Imaging of the aortic root on high-pitch non-gated and ECG-gated CT: awareness is the key!

An aortic aneurysm is defined as an abnormal permanent dilatation of the aorta to greater than 1.5 times its expected normal diameter [2]. Among all thoracic aortic aneurysms, 60% comprises of the aortic root, ascending aorta, or both [25]. True aneurysms involve all the layers of vessel wall while a false aneurysm/pseudoaneurysm represent a disruption of layers of the wall of the aorta with containment of extravasated blood by surrounding tissues forming a pseudo-capsule [26]. Based on the normal variation in the size of the proximal aorta (described in “Aortic anatomy—aortic root and ascending aorta” section), the American College of Radiology (ACR) white paper on management of incidentals on thoracic CT suggest a size of 5 cm as the cutoff for a proximal aortic aneurysm [27]. When using such approach of a cutoff number, it should be kept in mind that aortic diameter is a factor of sex, age, patient size, and the segment of the aorta [13]. If the maximum aortic diameter is between the upper limits of normal (SOV, STJ: 4 cm, 3.6 cm for males and 3.6, 3.2 cm for females [23, 24]) and not meeting the criteria for aneurysm, the abnormal segment of the aorta should be reported as dilated [27]. For increased accuracy, the maximum aortic diameter can be indexed to body surface area. For proximal aorta, the value of 2.1 cm/m² as an upper limit of normal and a value of 2.75 cm/m² as a cutoff for aneurysm has been described to have specificity of > 95% [14, 23]. The presence of connective tissue disease can be an important determinant in diagnosis and management of an aortic aneurysm. As per American Heart Association and European Society of Cardiology guidelines [13, 14], patients with known connective tissue disease should be treated early at smaller aortic diameters because of increased risk of aortic dissection and rupture. In patients without any aortopathy, guidelines suggest a cutoff of ≥ 5.5 cm. For patients with bicuspid aortic valve or Marfan syndrome, a cutoff for treatment is between 4.5 to 5.5 cm, depending on risk factors and clinical context [2, 13, 14]. Loeys-Dietz syndrome patients either follow similar criteria as Marfan syndrome patients or are treated at a diameter ≥ 4.2 cm, given lack of consensus [2, 13, 25].

Within the aortic root, aneurysms may be seen isolated at the SOV or involving the entire aortic root. Combined involvement of aortic annulus, SOV, and sinotubular junction, also known as annuloaortic ectasia, is characteristic for connective tissue disorders like Marfan syndrome and Ehlers-Danlos syndrome (Fig. 5) [15]. On the other hand, isolated SOV aneurysm is mostly congenital and less commonly associated with infection, atherosclerosis, degenerative disease, or trauma [28]. Congenital SOV aneurysm is seen in approximately 0.1% of the population, is common in Asians, and is most commonly seen at the right aortic sinus followed by the noncoronary sinus (Fig. 6) [15]. SOV aneurysm can also be associated with other congenital heart diseases like ventricular septal defect (30–60%), aortic insufficiency (20–30%), bicuspid aortic valve (10%), aortic stenosis, infundibular pulmonary stenosis, patent ductus arteriosus, left ventricular non-compaction, atrial septal defect, coronary artery anomalies, and persistent left-sided superior vena cava [15]. Commonly, these aneurysms are diagnosed incidentally. However, symptoms may be related to aneurysm rupture or mass effect on the adjacent structures [28]. The rupture most commonly happens into the right ventricle and the right atrium. Other less common sites include right ventricular outflow tract, left ventricle, interventricular septum, or left atrium [15, 28]. The rupture into the pericardium is very rare but has high mortality [29]. The other sites of rupture have relatively less mortality, with mean survival after diagnosis being 3.9 years [28]. The treatment and follow-up for unruptured SOV aneurysm are frequently debated. It is suggested that unruptured SOV aneurysms should be anticoagulated and followed up every 6 months [30].

Blunt thoracic trauma (related to motor vehicle accidents, falls, and sports injuries), post-surgical, and infection are the most common cause of the pseudoaneurysms of the heart or the thoracic aorta [31]. Pseudoaneurysms can be complicated with fatal rupture, fistula formation, and compression of surrounding structures (Fig. 7). Overt free rupture can occur due to complete disruption of all of three layers of the aortic wall leading to massive hematoma with resultant hemodynamic instability. Patients with an aortic pseudoaneurysm are characterized on imaging with perivascular hematoma sealed off by periaortic structures like mediastinum, pleura, or pericardium. Non-contrast scan is helpful to identify areas of contrast enhancement and to differentiate pseudoaneurysm from calcifications and prior surgical changes. Management of aortic pseudoaneurysm involves either endovascular intervention or open surgical repair and is independent of its size [14]. Rarely, cardiac pseudoaneurysm may be managed medically with serial imaging surveillance [31].

An AD is characterized by an intimomedial tear of the aortic wall with subsequent separation of the layers. Dissections most commonly arise in the ascending aorta 1 cm distal to the sinotubular junction or in the descending aorta at or just beyond the isthmus of thoracic aorta because of maximum wall shear stress [32]. Spontaneous dissections that originate in the aortic root are rare. Most AD with aortic root involvement is due to retrograde dissection from the ascending aorta which increases chances of rupture into the pericardial space causing cardiac tamponade, dissect into coronary artery origin, or create aortic valvular regurgitation [33]. These complications are life-threatening and therefore warrant urgent surgical repair. The involvement of the origin of coronary arteries can lead to ischemia from extension of the dissection into the ostia or by narrowing from the intimomedial flap within the aorta without extension into the coronary artery. The right coronary artery is most commonly affected [32].

CTA with ECG synchronization is the standard of care for the diagnosis of AD with very high sensitivity and specificity [12, 34]. However, the recent British Society of Cardiovascular Imaging/British Society of Cardiovascular CT guidelines mention that based on scanner capabilities, motion-free CT imaging without ECG synchronization on newer scanners may be appropriate for suspected acute aortic syndrome (AAS) [35]. On non-contrast or inappropriately timed contrast-enhanced CT, diagnosis of AD may be challenging (Fig. 8a). On non-ECG-gated CT/MRI images, complications of ascending aortic dissection (such as extension of dissection into the coronaries, or rupture into pericardial space) can be missed due to motion artifact, and ECG-gated images or high-pitch CTA (if scanner is capable) should be obtained whenever there is high suspicion (Fig. 8b, c).

Aortic IMH is pathologically characterized by a hematoma in the media of the aortic wall with an absence of a well-defined enhancing false lumen. Initially, these were thought to be from rupture of vasa vasorum but increasingly small intimal ulcer like projections are being recognized which is proposed to lead to hematoma within the wall [36]. As per the analysis of the International Registry of Acute Aortic Dissection [37], the clinical presentation and prognosis of type A (aortic root and ascending aortic) IMH is similar to type A AD. However, type A IMH patients were more likely to have periaortic hematoma and pericardial effusion which are important to recognize on imaging.

Non-contrast CT is very helpful in patients with IMH as high attenuation of wall hematoma is characteristic with an absence of intimal flap or enhancement on contrast-enhanced CT (Fig. 9). MRI using vessel wall imaging, black blood, and cine gradient echo sequences may be used as a problem-solving tool to differentiate IMH from atherosclerotic wall thickening and thrombus and to characterize the age of IMH [12, 36]. The complications and management of IMH is similar to AD.

Most commonly, proximal aortic infections are periannular and supraannular extensions from aortic valve endocarditis. Sometimes, aortic root infection may be incidentally seen in patients getting CT for fever or sepsis evaluation. Fat stranding around the proximal aorta, obliteration of pericardial and mediastinal fat, development of soft tissue attenuation at the aortic root (especially between the aortic wall and the left atrium, Fig. 10), and a frank fluid collection with enhancing rim are often the imaging features of a perivalvular extension of the infection. The complications include an extension to the mitral valve via aortomitral intervalvular fibrosa, thrombosis, or narrowing of the coronary artery (Fig. 10), fistulous connections with the adjacent cardiac chambers, or rupture [38‐40]. Aortic root involvement can also be seen as wall thickening or a pseudoaneurysm.

Imaging plays an important role in the diagnosis of these infections and is very helpful for pre-surgical planning. With an improved spatial and temporal resolution of CT, even valve vegetations can sometimes be recognized [41]. Although transthoracic and transesophageal echocardiograms are often the first imaging modalities in the diagnosis of infective endocarditis, ECG-gated CTA provides a comparatively better anatomic assessment in regards to the extension of periannular abscess into adjacent cardiac valves, myocardium, coronary arteries, and pericardial space which has been highlighted in the recent studies and guidelines [42‐45].

Inflammation of the aortic wall can be either infectious or non-infectious. Both infectious and non-infectious etiologies can involve proximal aorta. Among the inflammatory etiology, aortic involvement is classically seen with large vessel vasculitis most commonly giant cell arteritis (GCA) and Takayasu’s arteritis (TA). Chronic infectious causes of aortitis, including HIV, tuberculosis, and syphilis, demonstrate a non-specific imaging appearance of aneurysm formation with or without wall thickening which in the absence of other supporting imaging findings cannot be reliably distinguished based on their imaging appearance [46, 47]. Other etiologies of aortic root aortitis include radiation-induced vasculitis in patients with prior therapeutic radiation to the chest wall and are often encountered in the form of accelerated wall calcifications [48]. Within large vessel vasculitides, GCA is more likely to involve the aortic root and ascending aorta [49] while TA most commonly involves abdominal aorta followed by descending thoracic aorta and aortic arch [50, 51].

On imaging, it typically manifests as wall thickening and enhancement. On CT, the wall thickening can mimic intramural hematoma especially in the absence of non-contrast images; the differentiation of which is crucial. The inflammatory wall thickening is not hyperdense (< 40 HU) on non-contrast images and reveals enhancement on post-contrast and delayed images as compared to non-enhancing hyperdense (> 50 HU) wall thickening of IMH [52]. The pattern is also important; circumferential thickening is a feature of inflammatory aortitis while incomplete crescentic thickening is a feature of IMH. If confusion persists, T1-weighted black blood double inversion recovery images before and after contrast enhancement [53] or FDG-PET (Fig. 11) can be used as a problem-solving tool [54, 55]. Aortitis with coronary involvement can present as ostia narrowing leading to their ischemia or diffuse wall thickening. On imaging, the sequelae of aortitis include calcifications, aneurysm, pseudoaneurysm, and thrombosis [56].

Tumors and tumor-like conditions adjacent to the proximal aorta can mimic aortic root pathologies and may have overlapping clinical characteristics. Lymphomas, in particular, diffuse large B cell type, and melanoma have a predilection to pericardial space while the metastatic tumor deposits from these primary malignancies can be encountered at the aortic root in the form of enhancing mass like nodularity with associated pericardial effusion [57]. Mediastinal lymphoma can be seen as isodense to hyperdense thickening on CT with homogenous modest enhancement on post-contrast images [58]. Aortopulmonary window paragangliomas adjacent to the aortic root arise from the parasympathetic system and are rare tumors along the proximal aorta. They are usually benign, arterially enhancing homogenous tumors (Fig. 12). They can recruit vascular supply from coronaries arteries and maybe in close relation to the aortic root [59]. Their intense vascularity should warrant against percutaneous biopsy. These can be differentiated from true aortic root pathology based on maintenance of fat plane with aortic wall while confirmation of the diagnosis can reliably be obtained with an octreotide or ¹³¹I-metaiodobenzylguanidine scan [60].

Non-neoplastic aortic root pathologies include aortic valve pannus of the prosthetic valve and leaflet thrombus. Pannus is a diffuse or mass-forming inflammatory process often originating from a suture [61]. The fibrotic proliferation can often occur on the ventricular side and appear as a focal round hypodensity [62]. The distinction of pannus from thrombus may be difficult but is important as both the entities have different therapeutic approaches. The pannus characteristically occurs after 12 months, is underneath the valve surface extending from the sewing ring or the metal ring, may show contrast enhancement, and has CT attenuation of > 145 Hounsfield units (HU) (Fig. 13). The thrombus, on the other hand, can occur at any time, can be above or below the aortic prosthetic aortic leaflets, does not enhance, and has an attenuation of < 145 HU (Fig. 14) [63]. Thrombus with an attenuation of < 90 HU is associated with the higher success of lysis [64]. A mimic of aortic thickening and mediastinal lymph node is the superior aortic recess. It is the most cephalad portion of the transverse pericardial sinus. Fluid attenuation on non-contrast CT images, absence of contrast enhancement, and characteristic location (Fig. 15) helps to differentiate this entity from other pathologies [65].

A detail description of various aortic surgeries and their postoperative appearances is beyond the scope of this article and is well described in literature dedicated to this topic [66]. We discuss some expected post-surgical appearance of the proximal aorta that general radiologists may encounter and should be familiar with, as it may mimic abnormal conditions. Evaluation of post-operative thoracic aorta is typically performed with unenhanced and arterial phase CTA study. Knowledge of pertinent clinical information, including surgical notes, is important to recognize and differentiate normal from abnormal post-surgical appearances.

Felt pledgets or sutures used during surgery can be mistaken for small pseudoaneurysm on a contrast-enhanced study. CT imaging can differentiate these entities as felt appears hyperdense to blood pool on unenhanced CT (Fig. 16) while pseudoaneurysm appears isodense to the blood, and felt sutures are typically symmetric at the anastomosis. If incidentally seen in a patient with chest pain, comparison with prior exam or a repeat ECG-gated CT angiography (with non-contrast images) should be obtained for differentiation as management of these entities is different.

The cardiopulmonary bypass cannula required during various cardiothoracic surgical procedures is typically placed at the ascending aorta. An arterial perfusion cannula may also be placed through a graft side branch (if graft repair of ascending aorta is being performed) to allow antegrade systemic perfusion during the surgery till distal anastomosis is complete [67]. The bypass cannula site may appear as an outpouching at CT (Fig. 17), thereby mimicking a pseudoaneurysm or leak. Correlation with the surgical report or directly with the surgeon and correlation with patient symptoms is critical to avoid this confusion. On CT images, it appears as a well-defined, broad-based, outpouching of contrast beyond the normal contour of the aortic wall and in direct communication with the aortic lumen (Fig. 17) [60].

Infections related to synthetic materials (e.g., suture) or concomitant mediastinal infection may be seen on CT imaging as the following: (a) focal saccular outpouching (pseudoaneurysm), usually with a narrow neck, that contains contrast material and arises from the aortic wall (Fig. 10); (b) periaortic soft tissue stranding or edema; and (c) periaortic gas [66, 67]. An aortic pseudoaneurysm close to the aortic cannulation site may be difficult to differentiate but is a surgical emergency with associated high operative morbidity and mortality, given the high likelihood of a coexistent infectious state. However, it is critical to differentiate pseudoaneurysm from aortic cannulation because of difference in management. Pseudoaneurysm shows irregular outline and is associated with a surrounding hematoma (Fig. 18). Also, it is uncommon to develop a pseudoaneurysm in the middle of a surgical graft as they most commonly occur at the anastomotic suture lines.