The pathogenesis of aneurysm is extremely complex and not completely understood; most likely, aneurysms result from the interaction of multiple factors, systemic and local hemodynamic features (Michel et al. 2011). The elastic properties of the aorta are important for its normal function. The elasticity of the wall allows the aorta to accept the pulsatile output of the left ventricle in systole and to modulate continued forward flow during diastole. With aging, the medial elastic fibers become thinned and fragmented, with the increase in collagen and ground substance. Elasticity and compliance of the aortic wall are progressively lost, with the increase in pulse pressure and with progressive dilatation of the aorta (Goldstein et al. 2015). Many authors suggest that a group of enzymes called matrix metalloproteinases (MMPs) plays a significant role in the destruction of extracellular matrix in the aortic wall. MMPs lead to the degradation of these structural proteins with elastic fiber fragmentation and loss and degeneration of the tunica media with loss of elasticity and weakening of the aortic wall and consequent dilation (Sakalihasan et al. 2005). Hemodynamic factors probably play a fundamental role in the formation of aortic aneurysms. The human aorta is a relatively low-resistance circuit for circulating blood while the lower extremities have higher arterial resistance (Hoshina et al. 2003). Repeated hydrostatic trauma may injure a diseased aortic wall and contribute to aneurysmal development. In case of concomitant systemic hypertension, it can accelerate the expansion of known aneurysms and may contribute to their initial formation (Piccinelli et al. 2013). Wall rupture follows the basic principles of material failure, i.e., an aneurysm breaks open when mural stress or deformation meets an appropriate failure criterion. The law of Laplace states that wall tension is proportional to the pressure and to the radius of the arterial conduit (T = P × R). As diameter increases, wall tension increases, which contributes to increasing diameter and risk of rupture. Also increased pressure and aneurysm size aggravate wall tension and therefore increase the risk of rupture. The use of the law of Laplace to predict AAA rupture potential is erroneous, because the AAA wall geometry is not a simple cylinder or sphere with a single radius of curvature, and wall stress alone is not sufficient to predict AAA rupture. So AAA diameter cannot be considered as the only determinant factor of either wall strength or wall stress (Piccinelli et al. 2013). Various indices have been proposed to create a more reliable and patient-specific criteria for clinical practice. Despite many progresses, the majority of the proposed criteria for risk stratification and decision making still rely on empirical measurements rather than on the analysis of biomechanical properties (Creasy et al. 1997); among these are maximum AAA diameter (Greenhalgh 2004), currently the prevalent index for the evaluation of risk of rupture; AAA expansion rate (Hirose and Takamiya 1998); wall stiffness (Fillinger et al. 2003); intraluminal thrombus thickness (Speelman et al. 2010); and AAA wall peak stress (Raghavan et al. 2000; McGloughlin and Doyle 2010). The presence of anisotropic displacement of the AAA lumen boundary and their link to hemodynamic forces have been assessed, highlighting a new possible role for hemodynamics in the study of AAA progression (Piccinelli et al. 2013).

Atherosclerosis is the cause of approximately 70 % of all TAAs (Lesko et al. 1997) and contributes to the formation of aortic aneurysms secondary to the degeneration of the components of the aortic wall. Atherosclerotic aneurysms are the most common type; usually they are seen in the elderly and typically characterized by intimal calcification and fibrous plaques along the aorta. The majority of them are fusiform, up to 20 % may be saccular (Posniak et al. 1990), and they occur in the descending thoracic aorta and rarely involve only the ascending aorta (Lesko et al. 1997) (Fig. 4a, b).

Because an abdominal aortic aneurysm occurs in 28 % of patients with TAA, initial CT evaluation may include the entire thoracoabdominal aorta (Bickerstaff et al. 1982).

Many authors consider genetic causes as a leading factors in the formation and development of aortic aneurysms. Inherited disorders of connective tissue contribute to the formation of aortic aneurysms. Marfan syndrome is a multisystemic connective tissue, autosomal dominant inherited disorder (Goldstein et al. 2015). One of the hallmark features is dilatation or dissection of the aortic root (Daimon et al. 2008). Aortic aneurysms without annuloaortic ectasia are also common. Annuloaortic ectasia, especially with dilatation of the aortic root, is found in 60–80 % of adults with Marfan syndrome. It usually begins with dilatation of the aortic sinuses, which progresses first into the sinotubular junction, than into the aortic annulus. Dilatation of the aortic root causes aortic valve insufficiency that may progress to aortic root dissection or rupture (Vasan et al. 1995). The aortic aneurysms rarely show intimal calcification or atherosclerotic thrombosis; they occur commonly and develop more rapidly in younger patients. After the initial diagnosis of aortopathy by TTE, CT or MRI is recommended to confirm the size of the aorta and to document the diameters of the distal ascending aorta, aortic arch, and descending aortic segments. Repeated CT or MRI, about every 3 years, is recommended to reassess the aortic arch and the descending aorta. Prophylactic surgery for annuloaortic ectasia is recommended when the diameter of the Valsalva sinus exceeds 5.5 cm in an adult and 5.0 cm in a child, when the diameter of the Valsalva sinus is less than 5.0 cm but in a rapid rate of aortic aneurysm expansion (increase in dilatation by more than 1 cm per year), or when there is a family history of aortic dissection (Wolak et al. 2008; Kälsch et al. 2013; De Backer et al. 2006). Composite surgical replacement of the aortic root and valve is commonly performed with or without coronary reimplantation (Fig. 5a, b).

Other genetic conditions associated with aortic dilatation are bicuspid valve-related aortopathy (BAV), Loeys–Dietz syndrome, Turner syndrome, familial TAA, and Ehlers–Danlos syndrome. Ehlers–Danlos syndrome (EDS) is an autosomal dominant disorder with vascular involvement (arterial dilatation and rupture). The imaging characteristics of aortic aneurysms in Ehlers–Danlos syndrome resemble those in Marfan syndrome.

Aortitis is a general term that refers to a broad category of infectious or noninfectious conditions, in which there is abnormal inflammation of the aortic wall. These inflammatory conditions have different clinical and morphologic features and variable prognoses (Restrepo et al. 2011). The differentiation between infectious (mycotic) and noninfectious aneurysms is important because, as mycotic aneurysms are treated with antibiotic therapy and surgical debridement with or without surgical revascularization, noninfectious aneurysms are managed by steroids and immunosuppressive drugs (Nagpal et al. 2015).

Noninfectious aortitis may be part of a systemic disorder (Gornik and Creager 2008). The association between rheumatic diseases and aortic involvement is well known, but the prevalence of aortic involvement in the different rheumatic diseases is quite variable. Rheumatic diseases (Slobodin et al. 2006) with a high prevalence (>10 %) of aortic involvement include Takayasu arteritis (Fig. 6a–c), giant cell arteritis (GCA), long-standing ankylosing spondylitis, Cogan syndrome (interstitial keratitis, iritis, conjunctival or subconjunctival hemorrhage, fever, aortic insufficiency) (Fig. 7a, b), and relapsing polychondritis. Rheumatic diseases in which aortic involvement is an uncommon well-documented complication include rheumatoid arthritis, seronegative spondyloarthropathies, Behçet disease, and systemic lupus erythematosus (SLE). Aortitis most commonly affects the ascending aorta in rheumatoid arthritis (Fig. 8a, b), ankylosing spondylitis, giant cell arteritis, and relapsing polychondritis (Posniak et al. 1990). These conditions may also be associated with aortic valve insufficiency. Aortitis in rheumatic fever can be segmental, limited to the ascending aorta, or it can involve the abdominal aorta or the entire aorta (Lande and Berkmen 1976; Posniak et al. 1990). Takayasu arteritis commonly affects the aortic arch and its major branches, with variable involvement of the abdominal aorta and pulmonary arteries; although Takayasu arteritis typically causes arterial stenosis and occlusion, aneurysms may also occur. Inflammatory aneurysms differ from atherosclerotic aneurysms because of the presence of dense perianeurysmal fibrosis and thickened aortic wall (Restrepo et al. 2011). The prevalence is 5–25 % of all abdominal aortic aneurysms. Inflammatory aneurysms of the ascending aorta and aortic arch are much less frequent, usually associated with concomitant inflammatory aneurysms in the abdominal aorta (Girardi and Coselli 1997). CT shows a hypoattenuating mass with periaortic wall thickening that spares the posterior wall. After intravenous administration of contrast material, rapid luminal opacification is followed by delayed enhancement of the soft-tissue component.

Infected (mycotic) aneurysms. Infectious aortitis is secondary to gram-positive bacteria such as the Staphylococcal species, Enterococcus species, and Streptococcus pneumonia, responsible of 60 % of the infections. Gram-negative bacilli (Salmonella species in the majority of cases) are also a frequent cause of aortic infection; however, they are more prevalent in infectious abdominal aortitis (Revest et al. 2007). Aortic infection by Mycobacterium tuberculosis, an uncommon problem in developed countries, may occur as a result of the extension to the aortic wall of a contiguous infective focus such as infected mediastinal lymph nodes or lung lesions. Treponema pallidum is a rare etiology today, with a historical importance. Aortic infection by unusual bacteria and nonbacterial (fungi) microorganisms is extremely rare; however, this possibility must be considered in immunocompromised patients (Gornik and Creager 2008). Infected (mycotic) aneurysms are uncommon, and they usually involve segments not commonly involved by atherosclerosis (Lee et al. 2008). Although the infraabdominal aorta is the most frequently involved segment of the aorta, there is also a combined involvement of the descending thoracic, thoracoabdominal, and suprarenal aorta (Oderich et al. 2001). Early changes of aortitis preceding aneurysm formation include an irregular arterial wall, periaortic edema as fat stranding or a hypoattenuating concentric rim at CT, a periaortic soft-tissue mass, and periaortic gas. Concentric or eccentric periaortic inflammatory soft tissue can develop, as a homogeneous contrast-enhancing mass at CT (Ting and Cheng 1997; Tsao et al. 2002; Azizi et al. 2004). The inflammatory mass can develop necrosis with a heterogeneous attenuation at CT, with rim enhancement or poor enhancement after administration of contrast material (Gomes and Choyke 1992). Periaortic mass and fat stranding are the most common imaging findings of infected aortic aneurysms, and they are found in 48 % of cases (Macedo et al. 2004). Periaortic gas is an uncommon feature (Vogelzang and Sohaey 1988; Macedo et al. 2004). At CT, an infected aortic aneurysm appears as a focal, contrast-enhancing dilatation, usually saccular. The lumen can be central or eccentric and it can be a single compartment or multiloculated. Disrupted arterial wall calcifications can occur adjacent to the infected aneurysm (Sueyoshi et al. 1998; Azizi et al. 2004). Calcification within the aneurysmal wall and thrombus within an infected aneurysm are uncommon (Gomes and Choyke 1992). An infected aneurysm can rapidly develop and enlarge (Sueyoshi et al. 1998; Macedo et al. 2004), and subsequently it can rupture due to high systemic arterial pressure.

Posttraumatic aneurysms or, better, pseudoaneurysms, following blunt trauma may result from rapid deceleration, which is the most accepted mechanism of injury (Agarwal et al. 2009), with shearing and bending stress on the aortic wall. According to this theory, during the sudden deceleration, the distal transverse arch moves forward while the proximal descending thoracic aorta remains stationary, held back by the ligamentum arteriosum and the intercostal vessels (Javadpour et al. 2002). Another proposed mechanism is the “osseous pinch,” where an anteroposterior compression force results in posteroinferior displacement of the manubrium, first rib, and medial clavicle, which impinge on the aorta and compress it against the thoracic spine posteriorly (Crass et al. 1990). A third possible mechanism of aortic injury is represented by the sudden increase of intraluminal blood pressure due to trauma with the so-called water hammer effect (Creasy et al. 1997). The most common site of injury, seen in survived trauma victims, is the aortic isthmus (90 % of cases), followed by the ascending aorta and the descending aorta near the diaphragmatic hiatus (Mukherjee and Rajagopalan 2007). Chronic pseudoaneurysms develop in 2.5 % of patients who survive the initial trauma. These often calcify (Fig. 9), may contain thrombus (Heystraten et al. 1986), and have the potential to enlarge progressively, rupturing even years after the initial trauma (Posniak et al. 1990).

Aortic dissection is an abnormal passage of blood, through an intimal tear, into the media, producing a false lumen separated from the true lumen by an intimal flap. A previous aortic dissection, with a persistent false channel, may produce aneurysmal dilatation of the false lumen. These are false aneurysms, contained only by the outer media and adventitia, enlarging over time (Fig. 10).

Normal aortic variants can mimic an aortic aneurysm. Three of these are ductus diverticulum, Kommerell’s diverticulum, and aortic spindle.

Thoracic aortic aneurysms often grow slowly and usually without symptoms, making them difficult to detect. TAAs are usually found incidentally after chest radiographs or during other imaging studies. Most patients are hypertensive or can have a history of vascular disease in different districts, such as coronary or carotid arteries, but, in general, they remain relatively asymptomatic until the aneurysm expands.

The most common symptom is pain. Chest or back pain in presence of TAA is predictive of aortic rupture. Ascending aortic aneurysms tend to cause anterior chest pain, whereas arch aneurysms more likely cause pain radiating to the neck. Descending thoracic aneurysms more likely cause back pain localized between the scapulae. When located at the level of the diaphragmatic hiatus, the pain occurs in the mid-back and epigastric region; however, identifying the area of aortic involvement on the basis of the specific location of pain is not always reliable and can be misleading. Large ascending aortic aneurysms can cause superior vena cava obstruction with distended neck veins. Ascending aortic aneurysms also may develop aortic insufficiency and heart failure. Arch aneurysms, where stretching of the recurrent laryngeal nerves is present, may cause hoarseness. Descending thoracic aneurysms and thoracoabdominal aneurysms may compress the trachea and/or bronchi and cause dyspnea, stridor, wheezing, or cough. Compression of the esophagus results in dysphagia. They can also erode the surrounding structures and result in aortobronchial or aortoesophageal fistulas with, respectively, hemoptysis, hematemesis, or gastrointestinal bleeding. Erosion into the spine causes back pain or instability. Spinal cord compression or thrombosis of spinal arteries may result in paraparesis or paraplegia. Descending thoracic aneurysms may thrombose or embolize clot and atheromatous debris distally to visceral, renal, or lower extremity arteries.

The most common complications of TAAs are acute rupture and dissection with rupture into the pericardium, causing tamponade, or dissection into the aortic valve, causing acute aortic insufficiency, or into the coronary arteries, causing myocardial infarction (Safi et al. 2012; Upchurch 2014).

In the clinical setting of acute nontraumatic chest pain, case history, physical examination, ECG, and laboratory risk markers should be immediately obtained. Chest X-ray is useful to rule out many nonvascular causes of acute chest pain such as pleural effusions, pneumothorax, etc. In the field of nontraumatic vascular emergencies, CT has a primary role with a sensitivity and a specificity near 100 % for acute aortic pathologies, while MRI is indicated mainly for follow-up of aortic pathologies due to longer scan time, limited availability, and increased cost (Nagpal et al. 2015).

The CT protocol starts with unenhanced scan to visualize intramural hematoma that can be misdiagnosed as atherosclerotic thrombus in the arterial phase CT scan (Chiu et al. 2013). The scan should be extended above the aortic arch (for aortic branches) and caudally to the femoral heads to cover aorta entirely. Then contrast-enhanced CT with high infusion rate (4–5 mL/s) is performed to achieve a target opacification of the aorta >250 HU, obtained in the arterial phase with a test bolus or a bolus-tracking technique. The contrast should be injected through the right upper extremity to avoid artifacts from left innominate vein in the area of the aortic arch. A saline flush of 30–40 mL should be injected following contrast medium to minimize streak artifact from superior vena cava (Weininger et al. 2011). Delayed images are recommended after stent–graft repair of aortic aneurysm to detect endoleaks (Task et al. 2014). ECG gating when available is very useful to avoid cardiac motion artifacts impairing the evaluation of thoracic aorta and coronary origin. ECG-gated imaging both in precontrast and in postcontrast CT, from above aortic arch to diaphragmatic hiatus, should be included in the standard protocols for thoracic vascular emergency. Prospective ECG gating is performed at mid-diastole if heart rate is <70 beats per minute, while it is acquired in end-systole in patients with heart rate >70 beats per minute (Abbas et al. 2014). High-quality images of aortic root and proximal coronary tracts can be obtained without ECG gating using newer CT scanners (128 slice CT) with fast gantry rotation time and iterative reconstruction algorithm to reduce both X-ray dose and contrast media volume (Russo et al. 2015).

MDCT images are initially evaluated in axial sections, but two-dimensional and three-dimensional reformatting techniques such as multiplanar reformation (MPR), maximum intensity projection (MIP), and virtual rendering (VR) allow better diagnosis of aortic pathology. In particular to obtain correct measurement of the aortic diameters, it is necessary to use MPR images perpendicular to the centerline of the aortic lumen to avoid overestimation (Fig. 16a). Measurements of the aortic lumen on the transaxial CT images are incorrect when the central axis of the aorta is not perpendicular but oblique to the transaxial CT image (Fig. 16b).

MIP is a useful tool to display vascular map (Fig. 17) but doesn’t demonstrate the 3D relationship with nonvascular structures, while volume rendering (VR) allows to display vascular anatomy and provides, at the same time, definition of the surrounding structures (Fishman et al. 2006) (Fig. 18). It is the only way to transfer to the referring clinician the 3D perspective of the aortic aneurysm.

A standard report describing thoracic aorta should include the following measurements (Agarwal et al. 2009):

Every report of an aneurysmal thoracic aorta must include the relationships between the aneurysm and the aortic branches, to assess the possibility of endovascular repair. Minimum length of 15 mm from aortic lesion and left subclavian artery/celiac trunk and maximum aortic landing zone diameter of 40 mm without circumferential thrombus within the landing zone are essential to perform endovascular procedure (Garzón et al. 2005).