Atherosclerotic Aneurysm
                                           Most abdominal aortic aneurysms are atherosclerotic in origin. An occasional mycotic one is encountered. Screening for an abdominal aortic aneurysm is not widely practiced even in hypertensive patients. An abdominal aortic aneurysm in these patients is associated with claudication, and these patients appear to benefit from screening US. An occasional aortic aneurysm is associated with a coagulopathy, which often clears after aneurysm repair.
From a potential therapeutic perspective, an abdominal aortic aneurysm’s size and location are of obvious importance. One classification is into infrarenal, juxtarenal, and pararenal aneurysms. Most common are infrarenal ones, fusiform in shape. Pararenal aneurysms extend distal to the superior mesenteric artery and involve the renal arteries. The role of imaging is to establish that an aortic aneurysm is indeed present, provide information about its size and shape, detect complications, and outline the preoperative
Aneurysms vary in size considerably. Measurement of aneurysm dimensions before endovascular therapy is of obvious importance, yet DSA measurements of an aneurysm’s diameter and length are inaccurate by up to 15%; an indwelling catheter is the only available reference standard. Computed tomography, US, or MRA provides more reliable aneurysm Calcifications develop in long-standing aneurysms, but aortic calcifications do not imply that an aneurysm is present. An occasional aneurysm is suggested from a conventional abdominal radiograph, but this study is rarely employed when suspecting an aneurysm. In particular, measurement of a suspected 
aneurysm’s diameter is notoriously inaccurateon conventional radiographs, but if such a measurement is necessary, a lateral projection rather than a frontal one should be chosen. Computed tomography angiography is the current preoperative imaging study of choice. Numerous studies confirm accuracy of CT in measuring an aneurysm’s diameter.Multidetector CT in patients evaluated for aortoiliac aneurysms, performed with a 25-second delay after the start of IV contrast injection resulted in aortic enhancement of >200 Hounsfield units (HU) and no superimposed venous filling. A 3D shaded surface display is helpful in viewing aneurysms, with image rotation providing multidirectional images. In differentiating among suprarenal, juxtarenal, and
infrarenal aneurysms, the use of narrrow collimation and overlapping axial reconstructions aids in correctly classifying most aneurysms and identifies main and accessory renal arteries. Ultrasonography should identify an aortic aneurysm, outline its shape, and measure its width and rough length in most patients and is often the first screening examination obtained in a patient with a pulsatile abdominal mass.
Involvement of other vessels is difficult to evaluate. Obesity and bowel gas limit the information obtained. Doppler US does provide additional information but is generally superfluous if subsequent angiography (CT or other) is obtained.
Magnetic resonance imaging and MRA are evolving into primary imaging modalities for preoperative aneurysm study, at times providing information superior to CT. Potentially 3D MRA can provide definitive pretherapy studies and replace both CT and DSA. Some studies achieve almost perfect agreement between conventional angiography and MRA interpretations of aortic disease. Conventional angiography is often considered the gold standard in aneurysm evaluation primarily because many surgeons are comfortable with its results and rely on its information. Nevertheless, the use of angiography is declining for this indication and is gradually being relegated to those situations where CT and MR are inconclusive. Magnetic resonance imaging can identify an aneurysm’s size and shape. Transverse images tend to provide more information about the distal aorta and iliac vessels compared to
coronal images, but sagittal images are helpful in identifying major vessel origins. Gadoliniumenhanced MRA achieves high sensitivity and specificity in determining whether the renal arteries and iliac arteries are involved by an aneurysm; also, gadolinium aids in differentiating between slow blood flow and a mural thrombus. A 3D gadolinium-enhanced MRA technique identifies an aneurysm and aids in establishing its relationship to renal and other major arteries. Stenosis of major vessels is also detected and atherosclerotic plaques and thrombi are evaluated.
One inherent limitation of MRI is its inability to visualize calcifications. Also, multiple sequences are generally necessary. Thus although arterial-phase MRA visualizes a patent aortic lumen, it does not outline the aortic wall or identify thrombi; for the latter axial imaging is necessary.
Inflammatory Aneurysm An inflammatory abdominal aortic aneurysm is characterized by marked thickening and inflammation in the aneurysm wall. The etiology is unknown, although an immune response appears to be involved in some patients. Many inflammatory aneurysms are associated with extensive surrounding extraperitoneal fibrosis. At times the ureters become encased and obstruct. The duodenum or inferior vena cava can also be entrapped. The inflammation often subsides after aneurysm repair and ureteric obstruction is relieved. Computed tomography performed several years after inflammatory aortic aneurysm repair reveals no or little persisting inflammatory tissue in most
A MR study of an inflammatory aortic aneurysm reveals a complex, concentric, layered outline; homogeneous enhancement postcontrast distinguishes this condition from the more common atheromatous intima.
Dissecting Aneurysm Most dissecting aortic aneurysms are thoracic in origin and dissect into the abdomen. These aneurysms are generally subdivided into those involving the ascending aorta and those originating distal to the great vessels. A dissection is diagnosed by detecting both true and false lumens and identifying an intimal flap. Most
false aneurysms are saccular in outline and communicate with the aortic lumen. Some false lumens spiral around the aortic circumference; in these, dissection the inner lumen is invariably the true lumen. Aortography was the traditional gold standard in detecting a dissection, identifying an intimal flap, and establishing an entry site and possible exit site. Postcontrast CT and lately
MRI are evolving as noninvasive alternatives to aortography in suggesting the extent and length of a dissection. A helical CT study
evaluating various protocols concluded that an optimal CT study consists of two separate but adjacent scans—3-mm collimation for the aortic arch and 5-mm collimation for remaining aorta (22); such a study achieved almost a 100% sensitivity and specificity for detecting a dissection and identifying entry and exit sites.
Some acute dissecting aneurysms have an eccentric hyperdense aortic wall on precontrast CT. A hyperdense wall is also found in some intramural hematomas and nondissecting
aneurysms, but with the latter the hyperdensity is circumferential rather than eccentric. Contrast-enhanced CT in a minority of patients with a dissecting aneurysm reveals linear hypodense structures in the false lumen; these fibroelastic bands, described as aortic cobwebs, extend from the intimal flap and represent portions of aortic wall incompletely sheared during dissection. These cobwebs identify the false lumen and distinguish it from the true lumen.
Ultrasonography identifies most true and false lumens, separated by an intimal flap. Some false-positive Doppler imaging results are probably due to a heavily calcified aorta acting as a strong acoustic reflector, resulting in mirror image artifacts. At times a crescent-shaped  hypoechoic zone between a thrombus and aortic wall suggests dissection, but Doppler US does not detect any flow in this region, a condition called pseudodissection. Intravascular US also differentiates the true from the false lumen. It identifies an acute angle between the dissecting flap and false lumen outer wall, and differentiates the three-layered true lumen wall from single-layered false lumen wall. Intravascular US is also useful to differentiate causes of branch vessel compromise whether a dissection intersects a branch vessel origin or whether a vessel origin was spared but a dissection flap simply compresses the true
lumen and covers its origin.
Magnetic resonance detects an intimal flap and identifies both true and false lumens. Flowing blood results in a signal void in bothlumens with a SE technique, a finding modified by slow flow or a thrombus. As an aneurysm diameter increases, the false-to-true lumen cross-sectional area ratio also increases, but peak average velocity in the true lumen decreases, findings obtained from MR phasecontrast images. T1-weighted SGE images reveal slow flow in either a true or false lumen as high signal intensity, although, in general, postcontrast 2D or 3D techniques better identify both lumens and an intimal flap, and differentiate slow flow from a thrombus. Contrast-enhanced
MR establishes flow patterns in a dissection, including flow in major vessels, and is useful if iodine-based contrast agents are contraindicated.
Yet exceptions do occur; in an occasional patient aortic wall thickening is the only sign of dissection on T1-weighted MRI.
The diagnosis is difficult with a thrombosed false lumen; the appearance is similar to that of an eccentric mural thrombus. An intimal flap is not detected with a thrombosed false lumen. Inward displacement of intimal calcifications
suggests dissection, but this appearance is mimicked by calcifications within a thrombus and an intramural hematoma. A thrombosed false lumen does not enhance with contrast during an early phase; for reasons unknown, occasionally a false lumen enhances on latephase  images.
Dissecting abdominal aortic aneurysm.Transverse (A), coronal (B), and 3D (C) CT reconstructions each provide slightly different information about the shape and scope of this aneurysm.
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