Abdominal Aortic Aneurysm Imaging 

Approximately 40,000 patients undergo aneurysmorrhaphy each year. In the United States, 15,000 deaths per year are attributed to abdominal aortic aneurysms (AAAs).

Since 1951, when Dubost first performed repair of an AAA with a homograft, surgery has been the mainstay of treatment. Many refinements in technique have occurred during the interval, but none as significant as the stent-graft.

In 1991, Parodi et al described a novel, less invasive technique for repairing AAAs by placing a graft from within the vessel. This technique was labor intensive and involved the customized construction of the graft for each patient by sewing the graft material to a self-expanding metal skeleton. Today, designs are approved by the US Food and Drug Administration (FDA), and numerous devices are being used in clinical investigations.^{[1, 2, 3, 4, 5]}

The radiologic characteristics of AAAs are demonstrated in the images below.

Preferred examination

When an examination, especially a plain radiograph, is ordered for a reason other than the evaluation of AAA, curvilinear calcifications should be carefully assessed, because most AAAs are asymptomatic. When they are discovered, the referring clinicians should be notified of the abnormal and unexpected findings. In some cases, a referring clinician might be reminded of the need for appropriate follow-up and the time interval.

Because of portability, lack of ionizing radiation, cost, and availability, ultrasonography (US) should be the initial imaging modality when an asymptomatic, pulsatile abdominal mass is palpated.

If the aneurysm is approaching 5 cm or more or if rapid enlargement is seen on serial US images, a computed tomography (CT) or CT angiography (CTA) scan should be ordered to better delineate the extent of disease prior to conventional surgery or treatment with the insertion of an endovascular graft. In patients whose renal function does not permit the administration of iodinated contrast material, magnetic resonance imaging (MRI) and magnetic resonance angiography (MRA) provide good alternatives.

Angiographic examination may be requested because of a clinical concern that concomitant renal artery stenosis or peripheral vascular disease may require surgical intervention during repair of AAAs. At some institutions, CTA and MRA have replaced routine diagnostic angiography in the preoperative evaluation of AAA.

In urgent situations in which the clinical diagnosis is fairly certain or rupture is imminent or suspected and in which the patient's condition is stable, CT and/or CTA may be the initial and only examination required.

Imaging of the aorta does not end with the repair of the aneurysm. After repair with either a traditional open surgical procedure or an endovascular procedure, follow-up imaging is necessary. In the case of conventional surgical repair, follow-up imaging is performed yearly, usually with US. For endovascular grafts, the follow-up is more stringent, with immediate postprocedural CT scanning as well as 6-month and then yearly CT scan follow-up.

When the evaluation for AAA is performed with CT or MRI, note the extent of the aneurysm, any involvement of major branch vessels, and the existence of a retroaortic or circumaortic left renal vein. Note if the aneurysm has significant wall thickening, a typical characteristic of an inflammatory aneurysm, because the surgical approach for this condition differs from that needed for the more common, predominantly atherosclerotic aneurysm.

Limitations of techniques

With conventional radiography in the anteroposterior or lateral projection, calcification of both opposing abdominal aortic walls must be present to outline AAAs. However, this finding is present in less than 50% of cases. A tortuous, calcified aorta may mimic an AAA unless both walls can be seen clearly. The lack of overlying bony structures in the lateral projection may allow clearer definition of the aneurysm.

US is considered the screening examination of choice; however, it may not adequately depict the entire abdominal aorta if a large amount of bowel gas is present or if the patient is obese.

With or without contrast enhancement, CT is an excellent screening examination for AAA. CT depicts the absolute size of the aneurysm. However, the extent of mural thrombus and the presence of dissection cannot be evaluated without the administration of contrast material.

MRI with contrast enhancement provides an alternative to CT in patients with renal insufficiency. MRI has several absolute contraindications, including cardiac pacemakers and intracranial aneurysm clips. Claustrophobia and a patient's inability to remain motionless are likely to yield a nondiagnostic study. MRI is not as available as CT and US.

Angiography is also a safe procedure. However, because it is an invasive procedure, a small, but definite, risk to the patient exists. The true size of the aneurysm may not be discernible because of a mural thrombus; therefore, underestimation of the true extent of the aneurysm is possible. The role of angiography is in planning surgical or endovascular repair.

For excellent patient education resources, visit eMedicine's Circulatory Problems Center. Also, see eMedicine's patient education article Aortic Aneurysm.

Calcification of the abdominal aortic wall is frequently evident on plain radiographs of the abdomen, as demonstrated in the images below. Calcification is best seen on lateral views when the spine does not obscure the opposing walls of the vessel. When calcification can be clearly identified in the opposing aortic walls, abdominal aortic aneurysms (AAAs) can be diagnosed with the plain radiographic findings.

Degree of confidence

If the classic eggshell appearance is present, the degree of confidence is approximately 100%; however, this finding is present only in 50% of patients. Occasionally, only the anteroposterior or lateral abdominal image demonstrates the findings clearly. If AAAs are suspected, perform abdominal US or CT for confirmation. As such, negative plain radiographic findings do not exclude the diagnosis in any way.

False positives/negatives

A tortuous, calcified aorta can mimic AAA unless both walls can be seen clearly. If the opposing walls are not calcified, the diagnosis cannot be made with certainty. In these cases, US, CT, or MRI must be performed if AAA is clinically suspected.

CT scanning accurately demonstrates dilation of the aorta and involvement of major branch vessels proximally and distally. This information helps in determining the appropriate intervention, which may be either surgical or endovascular repair. (See the image below.)

CT also shows the other organs in the abdomen and demonstrates involvement or displacement of organs that can confuse the clinical picture. The location and number of the renal arteries, caliber of the aneurysm, degree of calcification, lengths of the neck and iliac artery, and presence of mural thrombus are readily assessed. CTA allows multiplanar assessment of the aneurysm and associated relevant vessels (visceral arteries, iliac and femoral arteries).

Degree of confidence

CT has emerged as the diagnostic imaging standard for the evaluation of AAA, with an accuracy that approaches 100%. A well-performed CT examination can reveal the extent of the aneurysm, as well as the involvement of other organs. Intravenously administered contrast agent is needed to obtain the full benefit of CT; however, a nonenhanced study accurately depicts AAAs. Three-dimensional reconstructions of state-of-the-art, multidetector-row, helical CT scans can help in preoperative planning and may replace the need for preoperative diagnostic angiography.

False positives/negatives

The administration of contrast material is essential for detecting dissection or ulceration of a vessel that might be missed without it. In the acute setting (eg, in a patient with back pain or an aneurysm), a false-positive diagnosis of rupture is possible if fluid resulting from another cause is seen in the abdomen. Conversely, an aneurysm or rupture can be missed in a patient who has recently undergone barium study, because artifact can obscure the aorta.

MRI and MRA can be used to define the extent of abdominal aortic aneurysms (AAAs). The absence of iodinated contrast material and radiation are advantages of this modality. However, MRI is more sensitive to motion than is CT, because a patient must remain motionless for a longer period than with current multidetector-row helical CT technology. In addition, the remaining organs in the abdomen are not seen as well on MRIs because of motion. (See the image below.)

Degree of confidence

In technically well-performed MRI and MRA, degree of confidence approaches 100%. These examinations clearly reveal the extent of the aneurysm.

False positives/negatives

If prior abdominal surgery has been performed and if metal clips or devices were used, MRI may not be possible. If the metal is close to the aneurysm or if branch vessels or heavy calcification is seen, artifacts may obscure the vessel and result in a nondiagnostic study.

US is the screening examination of choice as a result of its relative availability, speed, and low cost. US is operator dependent, unlike other modalities; therefore, operator experience is important. The abdominal aorta normally tapers as it extends distally. Any increase in its diameter is considered abnormal. (See the image below.)

Degree of confidence

If the abdominal aorta can be seen in its entirety, US provides a reliable, low-cost screening examination. However, in a patient who is obese or in whom the bowel is distended with gas, a complete examination of the aorta and proximal iliac arteries may not be technically possible. In such instances, another cross-sectional imaging study (eg, CT, MRI) should be obtained.

Angiography is often ordered for preoperative evaluation in patients with manifestations of atherosclerotic vascular disease, such as renal artery stenosis or peripheral vascular disease.

Compared with other images, arteriograms (shown below) currently enable better longitudinal measurements of aneurysms. The reason is that the catheters that are used to make these measurements follow the contour of the vessels and therefore allow better determination of the length of the aneurysm, as opposed to linear measurements obtained with CT. This information is not an issue in open surgical repair; however, it is important in endovascular repair.

Catheters with radiopaque graduated markers are used, allowing measurement of the lengths of the aneurysm and the caliber of the vessel. These data are occasionally important for endograft placement, and they may not be clear from CT and CTA. Typically, 7-10 measurements are required to size an endovascular graft to an abdominal aortic graft. Much, if not all, of the sizing can be accomplished with thin-section, contrast-enhanced CTA.

As CT algorithms for measuring aneurysms improve, the need for preoperative angiography will decrease. Angiography may be reserved for the most complex aneurysms in which endovascular repair is contemplated.

Angiography is often linked to embolization of the internal iliac artery in a patient in whom the procedure is necessary prior to endovascular repair. Examples of relevant conditions include an internal iliac artery aneurysm or an ectatic aneurysm ipsilateral to a common iliac artery that requires anchoring of the stent-graft in the ipsilateral external iliac artery.

Degree of confidence

When abdominal aortic aneurysm (AAA) is suspected, it is unlikely to be missed at angiography. In most cases, the morphology of an aneurysm can be clearly defined. If an aneurysm is suspected on the arteriogram, a cross-sectional image should be obtained. Not only will it confirm the existence of an aneurysm, but other pathologic conditions that may affect the surgical intervention can be detected.

If a large amount of luminal thrombus is present, the true diameter of the aneurysm may be obscured unless the wall of the aneurysm has a substantial amount of calcification. This limitation leads to significant underestimation of the diameter of the aneurysm.