Acute aortic syndromes (AAS) represent a spectrum of disorders with a common theme of disruption in aortic integrity. AAS are associated with high morbidity and mortality and warrant emergent medical or surgical intervention as delayed treatment is associated with worse outcomes. There are multiple advanced imaging modalities for the diagnosis and complimentary assessment of AAS, each with advantages and limitations. CT angiography remains the imaging modality of choice for diagnosis in the overwhelming majority of patients as it is rapidly acquired and widely available; however, transoesophageal echocardiogram also offers excellent diagnostic accuracy in addition to complimentary data for surgical repair in those with type A dissection. Transthoracic echocardiography and magnetic resonance angiography can also be valuable in select patients. Imaging is increasingly important for risk stratification in the subacute and chronic phases of AAS. Additionally, imaging is vital for planning of interventions in both acute and delayed intervention. Endovascular treatment options are used with increasing frequency—multimodality imaging during the procedure allows for optimisation of these increasingly complex procedures.
- aortic dissection or intramural haematoma
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Acute aortic syndromes (AAS) represent a spectrum of disorders with a common theme of disruption in the integrity of the aorta and are associated with high morbidity and mortality. Aortic dissection accounts for the majority of AAS, estimated at approximately 80%, followed by (~15%) intramural haematomas (IMHs) and penetrating aortic ulcers (PAUs, ~5%).1 Aortic dissection occurs when there is a tear between the intima and media layers of the aortic wall. The intimal tear can propagate proximally, or more commonly distally. The tear results in the formation of a false and true lumen separated by the dissection flap. There are growing AAS management options. As such, there is a recognition for the need of more detailed classification that better incorporate imaging findings including chronicity, maximum aortic diameter, segments involved, presence of complications and thrombosis status of false lumen; however, no contemporary classification has become widely used and the staple Stanford classification remains most common.2 Involvement of the ascending aorta is considered Stanford type A dissection regardless of distal extension, and involvement isolated to distal to the left subclavian origin are considered type B. IMH has traditionally been thought to be secondary to rupture of the vasa vasorum with formation of a haematoma within the media.1 In some instances, it may be difficult to differentiate a true IMH from a contained/non-propagating dissection that does not have a clear intimal tear on imaging. A PAU is a progressive atherosclerotic ulceration that disrupts the internal elastic lamina and results in an outpouching and potential pseudoaneurysm formation/contained rupture or an IMH. A PAU most commonly develops in the descending, less frequently in the abdominal aorta, and is rarely in the ascending aorta.3
AAS have an estimated incidence of 6 to 15 per 100 000 which is relatively rare compared with other causes of chest pain like myocardial infarction (200 per 100 000) or venous thromboembolism (108 per 100 000).4–6 Though infrequent, AAS is high on the differential of those presenting with acute chest pain given the high associated morbidity and mortality. Estimates suggest aortic dissections represent 7% of all out-of-hospital cardiac arrests.7 8 For those who survive to hospital presentation, in-hospital mortality for type A dissections is 24% and 11% for type B dissections.9
The role of imaging in the assessment and management is multiple—not only is it essential for diagnosis but also vital in the evaluation of potential complications, risk stratification and planning of acute intervention, and determining appropriate candidacy for intervention in the subacute and chronic phases.
Imaging in the diagnosis of AAS
Rapid diagnosis after presentation is important given the benefit of early intervention for AAS. Historical data estimate a 1%–2% mortality risk/hour prior to surgery for a type A dissection.8 10 11 Given the ramifications of a missed diagnosis, imaging for those with suspected AAS is common, but the yield remains low. In one study of 205 emergency department patients referred for CT angiography (CTA) for a query of aortic dissection, only 5.9% were found to have an AAS.12
CTA, transthoracic echocardiography (TTE) and transoesophageal echocardiography (TOE), and MRI are potential options for diagnosis (table 1, figure 1). Often findings from multiple modalities can complement each other. From the mid-1990s to the early 2010s, the International Registry of Acute Aortic Dissections (IRAD) has shown a consistent increased use of CTA as the initial modality for diagnosis, from 46% to 73% in the IRAD database.
CTA has numerous advantages that have accelerated its utilisation and is the guideline recommended first-line imaging modality for suspected AAS.13 14 A CTA protocol for the assessment of AAS contains both initial non-contrast and subsequent arterial iodinated phase contrast acquisition to opacify the aortic lumen.8 Generally, the field of view should include the proximal/cranial portion of the arch vessels through the femoral artery bifurcation to allow for adequate assessment of branch artery involvement and origination from the true or false lumen. Pulsation artefact, particularly within the aortic root, can occur in CTA performed without ECG gating. Such artefacts present as a hypodense curvilinear interface or flap along the aortic wall (figure 2).15 ECG gating nearly eliminates the artefact as images are captured during only one part of the cardiac cycle, thereby minimising aortic wall motion. Improving CTA technology with wide detector systems, rapid gantry rotation and dual-source tube configurations allow for more rapid image acquisition and improve temporal resolution, also decreasing the risk of motion artefact.16 High-pitch, dual-source, non-ECG-gated CTA protocols also decrease motion artefacts.17
Though non-contrast CT offers some information to suggest an AAS including IMH, presence of an aortic aneurysm, medial displacement of calcification, or haemopericardium or haemothorax, the sensitivity is insufficient. Thus, patients at high risk for contrast-induced nephropathy should have an alternative image modality assessment.
TTE is increasingly available and often used at the bedside in emergency departments and intensive care units. The aortic root and proximal ascending aorta can often be well visualised, but imaging of the more distal ascending aorta and segments of the descending and abdominal aorta is more variable. It is most valuable in the assessment of concomitant cardiac complications (eg, aortic regurgitation, haemopericardium, myocardial ischaemia). Artefacts masquerading as a dissection flap can occur, including reverberation (see below) and side lobe artefacts. Patients with a reflective and/or calcified sinotubular junction can have an arc-like side lobe artefact within the aortic lumen that can also mimic a dissection flap.18
TOE is the preferred echocardiographic modality for the assessment of AAS as it offers superior diagnostic accuracy with similar complimentary information. TOE is still the initial diagnostic test in almost 25% of patients.9 The overall diagnostic accuracy of TOE is excellent, though false positives can occur due to linear reverberation and motion artefacts, particularly in the setting of an aortic aneurysm or calcified atherosclerotic plaque. Reverberation can be present in the aortic root originating from the anterior left atrial wall. Motion will be present that is parallel to the posterior aortic wall and is usually located twice the distance from the transducer as the posterior aortic wall. Additionally, reverberation can occur from the posterior wall of the right pulmonary artery and can appear within the lumen of the aorta. Such a finding is similarly located at twice the distance from the right pulmonary artery posterior wall. In one study, linear reverberation artefacts were seen in 55% of those without an ascending dissection;19 however, in a review, the TEE clinical false positive rate was only 3.5%.20 Reverberation artefact is best delineated from a flap with the use of M-mode in which an intima flap is less likely to have movement that is in relation to the aortic wall.18 20 Interrogation of the concerning finding with colour Doppler can aid in differentiation as artefact will not accelerate or disturb flow. Additionally, three-dimensional TOE may aid in delineation of artefact for a true dissection flap.21
Magnetic resonance imaging
MRI offers similar diagnostic characteristics as CTA with the ability to identify dissection entry tears, extent of the dissection and branch vessel involvement. MR angiography (MRA) is less pragmatic for suspected AAS with scan time of 30–60 min. Both non-contrast and gadolinium contrast image acquisitions are recommended to optimise sensitivity.22 An advantage of MRI over CT is improved evaluation of aortic valve integrity and the aortic wall. A small study suggested three MRA parameters to identify IMH from type B dissection: (1) no visualised entry tear, (2) no contrast uptake in aortic lesion on first pass angiography, and (3) no contrast uptake in the aortic lesion on equilibrium phase T1-weighted sequence. All three were present in those with IMH while those with type B dissection did not have all three features.23 MRI may also provide a concomitant inflammatory assessment with enhancement and circumferential thickening of the arterial wall suggestive of vasculitis.
Additional imaging modalities
Invasive intravascular ultrasound (IVUS) offers excellent visualisation of the arterial wall from within the lumen and has the ability to evaluate dynamic motion of a dissection flap and to determine the true and false lumens. It is rarely the primary diagnostic modality, but is an option when there is concern for an iatrogenic dissection during coronary angiography or endovascular treatment of an aneurysm. Invasive aortography was the former gold standard, but it has been largely supplanted by CTA. It is now mostly limited to the setting of an endovascular treatment. In those with presumed acute coronary syndrome, aortography can be performed if there is raised clinical suspicion for AAS following negative coronary angiography. The sensitivity has ranged from 77% to 88% and a specificity of 95%.24 25 Positron emission tomography (PET)-CT plays little role in the initial AAS diagnosis, but can be helpful when there is concern for concomitant aortitis.26
Imaging in risk stratification and management
Imaging in initial management
Type A dissection warrants emergent operative repair with markedly improved outcomes with surgery compared with medical management (18% vs 56% based on IRAD data).9 Intraoperative TOE is a necessary adjunct in type A dissection for assessment of the aortic valve integrity, and anatomic assessment of the aortic root is essential for surgical planning (figure 3, table 2). An aortic valve-sparing procedure can be performed with preservation of the native aortic valve with reimplantation favoured over remodelling of the aortic root in addition to repair of the ascending aorta (±aortic arch) with a composite graft.27 If the aortic root and proximal ascending aorta are not aneurysmal with structurally normal valves, replacement of the segment involving the entry tear may be sufficient. Aortic valve-sparing procedures in the setting of type A dissection can be challenging with high rates of adverse outcomes in less experienced centres.27 28 Whether surgeon preference or dissection characteristics, the presentation may not be suitable for a valve-sparing procedure and thus a valve replacement with a prosthetic valve within a graft is often the most appropriate repair (Bentall procedure).
In patients with a type A dissection extending into the descending aorta, surgical interventions to decrease false lumen patency may be beneficial. After primary type A repair, the false lumen in the descending aorta has reported patency rate of 70%–77% with subsequent enlargement in 50%.29 30 When distal dissection is present, some advocate for more extensive repair at initial presentation with a haemiarch or total arch replacement to decrease the rate of neurological complications and residual patent false lumen.31 Additionally, such approaches may allow for a platform for potential future thoracic endovascular repair (TEVAR) with placement of an elephant trunk extension.
Understanding coronary artery involvement affects the surgical repair and when present, is associated with worse outcomes.32 In a postmortem study of patients with type A dissection, 7% of patients had extension of the tear into a coronary artery, most commonly the right coronary artery.10 ECG-gated CTA may allow for the assessment of extension of the tear into the ostium; however, TOE, particularly with three-dimensional image acquisition may better evaluate the dynamic relationship of the flap and coronary ostium including: (1) coronary artery origination from the true lumen (2) or false lumen, (3) extension of the dissection into the coronary lumen or (4) intimal flap overlying the coronary ostium.33 One study found 9% of type A dissection patients had coronary malperfusion due to a dissection flap involving the coronary ostium (34%), flap involving the coronary artery directly (42%) or complete avulsion of the coronary artery (24%). Optimal reperfusion strategy with bypass grafting versus direct repair is uncertain and dependent on the underlying mechanism.
The majority of type B dissections do not require emergent intervention with the exception of a minority of patients presenting with impending rupture or acute ischaemia; however, patients may warrant intervention later in their hospitalisation if there is development of malperfusion, rapid aneurysm growth, persistent pain or hypertension despite medical therapy. Over 40% of patients presenting with an acute type B dissection underwent TEVAR or open repair during initial hospitalisation in the contemporary era within IRAD.9 Malperfusion can occur either by true lumen compression, decreased flow within the false lumen or extension of the flap into the branch vessel with subsequent thrombus formation, which can be well visualised on CT, MR, TOE or IVUS. Repair is most frequently performed with TEVAR, though open repair is favoured in young patients and in those with connective tissue disorders.13 It is preferred to wait beyond the acute phase after presentation with uncomplicated type B dissection (>14 days) as complications are higher early in the presentation including increased risk of retrograde dissection.
Optimal management of IMH and PAU are less clear, though treatment generally follows a similar approach to dissection and is dependent on the location of aortic involvement. Accurate assessment of PAU depth and neck size has prognostic implications, with recommendations to intervene in those with a diameter >20 mm and a depth >10 mm.8 13 26 Identification of a rupture or contained rupture (ie, pseudoaneurysm) is also vital, as such findings warrant emergent intervention.
Imaging for risk stratification and chronic management
Though short-term mortality is comparably low for uncomplicated type B dissection, complications and need for follow-up intervention are high in those with residual descending dissection after type A surgical repair or isolated type B dissection.34 Aneurysmal degeneration can occur when there is persistent flow and pressurisation within the false lumen. Utilisation of TEVAR has rapidly increased since its introduction with improved aortic remodelling and decreased aortic-related mortality when performed in the subacute setting compared with medical therapy alone in appropriately selected patients (figure 4).35 36 Imaging is important both for identification of high-risk patients and in intervention planning. CT or MR are generally adequate for evaluation of the majority of parameters that have been identified as high risk (box 1). Clinical risk scores have been created incorporating both clinical and imaging findings to stratify patients including aortic diameter >40 mm and false lumen diameter greater than true lumen diameter, and number of vessels originating from the false lumen.37 38 Though not routinely performed, PET-CT demonstrating enhanced uptake is also associated with worse outcomes in both chronic dissection and intramural haematoma.39 40
High-risk imaging features of descending aortic dissection
Large primary intimal tear (>10 mm).
One entry tear.
Intimal tear on the inner curvature.
Patent or partially thrombosed false lumen.
False lumen diameter >22 mm in proximal descending aorta or >2/3 total aortic diameter.
Descending aortic dimension >35 mm.
Distal suture line leak (in a previously repaired type A dissection).
Elliptical configuration of true lumen/round configuration of false lumen.
Fusiform dilatation of proximal descending aorta (fusiform index ≥0.64).*
Helical flow in the false lumen on magnetic resonance angiography.41
Number of vessels originating from false lumen.38
Increased fluorodeoxyglucose uptake on positron emission tomography-CT.39
Adapted from Van Bogerijien, Tolenaar JL, Rampoldi V, et al: Predictors of aortic growth in uncomplicated type B aortic dissection. J Vasc Surg 59:1134, 2014.
*Fusiform index = (maximum diameter of proximal descending aorta)/(diameter of distal aortic arch +diameter of descending aorta at level of origin of main pulmonary artery)
MRI can additionally assess flow dynamics. Phase-contrast and four-dimensional flow-sensitive MRI has demonstrated that helical flow, high volume and velocity flow in the false lumen predict risk of aneurysmal degeneration.41 MRI can also evaluate for the presence of transient branch vessel obstruction with dynamic imaging of the dissection flap and can aid endovascular treatment planning.42 Dynamic MR may be used to observe movement (or lack thereof) of a dissection membrane. Decreased flap motion suggests dissection chronicity. Such findings can help predict the likelihood of the true lumen to expand after endograft placement and of favourable postintervention remodelling.
CTA is currently the gold standard for operative planning. Postprocessing centreline reconstructions are the most accurate for determining the aortic diameter at the intended TEVAR sealing zones (figure 5). This allows selection of the appropriate endograft diameter and length. CTA also allows assessment of the access vessels to determine the most appropriate pathway for delivery of stent grafts. CTA images are often fused with live fluoroscopy to create a roadmap to assist with the procedure. This minimises the need for additional contrast during endovascular interventions and aids in deployment accuracy and decreases both procedure time and radiation dose.43
IVUS is a necessity for the treatment of aortic dissection with an endograft. IVUS offers excellent delineation of the true from the false lumen to assure the endograft is placed in the desired lumen and demonstrates the extent of true lumen expansion after endograft placement. It also aids in fenestration of a dissection membrane with real-time guidance observing crossing from one lumen into the other. Additionally, IVUS is increasingly important with the growing utilisation of fenestrated and branch stenting, and assists with appropriate graft sizing.44 Intraoperative IVUS may also reduce iodinated contrast load.
TOE can also assist in endovascular repair of a type B dissection with identification of the entry tear location, the true lumen, graft positioning and early endoleaks after graft placement. For patients with focal lesions such as PAU or IMH that may not be well visualised with intraoperative angiography alone, adjunctive CTA fusion, IVUS or TOE may be employed to be certain that the lesion is appropriately treated.
Imaging for long-term follow up
After type A dissection surgical repair, intermittent follow-up imaging is appropriate to evaluate for the presence of pseudoaneurysms at suture lines as tissue may be more fragile in the acute setting and at increased propensity for leaks. Additionally, evaluation for subsequent aneurysm formation is warranted.
Patients with descending dissection warrant repeat imaging, generally with CTA prior to discharge, at 3–6 months, and annually thereafter to evaluate for the development of high-risk features and to determine appropriateness and timing of TEVAR for select patients. In patients that do undergo TEVAR, follow-up imaging is essential to monitor for the development of complications, particularly endoleaks. Subsequent imaging can assess for favourable or unfavourable aortic remodelling response to TEVAR. Though no consensus exists, a decrease of >10% in the false lumen diameter or an increase >10% in the true lumen diameter with stable or decreased total aortic size is favourable. Volumetric assessment may be of additional value and decreased variability, but is time intensive.45 Imaging may be able to identify an aetiology to unfavourable remodelling postrepair including unsealed landing zone of stent graft, uncovered primary entry site, defective stent graft, arch vessel leak or distal refill from intercostal arteries. MRA is useful for long-term follow-up in some patients who are relatively young and will need frequent lifelong surveillance imaging to minimise cumulative radiation with CT.
There are multiple high-quality options for the initial evaluation and diagnosis of AAS that can be tailored for a specific patients’ requirements and aid in early surgical planning and long-term monitoring. The future of imaging in AAS will focus on further characterising patients at highest risk for complications to aid in appropriate selection for intervention in the subacute and chronic phases. Additional study is required to incorporate such imaging findings with clinical features to optimise patient selection for intervention. Imaging advances will also allow for more precise application of the increasing complex endovascular treatment options.
Contributors BJC, MLS and WJM were involved in the drafting, writing and revision of this review.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests MLS has received consulting fees from Cook Medical, Medtronic and Endologix.
Patient consent for publication Not required.
Provenance and peer review Not commissioned; externally peer reviewed.
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