Abdominal aortic aneurysms (AAA) are most common in elderly patients with advanced atherosclerosis. The prevalence of AAA is estimated at 1.5% of the population over the age of 50 and higher in male patients over the age of 65.¹ An untreated AAA greater than 5 cm in diameter carries a five-year likelihood of rupture of 20%.²

This likelihood increases significantly with increasing aneurysm dimension. Acute rupture carries a 60% to 80% immediate mortality rate,³ and the mortality of emergency aortic aneurysmectomy approaches 50%. Major postoperative complications include renal failure, mesenteric ischemia, abdominal abscess, coronary insufficiency, and myocardial infarction. Respiratory complications also occur, related to anesthesia and abdominal surgery.

Avoidance of the high morbidity and mortality associated with acute aneurysm rupture places a premium on detection of AAA. In addition, imaging evaluation of patients with known aneurysms is critical in determining the appropriate mode of therapy: endovascular stent-grafting or surgical aneurysmectomy.

Abdominal aortic aneurysms may be suspected on physical examination and confirmed by sonography, or they may be incidental findings on other imaging examinations, including abdominal radiography, CT, and MR studies. Screening ultrasound studies can be considered for patients at risk, such as those with a family history of AAA and elderly patients with a diffuse atherosclerosis, particularly in cases with hypertension and a history of smoking.

An AAA is a fusiform or focal saccular dilatation, usually of the subrenal aorta, greater than 4 cm in external anteroposterior diameter.⁴ About 90% of AAAs are subrenal, the majority limited to the abdominal aorta, but extension to the common iliac arteries is not uncommon. Accessory renal arteries may arise from a subrenal aneurysm. Around 10% of aneurysms are juxtarenal, involving main renal arteries, or suprarenal, involving the celiac or superior mesenteric arteries. Suprarenal aneurysms may be associated in a “dumbbell” configuration with infrarenal aneurysms. Some patients may have complex thoracoabdominal aneurysms involving the descending thoracic, suprarenal, and infrarenal aorta.

Sonography is performed with sagittal and transverse projection and should evaluate the aorta from the diaphragmatic hiatus to the bifurcation with sequential imaging of the common iliac arteries (Figure 1). Patient obesity, overlaying bowel gas, and a relatively small left hepatic lobe may limit visualization of the aorta. Marginal calcification lining the thrombus can be detected.

The value of sonography is not only in detection and confirmation of suspected aneurysm, but also for performance of serial surveillance studies in patients whose aneurysm has not yet reached a critical dimension for intervention. Intervention is usually recommended in patients with aneurysms 5 to 5.5 cm or greater in diameter or an aneurysm greater than 4 cm in diameter with a 5-mm or greater increase in dimension within six months. Serial ultrasound studies are important in providing accurate comparative measurements.⁴

Pre-intervention planning can be performed with noninvasive angiography, using either CT angiography (CTA) or MR angiography (MRA). Both provide accurate measurements of aneurysm diameter and length and involvement of branch vessels. CTA is more commonly used, as the modality is readily available and the introduction of multidetector CT (MDCT) scanning has brought major improvements. In patients who have abdominal pain and suspected aneurysm disease, the contrast-enhanced CTA study should be preceded by a noncontrast survey study to detect the “crescent” sign and/or periaortic hematoma.⁵ The crescent sign is presumed secondary to fissurring and deposition of coagulum in lining mural thrombus. Periaortic hematoma reflects a contained aortic leak. Patients examined in an emergency situation with severe onset of abdominal pain, hypotension, and periaortic hematoma should proceed immediately to laparotomy without contrast injection.

The contrast-enhanced CTA study provides both axial and three-dimensional images; in the 3-D category, multiplanar reformation and volume rendering should be complemented with curved planar reformations through the aortoiliac system. Candidates for CTA should have normal renal function, adequate intravenous access, and no history of serious allergy to iodinated contrast material. Intravenous access is usually obtained via a 20-gauge plastic venous cannula in an antecubital vein. A preliminary minibolus measures circulation timing, to allow accurate estimate of injection-to-scan delay. Injection of 60% iodinated contrast material is performed at 5 cc per second, with the intent of elevating aortic attenuation above 250 HU throughout the duration of acquisition.

A general principle of CTA is to match the injection interval to the acquisition interval. The cephalocaudad dimension of the aortoiliac arterial tree from the supraceliac aorta to the proximal thigh approximates 30 cm. Four-channel MDCT systems with detector collimation of 1 or 1.25 mm and a scan rotation speed of 0.5 second can cover the aortoiliac arterial system in approximately 20 seconds. A contrast load of 28 g (100 cc of 60% iodinated contrast material), injected at 5 cc per second, with injection-to-scan delay determined by the preliminary minibolus, provides an adequately enhanced aorta throughout the duration of acquisition (Figure 2).

Radiologists should survey the CT image data in an axial cine paging mode as well as multiplanar reformations. CTA is displayed using a combination of maximum intensity projection (MIP) and volume rendering (VR). Besides determining aneurysm diameter and length, other important features are shown: length of superior neck between the renal vascular pedicle and the upper limit of the aneurysm, origin of accessory renal arteries from the aneurysm (Figure 3), presence or absence of a patent inferior mesenteric artery arising from the aneurysm (Figure 4), and distal extension of aneurysm to the iliac arteries (Figure 5). Associated renal, mesenteric, or celiac artery stenosis should be noted. A small percentage of AAAs may be juxtarenal or suprarenal and may be associated with thoracoabdominal aneurysmal disease.

Postprocessing software allows automated centerline tracking and vessel edge detection to provide true longitudinal and cross-sectional area measurements of a tortuous aortoiliac arterial system. This is of value in pre-stent-graft evaluation.

Multidetector CT scanner technology continues to advance, and the clinical advantage of improved multislice acquisition will be thinner and faster imaging resulting in an isotropic voxel size of about 0.6 mm without change in acquisition interval. Alternatively, for patients with marginal renal function, the contrast load of iodinated contrast material could be decreased to approximate the contrast volume of about 40 cc often used with gadolinium-enhanced abdominal MRA.

Similar principles of bolus contrast injection, accurate determination of injection-to-scan delay, and rapid acquisition also apply to abdominal MRA. Best results are obtained with 1.5-tesla scanners using T1-weighted gradient-echo techniques with rapid gradient switching and short rise times.⁶ The acquisition interval using a 3-D acquisition with 2-mm coronal plane partitions approximates that of a CTA study. Injection-to-scan delay can be determined with preliminary minibolus or with online bolus tracking, using MR fluoroscopy or automated triggering in a manner analogous to CT. MRA can be either performed as the preferred test or reserved for patients with marginal renal function or allergy to iodinated intravenous contrast material.

The 3-D MRA studies are displayed primarily with a MIP mode (Figure 6). The study should be supplemented with axial plane T1-weighted imaging to display lining mural thrombus. A deficiency of the 3-D MRA display is its inability to show mural or thrombus calcification. This is an important finding in relation to iliac access for endovascular stent-grafts and determination of a suitable distal placement zone for the endovascular graft.

Aortic Stent-Grafts

Stent-grafting is an endovascular procedure requiring accurate modeling of the aorta and aneurysm to select a stent-graft with appropriate dimensions for each individual patient and to plan insertion prior to the procedure. The cross-sectional and longitudinal information provided by a combination of 2-D and 3-D angiography should essentially replicate that provided by a combination of conventional catheter arteriography and intravascular ultrasound.^7-10

Infrarenal AAA are frequently tortuous. Conventional 3-D displays do not provide an accurate representation of the longitudinal dimensions of a tortuous aorta. Thus, discrepancies are to be expected between planar measurements on a CTA or MRA display and a measurement obtained at catheter arteriography using a calibrated catheter. Advanced vessel analysis (AVA) software uses centerline tracking and automatic edge detection to evaluate the aortoiliac system in potential stent-graft candidates. The software provides a true longitudinal dimension of tortuous aortoiliac vessels and diameter and cross-sectional area measurements that are orthogonal to the aorta and iliac arteries and not necessarily orthogonal to the patient’s longitudinal body axis. Software measurements include length and transverse dimensions of the superior neck; length, transverse dimension, and endoluminal volume of the aneurysm; and transverse dimensions of the inferior neck, if present (Figure 7).

Rotational projections of the aorta and each iliac artery using curved planar reformation are provided to assess iliac tortuosity and stenosis (Figure 8). The angle between the superior neck and the long access of the aneurysm is calculated automatically.

The AVA software addresses important morphologic and measurement issues related to feasibility of abdominal aortic endovascular stent-grafting in individual patients. These include iliac access, a suitable proximal placement zone in the superior neck, and an adequate distal landing zone in the common iliac arteries. The information set uses both curved planar reformations and true longitudinal and orthogonal aortic and iliac measurements.

Other important issues include the degree of atherothrombosis involving the superior neck, as adequate fixation may not occur there with asymmetric mural thrombus. In addition, proximal fixation may not be adequate with a conical superior neck. Patients with a patent inferior mesenteric artery off the aneurysm with a coexistent superior mesenteric artery or hypogastric artery stenosis are at risk for colon ischemia following stent-graft placement. Accessory renal arteries arising from the aneurysm or low in the superior neck may remain patent if overlain by the uncovered portion of the stent-graft, or they may be occluded by the covered portion of the stent-graft. This complication may be accepted depending on the patient’s general medical condition.^11,12

Postprocedural Evaluation

Postoperative imaging is useful in evaluation of several important issues. In addition to immediate postprocedure thrombosis, imaging allows serial measurement of aneurysm size, detection of aneurysm leak, assessment of stent-graft migration and kinking, and detection of branch vessel occlusion.^13-16 Aneurysms should remain stable and decrease in size in serial studies.

A type 1 endoleak occurs due to incomplete exclusion by the proximal or distal attachment site of the stent-graft. A type 2 endoleak results from retrograde inflow in branch vessels such as the lumbar or inferior mesenteric arteries (Figure 9). A type 3 endoleak is secondary to endograft disruption of either the metallic support or the fabric.

The usual post-stent-graft protocol requires a postprocedure baseline CTA several weeks following the procedure, with surveillance studies performed at intervals of three and six months within the first 18 months. Endoleak can be demonstrated on both 2-D and 3-D CTA. Some endoleaks may not be evident on the initial arterial phase sequence, and it is customary to perform an immediate post-arterial-phase CT to evaluate for possible endoleak.

References

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