Aortic dissection is characterized by a laceration of the aortic intima and inner layer of the aortic media that allows blood to course through a false lumen in the outer third of the media. Dissection can occur throughout the length of the aorta and the two most common classifications are based on the anatomic location and extension of intimal flap. According to the De Bakey classification, in type I dissection the intimal tear originates in the ascending aorta and the intimal flap extends below the origin of the left subclavian artery; type II dissection is confined to the ascending aorta; and in type III dissection the entry tear develops after the origin of the subclavian artery and extends distally. The Stanford classification simply classifies an aortic dissection irrespective of the site of the entry tear as type A, if the ascending aorta is involved, and as type B if the ascending aorta is spared. The Stanford classification is fundamentally based on prognostic factors: Type A dissection requires
urgent surgical repair whereas most of type B dissections can be successfully managed with medical therapy.
Acute aortic dissection is a life-threatening condition requiring prompt diagnosis and treatment (
50). The 14-day period after onset has been designated as an acute phase because the rates of morbidity and mortality are highest during this period. The estimated mortality rate of untreated aortic dissection is 1% to 2% per hour in the first 24 hours after onset and 80% within 2 weeks. Early and accurate detection of the dissection and a delineation of its anatomic details are critical for successful management. However, because physical findings may be absent or misleading and symptoms may mimic those of other disorders, such as myocardial ischemia and stroke, the diagnosis of aortic dissection is often missed at initial evaluation (
51,
52). The anatomic characteristics of the dissection indicate the type of surgical technique, and affect both the surgical success rate and long-term results. Thus, in dissection, the diagnostic goal, regardless the imaging modality, used a clear delineation not only of the intimal flap and its extension but also detection of the entry and re-entry sites, presence and degree of aortic insufficiency, and flow in the aortic branches (
53). Transcatheter endovascular reconstruction of type B aortic dissection is a new option for the treatment of both acute and chronic dissections (
54,
55). In endovascular techniques, the success of the procedure is strictly related to a detailed anatomic definition of the features of the dissected aorta. The identification of the entry and re-entry sites, the relationship between true and false lumina and the visceral vessels, and any involvement of the iliac arteries are crucial in patient selection and stent-graft design (
Fig. 32.4).
MRI Technique and Findings
In a suspected case of aortic dissection, the standard examination should begin with spin-echo sequences acquired with high-resolution parameters and preparatory pulses to nullify the blood signal and obtain a better definition of the aortic wall structures (
Table 32.4). In the axial plane the intimal flap is detected as a straight linear image inside the aortic lumen. The true lumen can be differentiated from the false by the anatomic features and flow pattern. The true lumen shows a signal void, whereas the false lumen has a higher signal intensity. In addition, the visualization of remnants of the dissected media as cobwebs adjacent to the outer wall of the lumen may help to identify the false lumen. The leakage of blood from the descending aorta into the periaortic space, which can appear with high signal intensity and result in a left-sided pleural effusion, is usually better visualized on axial images. A high signal intensity of a pericardial effusion indicates a bloody component and is considered a sign of impending rupture of the ascending aorta into the pericardial space. A detailed anatomic map of aortic dissection must indicate the type and extension of dissection but also distinguish the origin and perfusion of branch vessels (arch branches, celiac, superior mesenteric, renal arteries and coronary arteries) from the true or false channels. Therefore, a further spin-echo sequence on the sagittal plane should be performed to define the extension of the dissection in the thoracic and abdominal aortae and in the aortic arch branches (
Fig. 32.5).
Adjunctive gradient-echo sequences or phase-contrast images can be instrumental in identifying aortic insufficiency and entry or re-entry sites (
Fig. 32.6), as well as in differentiating
slow flow from thrombus in the false lumen (
56,
57). However, because the diagnosis of aortic dissection is not dependent on functional gradient-echo images, these sequences should be reserved to clinically stable patients.
The third step in the diagnosis of aortic dissection and the definition of its anatomic detail relies on the use of gadoliniumenhanced 3D MRA. Since 3D MRA is rapidly acquired without any need of ECG triggering, this technique may be used for even severely ill patients. Since it is not nephrotoxic and causes no other adverse effects, gadolinium can be used in patients with renal failure or low cardiac output. With spin-echo sequences, artifacts caused by imperfect ECG gating, respiratory motion, or slow blood pool can result in intraluminal signal, simulating, or obscuring an intimal flap. In gadolinium-enhanced 3D MRA, the intimal flap is easily detected and the relationship with aortic vessels clearly depicted (
Figs. 32.7 and
32.8). Entry and re-entry sites appear as a segmental interruption of the linear intimal flap on axial or sagittal images (
Fig. 32.9). The analysis of MRA images should not be limited to viewing MIP images or SSD; it should also include a complete evaluation of reformatted images in all three planes, to confirm or improve spin-echo information and exclude artifacts. In MRA postprocessing displays, the appearance of the dissected aorta is similar to that on conventional catheter angiograms, but diagnostic information such as the intimal flap can be masked. Combining the spin-echo with MRA images completes the diagnosis and anatomical definition (
58). However, two cases of intramural hematoma (IMH) missed by MRA (
28) raise concern about using MRA as the sole modality for suspected aortic dissection.
At present, MRI is one of the most accurate tools in the detection of aortic dissection. A high degree of spatial resolution and contrast and the capability for multiplanar acquisition provide excellent sensitivity and specificity that approximate 100% in the published series (
58,
59,
60 and
61). With modern scanner, a comprehensive study of the entire aorta is completed in less than 10 minutes, and patient’s ECG, blood pressure, and oxygen saturation can be monitored, even during assisted ventilation. The implementation of open systems may soon allow a wider use of MRI even in acute pathology.
Aortography has long been considered the method of choice in suspected aortic dissection, despite the risk of catheter manipulation and injection of high flow contrast in a dissected aorta. With the advent of noninvasive imaging modalities, its low accuracy has been demonstrated; the reported sensitivity is 77% to 90% and the specificity is 90% to 100%. The superiority of transesophageal echocardiography (TEE), CT, and MRI in comparison to angiography has been widely reported in the literature (
51,
52 and
53,
59).
In general, TEE is a reliable method with excellent sensitivity, and a great advantage is that it can be performed at the bedside in patients too unstable for transportation. However, artifacts and “blind areas,” such as the distal portion of the
ascending aorta, can influence specificity in an operatordependent manner. Since TEE information is limited to the thoracic aorta, sometimes with suboptimal display also of the aortic arch, a second imaging modality encompassing the entire aorta is advisable in stable patients.
CT Technique and Findings
CT has a difficult and crucial role in the diagnostic workup for aortic dissection. According to the results of the International Registry of Acute Aortic Dissection (
62), CT resulted to be the initial modality to confirm the clinical suspicion of aortic dissection in the majority of cases (63%), followed by TEE (32%), aortography (4%), and MRI (1%). Sensitivity rates of these four imaging modalities, in the set of suspected acute aortic dissection, are 100% for MRI, 93% for CT, 88% for angiography, and 87% for TEE.
The critical clinical issue required of any imaging test applied to a patient suspected of having an aortic dissection is the identification of an intimal flap and its localization to the ascending (type A) or descending (type B) aorta. This fundamental diagnostic feature that determines the need for emergent repair can be addressed by at least four imaging modalities: Angiography, CT, MRI, and TEE. The relative accuracy of these modalities has been debated in the medical literature and is confounded by the fact that technical improvements in CT, MRI, and TEE have outpaced our ability to compare them appropriately. Recent opinion has shifted
toward MRI or TEE as the most sensitive tests for aortic dissection. Unfortunately, much of this opinion is based upon comparative studies where MRI or TEE is compared with relatively obsolete conventional CT or SDCT techniques (
53,
59,
60,
63,
64 and
65) (
Table 32.5). To date there have been no comparisons of multidetector CT to either MRI or TEE and is useless to consider these prior studies while CT, more than MRI or TEE, got substantial advances and features.
Helical CT allows diagnosis of acute aortic dissection with sensitivity and specificity rates respectively of 83% to 94% and 87% to 100% (
66,
67 and
68). Imaging of dissection requires a volume of coverage from supra-aortic branches superiorly to the femoral arteries inferiorly, and MDCT in few seconds can acquire this extent. Imaging sensitivity is enhanced by greater temporal resolution and ECG-gating sequences which minimize pulsation artifacts at the aortic root (
Fig. 32.10). On unenhanced CT, it is possible to see internal displacement of intimal calcifications and this finding could be confused with an aneurysm with calcified mural thrombus. High attenuation of the false lumen at unenhanced CT may help differentiate between the two conditions (
69). The main and characteristic finding of aortic dissection on contrastenhanced CT scan is an intimal flap that separates true from the false lumen. This usually appears as a thin linear luminal filling defect and its appearance is determined by the circumference and length of dissection, the relative lumen flow, and aortic pulsation. Accurate differentiation between true and false lumen became fundamental with the advent of endovascular procedure for treatment planning (
70). The slender linear areas of low attenuation that occasionally appear in the false lumen on CT images, known as the cobweb sign, are specific to the false lumen and may aid in its recognition. These findings correspond to residual ribbons of the media, incompletely sheared away during the dissection process (
71). Two other useful indicators of the false lumen are a larger cross-sectional area and the beak sign. This one is the manifestation of the wedge of hematoma that cleaves a space for the propagation of the false lumen (
72). However, on most contrast-enhanced CT scans the true lumen may be identified by its continuity with an undissected portion of the aorta (
Fig. 32.11). Communicating and noncommunicating dissections are diagnosed based on identifiable flow in the false lumen. When slower flow is present, false lumen filling defects that represent strand of thrombus are observed, and complete thrombosis can be reliably diagnosed on follow-up examinations (
Fig. 32.12) (
71,
72). One unusual type of aortic dissection is the intimo-intimal intussusception produced by circumferential dissection of the intimal layer, which subsequently invaginates like a wind sock; CT scan may show one lumen wrapped around the other lumen in aortic arch, with the inner lumen invariably being the true one (
73,
74). Sometimes an aortic aneurysm with intraluminal thrombus may be difficult to distinguish from a dissection with a thrombosed false lumen; may help us the fact that dissection generally has a spiroidal shape, whereas a thrombus tends to maintain a constant circumferential relationship with the aortic wall and, furthermore, a mural thrombus usually has a smooth internal border. Calcification in aneurysm is typically located at the periphery of the aorta (
75). Visceral and supra-aortic vessel’s involvement can account for high mortality and MDCT has the spatial and contrast resolution to reliably diagnose branch vessel involvement and document true or false lumen supply (
Figs. 32.11 and
32.12). General strategy of CT study of the thoracic- abdominal aortae and main CT findings of aortic diseases are showed in
Table 32.6.
Apart from axial images and MPR, which provides an overall view of the aortic dissection and demonstrate the anatomic relationships between the flap and adjacent great vessels, VR is preferred to MIP and SSD for dissection 3D postprocessing as it preserve the variable enhancement patterns of the lumina and is more sensitive for visualization of the flap. The accurate localization of entry and re-entry sites remains a difficulty for all imaging techniques, but the high resolution of submillimeter MDCT acquisitions and cardiac gating may be enough reliable to depict them (
Figs. 32.11 and
32.12). The last generation of scanners may also depict coronary artery involvement but, to date, determinate or quantify aortic insufficiency with CT is still impossible.
In CT study for suspected aortic dissection, especially with SDCT scanners, there are a variety of pitfalls mimicking aortic
dissection. These pitfalls are attributable to technical factors (improper timing or rate of contrast administration), streak artifacts (high-attenuation materials, high-contrast interfaces, and cardiac motion), periaortic structures (aortic arch branches, mediastinal veins, pericardial recess, thymus, atelectasis, and pleural thickening or effusion), aortic wall motion, aortic anomalies, and the aforementioned aortic aneurysm with thrombus (
68,
76). At present with an optimal scan protocol which requires a precontrast study, an ECG-gated arterial phase sequence and a delayed scan for slower blood flows, most of these artifacts/pitfalls are minimized.