(Radiographics. 1999;19:45-60.)
© RSNA, 1999
Aortic Dissection: Diagnosis and Follow-up with Helical CT
Carmen Sebastià, MD1,
Esther Pallisa, MD1,
Sergi Quiroga, MD1,
Agustí Alvarez-Castells, MD1,
Rosa Dominguez, MD1 and
Arturo Evangelista, MD2
1 Departments of Radiology (C.S., E.P., S.Q., A.A., R.D.)
2 Cardiology (A.E.), Hospital General Universitari Vall d'Hebron, Paseo Vall d'Hebron 119–129, Barcelona 08035, Spain.
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Abstract
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Acute aortic dissection is a cardiovascular emergency that requires prompt diagnosis and treatment. Helical computed tomography (CT) allows diagnosis of acute aortic dissection with a sensitivity and specificity of nearly 100%. With helical CT, a dissection involving the ascending aorta (type A in the Stanford classification) can be differentiated from one distal to the left subclavian artery (type B). Helical CT can also be used to identify atypical forms of aortic dissection such as intramural hematoma, penetrating atherosclerotic ulcer, ruptured type B dissection, and atypical configurations of the intimal flap. Helical CT is useful in follow-up of aortic dissection by allowing assessment of early and late changes after surgery or medical treatment. Such changes include postoperative complications of type A dissection, healing of intramural hematoma, progression of intramural hematoma, and aneurysms of the true or false lumen. Helical CT can also be used to monitor potentially life-threatening ischemic complications of abdominal branch vessels.
Index Terms: Aorta, CT, 56.12115, 94.12915 • Aorta, dissection, 56.74, 94.74 • Computed tomography (CT), helical, 56.12115, 94.12915
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INTRODUCTION
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Acute aortic dissection (AAD) is one of the most dramatic cardiovascular emergencies. To limit the possibility of death, a detailed morphologic and functional diagnosis must be quickly obtained. Aortography has been the traditional method of assessing suspected AAD; however, concern over the low sensitivity of aortography has prompted the investigation of other imaging techniques for this purpose. Transesophageal echocardiography and magnetic resonance (MR) imaging are increasingly used in the evaluation of AAD and have sensitivities and specificities of 95%–100%. A recent study found that the sensitivity and specificity of helical computed tomography (CT) compare well with those of MR imaging and transesophageal echocardiography (1).
Various systems based on anatomic characteristics have been proposed to classify aortic dissection. In the widely used Stanford classification, type A dissections involve the ascending aorta and type B dissections are distal to the left subclavian artery (2). The risk of acute aortic insufficiency, occlusion of the coronary vessels, or rupture of the dissection into the pericardium is extremely high (~90%) in type A dissection and necessitates immediate replacement of the aorta. This risk is lower in type B dissection, which can be controlled medically unless there is aortic rupture or renal or visceral vascular compromise.
The dissection is termed acute when it is diagnosed within 14 days after the first symptoms appear; it is termed chronic when it is diagnosed later (3). Immediate appropriate treatment has improved the outcome of AAD: The overall in-hospital mortality rate is currently less than 30%. Patients with aortic dissection require continuous surveillance. Residual aortic disease deteriorates into life-threatening conditions that require surgery in 15%–30% of patients after 10 years (3). Dilatation of the dissected region and progressive reduction of organ perfusion are the most frequent such conditions.
In helical CT of AAD, it is important to evaluate the entire aorta to determine the distal extent of the dissection and to detect abdominal ischemic diseases that can increase the morbidity and mortality associated with this condition. Although primary reconstructed transverse sections remain the mainstay of CT angiographic interpretation, alternative visualization techniques, including multiplanar and three-dimensional reformation images, can substantially augment diagnosis and provide an efficient means of communicating critical anatomic relationships to referring clinicians.
In this article, we present the helical CT features of typical and atypical aortic dissection, changes during follow-up of aortic dissection, and abdominal complications of aortic dissection.
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TECHNIQUE
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Since 1986, 128 aortic dissections have been evaluated at our institution with CT. Beginning in 1994, helical CT performed with a Twin II scanner (Elscint, Haifa, Israel) has been used to diagnose new aortic dissections and to follow up chronic dissections. The examination begins with conventional unenhanced CT. Discontinuous images are obtained every 20 mm with 10-mm collimation in single mode; coverage begins 2 cm above the aortic arch and continues to the superior aspect of the femoral head. Unenhanced CT scans are useful in diagnosis of acute hemorrhage (pleural, mediastinal, or pericardial) and intramural hematoma, which are visualized as fluid collections of high attenuation (>50 HU), a finding consistent with fresh blood.
We then inject 100 mL of nonionic iohexol (Omnigraft 350; Juste, Madrid, Spain) at a rate of 2 mL/sec through a 20-gauge catheter positioned in the right arm. Helical CT is performed 30 seconds after administration of contrast material with the following parameters: 160 mA, 120 kV, pitch of 1.5, 5.5-mm collimation, and 3.5-mm reconstruction interval. Coverage begins 2 cm above the aortic arch and continues to the bifurcation of the iliac artery. The linear interpolation is 180°.
Multiplanar reformation (MPR) images in sagittal, coronal, oblique sagittal, and curved projections are generated on an independent workstation (Indy; Silicon Graphics, Mountain View, Calif). Maximum-intensity projection and shaded-surface display (SSD) reconstruction images of the target areas are also produced. In most cases of aortic dissection, axial images are sufficient to demonstrate the presence, location, and extent of an intimal flap. MPR images provide an overall view of the aortic dissection and demonstrate the anatomic relationships between the flap and the adjacent great vessels. We prefer SSD images to maximum-intensity projection images for evaluation of complex three-dimensional relationships, particularly in regions of vessel overlap. SSD images can be produced in several colors and provide a more realistic three-dimensional view; thus, they are more easily understood by the vascular surgeon.
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TYPICAL AORTIC DISSECTION
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The classic feature of aortic dissection is a partition between the true and false channels; such a partition, which is formed by the intimal flap, is found in approximately 70% of cases. Secondary findings include internal displacement of intimal calcifications or a hyperattenuating intima; delayed enhancement of the false lumen; widening of the aorta; and mediastinal, pleural, or pericardial hematoma (4).
In the Stanford classification, all dissections involving the ascending aorta are designated type A regardless of the site of the intimal tear or the distal extent of the dissection. Approximately 60% of aortic dissections are type A. There is general agreement that acute type A dissections require immediate surgical intervention. The most common complications are rupture of the dissection into the pericardium with progressive cardiac tamponade, occlusion of the coronary or supraaortic vessels, and severe aortic insufficiency with acute heart failure. The presence, location, and extent of an intimal flap may be readily determined with helical CT (Fig 1), but the technique is limited in assessment of coronary artery involvement and aortic insufficiency (5).
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Figure 1a. Stanford type A aortic dissection. (a, b) Enhanced CT scans show an intimal flap (arrow) in the ascending aorta (a) and brachiocephalic trunk (b). (c) Oblique sagittal two-dimensional reconstruction image shows the intimal flap in the ascending aorta (bottom arrow) with extension to the brachiocephalic trunk (top arrow).
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Figure 1b. Stanford type A aortic dissection. (a, b) Enhanced CT scans show an intimal flap (arrow) in the ascending aorta (a) and brachiocephalic trunk (b). (c) Oblique sagittal two-dimensional reconstruction image shows the intimal flap in the ascending aorta (bottom arrow) with extension to the brachiocephalic trunk (top arrow).
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Figure 1c. Stanford type A aortic dissection. (a, b) Enhanced CT scans show an intimal flap (arrow) in the ascending aorta (a) and brachiocephalic trunk (b). (c) Oblique sagittal two-dimensional reconstruction image shows the intimal flap in the ascending aorta (bottom arrow) with extension to the brachiocephalic trunk (top arrow).
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Dissections involving any portion of the aorta distal to the left subclavian artery are designated type B in the Stanford classification (Fig 2). Approximately 40% of aortic dissections are type B.
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Figure 2a. Stanford type B aortic dissection. (a) Enhanced CT scan shows an intimal flap in the descending aorta (arrow). There is flow within both lumina. (b) Oblique sagittal MPR image shows the aortic dissection (bottom arrow). The origin of the dissection is distal to the left subclavian artery (top arrow).
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Figure 2b. Stanford type B aortic dissection. (a) Enhanced CT scan shows an intimal flap in the descending aorta (arrow). There is flow within both lumina. (b) Oblique sagittal MPR image shows the aortic dissection (bottom arrow). The origin of the dissection is distal to the left subclavian artery (top arrow).
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PSEUDODISSECTION
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Helical CT Artifacts
Most of the interpretive limitations of thoracic CT aortography are the result of two artifacts: perivenous streaks and aortic motion artifact (6). Perivenous streaks are caused by a combination of beam hardening and motion due to transmitted pulsation in a vein carrying undiluted contrast medium to the heart. In practice, perivenous streaks are rarely confused with aortic dissection because the orientation of such streaks typically varies from section to section and extends beyond the confines of the aortic wall. We minimize perivenous streaks by performing bolus injection into the right arm at a rate of 2 mL/sec.
Aortic motion artifact simulates dissection of the ascending aorta and is related to movement of the aortic wall in the interval from the end of diastole to the end of systole (7). In most cases, the artifact is seen at the left anterior and right posterior margins of the aortic circumference and changes from one section to another (8). Use of a 180° linear-interpolation algorithm reduces the frequency of motion artifacts (9). When the findings on axial images are ambiguous, a serrated appearance of the left anterior ascending aorta on two- or three-dimensional reconstruction images provides clear evidence of a motion artifact (Fig 3).
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Figure 3a. Aortic motion artifact. (a) Enhanced CT scan shows an aorta with an artifactual rim of low attenuation (arrow). (b) Oblique sagittal MPR image shows a serrated appearance of the left anterior wall of the ascending aorta (arrow), particularly at the aortic root.
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Figure 3b. Aortic motion artifact. (a) Enhanced CT scan shows an aorta with an artifactual rim of low attenuation (arrow). (b) Oblique sagittal MPR image shows a serrated appearance of the left anterior wall of the ascending aorta (arrow), particularly at the aortic root.
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Diagnostic Pitfalls
The CT appearances of several entities can cause them to be mistaken for atypical AAD. A mural thrombus in a fusiform aneurysm (Fig 4); a focal periaortic soft-tissue mass such as periaortic fibrosis (Fig 5a) or a mediastinal, pulmonary, or retroperitoneal tumor (Fig 5b); and anemia with apparent high attenuation of the aortic wall are examples of conditions that may be difficult to distinguish from AAD. The vascular structures around the aorta (aortic sinus, pericardial recess, left brachiocephalic vein, left superior intercostal vein) can also cause confusion (5).
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Figure 4. Figures 4, 5. (4) Mural thrombus. CT scan shows an atheromatous thrombus with an irregular internal border in the thoracic descending aorta (arrow) and motion artifact in the ascending aorta. A thrombosed aortic dissection usually demonstrates a smooth internal border. (5) Focal periaortic soft-tissue mass. (a) CT scan shows idiopathic periaortic fibrosis (arrows). (b) CT scan shows periaortic lymphoma as a focal rounded mass at the aortic border (arrows). These periaortic masses have an irregular external border, whereas intramural hematoma appears as smooth, crescentic thickening of the aortic wall (see Fig 6).
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Figure 5a. Figures 4, 5. (4)Mural thrombus. CT scan shows an atheromatous thrombus with an irregular internal border in the thoracic descending aorta (arrow) and motion artifact in the ascending aorta. A thrombosed aortic dissection usually demonstrates a smooth internal border. (5) Focal periaortic soft-tissue mass. (a) CT scan shows idiopathic periaortic fibrosis (arrows). (b) CT scan shows periaortic lymphoma as a focal rounded mass at the aortic border (arrows). These periaortic masses have an irregular external border, whereas intramural hematoma appears as smooth, crescentic thickening of the aortic wall (see Fig 6).
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Figure 5b. Figures 4, 5. (4)Mural thrombus. CT scan shows an atheromatous thrombus with an irregular internal border in the thoracic descending aorta (arrow) and motion artifact in the ascending aorta. A thrombosed aortic dissection usually demonstrates a smooth internal border. (5) Focal periaortic soft-tissue mass. (a) CT scan shows idiopathic periaortic fibrosis (arrows). (b) CT scan shows periaortic lymphoma as a focal rounded mass at the aortic border (arrows). These periaortic masses have an irregular external border, whereas intramural hematoma appears as smooth, crescentic thickening of the aortic wall (see Fig 6).
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ATYPICAL AORTIC DISSECTION
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Intramural Hematoma
Intramural hematoma (aortic dissection without rupture of the intima) is caused by hemorrhage of the vasa vasorum weakening the media without intimal tears (10). Intramural hematoma accounts for approximately 13% of all AADs (11). Unenhanced CT shows a cuff or crescent of high attenuation and displacement of intimal calcifications. On enhanced CT scans, a smooth region of low attenuation can be seen (Fig 6). In an open dissection, these features are difficult to differentiate from those of an acutely thrombosed false lumen. An observation that may help one differentiate intramural hematoma from the thrombosed false lumen of classic intimal dissection is that the latter entity tends to spiral longitudinally around the aorta, whereas the former entity tends to maintain a constant circumferential relationship with the aortic wall (6).
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Figure 6a. Intramural hematoma of the descending aorta. (a) Unenhanced CT scan shows a crescentic area of high attenuation (left arrow) with medial displacement of intimal calcifications (right arrow). (b) Enhanced CT scan shows the crescentic area as hypoattenuating relative to the aortic lumen. Arrow = intimal calcification. (c) Oblique sagittal MPR image shows the intramural hematoma (bottom arrow). Top arrow = intimal calcification.
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Figure 6b. Intramural hematoma of the descending aorta. (a) Unenhanced CT scan shows a crescentic area of high attenuation (left arrow) with medial displacement of intimal calcifications (right arrow). (b) Enhanced CT scan shows the crescentic area as hypoattenuating relative to the aortic lumen. Arrow = intimal calcification. (c) Oblique sagittal MPR image shows the intramural hematoma (bottom arrow). Top arrow = intimal calcification.
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Figure 6c. Intramural hematoma of the descending aorta. (a) Unenhanced CT scan shows a crescentic area of high attenuation (left arrow) with medial displacement of intimal calcifications (right arrow). (b) Enhanced CT scan shows the crescentic area as hypoattenuating relative to the aortic lumen. Arrow = intimal calcification. (c) Oblique sagittal MPR image shows the intramural hematoma (bottom arrow). Top arrow = intimal calcification.
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Intramural hematoma of the ascending aorta has signs, symptoms, and a risk profile virtually identical to those of classic type A dissection and requires emergency surgical repair (12). Intramural hematoma can be detected and monitored with helical CT, MR imaging, and transesophageal echocardiography but not with aortography (Fig 7). It has been postulated that intramural hematoma is an early stage of classic aortic dissection; the prognosis is poor in cases that involve the ascending aorta (13).
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Figure 7a. Intramural hematoma of the ascending aorta. (a) Unenhanced CT scan shows a crescentic area of high attenuation along the walls of the ascending and descending aorta (arrows). (b) Enhanced CT scan does not show enhancement of the crescentic areas (arrows).
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Figure 7b. Intramural hematoma of the ascending aorta. (a) Unenhanced CT scan shows a crescentic area of high attenuation along the walls of the ascending and descending aorta (arrows). (b) Enhanced CT scan does not show enhancement of the crescentic areas (arrows).
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Penetrating Atherosclerotic Ulcer
Penetrating atherosclerotic ulcer is defined as an atherosclerotic lesion with ulceration that penetrates the internal elastic lamina; such penetration facilitates hematoma formation within the media of the aortic wall (14). The appearance of this lesion is similar to that of a peptic ulcer on images from a barium study. Typically, penetrating atherosclerotic ulcer occurs in the middle or distal third of the thoracic aorta; CT features include a focal ulcer with adjacent subintimal hematoma (15) (Fig 8). Penetrating atherosclerotic ulcer can be differentiated from aortic dissection by means of (a) the extensive atherosclerotic disease and ectasia in penetrating atherosclerotic ulcer and (b) the lack of compression of the aortic lumen in elderly persons with penetrating atherosclerotic ulcer. Extension of the ulcer can produce incomplete rupture (adventitial false aneurysm) or transmural rupture. Penetrating atherosclerotic ulcer is treated by replacing the ulcerated area with a graft (14).
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Figure 8a. Penetrating atherosclerotic ulcer. (a–c) CT angiograms (shown from superior [a] to inferior [c]) show a penetrating ulcer of the right lateral wall of the descending aorta (arrow in a and b) and distal intramural hematoma (arrowhead in c). (d) Sagittal MPR image shows the ulcer (arrow) and the intramural hematoma (arrowhead).
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Figure 8b. Penetrating atherosclerotic ulcer. (a–c) CT angiograms (shown from superior [a] to inferior [c]) show a penetrating ulcer of the right lateral wall of the descending aorta (arrow in a and b) and distal intramural hematoma (arrowhead in c). (d) Sagittal MPR image shows the ulcer (arrow) and the intramural hematoma (arrowhead).
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Figure 8c. Penetrating atherosclerotic ulcer. (a–c) CT angiograms (shown from superior [a] to inferior [c]) show a penetrating ulcer of the right lateral wall of the descending aorta (arrow in a and b) and distal intramural hematoma (arrowhead in c). (d) Sagittal MPR image shows the ulcer (arrow) and the intramural hematoma (arrowhead).
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Figure 8d. Penetrating atherosclerotic ulcer. (a–c) CT angiograms (shown from superior [a] to inferior [c]) show a penetrating ulcer of the right lateral wall of the descending aorta (arrow in a and b) and distal intramural hematoma (arrowhead in c). (d) Sagittal MPR image shows the ulcer (arrow) and the intramural hematoma (arrowhead).
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Ruptured Type B Dissection
The CT features of aortic rupture include irregularity of the aortic wall; extravasation of vascular contrast material; and hyperattenuating mediastinal, pericardial, or pleural fluid collections on unenhanced CT scans. These findings are consistent with mediastinal or pericardial hematoma or hemothorax (Fig 9). Most patients with type B AAD can be treated initially with medical therapy. The primary indications for immediate surgery are a ruptured aorta, a descending aortic diameter greater than 6 cm, malperfusion of the thoracoabdominal aorta, or pseudocoarctation syndrome with uncontrollable hypertension (16). The outcome seems to be worse in patients with retrograde involvement of the aortic arch and ascending aorta (16).
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Figure 9a. Ruptured type B dissection. (a) Unenhanced CT scan shows hyperattenuating periaortic mediastinal hematoma (arrow) and hyperattenuating pleural effusion (arrowheads). (b) Enhanced CT scan shows irregularity of the aortic wall with an ulcerlike appearance caused by extravasation of vascular contrast material (arrow).
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Figure 9b. Ruptured type B dissection. (a) Unenhanced CT scan shows hyperattenuating periaortic mediastinal hematoma (arrow) and hyperattenuating pleural effusion (arrowheads). (b) Enhanced CT scan shows irregularity of the aortic wall with an ulcerlike appearance caused by extravasation of vascular contrast material (arrow).
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Atypical Configuration of the Intimal Flap
In some cases, the intimal flap has an atypical configuration. These atypical configurations are as follows: (a) dissection of the entire intima with a circumferential intimal flap (Fig 10); (b) a filiform (extremely narrow) true lumen, which can have ischemic complications (Fig 11); (c) a calcified false lumen in chronic dissection (17) (Fig 12); (d) a three-channel aorta (Mercedes-Benz sign) (Fig 13) or an aorta with several false channels; and (e) intimointimal intussusception (18).
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Figure 10. Figures 10, 11. (10) Dissection of the entire intima. Enhanced CT scan shows dissection of the entire intima in the thoracic descending aorta (arrow) (11) Filiform true lumen. Enhanced CT scan shows a filiform true lumen in the thoracic descending aorta (arrow).
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Figure 11. Figures 10, 11. (10) Dissection of the entire intima. Enhanced CT scan shows dissection of the entire intima in the thoracic descending aorta (arrow). (11) Filiform true lumen. Enhanced CT scan shows a filiform true lumen in the thoracic descending aorta (arrow).
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Figure 12. Figures 12, 13. (12) Calcified false lumen. Venous-phase enhanced CT scan shows an abdominal aortic dissection with mural calcification of the false lumen (arrows). (13) Three-channel aorta. CT angiogram shows two false lumina (F) in the thoracic descending aorta. The intimal flap demonstrates the Mercedes-Benz sign.
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Figure 13. Figures 12, 13. (12) Calcified false lumen. Venous-phase enhanced CT scan shows an abdominal aortic dissection with mural calcification of the false lumen (arrows). (13) Three-channel aorta. CT angiogram shows two false lumina (F) in the thoracic descending aorta. The intimal flap demonstrates the Mercedes-Benz sign.
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Associated Diseases
Patients with hypertension or connective tissue disorders such as Marfan syndrome, cystic medial necrosis, Ehlers-Danlos syndrome, and Turner syndrome are at risk for aortic dissection. Pregnancy, aortic stenosis, bicuspid aortic valve, and aortic coarctation (Fig 14) are other risk factors. In terms of symptoms, aortic dissection can mimic many other entities, including myocardial infarction, pericarditis, pulmonary thromboembolism, acute cholecystitis, and inflammatory conditions involving the costochondral region. Moreover, a patient can have more than one of these diseases.
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Figure 14a. Aortic coarctation and dissection. (a) Oblique sagittal MPR image shows focal narrowing of the thoracic descending aorta (long arrow) and an intimal flap in the ascending aorta (short arrow). (b) SSD image shows a dilated ascending aorta and aortic narrowing immediately distal to the left subclavian artery (arrow). Dissection is not seen because an SSD image shows only the aortic surface.
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Figure 14b. Aortic coarctation and dissection. (a) Oblique sagittal MPR image shows focal narrowing of the thoracic descending aorta (long arrow) and an intimal flap in the ascending aorta (short arrow). (b) SSD image shows a dilated ascending aorta and aortic narrowing immediately distal to the left subclavian artery (arrow). Dissection is not seen because an SSD image shows only the aortic surface.
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CHANGES DURING FOLLOW-UP
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Postoperative Complications of Type A Dissection
Surgical treatment of type A dissection consists of replacing the ascending aorta, reconstructing the aortic root to restore aortic valve competence, and directing blood flow preferentially to the true lumen. The mortality rate for surgical treatment of type A dissection is 10%–35% (19). Early postoperative complications include myocardial infarction, stroke, respiratory insufficiency, pulmonary embolism, aortic rupture, pseudoaneurysm (Fig 15), and graft infection (20). In a series of patients who underwent surgical repair of a type A dissection, the survival rate at 5 years was 95% when the false lumen was thrombosed and 76% when the false lumen was patent (21). At 10 years, 15%–30% of patients require surgery for life-threatening conditions such as dilatation of the dissected region with risk of rupture (Fig 16) and progressive reduction of myocardial perfusion with development of severe aortic insufficiency (19).
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Figure 15a. Figures 15, 16. (15) Pseudoaneurysm after surgery for type A dissection in a 28-year-old man with Marfan syndrome. The surgery involved replacement of the aortic valve and placement of a Dacron graft in the aortic root. (a) Enhanced CT scan shows a pseudoaneurysm of the aortic root (arrows). Note the aortic motion artifacts (arrowheads). (b) SSD image shows the pseudoaneurysm of the aortic root (arrow). (16) Aneurysm after surgery for type A dissection 9 years earlier. The surgery involved replacement of the aortic valve and angioplasty of the intimal tear with a patch graft. (a) Enhanced CT scan shows an aneurysm of the ascending aorta with a persistent intimal flap (arrow). (b) SSD image shows the aneurysm of the ascending aorta with extension to the aortic arch.
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Figure 15b. Figures 15, 16. (15) Pseudoaneurysm after surgery for type A dissection in a 28-year-old man with Marfan syndrome. The surgery involved replacement of the aortic valve and placement of a Dacron graft in the aortic root. (a) Enhanced CT scan shows a pseudoaneurysm of the aortic root (arrows). Note the aortic motion artifacts (arrowheads). (b) SSD image shows the pseudoaneurysm of the aortic root (arrow). (16) Aneurysm after surgery for type A dissection 9 years earlier. The surgery involved replacement of the aortic valve and angioplasty of the intimal tear with a patch graft. (a) Enhanced CT scan shows an aneurysm of the ascending aorta with a persistent intimal flap (arrow). (b) SSD image shows the aneurysm of the ascending aorta with extension to the aortic arch.
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Figure 16a. Figures 15, 16. (15) Pseudoaneurysm after surgery for type A dissection in a 28-year-old man with Marfan syndrome. The surgery involved replacement of the aortic valve and placement of a Dacron graft in the aortic root. (a) Enhanced CT scan shows a pseudoaneurysm of the aortic root (arrows). Note the aortic motion artifacts (arrowheads). (b) SSD image shows the pseudoaneurysm of the aortic root (arrow). (16) Aneurysm after surgery for type A dissection 9 years earlier. The surgery involved replacement of the aortic valve and angioplasty of the intimal tear with a patch graft. (a) Enhanced CT scan shows an aneurysm of the ascending aorta with a persistent intimal flap (arrow). (b) SSD image shows the aneurysm of the ascending aorta with extension to the aortic arch.
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Figure 16b. Figures 15, 16. (15) Pseudoaneurysm after surgery for type A dissection in a 28-year-old man with Marfan syndrome. The surgery involved replacement of the aortic valve and placement of a Dacron graft in the aortic root. (a) Enhanced CT scan shows a pseudoaneurysm of the aortic root (arrows). Note the aortic motion artifacts (arrowheads). (b) SSD image shows the pseudoaneurysm of the aortic root (arrow). (16) Aneurysm after surgery for type A dissection 9 years earlier. The surgery involved replacement of the aortic valve and angioplasty of the intimal tear with a patch graft. (a) Enhanced CT scan shows an aneurysm of the ascending aorta with a persistent intimal flap (arrow). (b) SSD image shows the aneurysm of the ascending aorta with extension to the aortic arch.
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Healing of Intramural Hematoma
Intramural hematomas can decrease in size and even disappear (22) (Fig 17). An intramural hematoma of the descending aorta can be safely managed with observation and does not necessarily require an early operation. Surgical intervention can be avoided if the hematoma resolves or if the symptoms disappear with negatively inotropic and hypotensive therapy. After several weeks, the hyperattenuating crescentic area on unenhanced CT scans becomes hypoattenuating.
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Figure 17a. Healing of intramural hematoma. (a, b) Unenhanced (a) and enhanced (b) CT scans obtained at the level of the aortic arch show a crescentic area (top arrow) and displacement of calcifications (bottom arrow), findings consistent with a fresh intramural hematoma. (c) Follow-up CT scan obtained 7 months later shows that the intramural hematoma has been absorbed.
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Figure 17b. Healing of intramural hematoma. (a, b) Unenhanced (a) and enhanced (b) CT scans obtained at the level of the aortic arch show a crescentic area (top arrow) and displacement of calcifications (bottom arrow), findings consistent with a fresh intramural hematoma. (c) Follow-up CT scan obtained 7 months later shows that the intramural hematoma has been absorbed.
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Figure 17c. Healing of intramural hematoma. (a, b) Unenhanced (a) and enhanced (b) CT scans obtained at the level of the aortic arch show a crescentic area (top arrow) and displacement of calcifications (bottom arrow), findings consistent with a fresh intramural hematoma. (c) Follow-up CT scan obtained 7 months later shows that the intramural hematoma has been absorbed.
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Progression of Intramural Hematoma
Although an intramural hematoma usually decreases in size, in some cases ulcerlike projections, an aneurysm, or open dissection develops in the affected segment of the aorta (Fig 18). Patients with intramural hematoma are at high risk for developing a saccular or fusiform aneurysm. Saccular aneurysms develop from ulcerlike projections (22).
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Figure 18a. Ulcerlike projection after intramural hematoma. (a) Enhanced CT scan obtained at admission shows an intramural hematoma of the descending aorta (arrow). Ten months later, the patient developed a type B open dissection. (b) CT scan shows a double-lumen dissection of the descending aorta with dilatation and mural thrombosis of the false lumen (arrow). (c) Sagittal MPR image shows the open dissection with a distal intimal tear and retrograde dissection (arrow).
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Figure 18b. Ulcerlike projection after intramural hematoma. (a) Enhanced CT scan obtained at admission shows an intramural hematoma of the descending aorta (arrow). Ten months later, the patient developed a type B open dissection. (b) CT scan shows a double-lumen dissection of the descending aorta with dilatation and mural thrombosis of the false lumen (arrow). (c) Sagittal MPR image shows the open dissection with a distal intimal tear and retrograde dissection (arrow).
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Figure 18c. Ulcerlike projection after intramural hematoma. (a) Enhanced CT scan obtained at admission shows an intramural hematoma of the descending aorta (arrow). Ten months later, the patient developed a type B open dissection. (b) CT scan shows a double-lumen dissection of the descending aorta with dilatation and mural thrombosis of the false lumen (arrow). (c) Sagittal MPR image shows the open dissection with a distal intimal tear and retrograde dissection (arrow).
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Aneurysm of the False Lumen
One of the most significant findings during follow-up of an aortic dissection is an aneurysm of the false lumen (20) (Fig 19). A complication of continuous dilatation of the false lumen is aortic rupture. Complete thrombosis and reduced flow in the false lumen decrease the risk of subsequent aortic dilatation (21). Elective resection is advisable if the aneurysm exceeds 5–6 cm in diameter or symptoms are present (23).
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Figure 19a. Aneurysm of the false lumen in a patient who had type B AAD 10 years earlier. (a) Enhanced CT scan obtained 10 years earlier shows an intimal flap (arrow) in a descending aorta of normal diameter. (b) Helical CT scan shows dilatation and mural thrombosis of the false lumen (arrow). (c) Oblique sagittal MPR image shows irregular mural thrombosis of the false lumen (F), which compresses the true lumen.
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Figure 19b. Aneurysm of the false lumen in a patient who had type B AAD 10 years earlier. (a) Enhanced CT scan obtained 10 years earlier shows an intimal flap (arrow) in a descending aorta of normal diameter. (b) Helical CT scan shows dilatation and mural thrombosis of the false lumen (arrow). (c) Oblique sagittal MPR image shows irregular mural thrombosis of the false lumen (F), which compresses the true lumen.
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Figure 19c. Aneurysm of the false lumen in a patient who had type B AAD 10 years earlier. (a) Enhanced CT scan obtained 10 years earlier shows an intimal flap (arrow) in a descending aorta of normal diameter. (b) Helical CT scan shows dilatation and mural thrombosis of the false lumen (arrow). (c) Oblique sagittal MPR image shows irregular mural thrombosis of the false lumen (F), which compresses the true lumen.
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Aneurysm of the True Lumen
Degenerative aneurysms associated with atheromatosis of the aorta most commonly involve the thoracic descending and abdominal segments. During follow-up of aortic dissection, an aneurysm of the true lumen can develop (Fig 20), particularly in older, hypertensive patients
with advanced atherosclerosis of the intima. Surgery must be considered when the total aortic diameter reaches 6 cm (24).
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Figure 20a. Aneurysm of the true lumen in a patient with polycystic kidney disease. (a) Enhanced CT scan shows an abdominal aortic dissection (arrow). (b) Enhanced CT scan obtained 11 years later shows an aneurysm of the true lumen (arrows). (c) Oblique sagittal MPR image shows the anterior abdominal aneurysm of the true lumen (large arrow) and slow flow in the false lumen (F) (small arrows).
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Figure 20b. Aneurysm of the true lumen in a patient with polycystic kidney disease. (a) Enhanced CT scan shows an abdominal aortic dissection (arrow). (b) Enhanced CT scan obtained 11 years later shows an aneurysm of the true lumen (arrows). (c) Oblique sagittal MPR image shows the anterior abdominal aneurysm of the true lumen (large arrow) and slow flow in the false lumen (F) (small arrows).
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Figure 20c. Aneurysm of the true lumen in a patient with polycystic kidney disease. (a) Enhanced CT scan shows an abdominal aortic dissection (arrow). (b) Enhanced CT scan obtained 11 years later shows an aneurysm of the true lumen (arrows). (c) Oblique sagittal MPR image shows the anterior abdominal aneurysm of the true lumen (large arrow) and slow flow in the false lumen (F) (small arrows).
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ABDOMINAL COMPLICATIONS
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Obstruction of abdominal branch vessels adds substantially to the mortality and morbidity rates in patients with aortic dissection and is a challenge to medical and surgical treatment of this disease. The frequency of such obstruction after AAD is as high as 27% (25). Infradiaphragmatic ischemic complications related to the main abdominal arterial branches (celiac trunk, superior mesenteric artery, main renal artery, and common iliac artery) can be demonstrated in the arterial phase with our helical CT protocol.
There are two types of branch-vessel occlusion. In static obstruction, the intimal flap intersects or enters the branch-vessel origin (Fig 21). Static obstruction is treated locally with an intravascular stent. In dynamic obstruction, the intimal flap spares the branch-vessel wall but prolapses across the branch-vessel origin and covers it like a curtain (Fig 22). Dynamic obstruction is treated with a fenestration procedure (26,27). In dynamic obstruction, the intimal flap has an ischemic configuration: The true lumen resembles a C-shaped envelope that is predominantly concave toward the false lumen. Presence of an ischemic configuration should be confirmed with manometry of the true and false lumina.
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Figure 21a. Static obstruction of the mesenteric artery. (a) Enhanced CT scan shows that the intimal flap enters the superior mesenteric artery. The true lumen (black arrow) is narrowed by a circumferential thrombosed false lumen that extends to the superior mesenteric artery (white arrows). (b) Sagittal MPR image shows the intimal flap (black arrow) and thrombosed false lumen in the superior mesenteric artery (white arrows).
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Figure 21b. Static obstruction of the mesenteric artery. (a) Enhanced CT scan shows that the intimal flap enters the superior mesenteric artery. The true lumen (black arrow) is narrowed by a circumferential thrombosed false lumen that extends to the superior mesenteric artery (white arrows). (b) Sagittal MPR image shows the intimal flap (black arrow) and thrombosed false lumen in the superior mesenteric artery (white arrows).
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Figure 22a. Dynamic obstruction of the renal artery. (a) Enhanced CT scan shows dynamic obstruction of the right renal artery (black arrow) and infarction of the right kidney (white arrows). (b) Coronal MPR image obtained at the level of the renal arteries shows a collapsed true lumen (black arrow) at the right renal ostium (white arrow).
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Figure 22b. Dynamic obstruction of the renal artery. (a) Enhanced CT scan shows dynamic obstruction of the right renal artery (black arrow) and infarction of the right kidney (white arrows). (b) Coronal MPR image obtained at the level of the renal arteries shows a collapsed true lumen (black arrow) at the right renal ostium (white arrow).
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Aneurysms of the abdominal branch vessels can be seen during follow-up of dissected vessels.
Compromise of the Celiac Trunk and Mesenteric Artery
The celiac trunk and superior mesenteric artery almost invariably originate from the true lumen. Obstruction of the celiac trunk due to aortic dissection can lead to hepatic or splenic infarction. Obstruction of the mesenteric artery (Fig 21) can lead to mesenteric ischemia. Clinical suspicion of mesenteric ischemia is based on the presence of abdominal pain, bloody diarrhea, recurrent sepsis, or elevated levels of hepatic or pancreatic enzymes. However, to establish the diagnosis of arterial compromise, there must be definite findings from manometry, intravascular ultrasound, or other radiologic studies (26).
Compromise of the Renal Arteries
In assessment of aortic dissection, it is not uncommon to encounter extension of the intimal flap into the renal artery (Fig 22), particularly in patients with acute azotemia. Nephrographic asymmetries can be caused by renal obstruction; however, they can also be caused by delayed delivery of contrast medium to a kidney predominantly supplied by the false lumen or by acute tubular necrosis due to transient ischemia that occurred early in the dissection process. Arteries supplied exclusively by the false lumen are rarely compromised. Demonstration of an ischemic configuration of the lumina is approximately 80% specific for a pressure deficit in branches that arise from the true lumen (27). Complete healing of renal artery dissection has been reported (28).
Compromise of the Iliac Arteries
Many type B aortic dissections demonstrate extension to the iliac arteries without clinical repercussions, but thrombosis (Fig 23) or rupture of the iliac arteries can be seen. Clinical suspicion of lower-extremity ischemia is based on the results of clinical examination of the legs; a confident diagnosis can be made with helical CT.
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Figure 23a. Thrombosis of the iliac artery. (a) Enhanced CT scan shows an abdominal aortic dissection with obliteration of the true lumen (arrow). (b) CT scan shows thrombosis of the right common iliac artery (arrow). (c) Maximum-intensity projection image of the abdominal aorta shows obstruction of the right common iliac artery.
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Figure 23b. Thrombosis of the iliac artery. (a) Enhanced CT scan shows an abdominal aortic dissection with obliteration of the true lumen (arrow). (b) CT scan shows thrombosis of the right common iliac artery (arrow). (c) Maximum-intensity projection image of the abdominal aorta shows obstruction of the right common iliac artery.
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Figure 23c. Thrombosis of the iliac artery. (a) Enhanced CT scan shows an abdominal aortic dissection with obliteration of the true lumen (arrow). (b) CT scan shows thrombosis of the right common iliac artery (arrow). (c) Maximum-intensity projection image of the abdominal aorta shows obstruction of the right common iliac artery.
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CONCLUSIONS
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Helical CT can be used to study the entire aorta in AAD and for follow-up of chronic aortic dissection. This technique allows accurate diagnosis of the type of dissection, atypical forms of aortic dissection, and early and late complications after surgery or medical treatment. In addition, helical CT is useful in surveillance of abdominal branch-vessel compromise, which can be life threatening in the acute or chronic phase of aortic dissection.
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Acknowledgments
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The authors thank Celine Cavallo for linguistic advice and Xesco Guarné for help in photography.
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Footnotes
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Address reprint requests to C.S.
Presented as a scientific exhibit at the 1997 RSNA scientific assembly.
Abbreviations: AAD = acute aortic dissection
MPR = multiplanar reformation
SSD = shaded-surface display
CME FEATURE This article meets the criteria for 1.0 credit hour in category 1 of the AMA Physician's Recognition Award. To obtain credit, see the questionnaire on pp 147–154.
LEARNING OBJECTIVES After reading this article and taking the test, the reader will:
• Be able to identify the helical CT features of aortic dissection, intramural hematoma, and penetrating atherosclerotic ulcer.
• Be familiar with the changes that occur during follow-up of aortic dissection.
• Be aware of the potential abdominal complications of aortic dissection.
Received for publication February 23, 1998.
Revision received April 17, 1998. June 12, 1998.
Accepted for publication June 15, 1998.
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Top
Abstract
INTRODUCTION
