Dr. Collins is Associate Professor of Radiology and Medicine, and Assistant Dean of Graduate Medical Education in the Department of Radiology at the University of Wisconsin Hospital and Clinics, Madison, WI. Dr. Primack is Associate Professor and Vice Chair of Radiology in the Department of Radiology at Oregon Health Sciences University, Portland, OR.

Each year in the United States, more than 300,000 patients are hospitalized ¹ and 25,000 people die as a direct result of chest trauma. ^2,3 Blunt, or nonpenetrating injury accounts for 90% of chest trauma seen in civilian populations, and most injuries are due to motor vehicle accidents and falls. ⁴

After the patient has been stabilized, a chest radiograph is usually performed as the initial imaging study. Because of the critical nature of the patient's condition, radiographs are limited to portable, bedside examinations. Portable anteroposterior chest radiographs are limited by patient positioning, inconsistent exposure technique, obscuration of thoracic anatomy secondary to portions of external monitoring and support devices overlying the patient, limited exposure capability, expiratory views, and magnification and distortion of the mediastinum.

Chest computed tomography (CT) is becoming an established modality in the evaluation of trauma patients. CT scanning is more accurate and detects significantly more injuries than chest radiography, obviates the need for aortography in many patients with aortic injury, and affects patient management in a significant number of patients. ⁵ This paper provides an overview of the CT findings in patients who have suffered from acute nonpenetrating injury to the chest.

Aortic laceration

Approximately 8,000 cases of traumatic aortic rupture occur in the United States each year, and traumatic aortic rupture is responsible for 15% to 20% of all fatalities associated with motor vehicle accidents. ⁴ Approximately 90% of patients with traumatic aortic rupture die before emergency treatment can be instituted. Most aortic injuries involve the descending aorta and occur just distal to the origin of the left subclavian artery. ⁶

Several radiographic signs have been described as indicators of aortic injury, the most sensitive being widening of the mediastinum and loss of definition of the aortic arch. However, no single radiographic sign or combination of radiographic signs demonstrates sufficient sensitivity to detect all cases of traumatic aortic rupture on chest radiographs without the performance of a large number of normal aortograms. ⁷ A normal chest radiograph has a high negative predictive value (98%) but a low positive predictive value for aortic injury. ⁷ When aortography is performed because of findings on a chest radiograph, only 10% to 20% of patients will have an aortic injury. ^7,8

In the past decade, several studies have found CT to be 92% to 100% sensitive and 62% to 100% specific in detecting aortic injury. ^9-13 CT findings of aortic injury include: the indirect sign of hemomediastinum and direct signs such as aortic contour deformity (figure 1), intimal flap, thrombus or debris protruding into the aortic lumen, pseudoaneurysm (figure 2), abrupt tapering of the diameter of the descending aorta compared with the ascending aorta ("pseudocoarctation"), and extravasation of intravenous contrast material. In one study, if the criteria for a positive CT scan included only direct signs of aortic injury (and excluded hemomediastinum), the sensitivity and negative predictive value remained 100%, whereas the specificity increased to 96% and the positive predictive value increased to 40%. ¹³ None of the patients with isolated anterior hemomediastinum at CT had evidence of aortic injury at follow-up aortography. ¹³ In this same study, at least 638 (80%) of 795 patients would have been spared aortography by undergoing CT first. ¹³

Prior to spiral CT studies, the false-positive rate for CT in the detection of aortic injury was 0% to 39% and the false-negative rate was 0.7%. ¹⁴ Potential pitfalls in CT interpretation include hemomediastinum due to sternal or vertebral body fracture; left pleural effusion with left lower lobe subsegmental atelectasis "surrounding" the aorta; intraluminal artifacts; atherosclerotic plaques; prominent ductus arteriosus; and pseudointimal flaps secondary to volume averaging of the left brachiocephalic vein as it crosses in front of the aortic arch, the left superior intercostal vein, and right bronchial arteries branching off the descending aorta. ⁹ In one study, if aortography had been reserved for patients whose chest CT showed hematoma only in a periaortic location, the negative rate of aortography would have been reduced from 62% to 27%. ¹⁰

CT is useful in detecting other injuries to the chest in addition to aortic injury, as well as showing alternative causes for mediastinal widening on chest radiography. The latter include paramediastinal atelectasis or pleural effusion, residual thymic tissue, ¹⁵ mediastinal lipomatosis, tortuous vessels, vascular anomalies, ¹⁶ and lymphadenopathy.

The best technique for performing CT to detect aortic injury has yet to be determined. The following protocol has been offered: ^15,16 helical scanning mode with single breath-hold acquisition (if patient is able), 5-mm thick collimation (reconstructed every 3 mm) with a pitch of 1.5 to 2, and 150 mL of intravenous contrast material administered at a rate of 2 to 3 mL/sec after a delay of 30 to 40 seconds (or per software provided with the CT scanner to monitor contrast material enhancement; Smart prep, GE Medical Systems, Milwaukee, WI). Scanning begins at the level of the diaphragm and progresses cephalad to above the aortic arch. The remainder of the chest is scanned with 7-mm collimation.

Patients with no direct evidence of aortic injury or hemomediastinum on CT do not require further evaluation unless serial chest radiographs show progressive mediastinal widening. Dyer et al ¹³ also recommend no further evaluation if isolated anterior hemomediastinum is present. Patients with direct signs of aortic tear either go directly to surgery or confirmatory conventional aortography, depending upon the preference of the surgeon. Patients with hemomediastinum adjacent to the aorta in which no cause for the hematoma is identified should undergo conventional aortography as should patients with indeterminate or inadequate CT studies.

Pulmonary parenchymal injury

Pulmonary contusion--Pulmonary contusion is defined as traumatic extravasation of blood and edema fluid into the adjacent interstitial and air spaces as a result of torn vessels but without substantial tissue disruption. ¹⁷ Findings on chest radiography vary from irregular, patchy areas of consolidation to diffuse and extensive homogeneous consolidation. Extensive bilateral contusion may lead to respiratory failure and adult respiratory distress syndrome. ¹⁸ Radiographic changes of contusion are evident within 6 hours after trauma to the chest, and resolve rapidly, typically within 3 to 10 days. ¹⁹

CT findings of contusion consist of nonsegmental areas of consolidation and ground-glass opacification that predominantly involve the lung directly deep to the area of trauma, often sparing 1 to 2 mm of subpleural lung parenchyma adjacent to the injured chest wall ²⁰ (figure 3).

Pulmonary laceration--A laceration is defined as an abnormal intraparenchymal collection of air resulting from traumatic disruption of the lung architecture. ²¹ Wagner et al ²¹ described 4 types of laceration: Type 1 is an air-filled cavity with or without an air-fluid level, resulting from sudden compression of a pliable chest wall wherein the air-containing lung ruptures. Type 2 is an air-containing cavity in a paravertebral location, resulting from severe compression of the more pliable lower chest wall and sudden shifting of the lower lobe across the vertebral body causing a shearing type of injury. Type 3 is a small peripheral cavity or peripheral linear radiolucency that is always close to the chest wall where a rib has been fractured, resulting from a fractured rib that has punctured the lung. Type 4 is a result of previously formed, firm pleuropulmonary adhesions causing the lung to tear when the overlying chest wall is violently moved inward or fractures, diagnosed only at surgery or autopsy.

The intraparenchymal collections of air described by Wagner are also termed pneumatoceles (figure 4). When traumatic cavities fill with blood, a hematoma forms. Radiographically, traumatic pneumatoceles and hematomas are not usually seen until a few hours or even several days after trauma, initially obscured by surrounding contusion. The size, shape, thickness of the wall, and number of pneumatoceles varies widely from patient to patient. Unlike simple contusion, which resolves fairly quickly and completely, a laceration generally takes weeks to months to resolve and may result in residual scarring. Occasionally, pneumatoceles can become secondarily infected, resembling formation of a hematoma. ²²

Tracheobronchial injury

The incidence of tracheobronchial injury (TBI) was reported as 0.4% to 1.5% in clinical series of major blunt trauma and 2.8% to 5.4% in autopsy series of trauma victims. ²³ Because TBI is uncommon, and there are usually other associated injuries, it often goes unrecognized. The clinical presentation is varied, and the initial diagnostic evaluation may be misleading. There is no initial radiographic evidence of TBI in 10% of patients, ¹⁹ and patients who present with radiographic findings of hemothorax or pneumothorax may initially respond well to treatment of these conditions, delaying diagnosis of TBI. Definitive diagnosis usually requires demonstration of injury bronchoscopically. ²⁴ Failure to recognize TBI may result in death or allow cicatrization to occur with airway obstruction occurring days or months after initial injury (figure 5).

More than 80% of tracheobronchial injuries occur within 2.5 cm of the carina. ^25,26 There is equal incidence of rupture of the right and left mainstem bronchi. ^27,28 The most common radiographic findings are subcutaneous and mediastinal air due to air leakage into surrounding tissue planes with dissection into the neck. If the trachea or the proximal left main bronchus is torn, the air commonly dissects centrally, producing mediastinal and cervical air collections without pneumothorax or hemothorax. Although most patients with TBI will have an abnormal chest radiograph, occasionally the initial radiograph can be normal. ²⁹ Pneumomediastinum occurs in most cases of tracheal rupture and in 20% to 77% of all airway injuries, ³⁰ but is a nonspecific finding and can occur from alveolar rupture secondary to blunt trauma, esophageal rupture, or positive pressure ventilatory support. Pneumothorax occurs in 63% to 87% of tracheobronchial injuries. A pneumothorax that does not resolve with functioning tube drainage is the sine qua non of mediastinal tracheal and major bronchial injury. ³¹ However, since up to 79% of pneumothoraces due to TBI will respond to initial treatment with chest tubes, ²⁸ complete re-expansion of the lung with chest tube does not exclude TBI. Furthermore, delayed pneumothorax has been reported as late as 13 days following injury. ³² Late sequelae of partial rupture of a main bronchus are granulation tissue formation leading to fibrous stricture, bronchiectasis, atelectasis, and pulmonary fibrosis.

The "fallen lung" sign where the collapsed lung falls away from the hilum, is very suggestive, if not pathognomonic, of bronchial tear (figure 6). In the supine position, the lung falls laterally and posteriorly, and in the upright position, inferiorly away from the hilum. This is the reverse of simple pneumothorax unrelated to TBI, in which the lung collapses toward the hilum. Air surrounding a sharply angulated bronchus, discontinuity, or bronchial air column truncation (so called "bronchus cut-off" sign), sometimes with a smooth rounded termination, are other signs of TBI. Endotracheal tube balloon diameter may be greater than normal in tracheal injuries because an increased amount of air is required to raise the cuff-to-tracheal wall pressure required to seal the airway. The balloon may herniate through the tracheal tear into the mediastinum. CT scanning can show communications between the mediastinum and the airway. Subtle signs of pneumomediastinum by CT may be the only indication of airway injury.

Skeletal trauma

Rib fractures occur in about half of all patients who have had major blunt chest trauma. ^33,34 The fractures are often missed on the anteroposterior chest radiograph because the lateral portions of the ribs are frequently involved and the fracture line is not tangential to the x-ray beam. CT may demonstrate rib fractures not evident on the radiograph as well as complications such as pneumothorax and hemothorax. ³³ Fractures of the 9th, 10th, or 11th ribs are often associated with splenic (figure 7), renal, or hepatic injury. Because they are relatively protected, fractures of the 1st, 2nd, and 3rd ribs usually imply severe trauma to the chest. Fracture of 5 contiguous ribs or 3 contiguous segmental rib fractures may result in focal chest wall instability, in which case paradoxical motion of the "flail" chest may lead to respiratory failure.

Potentially serious morbidity and even death have been associated with posterior dislocation of the clavicle at the sternoclavicular joint. ³⁵ The displaced clavicle may impinge on the trachea, esophagus, or great vessels or major nerves in the superior mediastinum. This injury is better depicted on CT than radiography. ³⁶

Fractures of the thoracic spine account for 15% to 30% of all spine fractures. ³⁷ About 70% to 90% of fractures are visible on radiographs. ⁴ CT and MR imaging allow detection of otherwise occult fractures and assessment of the relationship between the fracture fragments and the spinal cord. Radiographs do not reliably distinguish unstable burst fractures from the usually stable, simple, anterior wedge compression fractures. CT and MR are more helpful in making this distinction and aid in detecting injuries such as retropulsed fracture fragments and extradural hematomas (figure 8). Spiral CT allows rapid thin-section imaging of the spine with high-quality sagittal and coronal reconstructions.

Usually, sternal fractures are not evident on bedside chest radiographs, but are almost always visible on CT. CT findings include direct evidence of fracture with or without significant displacement of fracture fragments and associated retrosternal hematoma (figure 9). The presence of a fat plane between the hematoma and the aorta implies that the hematoma is not aortic in origin.

Scapular fractures are diagnosed on the initial chest radiograph in only a little more than half of patients. ³⁸ When scapular fractures are not seen on the initial chest radiograph, they are visible in retrospect in 72% of cases, not included on the examination in 19%, and obscured by superimposed structures or artifacts in 9%. ³⁸ CT of the chest should demonstrate most scapular fractures (figure 10), especially if utilized in combination with conventional radiographs.

Pleural manifestations of chest trauma

Hemothorax and pneumothorax are common manifestations of nonpenetrating trauma. Blood can enter the pleural space from injury to vessels of the chest wall, diaphragm, lung, or mediastinum. Pneumothorax can occur as a result of lung punctured by a fractured rib, pulmonary interstitial emphysema, TBI, and esophageal rupture. On CT, acute hemorrhage into the pleural space may be recognized by the increased attenuation of the pleural fluid (figure 11) or by the presence of a fluid-fluid level. Loculation tends to occur early in hemothorax. Pneumothorax occurs in 15% to 40% of patients with nonpenetrating chest trauma, ³⁹ and is more commonly detected on CT than chest radiography ²¹ (figures 3, 4, and 6).

Diaphragm injury

Diaphragmatic rupture is diagnosed in 1% to 4% of patients admitted to the hospital with blunt trauma, ^40,41 and in about 5% of patients undergoing laparotomy or thoracotomy for trauma. ⁴⁰ The most commonly accepted mechanism postulated for the development of diaphragmatic rupture during blunt trauma is sudden increase in intrathoracic or intra-abdominal pressure against a fixed diaphragm. Although there is a reportedly higher incidence of left-sided injuries, right-sided injuries are thought to be underdiagnosed. ⁴⁰ If diaphragm rupture is not promptly diagnosed, the patient may remain asymptomatic or develop incarceration of herniated abdominal viscera, which can occur at a time remote from the incidence of trauma.

Preoperative diagnosis based on radiographic findings ranges from 4% to 63% of cases. ^40,42,43 The radiographic findings include normal, hemothorax, pneumothorax, loss of visualization of the diaphragm, apparent elevation of the diaphragm, visualization of herniated viscera into the chest, cephalad extension of an intragastric tube above the level of the diaphragm, and contralateral shift of the mediastinum in the absence of a large pleural effusion or pneumothorax. CT findings include focal constriction ("collar sign") of herniated stomach or bowel (figure 12), herniation of other abdominal viscera, visualization of peritoneal fat, bowel, or viscera lateral to the lung or diaphragm or posterior to the crus of the hemidiaphragm, and sharp discontinuity of the diaphragm (figure 13). Diaphragm injuries can also be suspected when the diaphragm is not visualized ("absent diaphragm" sign). Most ruptures involve the posterolateral portion of the diaphragm at the junction of its central tendon and posterior leaves and are therefore well seen on CT. ⁴⁴ Although focal discontinuity of the diaphragm is said to be the most common finding in patients with a diaphragmatic tear, it should be noted that there is a normal increase in diaphragmatic defects with age, not related to trauma. ⁴⁵ Optimal assessment of diaphragmatic dome rupture is obtained using spiral CT with multiplanar coronal and sagittal reconstructions. ^46,47 Individual diagnostic sensitivity for detecting diaphragmatic rupture on CT is 50% to 100% and specificity is 86% to 100%. ^48,49

Cardiac trauma

The heart and pericardium are fairly well protected from nonpenetrating injury, and documented traumatic injury is uncommon. Cardiac injuries caused by blunt chest trauma include cardiac contusion, cardiac rupture, pneumopericardium, hemopericardium, cardiac tamponade, and cardiac valve dysfunction. Rapid accumulation of blood or air in the pericardial space can cause cardiac tamponade and severe hemodynamic compromise (figure 14). Bedside sonographic evaluation of the heart is the study of choice to detect pericardial fluid quickly and noninvasively. CT is also very sensitive for detecting pericardial fluid or air and may indicate pericardial hemorrhage as determined by the high CT attenuation value of the fluid.

Soft-tissue injuries of the chest wall

CT scanning can easily distinguish chest wall from parenchymal or mediastinal injury, whereas this differentiation may not be possible with chest radiography. Soft-tissue hematomas of the chest wall are readily distinguished from parenchymal injury, and subcutaneous air will not be confused with pneumothorax on CT. CT scanning can show a broncho-pleural-cutaneous fistula (figure 15), which may not be appreciated on the chest radiograph.

Role of CT in nonpenetrating chest trauma

CT is the imaging modality of choice in the assessment of patients with clinical or radiographic findings suggestive of aortic injury, thoracic spine fracture, or diaphragmatic tear. Other injuries that can be diagnosed with CT include TBI, esophageal tear, lung parenchymal injuries, hemothorax, pneumothorax, and fractures of ribs, sternum, scapula, and clavicle. Optimal assessment requires careful attention to technique, including the use of intravenously administered contrast material and multiplanar reconstructed images, and an awareness of potential pitfalls in interpretation. AR

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