Sonographic Appearance
At the diaphragm, normal aortic diameters have been cited at approximately 2.5 cm.
6 During its course, inferiorly the aorta tapers, reaching a diameter of about 1.5 to 2.0 cm at the level of the iliac arteries.
6 Ectasia of the aorta, as seen with atherosclerosis, is manifested by a slight widening of the normal aortic diameter up to 3.0 cm. There are also aortic wall irregularities, owing to the atherosclerotic changes that take place in this disease process. A true aneurysm is identified sonographically as a dilatation of the aorta ≥3.0 cm near its bifurcation point, a focal dilatation along the course of the aorta, or lack of normal tapering of the aorta.
14,
15,
16 and
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Aneurysms vary in size and can range from 3 to 20 cm as a result of the abnormal blood flow patterns within an aneurysm. If thrombus is formed, it can usually be detected by sonographic techniques and is a common finding. Sonographically, thrombus typically produces a low-level echo pattern and tends to accumulate along the anterior and lateral walls of the aortic lumen (
Fig. 6-13A-H).
14,15 Adequate demonstration of the thrombus may require that gain settings be increased from initial settings to display the low-level echoes associated with thrombus. It may also be necessary to scan coronally or obliquely through the aorta to demonstrate thrombus. These maneuvers may help reduce confusion between reverberation artifacts and actual thrombus. Occasionally, there may be calcification within the thrombus. An interesting phenomenon that has been reported in association with aortic aneurysm thrombus is that of an anechoic crescent sign (
Fig. 6-13C). In these instances, the anechoic area within the lumen of the aneurysm was found at surgery to be serosanguineous fluid or liquefying clot. When evaluating the aorta for aneurysm formation, it is important to distinguish this finding from aortic dissection because the surgical treatments are different.
Thrombus within the aorta may be difficult at times to visualize, especially in an obese or a gassy patient. Anterior reverberation artifacts from a calcific anterior aortic wall may obscure the clot as well. Instances have been reported in which an obstruction clot of the aorta was not detected sonographically.
18 If an obstructing clot is suspected on clinical grounds, Doppler examination of the aorta can confirm the presence or absence of flow within it, thereby solving the problem (
Fig. 6-13D).
If an aneurysm is detected during sonographic examination, it is prudent to attempt to identify the origins of the renal arteries as well as to extend the examination to the iliac arteries to look for aneurysmal involvement in these areas.
Associated renal artery aneurysm in conjunction with abdominal aortic aneurysm has been reported to be 1% or less.
19 Nonetheless, it is important for the surgeon to know of this coexistence because of the difference in treatment procedures. When the renal arteries are involved in an aneurysm, renal artery enlargement generally coexists with aortic dilatation. Demonstration of this complication, however, can be quite difficult because large aneurysms tend to displace surrounding bowel superiorly and subsequently cover the renal artery origins.
19 Consequently, diligent scanning techniques involving multiple patient positions and numerous transducer angulations may be necessary before adequate visualization of the renal artery origins is achieved. Color-flow Doppler may also aid visualization of the renal arteries in such patients. If efforts to identify the renal arteries are unsuccessful, an attempt should be made to visualize the SMA. Because of the proximity of the renal arteries to the SMA, any aneurysm shown to involve the SMA also involves the renal arteries.
Abdominal aneurysms may also extend into the iliac arteries. When this occurs, the iliac arteries will be abnormally dilated, and the thrombus may or may not be present in the dilated areas. Isolated iliac artery aneurysms are an occasional finding.
2 On occasion, aneurysm thrombosis may lead to peripheral thromboembolism or acute limb ischemia.
The accuracy rate for the sonographic detection of aortic aneurysms approaches 100% in most reports.
5,20,
21 and
22 Because of this, sonography is a very good screening tool as the first step
in the evaluation of suspected aortic aneurysm and can be used to monitor the growth of aneurysms over time.
23 However, there are some important considerations to keep in mind to avoid misdiagnosis of an aneurysm. Tortuosity may make the aortic diameter appear larger than it is. This occurs when the plane of imaging is not truly perpendicular to the aortic walls. Therefore, careful observations should be made of the aortic curvature in these instances to avoid misrepresentation of a tortuous aortic segment as an aortic aneurysm. Excessive air in the abdomen or obesity may obscure the distal aorta and iliac vessels and render some aneurysms invisible. Lymphadenopathy may also confound the picture.
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Normally, the abundant lymph nodes that are linked together chainlike along the anterior and lateral aspects of the aorta are invisible sonographically. When enlarged, however, their appearance can be dramatic—and initially confusing. Sonographically, enlarged lymph nodes are echo poor, but with increased gain settings, fine internal echoes may be appreciated. Several patterns of lymph node enlargement have been described: isolated large masses, which tend to develop along the aortic chain; mantle-like distributions of enlarged nodes draped atop the aorta and IVC; symmetric nodal enlargement along the aortic chain bilaterally; multiple spindle-shaped nodes dispersed in the mesentery; and large, confluent masses surrounding the aorta and IVC.
14 It is conceivable that the mantle-like configurations and the confluent mass effects may be confused with aortic aneurysm with thrombus. Close inspection of the area should reveal linear separations between lymph node masses. In addition, the general appearance of extensive lymph node enlargement seems to be slightly more irregular, or “lumpy,” than an aortic aneurysm. The most reliable and accurate tool to use to determine artery versus vein versus lymph node is to Doppler sample for the presence or absence of arterial or venous flow patterns.
The rates with which sonography can accurately detect renal artery involvement and other abnormalities (ruptured aneurysm) are unfortunately not as high as those for aneurysm detection. Therefore, other diagnostic imaging tests may be necessary to further evaluate these complications if they are suspected.
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Although other imaging procedures may be the primary tool used to evaluate the extent of various aortic pathologies, sonographic evaluation is a sensitive and specific imaging technique of choice for screening patients suspected of having aortic aneurysms. The sonographic measurements are accurate, repeatable, and noninvasive and do not involve ionizing radiation.
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