Classic aortic dissection is a longitudinal split or partition in the media of the aorta. An intimal tear connects the media with the aortic lumen, and an exit tear creates a true and a false lumen. The smaller true lumen is lined by intima, and the false lumen is lined by media. Typically, flow in the false lumen is slower than in the true lumen, and the false lumen often becomes aneurysmal when subjected to systemic pressure. An acute aortic dissection is considered chronic at 2 weeks. The dissection usually stops at an aortic branch vessel or at the level of an atherosclerotic plaque. [1, 2, 3]
See the images of aortic dissection below.
Most classic aortic dissections begin at 3 distinct anatomic locations: the aortic root; 2 cm above the aortic root; and just distal to the left subclavian artery. Ascending aortic involvement may result in death from wall rupture, hemopericardium and tamponade, occlusion of the coronary ostia with myocardial infarction, or severe aortic insufficiency.
Aortic intramural hematoma (AIH) is a more recently described entity in which no intimal flap is present. It results in a spontaneous medial hematoma that may be secondary to an infarction of the vasa vasorum of the adventitia. Aortic intramural hematoma accounts for approximately 25% of aortic dissections. Involvement of the ascending aorta, especially if the overall aortic diameter is greater than 5 cm, should be treated surgically to prevent rupture or progression to a classic dissection with intimal tear. Conservative management is indicated for AIH of the descending aorta. [4, 5, 6, 7, 8, 9]
Preferred examinations for aortic dissection include contrast-enhanced spiral CT transesophageal echocardiography (TEE) in the emergency setting and MRI for hemodynamically stable patients. TEE has an advantage over CT and MRI in its ability to evaluate the status of the aortic valve and the ostia of the coronary arteries. CT and MR angiography have largely replaced conventional diagnostic angiography in the assessment of aortic dissection. [10, 11, 12, 13, 14, 15, 16, 3]
Several factors determine the best modality for the initial evaluation and postoperative follow-up. These factors include the following: stability of the patient's condition, the patient's renal function, suspected postoperative complication, and the availability of each imaging modality.
Maffei et al performed a randomized, controlled trial in which 44 patients (252 evaluations) were examined with TEE and CT.  The authors concluded that both TEE and CT are atraumatic, safe, and accurate techniques for serial follow-up studies of patients treated for aortic dissection.
Three noninvasive studies are associated with high specificity and sensitivity for aortic dissection. CT and MRI are associated with a sensitivity and a specificity of 94-100% and 95-100%, respectively. TEE is less sensitive and specific than spiral CT or MR, and TEE is operator-dependent. In addition, because of tracheal interposition, there is a 2 cm "blind spot" for TEE just proximal to the innominate arteries. Also, TEE is contraindicated in approximately 1% of patients (eg, TEE is contraindicated in patients with esophageal varices).
Mediastinal widening is the most common plain radiographic finding in aortic dissection; it is noted in 80% of patients (see image below).
Other radiographic findings include the following:
Double aortic knob sign (present in 40% of patients)
Diffuse enlargement of the aorta with poor definition or irregularity of the aortic contour
Inward displacement of aortic wall calcification by more than 10 mm
Tracheal displacement to the right
Pleural effusion (more common on the left side; suggests leakage)
Displacement of a nasogastric tube
Left apical opacity
All findings on plain images are nonspecific but may help in determining the need for further workup. Mediastinal fat commonly causes a widening of the mediastinum, which may lead to a false-positive diagnosis of aortic dissection.
Since its introduction in the 1970s, CT has become a widely used technology, particularly in the ED. With the advent of spiral CT, studies may be performed in less time than before, with less patient discomfort, greater accuracy, and lower iodine load. Spiral CT permits patient translation and data acquisition simultaneously. A major advantage of this technology is in the evaluation of thoracic trauma, which enables the rapid diagnosis of thoracic injury. Multislice or multidetector CT may be used for faster imaging or to acquire thinner slices that may be reconstructed in multiple planes. [18, 19, 20, 21, 22, 23, 24, 25, 26, 14]
The sensitivity of CT for aortic dissection is 87-94%, and the specificity is 92-100%.
A typical helical scanning protocol for aortic dissection includes the following parameters: 5-mm collimation, 1.5 pitch, and 7.5-mm imaging spacing. Multidetector CT may be performed with 1-2.5 mm collimation. Initial nonenhanced CT is used for the diagnosis of acute hemorrhage and aortic rupture. This is followed by helical CT performed approximately 25-30 seconds after the injection of contrast material. Nonionic contrast material (120-135 mL) is power injected via a peripheral intravenous site at a rate of 3-4 mL/s.
Because cardiac output is quite variable in these sick patients, a test injection of contrast should be used to determine circulation time; alternatively, an automated bolus detection scheme may be used. One advantage of the test injection method is that one may visually differentiate the true and false lumen on the basis of contrast arrival time.
Usually, scanning is performed from the thoracic inlet to the common femoral arteries. When a dissection is identified, repeat scanning may be performed to obtain delayed images of the false lumen and aortic branches. Multiplanar reformation images are obtained in sagittal, coronal, oblique sagittal, and curved projections generated with an independent workstation. The use of volume rendering may be helpful for planning surgery.
Typical CT findings in acute dissection or intramural hematoma include the following:
Aortic intramural hematoma: Crescentic high-attenuating clot within the media, with internally displaced calcification (see the image below)Nonenhanced CT scan of the chest demonstrates a type B acute aortic intramural hematoma with displacement of intimal calcification and a crescentic high-attenuating clot without mass effect on the aortic lumen. Courtesy of Joel L. Fishman, MD.
Intimal flap separating the two aortic channels (see the images below)Contrast-enhanced axial CT image demonstrates an intimal flap that separates the 2 channels in the ascending and descending aorta diagnostic of a Stanford type A dissection. Courtesy of Joel L. Fishman, MD.Contrast-enhanced axial CT image demonstrates an intimal flap that separates the 2 channels in the ascending aorta and descending aorta and begins at the level of the aortic root. Courtesy of Joel L. Fishman, MD.
Hemorrhagic pleural and pericardial effusions and mediastinal hemorrhage may be seen.
CT is helpful in postoperative follow-up. It may accurately depict associated complications, including the following:
Inadequate contrast opacification may lead to false-negative findings of aortic dissection. Aortic intramural hematoma may be misinterpreted as an aneurysm with thrombus or arteritis.
Spiral CT artifacts include perivenous streaks and motion artifacts. The perivenous streaks are caused by beam hardening and motion resulting from transmitted pulsation in a vein that carries undiluted contrast medium into the heart. Some authors recommend injecting the contrast agent at a rate of 2 mL/s via a peripheral intravenous site in the right arm. Aortic motion artifact is produced by the aortic wall motion from the end of diastole to the end of systole. Typically, this artifact is seen in the left anterior and right posterior margins of the aortic circumference.
In some patients, especially those with cystic medial necrosis, the intimal flap may be subtle.
CT scanning for patients with type B intramural hematoma is useful in determining morphologic evolution and intimal erosion. It also helps assess predictive factors that would allow for better endovascular prognosis and treatment.
In a study by Schlatter et al , it was determined that complications or morphologic progression were tied to preexisting intimal anomalies seen on the initial CT.  Increasing the detection rate of intimal anomalies, such as intimal erosion and aortic branch artery lesions, was done by using multislice CT, with systematically delayed phase and millimetric thin slices.
See the images below.
Magnetic Resonance Imaging
MRI is an accurate tool for use in diagnosis of aortic dissection, but it may not be readily available in the acute setting. In addition, unstable patients with Swan-Ganz catheters should not undergo MRI. [28, 29, 30, 31, 32, 33, 13, 24, 34, 26, 16] The sensitivity and specificity of MRI for aortic dissection are both more than 90%.
MRI findings of aortic dissection include the following:
An intimal flap of medium signal intensity is surrounded by a signal void of fast-flowing blood on "black blood" echocardiogram (ECG)-gated spin-echo or double inversion recovery single-shot fast spin-echo MRI (see the image below).Sagittal gradient-echo MRI image obtained in early systole shows a jet of blood flowing through the intimal tear from the smaller anterior true lumen into the larger posterior false lumen. The intimal flap is recognized as the medium signal intensity linear structure that divides the true and false lumens. An aortic stent-graft could be placed across the intimal/entry tear in the descending thoracic aorta to redirect blood into the anterior true lumen. Courtesy of Joel L. Fishman, MD.
With cine gradient echo imaging, the intimal flap appears as a dark line against the high signal intensity of the flowing blood; it may change configuration during the cardiac cycle. It is important that a careful examination of the aortic flap be conducted during the cardiac cycle with cine MRI so as to detect the presence of "true lumen collapse," which may be associated with end-organ ischemia. In some cases, the intima is stripped 360° from the media and is essentially "free floating"; this may result in catastrophic intimo-intimo intussusception.
Newer pulse sequences such as true fisp or Fiesta offer very fast cine imaging.
Basic MRI sequences for evaluating aortic dissection include spin-echo T1-weighted or breath-hold double inversion recovery sequences, cardiac-gated gradient-echo sequences, and three-dimensional (3D) thin-section MR angiography with a bolus injection of a single or double dose of gadolinium-based contrast agent.
MRI findings of AIH include a crescent of blood surrounding but not compressing the aorta. The signal intensity of the crescent varies with age on T1-weighted imaging: it is isointense to muscle in the acute setting and is markedly hyperintense after 3-7 days.
MRI is helpful in postoperative follow-up. It may accurately depict associated complications, including the following:
Potential drawbacks of MRI include reported artifacts on cardiac-triggered thoracic spin-echo phase images. These may appear as an artifactual borderlike feature across the aorta (caused by helical flow in the aorta) that may be interpreted as a dissection. Other potential causes of misinterpretation include an atypical configuration of the intimal flap seen in short dissections and multiple false channels in cases in which the flaps are complex.
Aortic anomalies also may cause confusion. False-positive findings seen in gadolinium-enhanced MRA include a central line or "maki" artifact. This occurs when the acquisition is performed too early as intra-aortic gadolinium concentration is rising. This artifact may be readily differentiated from an aortic dissection because it does not take a spiral course like a true intimal flap.
Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the Medscape Reference topic Nephrogenic Systemic Fibrosis. NSF/NFD disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.
ECG is helpful in the diagnosis of aortic dissections. It is particularly helpful in cases of ascending thoracic dissections, cardiac tamponade, and aortic regurgitation; transesophageal echocardiography (TEE) has a greater sensitivity and specificity than CT or MRI in detecting coronary arterial occlusion, aortic insufficiency, and cardiac tamponade. The sensitivity is 97-99%. The specificity is in the range of 97-100%. [35, 36]
According to a study by Mastrogiovanni, TEE was the favored study for the evaluation of aortic dissection.  In their report of 54 patients, TEE findings confirmed the diagnostic dissection in all patients but one. TEE noted the site of the intimal tear; the extension of the dissection, pericardial effusion, aortic incompetence; and left ventricular function. Because of the high level of correspondence between the diagnosis made at TEE and the surgical anatomic findings, the authors favored the use of TEE—for many cases, as the sole diagnostic modality.
A tortuous aorta may result in a false-positive diagnosis of dissection. In cases involving a massive dilated ascending aorta (usually occurring as a result of cystic medial necrosis), it may be difficult to identify a small intimal flap.
Intravascular ultrasonography (IVUS) plays an essential role in the diagnosis and classification of aortic dissection, evaluation of dissection flap, and whether or not it exhibits dynamic or static obstruction to the aortic branches. IVUS is an imaging guidance for fenestration for the treatment of aortic dissection and endograft placement.
Aortography was the reference standard for the preoperative evaluation and diagnosis of aortic dissection. With the advent of transesophageal echocardiography (TEE), CT, and MRI, its role has become important only if nonsurgical interventional procedures are indicated. Aortographic findings are less sensitive than those of newer noninvasive techniques, especially for aortic intramural hematoma.
It is quite controversial whether coronary angiography should be performed before sternotomy in a stable patient with aortic dissection, inasmuch as concomitant coronary bypass grafting may be performed if diseased vessels are present.
Diagnostic criteria include visualization of a lucent flap and delayed filling and washout of the false lumen. The expanding false lumen may compress the true lumen and cause it to become narrowed. A dual lumen aorta is noted when both the true and false lumens are opacified (see the image below). Aortic intramural hematoma (AIH) is almost impossible to diagnose with aortography because no compression of the lumen exists. 
During aortography, overinjection of the false lumen should be avoided if it is entered during the procedure. The operator should be suspicious if he or she has difficulty advancing the guidewire into the aortic valve. Abdominal and pelvic aortography should be included in the diagnostic study to assess the level of the reentry site. Obstruction of the aortic branches may be noted (most commonly in the left renal artery, which is the site in approximately 25-30% of patients). Visceral and extremity ischemia may occur when the superior or inferior mesenteric arteries and the iliac arteries are compromised.
Pitfalls of angiography include a lack of visualization of the false lumen because of thrombosis or inadequate opacification with contrast material. Streak artifacts secondary to aortic or cardiac motion or opacification of the sinus of Valsalva may be confused with thrombus. Pitfalls also include missing the diagnosis of an intramural hematoma (frequently associated with progression to frank dissection) and misdiagnosis when the false lumen is thrombosed.