SYMPOSIUM REVIEW ARTICLE
Acute Traumatic Aortic Injury Practical Considerations for the Diagnostic Radiologist Constantine A. Raptis, MD,* Mark M. Hammer, MD,* Kathleen G. Raman, MD, MPH,w Vincent M. Mellnick, MD,* and Sanjeev Bhalla, MD*
Abstract: The diagnosis of acute traumatic aortic injury (ATAI) relies heavily on accurate and efficient imaging interpretation, thereby making the radiologist integral to the care of patients in whom these life-threatening lesions are suspected. Typically, this evaluation begins with the initial trauma radiograph, in which findings suggestive of mediastinal hematoma or ATAI can be detected. Definitive diagnosis of ATAI is made with the current gold standard, computed tomography, wherein indirect and direct signs of ATAI provide the means for sensitive and specific diagnosis. Although the diagnosis of ATAI on computed tomography can be straightforward, technical and anatomic pitfalls can complicate interpretation and must be understood. Once the diagnosis is made, the radiologist needs to provide a meaningful report that includes an appropriate description of the lesion location and characteristics. The purpose of this article is to review the key aspects of the imaging evaluation of ATAI with a focus on factors that affect the management of these patients. Key Words: acute traumatic aortic injury, trauma, computed tomography, emergency radiology
(J Thorac Imaging 2015;30:202–213)
A
cute traumatic aortic injury (ATAI) is a potential lifethreatening complication of blunt and penetrating trauma. In blunt trauma, motor vehicle collisions account for the majority of ATAI.1–5 On the basis of autopsy series, blunt thoracic aortic injuries can be classified by location into several groups, including intrapericardial/ascending aorta (8% to 27%), aortic arch (8% to 18%), and descending thoracic aorta (47% to 75%).6 ATAI due to blunt trauma carries a dismal prognosis, as most patients expire in the field, often immediately.3,7–9 Patients with penetrating aortic trauma, most commonly due to gunshot or stab wounds, also have poor prognoses; many patients with penetrating aortic trauma who reach the hospital are unstable at presentation and require emergent thoracotomy before imaging.10 Prompt diagnosis of ATAI is essential, as 70% to 90% of patients reaching the hospital alive will survive repair.7,11,12 Clinical symptoms of ATAI include chest pain, back pain, or difficulty breathing, but these are neither sensitive nor specific for ATAI.13 Similarly, clinical signs of ATAI, which may be absent in up to one third of patients, From the *Mallinckrodt Institute of Radiology; and wDepartment of Surgery, Washington University, St Louis, MO. The authors declare no conflicts of interest. Correspondence to: Constantine A. Raptis, MD, Mallinckrodt Institute of Radiology, 510 S. Kingshighway Boulevard, Campus Box 8131, St Louis, MO 63101 (e-mail: [email protected]). Copyright r 2015 Wolters Kluwer Health, Inc. All rights reserved.
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are also not specific; these include external chest wall injuries, systemic hypotension, concomitant upper limb hypertension with lower limb hypotension, or a substantial difference in blood pressures between the right and left brachial arteries (present in up to 50% of patients with ATAI). Some patients may present initially with no clinical signs but rapidly develop hemodynamic instability due to an underlying ATAI.11,13,14 Given that the clinical signs and symptoms of ATAI are unreliable, clinicians rely heavily on imaging by chest radiography and computed tomography (CT) to make the diagnosis of ATAI. Although most patients involved in trauma undergo a portable anteroposterior chest radiograph, the decision of which patients to image with CT rests upon an assessment of clinical signs and symptoms, the mechanism of injury, and findings on the chest radiograph. The diagnosis of ATAI relies heavily on accurate and efficient imaging interpretation, thereby making the radiologist integral to the care of these patients. The purpose of this article is to review the key aspects of imaging interpretation in the setting of ATAI, focusing on findings seen on chest radiograph and CT. Special consideration will be given to factors that may affect the management of patients with ATAI.
CHEST RADIOGRAPH The portable, supine anteroposterior chest radiograph remains the first imaging study trauma patients typically undergo upon reaching the emergency department. The primary goal of the chest radiograph in the setting of trauma is to identify immediately life-threatening injuries that require prompt treatment, such as tension pneumothorax or large hemothorax. Although not the primary purpose of the chest radiograph, findings that suggest a possible underlying mediastinal hematoma are useful in directing further investigation for underlying ATAI. Evaluation for mediastinal hematoma on a chest radiograph begins with assessment of the width of the superior mediastinum. Mediastinal widening can be defined as a width of 8 cm (at the level of the left subclavian artery origin) on an anteroposterior view of the chest or a mediastinum to chest width ratio of >0.25.15 Superior mediastinal widening is a fairly sensitive indicator (sensitivity 81% to 100%) of underlying ATAI.14,16,17 Unfortunately, mediastinal widening is not specific (specificity 34% to 60%) for mediastinal hematoma or ATAI, as there are many other causes of a wide mediastinum, including mediastinal lipomatosis, vascular anomalies, lymphadenopathy, and even the supine anteroposterior technique itself (Fig. 1). To further complicate matters, a mediastinal J Thorac Imaging
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Volume 30, Number 3, May 2015
FIGURE 1. Initial anteroposterior, supine chest radiograph (A) demonstrates a wide mediastinum, poor visualization of the normal aortic contour, deviation of the trachea to the right, and depression of the left mainstem bronchus, all findings concerning for mediastinal hematoma in this patient status post motor vehicle collision. Transaxial CT of the chest (B) in the same patient shows no mediastinal hematoma and a normal aorta. The cause of the abnormal widening on the chest radiograph is shown to be abundant mediastinal lipomatosis.
hematoma can be due to causes other than ATAI, such as venous bleeding or an adjacent spine or sternal fracture.15,18,19 These false positives were reported in a study by Mirvis et al20 of 677 patients with an abnormal chest radiograph after blunt trauma, in which 570 (84%) patients had no mediastinal hematoma at CT, and only 21 (3%) patients were ultimately diagnosed with an ATAI after CT and angiography. Of note, mediastinal widening from an ATAI typically involves the left paratracheal region or entire superior mediastinum. In our experience, isolated right-sided mediastinal widening in the setting of trauma is more typically associated with brachiocephalic artery injury or, rarely, esophageal or ascending aortic injury (Fig. 2). Despite the considerable attention that mediastinal width receives in the assessment of a chest radiograph in trauma, a thorough evaluation of the aortic contour is also Copyright
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FIGURE 2. Initial anteroposterior, supine chest radiograph (A) in this patient status post motor vehicle collision shows right-sided mediastinal widening with a preserved aortic knob and descending thoracic aortic contour. The isolated right-sided mediastinal widening was suspicious for a mediastinal hematoma due to brachiocephalic artery injury. B, A transaxial CT image including the superior mediastinum of the same patient confirms the presence of a mediastinal hematoma and also demonstrates direct signs of arterial injury manifested by a posterior outpouching filled with contrast from the brachiocephalic artery (arrow).
helpful in screening patients for mediastinal hematoma. In a truly normal study, the transverse and descending aortic contour should be crisp, with normal interfaces with the lung and aorticopulmonary window. Any obscuration of the aortic contour should be viewed with suspicion and should be evaluated with a follow-up upright chest radiograph or CT. An indistinct aortic contour has a sensitivity of 53% to 100% and a specificity of 21% to 55% for mediastinal hematoma. Loss of the aorticopulmonary window has a sensitivity of 40% to 100% and a specificity of 56% to 83%.14,21,22 Other findings that can be seen in the setting of a mediastinal hematoma or ATAI include depression of the left mainstem bronchus, deviation of the trachea, deviation of the nasogastric tube to the right of the T3 or T4 spinous processes, and increased density of the left subclavian artery reflection. The latter is indicative of blood tracking in the mediastinum to the extrapleural left apex, a
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FIGURE 3. A, A supine, anteroposterior radiograph of the chest in a patient involved in a motor vehicle collision demonstrates multiple findings concerning for mediastinal hematoma and ATAI, including widening of the mediastinum, loss of the normal aortic knob, depression of the left mainstem bronchus (arrow), and left apical capping (arrowhead). There are also rib fractures and subcutaneous gas, indicative of severe blunt force chest trauma in this patient who has already had bilateral pneumothoraces treated with thoracostomy tube placement. B (transaxial CT of the chest) and C (oblique sagittal reconstruction of the chest) demonstrate periaortic mediastinal hematoma as well as flaps of tissue within the aortic lumen (arrows). In addition, there is mild focal dilatation of the aorta between the flaps of tissue on image B. These CT findings confirmed the presence of an ATAI, which was treated with an endovascular stent.
finding often referred to as left apical capping11 (Fig. 3). The chest radiograph should also be examined for findings of substantial thoracic injury, including fractures involving the sternum, scapula, clavicle, first rib, or multiple left-sided ribs, diaphragmatic injury, and pneumomediastinum. Although not sensitive or specific for underlying mediastinal hematoma or ATAI, these imaging findings indicate that a substantial blunt force was applied to the chest and may be particularly helpful in patients who cannot provide any history. Given that the chest radiograph findings of ATAI are often nonspecific and not entirely sensitive, rather than focusing on specific measurements or findings, a general assessment of the mediastinum is a more consistent and sensitive means of detecting an underlying ATAI. A study by Ho et al23 demonstrated that interobserver agreement and sensitivity for ATAI were higher for a general “normal
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or abnormal” assessment of the mediastinum rather than a determination of whether the mediastinum was wide. Because of the gravity of missing an underlying ATAI, any suspicious findings on a chest radiograph, even if questionable, at least warrant additional imaging. In stable patients, an upright chest radiograph may help clear an abnormal mediastinum, as up to 38% of patients in a study by Schwab et al24 demonstrated normalization of an abnormal mediastinal width when obtaining an upright radiograph after the initial supine examination. A minority of patients (0.5% to 7%) may have a normal chest radiograph in the setting of occult mediastinal hematoma or ATAI.5,11,25,26 Therefore, for patients with abnormal chest radiograph findings, patients with a high index of clinical suspicion for aortic injury, or patients with chest pain out of proportion to their known injuries, our recommendation is to proceed to chest CT.
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CT Although CT was used in conjunction with catheter angiography for the evaluation of ATAI in the 1980s and 1990s, advances in scanner technology, particularly multidetector row CT (MDCT), have allowed CT to become the primary diagnostic tool in the evaluation of ATAI today.12,20,27–30 CT examinations in patients with thoracic trauma should be performed with intravenous contrast to allow for opacification of the vasculature. Unlike our nontraumatic acute aorta protocol for aortic dissection, noncontrast imaging is not routinely performed. Whereas some institutions obtain CT data sets in the arterial and portal venous phases, at our institution, only 1 set of images is obtained with a fixed delay at 70 seconds. This delay allows for a venous phase, which is optimal for scanning the entire thorax, abdomen, and pelvis. With the high spatial resolution available on today’s MDCT scanners, it is probably no longer necessary to obtain a dedicated arterial phase of the thorax as almost all vascular injuries can be detected on venous phase imaging. Detection of an ATAI on CT requires the identification of direct and indirect signs of vascular injury. The main indirect sign of major vascular injury is a mediastinal hematoma that effaces the fat plane with a vessel. A mediastinal hematoma presents as an area of hyperattenuating material (typically 40 to 70 HU) that displaces and infiltrates the normal mediastinal fat. The location of the mediastinal hematoma is critical, however, as a hematoma that is not in direct contact with the aorta is not a sign of an ATAI and may be related to venous bleeding or an adjacent fracture31–33 (Fig. 4). A mediastinal hematoma that does abut the aorta or any arch vessel is an indirect sign of vascular injury and should prompt a close inspection for direct signs of vascular injury. It is important to recognize that the mediastinal hematoma associated with an injured artery usually does not result from extravasated blood from the lumen of the vessel itself but rather is due to avulsion of small perivascular veins or possibly from the arterial vasovasorum.15 Consequently, the absence of contrast extravasation into a periaortic hematoma should not be used as a finding to suggest that the aorta has not been injured. Utilization of periaortic mediastinal hematoma alone as a diagnostic criterion results in false-positive examinations and decreases the specificity of CT.28,34 It should also be noted that a periaortic mediastinal hematoma is not a requirement for the diagnosis of ATAI, as direct signs of ATAI can be present in the absence of a periaortic hematoma.35,36 Direct signs of ATAI are changes in the normal imaging appearance of the aorta itself. Although transaxial images can be useful in this regard, it is important to examine the aorta in multiple planes when evaluating for direct findings of ATAI. The simplest direct finding of ATAI on chest CT is disruption of the aortic wall. This can manifest as a focal outpouching of the aortic wall containing contrast, a contour irregularity in the aortic wall, or, in rare cases, a focal defect with extravasation of contrast from the aorta into the adjacent mediastinum. In some cases of ATAI, the normal aortic contour may be preserved, but there will be strands of tissue (either intimal or intimomedial) within the aortic lumen due to injury. An ATAI involving the luminal side of the wall can form a nidus for thrombus; indeed, intraluminal thrombus is another direct finding of ATAI.11,35,37 In some cases, ATAI can result in an abrupt caliber change in the aorta, often Copyright
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due to blood within the wall of the aorta or due to compression of the aorta by adjacent hematoma. In our practice, we avoid the use of terms such as pseudoaneurysm or dissection when describing aortic injuries on CT. Although these processes can certainly occur in the setting of ATAI, CT can often underestimate the extent of vascular injury, and full transections of the aorta may have the imaging appearance of what might be described as a pseudoaneurysm or even dissection. Therefore, we focus on diagnosing ATAI and providing an accurate description of its location and appearance without using specific descriptors that may be inaccurate or confusing (Figs. 5–7). For patients with direct findings of ATAI on CT, no confirmation with conventional angiography is required and will only serve to delay appropriate management.30 In fact, in cases of subtle injuries, CT can identify ATAI that would be very difficult or impossible to detect on conventional angiography.38 Using direct signs, the sensitivity of CT for ATAI is >96%, with several studies placing the sensitivity at 100%.20,28–30 When relying on direct signs only, specificity is also excellent, with values >99%.20,30 For cases in which there is a periaortic hematoma and no direct findings indicating an ATAI, there are several options for follow-up. In stable patients, one option would be to repeat a CT scan in 48 to 72 hours. An alternative approach would involve transcatheter intravascular ultrasound or transesophageal echocardiography to look for a subtle, CT-occult injury. Previously, conventional angiography was used in cases such as these, but in the era of high–spatial resolution MDCT, angiography is unlikely to provide additional useful information.39 Disadvantages of using angiography for diagnosis include its invasive nature, cost, access site complications, and the need to transport the patient to an angiography suite or operating room. It should be noted, however, that angiograms are still performed by vascular surgeons in the operating room at the time of definitive endoluminal repair. The literature discussed above pertains mainly to blunt trauma causing traumatic aortic injury. CT is also an excellent modality for diagnosing penetrating aortic injury in hemodynamically stable patients. In cases of penetrating trauma, in addition to the findings of a periaortic hematoma and direct signs of ATAI, wound trajectory is critical. In cases in which the wound tract passes through or near the aorta, suspicion should be very high for an aortic injury. If a periaortic hematoma alone is seen without a direct sign of aortic injury, it is our recommendation that follow-up imaging be obtained within 24 to 48 hours to evaluate for any evolution in the appearance of the aorta. Patients with wound trajectories not intersecting or near the aorta and no other concerning findings of ATAI can be considered to not have an aortic injury40 (Fig. 8).
CT PITFALLS Understanding the potential pitfalls in CT imaging of the thoracic aorta is crucial for accurate evaluation for ATAI. Several technical artifacts can complicate evaluation for ATAI. Ideally, the patient’s arms should be placed over their head for a trauma CT of the chest. If the patient’s arms cannot be placed over their head and are at their side, beam hardening, photon-starvation, and reconstruction artifacts may occur, which will result in linear streak artifact across the aorta as well as worsened quantum mottle, potentially
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FIGURE 4. A, An anteroposterior, supine chest radiograph in this patient status post motor vehicle collision demonstrates predominantly right-sided mediastinal widening concerning for mediastinal hematoma. In addition, the aortic knob was indistinct, further raising concern for vascular injury. B, A transaxial CT image of the chest confirms the presence of a hematoma in the anterior mediastinum, deep to the manubrium. There is, however, a preserved fat plane (arrow) between the hematoma and the aorta, and no direct signs of aortic injury were found. This anterior mediastinal hematoma was found to be secondary to a manubrial fracture, seen in C (arrow), a sagittal reconstruction of the CT using bone windows.
complicating evaluation for mediastinal hematoma and intraluminal flaps (Fig. 6). If there is a problem with the contrast injection (incorrect scan delay, bolus extravasation), mixing and flow artifacts can occur in the aorta, which may simulate strands of intimomedial tissue in the lumen. If flow artifact is seen in real time at the scanner and a timing problem is felt to be the cause, an immediate repeat scan of the patient will allow for contrast to fully opacify the aorta and eliminate the artifact. If the artifact is only seen later, at the reading station, and raises the possibility of an ATAI, there should be a low threshold for repeating the scan with a second bolus of contrast to fully evaluate for ATAI (Fig. 9). Although trauma CT examinations are ideally performed during a breath-hold, incomplete breath-holds are common in critically ill patients. Breath-hold artifacts can produce apparent discontinuities in the walls of vessels or simulate fractures of bones, particularly within the sternum. Breathing artifacts can be recognized by examining lung windows to confirm the presence of motion. Sagittal reconstructions can also be helpful in showing the nonphysiologic, artifactual
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step-off caused by an incomplete breath-hold. Finally, pulsation artifact can result in blurring of the aortic wall or curvilinear defects within the aortic lumen by superimposition of the moving aortic wall with the lumen.41 Pulsation artifacts are typically most pronounced in the ascending aorta and can be recognized by noting that the wall artifact will appear both in the lumen and the adjacent mediastinal fat. In addition, the interface between the artifact and the contrast in the aortic lumen is typically indistinct, whereas the interface between true intimomedial tissue and luminal contrast is often sharp. If a pulsation artifact is suspected but cannot be confirmed, options include repeating the scan with cardiac gating or using an ultrafast high-pitch technique (eg, on a dual-source scanner) to eliminate the effects of motion in the aorta (Fig. 10). In addition to technical artifacts, several anatomic pitfalls can complicate evaluation for ATAI. Variants in the normal appearance of the aorta are one potential source of confusion. One of the most commonly confusing anatomic variants is the ductus diverticulum. A ductus diverticulum
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FIGURE 5. A, A transaxial CT image of the chest in a patient status post motor vehicle collision demonstrates a left hemothorax and a periaortic hematoma surrounding the descending thoracic aorta with an outpouching filled with contrast arising from the anterior contour of the aorta (arrows). B, Sagittal reconstruction of the CT again demonstrates this outpouching (arrows), consistent with an ATAI. Although this was described as a pseudoaneurysm in the report, the patient was found to have a complete transection of the anterior wall of the aorta (involving all 3 components of the wall) at open repair. Given that CT can underestimate the degree of injury, we prefer to describe the injury using generic terms such as outpouching rather than using specific terms such as pseudoaneurysm, which can be inaccurate and misleading.
is a focal smooth bulge along the undersurface of the aorta at the level of the isthmus. The typical ductus diverticulum forms obtuse angles with the remainder of the aorta, but an atypical ductus diverticulum has a shorter and steeper neck. By virtue of their location at the isthmus, which is the most common site of blunt ATAI (>90% by surgical series, 36% to 54% by autopsy series), ductus diverticula are frequently heavily scrutinized as possible injuries.1,3,6,42,43 In contrast to an ATAI, which often has an irregular or discontinuous margin with acute angles at its base, a ductus diverticulum should have smooth margins. Other findings that favor a Copyright
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FIGURE 6. Transaxial (A) and oblique sagittal (B) CT images from a patient involved in a motor vehicle collision demonstrate circumferential high-attenuating thickening of the wall of the descending aorta that can be seen over a long length on the oblique sagittal reconstructions (B, arrowheads). These findings were consistent with an ATAI manifested by an intramural hematoma, which narrowed the lumen of the aorta. Intramural hematoma is an infrequently seen but important direct finding for ATAI. Note also linear beam hardening artifact degrading the image secondary to scanning the patient with his arms at his sides.
ductus diverticulum over an ATAI include the absence of periaortic mediastinal hematoma or intimomedial strands in the aortic lumen.35,44 Occasionally, a fibrotic remnant of the ductus arteriosus can persist and may calcify along the undersurface of the aorta at the isthmic region. A small ductus diverticulum or irregularity may be present along the aorta at the site of the ductus remnant and should also not be confused for an ATAI.
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FIGURE 8. A transaxial CT image of the thorax shows a thoracostomy tube in place for a hemopneumothorax. There is gas and blood in the mediastinum directly abutting the aorta along the trajectory (arrow) of a bullet that passed through the thorax. There is an adjacent contour bulge (arrowhead) of the aorta, representing traumatic aortic injury.
FIGURE 7. A, A transaxial CT image from a patient involved in a motor vehicle collision demonstrates a focal discontinuity of the anterior wall of the aorta with a focal outpouching consistent with an ATAI. In addition, more amorphous high-attenuating material is seen lateral to this outpouching consistent with actively extravasating contrast (arrowheads). B, Another transaxial CT image from the same patient; this contrast can be seen actively extravasating (arrowheads) into a left hemothorax. Although rarely seen on imaging, active extravasation of contrast is an important direct finding of ATAI, which indicates a ruptured aorta that requires emergent treatment.
The aortic spindle is another anatomic variant that can be mistaken for an ATAI. The aortic spindle is defined as an area of localized mild dilatation of the descending aorta just beyond the isthmus and occurs as a remnant of the appearance of the fetal aorta. In contrast to the ductus diverticulum, which results in an eccentric outpouching, the aortic spindle results in a fusiform dilatation of the aorta. Unlike a true ATAI, the aortic spindle has smooth margins.44 Oblique sagittal reconstructions can be helpful to confirm the smooth contour of the wall of an aortic spindle. Occasionally, at the origins of branch vessels, such as the intercostal and bronchial arteries, small infundibula may be present. These conical outpouchings can be confirmed as vascular infundibula and not an ATAI by noting the absence of surrounding hematoma, appreciating their smooth margins, and, most importantly, by identifying the small vessel arising from their apex35,44 (Fig. 11). Normal structures adjacent to the aorta can occasionally cause confusion in the evaluation for ATAI. When the superior intercostal vein is opacified with contrast, it is
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closely apposed to the aortic arch and can simulate an outpouching on single slices. Knowledge of its anatomic location and scrolling at the workstation should eliminate any confusion in this regard. In some patients, particularly younger adults, normal thymic tissue is often present in the anterior mediastinum and may be confused for a mediastinal hematoma. In contrast to a mediastinal hematoma, which is typically a contiguous and often irregularly shaped collection of intermediate-attenuation blood products, the thymus has a stippled or nodular appearance due to interspersed fat, is often triangular, and is located in the anterior mediastinum. In addition, a mediastinal hematoma is usually surrounded by fat stranding in the mediastinum, whereas the fat adjacent to the thymus will have no stranding.
REPORTING OF POSITIVE RESULTS The treatment of ATAI has evolved over the past two decades to now include a combination of open repair, endovascular repair, and conservative management.12,45–48 Although the decision as to which type of repair to pursue involves multiple clinical factors, the imaging appearance of the ATAI also plays an important role. As CT emerged as the imaging test of choice for ATAI, it became evident that subtle ATAI that would previously have gone undetected was now being recognized.35,49–51 As a result, in the late 1990s, the concept of minimal aortic injury was introduced.49,52 Whereas the majority of authors chose to define minimal aortic injury as a lesion measuring
Acute Traumatic Aortic Injury Practical Considerations for the Diagnostic Radiologist Constantine A. Raptis, MD,* Mark M. Hammer, MD,* Kathleen G. Raman, MD, MPH,w Vincent M. Mellnick, MD,* and Sanjeev Bhalla, MD*
Abstract: The diagnosis of acute traumatic aortic injury (ATAI) relies heavily on accurate and efficient imaging interpretation, thereby making the radiologist integral to the care of patients in whom these life-threatening lesions are suspected. Typically, this evaluation begins with the initial trauma radiograph, in which findings suggestive of mediastinal hematoma or ATAI can be detected. Definitive diagnosis of ATAI is made with the current gold standard, computed tomography, wherein indirect and direct signs of ATAI provide the means for sensitive and specific diagnosis. Although the diagnosis of ATAI on computed tomography can be straightforward, technical and anatomic pitfalls can complicate interpretation and must be understood. Once the diagnosis is made, the radiologist needs to provide a meaningful report that includes an appropriate description of the lesion location and characteristics. The purpose of this article is to review the key aspects of the imaging evaluation of ATAI with a focus on factors that affect the management of these patients. Key Words: acute traumatic aortic injury, trauma, computed tomography, emergency radiology
(J Thorac Imaging 2015;30:202–213)
A
cute traumatic aortic injury (ATAI) is a potential lifethreatening complication of blunt and penetrating trauma. In blunt trauma, motor vehicle collisions account for the majority of ATAI.1–5 On the basis of autopsy series, blunt thoracic aortic injuries can be classified by location into several groups, including intrapericardial/ascending aorta (8% to 27%), aortic arch (8% to 18%), and descending thoracic aorta (47% to 75%).6 ATAI due to blunt trauma carries a dismal prognosis, as most patients expire in the field, often immediately.3,7–9 Patients with penetrating aortic trauma, most commonly due to gunshot or stab wounds, also have poor prognoses; many patients with penetrating aortic trauma who reach the hospital are unstable at presentation and require emergent thoracotomy before imaging.10 Prompt diagnosis of ATAI is essential, as 70% to 90% of patients reaching the hospital alive will survive repair.7,11,12 Clinical symptoms of ATAI include chest pain, back pain, or difficulty breathing, but these are neither sensitive nor specific for ATAI.13 Similarly, clinical signs of ATAI, which may be absent in up to one third of patients, From the *Mallinckrodt Institute of Radiology; and wDepartment of Surgery, Washington University, St Louis, MO. The authors declare no conflicts of interest. Correspondence to: Constantine A. Raptis, MD, Mallinckrodt Institute of Radiology, 510 S. Kingshighway Boulevard, Campus Box 8131, St Louis, MO 63101 (e-mail: [email protected]). Copyright r 2015 Wolters Kluwer Health, Inc. All rights reserved.
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are also not specific; these include external chest wall injuries, systemic hypotension, concomitant upper limb hypertension with lower limb hypotension, or a substantial difference in blood pressures between the right and left brachial arteries (present in up to 50% of patients with ATAI). Some patients may present initially with no clinical signs but rapidly develop hemodynamic instability due to an underlying ATAI.11,13,14 Given that the clinical signs and symptoms of ATAI are unreliable, clinicians rely heavily on imaging by chest radiography and computed tomography (CT) to make the diagnosis of ATAI. Although most patients involved in trauma undergo a portable anteroposterior chest radiograph, the decision of which patients to image with CT rests upon an assessment of clinical signs and symptoms, the mechanism of injury, and findings on the chest radiograph. The diagnosis of ATAI relies heavily on accurate and efficient imaging interpretation, thereby making the radiologist integral to the care of these patients. The purpose of this article is to review the key aspects of imaging interpretation in the setting of ATAI, focusing on findings seen on chest radiograph and CT. Special consideration will be given to factors that may affect the management of patients with ATAI.
CHEST RADIOGRAPH The portable, supine anteroposterior chest radiograph remains the first imaging study trauma patients typically undergo upon reaching the emergency department. The primary goal of the chest radiograph in the setting of trauma is to identify immediately life-threatening injuries that require prompt treatment, such as tension pneumothorax or large hemothorax. Although not the primary purpose of the chest radiograph, findings that suggest a possible underlying mediastinal hematoma are useful in directing further investigation for underlying ATAI. Evaluation for mediastinal hematoma on a chest radiograph begins with assessment of the width of the superior mediastinum. Mediastinal widening can be defined as a width of 8 cm (at the level of the left subclavian artery origin) on an anteroposterior view of the chest or a mediastinum to chest width ratio of >0.25.15 Superior mediastinal widening is a fairly sensitive indicator (sensitivity 81% to 100%) of underlying ATAI.14,16,17 Unfortunately, mediastinal widening is not specific (specificity 34% to 60%) for mediastinal hematoma or ATAI, as there are many other causes of a wide mediastinum, including mediastinal lipomatosis, vascular anomalies, lymphadenopathy, and even the supine anteroposterior technique itself (Fig. 1). To further complicate matters, a mediastinal J Thorac Imaging
Volume 30, Number 3, May 2015
J Thorac Imaging
Volume 30, Number 3, May 2015
FIGURE 1. Initial anteroposterior, supine chest radiograph (A) demonstrates a wide mediastinum, poor visualization of the normal aortic contour, deviation of the trachea to the right, and depression of the left mainstem bronchus, all findings concerning for mediastinal hematoma in this patient status post motor vehicle collision. Transaxial CT of the chest (B) in the same patient shows no mediastinal hematoma and a normal aorta. The cause of the abnormal widening on the chest radiograph is shown to be abundant mediastinal lipomatosis.
hematoma can be due to causes other than ATAI, such as venous bleeding or an adjacent spine or sternal fracture.15,18,19 These false positives were reported in a study by Mirvis et al20 of 677 patients with an abnormal chest radiograph after blunt trauma, in which 570 (84%) patients had no mediastinal hematoma at CT, and only 21 (3%) patients were ultimately diagnosed with an ATAI after CT and angiography. Of note, mediastinal widening from an ATAI typically involves the left paratracheal region or entire superior mediastinum. In our experience, isolated right-sided mediastinal widening in the setting of trauma is more typically associated with brachiocephalic artery injury or, rarely, esophageal or ascending aortic injury (Fig. 2). Despite the considerable attention that mediastinal width receives in the assessment of a chest radiograph in trauma, a thorough evaluation of the aortic contour is also Copyright
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FIGURE 2. Initial anteroposterior, supine chest radiograph (A) in this patient status post motor vehicle collision shows right-sided mediastinal widening with a preserved aortic knob and descending thoracic aortic contour. The isolated right-sided mediastinal widening was suspicious for a mediastinal hematoma due to brachiocephalic artery injury. B, A transaxial CT image including the superior mediastinum of the same patient confirms the presence of a mediastinal hematoma and also demonstrates direct signs of arterial injury manifested by a posterior outpouching filled with contrast from the brachiocephalic artery (arrow).
helpful in screening patients for mediastinal hematoma. In a truly normal study, the transverse and descending aortic contour should be crisp, with normal interfaces with the lung and aorticopulmonary window. Any obscuration of the aortic contour should be viewed with suspicion and should be evaluated with a follow-up upright chest radiograph or CT. An indistinct aortic contour has a sensitivity of 53% to 100% and a specificity of 21% to 55% for mediastinal hematoma. Loss of the aorticopulmonary window has a sensitivity of 40% to 100% and a specificity of 56% to 83%.14,21,22 Other findings that can be seen in the setting of a mediastinal hematoma or ATAI include depression of the left mainstem bronchus, deviation of the trachea, deviation of the nasogastric tube to the right of the T3 or T4 spinous processes, and increased density of the left subclavian artery reflection. The latter is indicative of blood tracking in the mediastinum to the extrapleural left apex, a
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FIGURE 3. A, A supine, anteroposterior radiograph of the chest in a patient involved in a motor vehicle collision demonstrates multiple findings concerning for mediastinal hematoma and ATAI, including widening of the mediastinum, loss of the normal aortic knob, depression of the left mainstem bronchus (arrow), and left apical capping (arrowhead). There are also rib fractures and subcutaneous gas, indicative of severe blunt force chest trauma in this patient who has already had bilateral pneumothoraces treated with thoracostomy tube placement. B (transaxial CT of the chest) and C (oblique sagittal reconstruction of the chest) demonstrate periaortic mediastinal hematoma as well as flaps of tissue within the aortic lumen (arrows). In addition, there is mild focal dilatation of the aorta between the flaps of tissue on image B. These CT findings confirmed the presence of an ATAI, which was treated with an endovascular stent.
finding often referred to as left apical capping11 (Fig. 3). The chest radiograph should also be examined for findings of substantial thoracic injury, including fractures involving the sternum, scapula, clavicle, first rib, or multiple left-sided ribs, diaphragmatic injury, and pneumomediastinum. Although not sensitive or specific for underlying mediastinal hematoma or ATAI, these imaging findings indicate that a substantial blunt force was applied to the chest and may be particularly helpful in patients who cannot provide any history. Given that the chest radiograph findings of ATAI are often nonspecific and not entirely sensitive, rather than focusing on specific measurements or findings, a general assessment of the mediastinum is a more consistent and sensitive means of detecting an underlying ATAI. A study by Ho et al23 demonstrated that interobserver agreement and sensitivity for ATAI were higher for a general “normal
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or abnormal” assessment of the mediastinum rather than a determination of whether the mediastinum was wide. Because of the gravity of missing an underlying ATAI, any suspicious findings on a chest radiograph, even if questionable, at least warrant additional imaging. In stable patients, an upright chest radiograph may help clear an abnormal mediastinum, as up to 38% of patients in a study by Schwab et al24 demonstrated normalization of an abnormal mediastinal width when obtaining an upright radiograph after the initial supine examination. A minority of patients (0.5% to 7%) may have a normal chest radiograph in the setting of occult mediastinal hematoma or ATAI.5,11,25,26 Therefore, for patients with abnormal chest radiograph findings, patients with a high index of clinical suspicion for aortic injury, or patients with chest pain out of proportion to their known injuries, our recommendation is to proceed to chest CT.
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CT Although CT was used in conjunction with catheter angiography for the evaluation of ATAI in the 1980s and 1990s, advances in scanner technology, particularly multidetector row CT (MDCT), have allowed CT to become the primary diagnostic tool in the evaluation of ATAI today.12,20,27–30 CT examinations in patients with thoracic trauma should be performed with intravenous contrast to allow for opacification of the vasculature. Unlike our nontraumatic acute aorta protocol for aortic dissection, noncontrast imaging is not routinely performed. Whereas some institutions obtain CT data sets in the arterial and portal venous phases, at our institution, only 1 set of images is obtained with a fixed delay at 70 seconds. This delay allows for a venous phase, which is optimal for scanning the entire thorax, abdomen, and pelvis. With the high spatial resolution available on today’s MDCT scanners, it is probably no longer necessary to obtain a dedicated arterial phase of the thorax as almost all vascular injuries can be detected on venous phase imaging. Detection of an ATAI on CT requires the identification of direct and indirect signs of vascular injury. The main indirect sign of major vascular injury is a mediastinal hematoma that effaces the fat plane with a vessel. A mediastinal hematoma presents as an area of hyperattenuating material (typically 40 to 70 HU) that displaces and infiltrates the normal mediastinal fat. The location of the mediastinal hematoma is critical, however, as a hematoma that is not in direct contact with the aorta is not a sign of an ATAI and may be related to venous bleeding or an adjacent fracture31–33 (Fig. 4). A mediastinal hematoma that does abut the aorta or any arch vessel is an indirect sign of vascular injury and should prompt a close inspection for direct signs of vascular injury. It is important to recognize that the mediastinal hematoma associated with an injured artery usually does not result from extravasated blood from the lumen of the vessel itself but rather is due to avulsion of small perivascular veins or possibly from the arterial vasovasorum.15 Consequently, the absence of contrast extravasation into a periaortic hematoma should not be used as a finding to suggest that the aorta has not been injured. Utilization of periaortic mediastinal hematoma alone as a diagnostic criterion results in false-positive examinations and decreases the specificity of CT.28,34 It should also be noted that a periaortic mediastinal hematoma is not a requirement for the diagnosis of ATAI, as direct signs of ATAI can be present in the absence of a periaortic hematoma.35,36 Direct signs of ATAI are changes in the normal imaging appearance of the aorta itself. Although transaxial images can be useful in this regard, it is important to examine the aorta in multiple planes when evaluating for direct findings of ATAI. The simplest direct finding of ATAI on chest CT is disruption of the aortic wall. This can manifest as a focal outpouching of the aortic wall containing contrast, a contour irregularity in the aortic wall, or, in rare cases, a focal defect with extravasation of contrast from the aorta into the adjacent mediastinum. In some cases of ATAI, the normal aortic contour may be preserved, but there will be strands of tissue (either intimal or intimomedial) within the aortic lumen due to injury. An ATAI involving the luminal side of the wall can form a nidus for thrombus; indeed, intraluminal thrombus is another direct finding of ATAI.11,35,37 In some cases, ATAI can result in an abrupt caliber change in the aorta, often Copyright
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due to blood within the wall of the aorta or due to compression of the aorta by adjacent hematoma. In our practice, we avoid the use of terms such as pseudoaneurysm or dissection when describing aortic injuries on CT. Although these processes can certainly occur in the setting of ATAI, CT can often underestimate the extent of vascular injury, and full transections of the aorta may have the imaging appearance of what might be described as a pseudoaneurysm or even dissection. Therefore, we focus on diagnosing ATAI and providing an accurate description of its location and appearance without using specific descriptors that may be inaccurate or confusing (Figs. 5–7). For patients with direct findings of ATAI on CT, no confirmation with conventional angiography is required and will only serve to delay appropriate management.30 In fact, in cases of subtle injuries, CT can identify ATAI that would be very difficult or impossible to detect on conventional angiography.38 Using direct signs, the sensitivity of CT for ATAI is >96%, with several studies placing the sensitivity at 100%.20,28–30 When relying on direct signs only, specificity is also excellent, with values >99%.20,30 For cases in which there is a periaortic hematoma and no direct findings indicating an ATAI, there are several options for follow-up. In stable patients, one option would be to repeat a CT scan in 48 to 72 hours. An alternative approach would involve transcatheter intravascular ultrasound or transesophageal echocardiography to look for a subtle, CT-occult injury. Previously, conventional angiography was used in cases such as these, but in the era of high–spatial resolution MDCT, angiography is unlikely to provide additional useful information.39 Disadvantages of using angiography for diagnosis include its invasive nature, cost, access site complications, and the need to transport the patient to an angiography suite or operating room. It should be noted, however, that angiograms are still performed by vascular surgeons in the operating room at the time of definitive endoluminal repair. The literature discussed above pertains mainly to blunt trauma causing traumatic aortic injury. CT is also an excellent modality for diagnosing penetrating aortic injury in hemodynamically stable patients. In cases of penetrating trauma, in addition to the findings of a periaortic hematoma and direct signs of ATAI, wound trajectory is critical. In cases in which the wound tract passes through or near the aorta, suspicion should be very high for an aortic injury. If a periaortic hematoma alone is seen without a direct sign of aortic injury, it is our recommendation that follow-up imaging be obtained within 24 to 48 hours to evaluate for any evolution in the appearance of the aorta. Patients with wound trajectories not intersecting or near the aorta and no other concerning findings of ATAI can be considered to not have an aortic injury40 (Fig. 8).
CT PITFALLS Understanding the potential pitfalls in CT imaging of the thoracic aorta is crucial for accurate evaluation for ATAI. Several technical artifacts can complicate evaluation for ATAI. Ideally, the patient’s arms should be placed over their head for a trauma CT of the chest. If the patient’s arms cannot be placed over their head and are at their side, beam hardening, photon-starvation, and reconstruction artifacts may occur, which will result in linear streak artifact across the aorta as well as worsened quantum mottle, potentially
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FIGURE 4. A, An anteroposterior, supine chest radiograph in this patient status post motor vehicle collision demonstrates predominantly right-sided mediastinal widening concerning for mediastinal hematoma. In addition, the aortic knob was indistinct, further raising concern for vascular injury. B, A transaxial CT image of the chest confirms the presence of a hematoma in the anterior mediastinum, deep to the manubrium. There is, however, a preserved fat plane (arrow) between the hematoma and the aorta, and no direct signs of aortic injury were found. This anterior mediastinal hematoma was found to be secondary to a manubrial fracture, seen in C (arrow), a sagittal reconstruction of the CT using bone windows.
complicating evaluation for mediastinal hematoma and intraluminal flaps (Fig. 6). If there is a problem with the contrast injection (incorrect scan delay, bolus extravasation), mixing and flow artifacts can occur in the aorta, which may simulate strands of intimomedial tissue in the lumen. If flow artifact is seen in real time at the scanner and a timing problem is felt to be the cause, an immediate repeat scan of the patient will allow for contrast to fully opacify the aorta and eliminate the artifact. If the artifact is only seen later, at the reading station, and raises the possibility of an ATAI, there should be a low threshold for repeating the scan with a second bolus of contrast to fully evaluate for ATAI (Fig. 9). Although trauma CT examinations are ideally performed during a breath-hold, incomplete breath-holds are common in critically ill patients. Breath-hold artifacts can produce apparent discontinuities in the walls of vessels or simulate fractures of bones, particularly within the sternum. Breathing artifacts can be recognized by examining lung windows to confirm the presence of motion. Sagittal reconstructions can also be helpful in showing the nonphysiologic, artifactual
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step-off caused by an incomplete breath-hold. Finally, pulsation artifact can result in blurring of the aortic wall or curvilinear defects within the aortic lumen by superimposition of the moving aortic wall with the lumen.41 Pulsation artifacts are typically most pronounced in the ascending aorta and can be recognized by noting that the wall artifact will appear both in the lumen and the adjacent mediastinal fat. In addition, the interface between the artifact and the contrast in the aortic lumen is typically indistinct, whereas the interface between true intimomedial tissue and luminal contrast is often sharp. If a pulsation artifact is suspected but cannot be confirmed, options include repeating the scan with cardiac gating or using an ultrafast high-pitch technique (eg, on a dual-source scanner) to eliminate the effects of motion in the aorta (Fig. 10). In addition to technical artifacts, several anatomic pitfalls can complicate evaluation for ATAI. Variants in the normal appearance of the aorta are one potential source of confusion. One of the most commonly confusing anatomic variants is the ductus diverticulum. A ductus diverticulum
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FIGURE 5. A, A transaxial CT image of the chest in a patient status post motor vehicle collision demonstrates a left hemothorax and a periaortic hematoma surrounding the descending thoracic aorta with an outpouching filled with contrast arising from the anterior contour of the aorta (arrows). B, Sagittal reconstruction of the CT again demonstrates this outpouching (arrows), consistent with an ATAI. Although this was described as a pseudoaneurysm in the report, the patient was found to have a complete transection of the anterior wall of the aorta (involving all 3 components of the wall) at open repair. Given that CT can underestimate the degree of injury, we prefer to describe the injury using generic terms such as outpouching rather than using specific terms such as pseudoaneurysm, which can be inaccurate and misleading.
is a focal smooth bulge along the undersurface of the aorta at the level of the isthmus. The typical ductus diverticulum forms obtuse angles with the remainder of the aorta, but an atypical ductus diverticulum has a shorter and steeper neck. By virtue of their location at the isthmus, which is the most common site of blunt ATAI (>90% by surgical series, 36% to 54% by autopsy series), ductus diverticula are frequently heavily scrutinized as possible injuries.1,3,6,42,43 In contrast to an ATAI, which often has an irregular or discontinuous margin with acute angles at its base, a ductus diverticulum should have smooth margins. Other findings that favor a Copyright
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FIGURE 6. Transaxial (A) and oblique sagittal (B) CT images from a patient involved in a motor vehicle collision demonstrate circumferential high-attenuating thickening of the wall of the descending aorta that can be seen over a long length on the oblique sagittal reconstructions (B, arrowheads). These findings were consistent with an ATAI manifested by an intramural hematoma, which narrowed the lumen of the aorta. Intramural hematoma is an infrequently seen but important direct finding for ATAI. Note also linear beam hardening artifact degrading the image secondary to scanning the patient with his arms at his sides.
ductus diverticulum over an ATAI include the absence of periaortic mediastinal hematoma or intimomedial strands in the aortic lumen.35,44 Occasionally, a fibrotic remnant of the ductus arteriosus can persist and may calcify along the undersurface of the aorta at the isthmic region. A small ductus diverticulum or irregularity may be present along the aorta at the site of the ductus remnant and should also not be confused for an ATAI.
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FIGURE 8. A transaxial CT image of the thorax shows a thoracostomy tube in place for a hemopneumothorax. There is gas and blood in the mediastinum directly abutting the aorta along the trajectory (arrow) of a bullet that passed through the thorax. There is an adjacent contour bulge (arrowhead) of the aorta, representing traumatic aortic injury.
FIGURE 7. A, A transaxial CT image from a patient involved in a motor vehicle collision demonstrates a focal discontinuity of the anterior wall of the aorta with a focal outpouching consistent with an ATAI. In addition, more amorphous high-attenuating material is seen lateral to this outpouching consistent with actively extravasating contrast (arrowheads). B, Another transaxial CT image from the same patient; this contrast can be seen actively extravasating (arrowheads) into a left hemothorax. Although rarely seen on imaging, active extravasation of contrast is an important direct finding of ATAI, which indicates a ruptured aorta that requires emergent treatment.
The aortic spindle is another anatomic variant that can be mistaken for an ATAI. The aortic spindle is defined as an area of localized mild dilatation of the descending aorta just beyond the isthmus and occurs as a remnant of the appearance of the fetal aorta. In contrast to the ductus diverticulum, which results in an eccentric outpouching, the aortic spindle results in a fusiform dilatation of the aorta. Unlike a true ATAI, the aortic spindle has smooth margins.44 Oblique sagittal reconstructions can be helpful to confirm the smooth contour of the wall of an aortic spindle. Occasionally, at the origins of branch vessels, such as the intercostal and bronchial arteries, small infundibula may be present. These conical outpouchings can be confirmed as vascular infundibula and not an ATAI by noting the absence of surrounding hematoma, appreciating their smooth margins, and, most importantly, by identifying the small vessel arising from their apex35,44 (Fig. 11). Normal structures adjacent to the aorta can occasionally cause confusion in the evaluation for ATAI. When the superior intercostal vein is opacified with contrast, it is
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closely apposed to the aortic arch and can simulate an outpouching on single slices. Knowledge of its anatomic location and scrolling at the workstation should eliminate any confusion in this regard. In some patients, particularly younger adults, normal thymic tissue is often present in the anterior mediastinum and may be confused for a mediastinal hematoma. In contrast to a mediastinal hematoma, which is typically a contiguous and often irregularly shaped collection of intermediate-attenuation blood products, the thymus has a stippled or nodular appearance due to interspersed fat, is often triangular, and is located in the anterior mediastinum. In addition, a mediastinal hematoma is usually surrounded by fat stranding in the mediastinum, whereas the fat adjacent to the thymus will have no stranding.
REPORTING OF POSITIVE RESULTS The treatment of ATAI has evolved over the past two decades to now include a combination of open repair, endovascular repair, and conservative management.12,45–48 Although the decision as to which type of repair to pursue involves multiple clinical factors, the imaging appearance of the ATAI also plays an important role. As CT emerged as the imaging test of choice for ATAI, it became evident that subtle ATAI that would previously have gone undetected was now being recognized.35,49–51 As a result, in the late 1990s, the concept of minimal aortic injury was introduced.49,52 Whereas the majority of authors chose to define minimal aortic injury as a lesion measuring
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