The development of percutaneous treatments for aortic disease has led to a need for more precise definition of the aorta and a better understanding of three-dimensional (3D) anatomy. Both CT angiography and MR angiography can produce good 3D images of the aorta, although CT is more usable for this application at present. A wider understanding of the capabilities of CT angiography (CTA) and MR angiography (MRA) and choosing the best protocols will further expand the use of these techniques and replace conventional angiography or digital subtraction angiography (DSA).
The diagnosis of aortic aneurysms and aortic dissection has been revolutionized by developments in cross-sectional imaging. Traditionally investigated by contrast angiography, the last two decades have seen considerable developments in the diagnosis of aortic disease by echocardiography, CT, and MRI. These developments have led to a complete change in the way in which aorta disease is evaluated and have contributed to an improvement in treatment outcome (1). The purpose of this review is to discuss the current techniques and applications of CT and MRI for imaging aorta aneurysms and aortic dissection. It should be remembered, however, that the relative merits of the different available techniques do not apply in all situations.
Local differences in the quality of equipment, skill of interpretation, and choice of appropriate protocols can make a substantial difference in the accuracy of each imaging technique. For instance, echocardiography is heavily operator dependent, whereas CTA, providing that the correct protocol is used, although relatively operator independent, is subject to errors of interpretation by the unskilled. There is more variation in the conduct of MRI and MRA. Although less operator skill is required to perform high-quality MRI than high quality echocardiography, it is still more critical than with CT. Skill and experience in vascular image interpretation as well as an understanding of the range of aortic pathologic conditions and the therapeutic options is crucial. Although this should be self-evident, all too often it seems that patients with aortic disease are evaluated with inappropriate protocols, and images are interpreted by those who have a poor understanding of aortic pathology. The appropriate choice of technique may be dependent also on availability of the technique at the time of patient presentation. Frequently, patients with aortic disease present acutely, and operators to perform the best possible diagnostic technique may not be immediately available at the appropriate time. It therefore may be that clinical urgency dictates that a less optimal imaging technique is required in these very sick patients.
Similar considerations apply when comparing the relative performance of imaging techniques reported in the scientific literature. It is relatively rare for the best quality techniques to be compared with each other. Most published studies report the experience of experts with one imaging technique, who compare their performance with the performance of less expert operators performing less than optimal comparative techniques. One should be aware of the limitations of these studies and understand how little some studies may be applicable to possible local practice. Although one technique may be useful for establishing a diagnosis, it may not provide all of the information concerning the extent of aortic disease needed to make management decisions. For instance, transesophageal echocardiography usually can make the diagnosis of aortic dissection. However, it is frequently unsatisfactory for showing the full extent of the dissection and documenting branch vessel involvement. The same applies to conventional spin echo MRI, which accurately shows aortic dissection, aneurysms, and aortic dimensions but is of limited use in showing branch vessel involvement or similar complications.
Those performing the imaging must be fully aware of the differential diagnosis of aortic disease and also be aware of the adjunct information required to determine appropriate therapy. Merely diagnosing an aortic aneurysm or dissection is not enough. It is necessary to demonstrate the full extent of the dissection or aneurysm, to document dimensions accurately, to assess branch vessel involvement, and to relate the level of the neck of the aneurysm to major branch vessels such as the renal arteries. In addition, hemopericardium (Fig. 1) and any evidence of old or recent rupture also should be assessed. With the increasing availability of percutaneous methods of repair, assessment of suitability for this along with appropriate measurements to plan percutaneous repair is required.
AVAILABLE TECHNIQUES FOR DIAGNOSIS OF AORTIC ANEURYSM AND AORTIC DISSECTION
Although echocardiography is not a major part of this discussion, it is important to review its role relative to CT and MRI. Both transthoracic echocardiography (TTE) and transesophageal echocardiography (TEE) have an important role in aortic imaging. Echocardiography is widely available, noninvasive (TTE) or minimally invasive (TEE), and relatively inexpensive. Although most patients with acute thoracic aortic disease should be evaluated by using TEE, the performance of TTE in skilled hands is often adequate for extensive evaluation of aortic disease. Transthoracic echocardiography can usually accurately visualize the aortic root, segments of the aortic arch, the descending aorta posterior to the left atrium, as well as the abdominal aorta. Transthoracic echocardiography accurately shows aortic dimensions, is sensitive for detecting aortic dissection close to the aortic root, and is very suitable for assessing cardiac complications such as aortic regurgitation, impaired left ventricular function, and hemopericardium. The limitations of TTE imposed by poor acoustic access are a particular problem in sick patients, patients who are ventilated or are unable to cooperate with appropriate positioning.
TTE is less than ideal for showing the full extent of the aortic arch and evaluating the origins of the head and neck vessels. In situations in which TTE is unsatisfactory, it is usually easy to convert the study to a TEE approach. Alternatively, a combination of TTE and CT provides a high degree of accuracy (2,3). In skilled hands, TEE is quick and safe. The accuracy of the best TEE is similar to the best quality CTA or MRI/MRA (4,5). TEE can accurately detect aortic dissection, evaluate aortic root complications, identify entry sites in the intimal flap, and assess aortic dimensions. Current multiplanar TEE transducers have few blind areas (i.e., posterior to the trachea) that limit evaluation. The main limitation of TEE is its inability to evaluate the infradiaphragmatic aorta, but this can usually be done by use of transabdominal ultrasound.
Computed Tomography and CTA
In many institutions, CT/CTA is the investigation of choice for evaluating the aorta. With use of spiral acquisition, particularly multidetector arrays, very accurate imaging of the aorta is possible with reasonable doses of iodinated contrast medium (6). Studies can be conducted quickly and usually at any time of day or night. Limitations on contrast use because of renal insufficiency or a history of allergy usually can be dealt with, particularly when there is an urgent need to make a definitive diagnosis. CT/CTA is not subject to the limitations imposed on TTE by limited acoustic access. CTA accurately shows branch vessel anatomy as well as the configuration of complex aortic aneurysms and dissections, provided appropriate protocols and reconstruction methods are used (Fig. 2). With older machines, limitations on tube heat capacity limit the area of anatomic coverage, especially when using thin slices. Newer CT scanners, especially those that use multidetector arrays, can cover a larger anatomic area with good spatial resolution. CT is the best method for showing calcification in the aortic wall when this is relevant to surgical planning. In many situations, CT is the most appropriate method for the urgent evaluation of patients with acute aortic syndromes (7). In trauma patients, CT is useful for predicting the presence of aortic rupture and may help to determine which patients require further evaluation by aortography (8,9). Neither MRI nor TTE are suitable for this function, although there may be place for TEE in difficult cases.
Some studies determining the value of CT for evaluating aortic disease used equipment that no longer represent the state-of-the-art, or they used inappropriate protocols. The best current CT/CTA should have an accuracy approaching 100% in assessing the size of the aorta, detecting dissection, and showing branch vessel anatomy, provided that motion artifacts related to aortic root pulsation (Fig. 2) seen in up to 57% of studies are recognized (10). Older spiral CT scanners should have greater than 95% accuracy in this application, but they require conscientious technique for optimum sensitivity and accuracy (11). Conventional CT scanners without spiral acquisition produce images over a period of several seconds and yield less satisfactory results. For instance, there is a greater risk of missing rapidly moving dissection flaps while using incremental scanners (2,12). Frequently, with older CT scanners, it is necessary to perform another diagnostic test (such as echocardiography) to ensure sufficient diagnostic accuracy (2,3). Measurements of aortic dimensions by CT should be accurate and reproducible (13).
In the abdomen, an important consideration is not only detecting major branch vessels but also defining other vascular abnormalities that may affect aortic surgery, such as accessory renal arteries or renal artery stenosis. In this respect, CTA seems to be as accurate as DSA, provided a thin slice (2–3 mm) protocol is used (14–16). Spiral CT is also important in the assessment of the aneurysm neck before surgery or stent grafting. Precise definition of the aneurysm neck relative to the renal arteries (for abdominal aortic aneurysm) or the head and neck vessels (for thoracic aneurysms) is best achieved with thin slice CTA and review of both the 3D images and two-dimensional source images (Fig. 3). This is especially important before stent-grafting or other interventions (17). Detection of inflammatory changes also may be important, especially in determining surgical approach. In this respect, CT is inferior to MRI but superior to ultrasound (18). However, CT may be superior to MRI in defining bony changes that complicate aortic infection (Fig. 4).
After surgery or stent grafting, CTA is probably the most useful routine method for detecting complications such as false aneurysms, leaks, infection, and stent deformity (19,20). For patients with stent-grafts, a biphasic acquisition protocol may be needed to image small, late-appearing leaks (21). Ultrasound may be limited by acoustic shadowing, and MRA can be somewhat limited by the effects of peristalsis, limited spatial resolution for leaks, and metal artifact from some surgical clips and some stent-grafts.
Magnetic Resonance Imaging
The imaging of aortic disease was one of the earliest accepted cardiovascular applications for MRI. Even with early electrocardiogram (ECG)-gated spin-echo sequences, it was possible to determine aortic dimensions accurately and to detect dissections and intraluminal thrombus. If this were the only information required, ECG-gated spin-echo MRI would have near 100% accuracy (5). This technique remains the basis for many MRI imaging algorithms for diagnosing thoracic and abdominal aortic disease (22). More recent developments such as rapid breath hold MRI (Fig. 5) and breath hold MRA (Fig. 6) allow faster and more comprehensive examination of the aorta (23). This has overcome most if not all of the reservations about the limitations of spin-echo MRI for aortic imaging (24). Both fast MRI and non-contrast breath hold MRA accurately define many types of aortic pathology (23,25).
The rapid breath hold MRI techniques now available include variations on a theme of turbo-spin echo and half Fourier single shot turbo spin echo (HASTE) (Fig. 5). Although these may be less susceptible to respiratory artifact than conventional spin-echo MRI (26) limited data are available regarding the reliability of these techniques for showing smaller abnormalities, and some produce too many artifacts for routine use (23). Breath-hold MRI with contrast enhancement may be useful (27) but has been superseded by 3D MRA techniques. Although contrast-enhanced 3D MRA can be implemented with single-dose contrast at 1 T, the best images are obtained with double-or triple-dose regimes at 1.5 T (28–30).
Conventional MRA without contrast enhancement is excellent for showing the size of aortic aneurysms and detecting dissections, and it is superior to spin-echo MRI for relating the position of the aortic abnormalities to major branch vessels (23). Both ECG-gated two-dimensional time-of-flight (TOF) and phase contrast techniques are useful. Phase contrast techniques have been less widely used but are useful for identifying flow in false channels and measuring flow (31). The greater robustness and wider experience with TOF allow it to be used with a high degree of confidence for assessing aneurysms, dissections and, in its cine form, flow and cardiac function (23,32). Non-contrast MRA can be time consuming, may produce images that are diagnostic but unconvincing to the nonradiologist, and often is poor at showing branch vessel anatomy. For these reasons, breath hold 3D contrast-enhanced MRA has tended to replace TOF MRA (Fig. 7) and has further strengthened the capability of MRI to evaluate the aorta (33–35). Demonstration of small branches such as accessory renal, distal parts of larger branches such as the carotid arteries, and involvement of branch vessel origins by dissection (Fig. 8) is substantially improved with contrast-enhanced 3D MRA (36, 37). Breath-hold 3D MRA produces better image quality than non–breath-hold 3D MRA (36).
Contrast-enhanced MRA is less prone than unenhanced MRA to the susceptibility effects from metal in stent-grafts (Fig. 9) or adjacent to surgical grafts. In many complicated cases, contrast-enhanced 3D MRA can be the best imaging investigation and clarifies the findings of angiography or CTA when these are inconclusive (Fig. 10). Although contrast-enhanced MRA is probably less robust than CTA for evaluation of stent-grafts (38), it seems to have sufficient accuracy for routine use when conventional contrast agents should be avoided (39).
In many institutions, breath-hold contrast-enhanced 3D MRA has become the imaging investigation of choice for patients with stable aortic disease. The great increase in contrast that gadolinium enhancement provides when compared with unenhanced MRA allows faster imaging with improved spatial resolution (typically nearly isometric voxels smaller than 2 mm3) and reduced movement or susceptibility artifacts. These advantages apply to both thoracic and abdominal aortic imaging. When evaluating abdominal aortic aneurysms, both contrast-enhanced 3D and two-dimensional TOF MRA are accurate for evaluating the aneurysm neck but are currently inferior to CTA for evaluating accessory renal arteries (26,39).
Although patients with acute aortic problems can be safely evaluated in MRI scanners (5), most MRI units still refer these patients to CT and limit aortic imaging to patients with stable pathology. In this patient population, the accuracy of contrast-enhanced MRA is essentially the same as for conventional contrast angiography. For high-risk patients in whom iodinated contrast should not be used for angiography, and carbon dioxide has to be used as a contrast agent, the quality of contrast-enhanced 3D MRA is superior to DSA (Fig. 11). Which investigation is more appropriate depends on the relative availability and quality of contrast angiography, MRA, and, for that matter, CTA. One advantage of MRA over CTA is the ability to evaluate complications such as hemopericardium, left ventricular dysfunction, and valve regurgitation. All of these can be evaluated at the time of contrast-enhanced MRA, ideally before giving contrast.
The advantages of MRI for vascular diagnosis are well known. They include safety, noninvasive nature (except for giving intravenous contrast), a wide field-of-view unimpaired by acoustic shadowing or beam-hardening artifact, multiplanar imaging, and an ability to show complicated 3D relationships. The latter is not possible with conventional angiography and is difficult to do with echocardiography. MRI has a significant advantage when compared with CT in that it does not require large volumes of iodinated contrast (a benefit also compared with angiography). Cine MRA can provide all of the functional information provided by echocardiography.
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Editor: Jeffrey S. Klein
Associate Editors: Ann Leung, MD
David Lynch, MD, Jung-Gi Im, MD
Michio Kono, MD, Charles White, MD
Guest Editor: William Stanford, M.D.