MDCT PROTOCOL FOR SUSPECTED THORACIC INJURY

Chest CT scans for suspected thoracic trauma should be performed with intravenous contrast material in order to demonstrate any sites of active bleeding and to opacify the heart, aorta, and thoracic blood vessels. CT scanning of the chest following trauma is best performed in the arterial phase after a 25- to 30-second delay. Scans should be acquired at thin detector configurations so that high-quality multiplanar volumetric reformations and a CT arteriogram can be produced from the thin axial images. Routine coronal and sagittal reformations of the chest are recommended on all chest trauma patients in order to better demonstrate injuries, particularly injuries to the major vessels, diaphragm, or spine, which are best shown on these planes. A detector configuration of 16 • 1.25 mm is preferable for 16-slice scanners, while 64-slice scanners would utilize axial slices having less than 1-mm thickness. The thin slices may be reformatted to 2.5 mm for viewing axial slices and transmitted to the Picture Archival Computer System (PACS) along with the coronal and sagittal reformations for formal examination interpretation. All axial and multiplanar images should be reviewed on soft tissue, lung, and bone windows.

INJURIES OF THE PLEURAL SPACE

A hemothorax, blood in the pleural space, may result from injury to the chest wall, diaphragm, lung, or mediastinal structures. CT may confirm a hemothorax when a pleural fluid collection in a trauma patient seen on CT measures blood density over 35 to 40 Hounsfield units (HU) (Fig. 2-1). A pneumothorax, air in the pleural space, may result from a lung injury, tracheobronchial injury, or esophageal rupture. The most common cause is lung injury associated with a rib fracture. A pneumothorax occurs in about 15% to 40% of patients with acute chest trauma. Many small and even moderate-sized pneumothoraces that are not visible on the supine chest film may be identified on CT. A pneumothorax seen on CT that cannot be identified on a supine chest film is referred to as an “occult” pneumothorax. Studies estimate that 10% to 50% of pneumothoraces seen on CT are not evident on the supine anteroposterior film. Radiographic signs of pneumothorax may be subtle. In the supine patient air collects in nondependent locations such as the anterior costophrenic sulcus. This region extends from the seventh costal cartilage to the eleventh rib in the midaxillary line. The air collection appears as an abnormal lucency in the lower chest or upper abdomen, frequently referred to as the “deep sulcus” sign. Additional signs of pneumothorax in a supine patient include a sharply outlined cardiac or diaphragmatic border and depression of the hemidiaphragm (Fig. 2-2). Detection of even a small pneumothorax is important as it may enlarge during positive-pressure ventilation or general anesthesia. A tension pneumothorax is an emergency condition resulting from a lung or airway injury associated with a one-way accumulation of air within the pleural space. As intrapleural pressure rises the mediastinal structures are compressed, decreasing venous return to the heart, leading to hemodynamic instability. Radiography and CT will show mediastinal shift to the contralateral hemithorax, hyperexpansion of the ipsilateral thorax, and depression of the ipsilateral hemidiaphragm.

Figure 2-1 A, Axial CT shows high-density blood layering within the pleural space consistent with a hemothorax (black arrow). Red arrow indicates adjacent opacified lung. B, Sagittal image shows both hemothorax (black arrow) and opacified lung (white arrow).

Figure 2-2 A, Anteroposterior chest radiograph findings of tension pneumothorax including the “deep sulcus” sign (black arrow), sharply outlined cardiac and diaphragmatic border (arrowhead), and depression of the hemidiaphragm (white arrow). Note rightward deviation of the mediastinum consistent with tension. B, Axial image from MDCT demonstrates air collecting nondependently in the anterior costophrenic sulcus. Note rightward deviation of the heart indicating developing tension. C, Coronal image demonstrates depression of the hemidiaphragm and rightward deviation of the mediastinum. D, Chest tube placement results in normal positioning of the hemidiaphragm and mediastinum.

ESOPHAGEAL INJURIES

Blunt trauma to the esophagus is extremely rare since this structure is well protected in the posterior mediastinum. Most esophageal injuries occur from penetrating or iatrogenic trauma. Blunt trauma may result in rupture or intramural hematoma. These injuries normally involve the upper thoracic esophagus or the lower esophagus just above the gastroesophageal junction. The most commonly accepted theory regarding the pathophysiology of rupture is similar to the mechanism in Boerhaave syndrome in that an increase in intraluminal pressure against a closed glottis results in a tear at the weakest point of the esophageal wall, usually the distal third of the esophagus on the left where there is less protection from the pleural lining and the heart. Other etiologies for injury include disruption of the esophageal blood supply, resulting in ischemia and late perforation, and a blast effect caused by a concomitant tracheal injury. Direct injury may also result from adjacent thoracic spine fractures or compression between the sternum and thoracic spine, as observed in high-speed road traffic accidents. Esophageal injuries are often associated with clinical symptoms, including blood in the esophagus or pain on swallowing. CT may suggest the diagnosis of traumatic esophageal perforation with the presence of pneumomediastinum, mediastinitis, hydropneumothorax, or leakage of oral contrast medium into the mediastinum or the pleural space. Water-soluble contrast esophagography, followed by flexible esophagoscopy, may be required to fully evaluate the site of injury.

CARDIAC INJURIES

Cardiac and pericardial injuries are uncommon with blunt thoracic trauma but do occur with severe blows to the anterior chest. The diagnosis of blunt cardiac injury relies on a high clinical suspicion. Cardiac injuries often occur in conjunction with sternal fractures. The anterior aspect of the heart that abuts the sternum is most vulnerable to injury. Patients with cardiac injuries may have an abnormal electrocardiogram and elevated cardiac enzymes. Cardiac injuries include cardiac contusion, cardiac rupture, pneumopericardium, hemopericardium, cardiac tamponade, and cardiac valve injury. Hemopericardium from a cardiac injury or cardiac rupture may quickly produce cardiac tamponade with hemodynamic compromise (Fig. 2-3). Cardiomegaly may be seen on the plain chest radiograph, while CT or cardiac sonography may confirm hemopericardium.

Figure 2-3 A 28-year-old male with a gunshot injury. A, Anteriorposterior view of the chest demonstrates bullet fragments projecting over the cardiac silhouette. B, Axial CT demonstrates a bullet fragment in the myocardium of the left ventricle (black arrow). Note hemopericardium (black arrowhead) and adjacent lung contusion (white arrow). C, Axial CT in lung windows better demonstrates adjacent lung contusion. D, Coronal CT demonstrates multiple rib fractures resulting from penetrating injury.

AORTIC AND GREAT VESSEL INJURIES

Injuries of the aorta account for a significant number of fatalities following blunt trauma. Seventy percent of all thoracic aortic injuries are fatal at the scene of trauma. Of patients who are transported to the hospital, 90% of aortic rupture occurs at the aortic isthmus, located at the junction of the posterior aortic arch and descending aorta, just distal to the origin of the left subclavian artery. The proposed mechanism of injury is rapid deceleration producing shear injury at the site where the rate of deceleration of the mobile aortic arch differs from that of the relatively fixed descending aorta. In addition, bending stress occurs because the aorta is flexed over the left pulmonary artery and left mainstem bronchus. Only 5% of aortic injuries in clinical series involve the ascending aorta, and these injuries may be associated with life-threatening cardiac and pericardial injuries. Rarely, aortic injuries may involve the descending aorta at the level of the diaphragmatic hiatus.

A normal chest radiograph has a high negative predictive value (98%) but a low positive predictive value for aortic injury. The chest film findings suggestive of aortic injury include mediastinal widening greater than 8 cm, loss of the normal aortic arch contour, a left apical pleural cap, displacement of the nasogastric tube to the right, widened paraspinal lines, and loss of the descending aortic line. Most of the plain film findings of aortic injury are nonspecific. The gold standard for the diagnosis of aortic injury has traditionally been aortography; however, at most trauma centers today, aortography has been replaced with MDCT.

The sensitivity of CT has been reported to be 92% to 100% and specificity 62% to 100% for the detection of aortic injury. The accuracy of aortic trauma detection with CT has been improving in parallel with technological improvements in CT scanning. Current fast MDCT scanners decrease motion artifact and provide higher-quality two- and three-dimensional reformations for diagnosis and treatment planning.

The CT findings of aortic trauma include indirect signs, such as mediastinal hematoma surrounding the posterior aortic arch and proximal descending aorta, as well as the direct signs of intimal tear/flap, aortic contour abnormality, thrombus protruding into the aortic lumen, false aneurysm formation, pseudocoarctation, and extravasation of intravenous contrast material. If only direct signs are utilized, the sensitive and negative predictive value remains at 100% but the specificity increases to 96% (Fig. 2-4).

Figure 2-4 A, Portable chest radiograph shows an enlarged indistinct aortic arch (black arrow), a left apical pleural cap, and upper rib fractures (white arrows). B, Axial CT demonstrates mediastinal hematoma, intimal flap, and pseudoaneurysm (arrow). C, Oblique coronal reformation shows the extent of pseudoaneurysm (arrow). D and E, Thoracic aortography before and after treatment with an aortic stent.

A common aortic injury is a traumatic false aneurysm resulting from disruption of the vessel intima and media while the adventitia remains intact. The intravascular blood confined by only the adventitia bulges outward forming a pseudoaneurysm. In many cases, the aortic injury may be limited to a partial circumferential tear. CT findings typically consist of a saccular out-pouching demarcated from the aortic lumen by torn intima. It frequently results in hemomediastinum. Treatment of a pseudoaneurysm may today be performed with intravascular stent grafting. False positive examinations may be related to a prominent ductus diverticulum or an ulcerated atheromatous plaque. A traumatic pseudoaneurysm is usually surrounded by mediastinal blood whereas a ductus diverticulum and an ulcerated atheromatous plaque are not (Figs. 2-5 to 2-7).

Figure 2-5 A 39-year-old female involved in a rollover motor vehicle accident. A, Portable chest radiograph demonstrates widened superior mediastinum (arrow). B, Axial CT section shows intimal flap and pseudoaneurysm formation with extraluminal extension of contrast (arrow). C, Lower axial section shows hemopericardium; the patient had cardiac enzyme elevation consistent with cardiac contusion (arrow). D, Sagittal volume-rendered image shows pseudoaneurysm more clearly (arrow). E, Three-dimensional images demonstrate pseudoaneurysm formation.

Figure 2-6 A 53-year-old female following a motor vehicle accident. A and B, Axial and coronal sections at the level of the diaphragm demonstrate intimal flap formation (arrow)

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