.

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