Comparison of imaging modalities for diagnosis and assessment of AAS
| CTA | TOE | TTE | MRI | |
| General advantages | Wide availability. Complete evaluation of thoracic aorta. Rapid acquisition. High resolution. Reproducible between institutions. Assessment of spectrum of AAS. | Assessment of aorta in near field. Performed at bedside. No contrast or radiation. Evaluate alternative cardiac causes of hypotension, aortic valve integrity and cardiac function. | Available at the bedside. No contrast or radiation. No sedation. Assessment of alternative cardiac causes of hypotension, aortic valve integrity and cardiac function. | No radiation exposure. Excellent evaluation of aortic wall. Dynamic assessment of flow. |
| Limitations | Radiation exposure (can be decreased with ECG gating). Risk of iodinated contrast-induced nephropathy. | ‘Blind spot’ in distal ascending aorta.13
Suboptimal assessment of branch vessels. Requires sedation. Operator dependent. | Aortic arch and descending aorta poorly visualised. Operator dependent. | Prolonged scan time. Risk of nephrogenic systemic fibrosis with GFR <30 mL/min/1.73 m. Possible long-term effects of gadolinium retention. Poor assessment of arterial wall calcification important for planning cross-clamp location for open surgery. |
| Sensitivity20 24 46* | 100% | 98% | 78%–90% (type A); 31%–55% (type B) | 98% |
| Specificity* | 98%–99% | 95% | 87%–96% (type A); 60%–83% (type B) | 98% |
| Detection of intimal tear/false lumen | Excellent—false lumen tends to be larger and crescent shape, darker relative to true lumen in arterial phase. Thrombus may be present in false lumen.47 | Visualisation of tear and evaluation of flow within true/false lumen, but with lower sensitivity than CTA. | Visualisation of tear and evaluation of flow within true/false lumen, but with lower sensitivity than with CTA and generally limited to the proximal ascending aorta. | Excellent —similar to CTA with additional evaluation flow dynamics and mobility of flap to assess chronicity.42 |
| Artefacts | Pulsation artefact in ascending aorta with ‘pseudoflap’. ECG gating essentially eliminates such artefacts. High-pitch, dual-source CTA also limits motion artefact. | Reverberation, mirror, and side lobe artefacts. 3D assessment may aid in evaluating potential artefact.21 Contrast-enhancement can improve diagnostic quality.48 | Reverberation, mirror, and side lobe artefacts. Side lobe artefact from reflective, calcified sinotubular junction. Artefacts do not alter velocity or colour Doppler findings. Contrast-enhancement can improve diagnostic quality.48 | Chemical shift artefact between aortic wall and surrounding fat. Signal loss within endovascular stent due lack of radiofrequency penetration. |
| Detection of PAU | Preferred modality. Contrast filling of an outpouching with irregular walls and associated atheroma – differentiated from atheromatous ulcer by presence of saccular disruption of outer aortic wall. | Outpouching with irregular edges and colour flow within the sac can be visualised. Inferior assessment of extraluminal abnormalities (eg, pseudoaneurysm). | Outpouching with irregular edges and colour flow within the sac can be visualised, but relatively rare in the ascending aorta. | Excellent detection of associated IMH or differentiating from IMH, but CTA preferred.14 |
| Detection of IMH | Contrast and non-contrast acquisition delineate IMH from non-calcified plaque or intraluminal thrombus. IMH smooth, crescent shaped and hyperdense on non-contrast CT. | Sub-optimal to CTA and MRI. Circular or crescentic thickening of the aortic wall >5 mm, displacement of intimal calcification.20 | Sub-optimal to CTA and MRI. Circular or crescentic thickening of the aortic wall >5 mm, displacement of intimal calcification. | T2-weighted images have hyperintense signal within 24 hours of occurrence with T1-weighted sequences demonstrating an isointense signal. After 24 hours, both T1- and T2-weighted images are hyperintense.48 |
| Aortic regurgitation | Unable to evaluate. | Preferred modality (see table 2). | Adequate assessment for many, but TOE superior in evaluating mechanism. | Can quantify severity if appropriate protocol, but decreased assessment of mechanism compared with echocardiography. |
| Coronary artery involvement | Limited to proximal portions, though improved with ECG-gating. | Limited to proximal assessment with optimal acquisition. | Coronary arteries poorly visualised; evaluation of left ventricular focal wall motion abnormalities can suggest ischaemia. | Proximal assessment if dedicated cardiac MRI. |
| Pericardial effusion/haemopericardium | Evaluate presence and size but not haemodynamic effect/tamponade. | Excellent for evaluation of presence effusion and tamponade. | Excellent for evaluation of presence and presence of tamponade. | Evaluate presence and size but not haemodynamic effect unless dedicated cardiac MRI. |
| Branch vessel involvement | Determination of branches originating from false lumen and extension into branch. | Assess proximal branch involvement, but inferior to MR/CT. | Minimal assessment and limited to aortic arch vessels. | Similar to CT with additional dynamic assessment of transient branch vessel occlusion and flow dynamics within branch vessel. |
Adapted from Goldstein, et al 14 and Isselbacher, EM. Circulation 2005;111:816–28.
*May not be representative of real-world findings as studies included in the meta-analysis were from tertiary centres.
AAS, acute aortic syndromes; CTA, CT angiography;3D, three dimensional; GFR, glomerular filtration rate; IMH, intramural haematoma;MR, magnetic resonance; MRA, magnetic resonance angiography; PAU, penetrating aortic ulcer; TOE, transoesophageal echocardiogram; TTE, transthoracic echocardiogram.