Abdominal Aortic Aneurysm (AAA)
- Abdominal aorta lies slightly to left of midline and bifurcates at the level of the 4 th or 5 th lumbar vertebral body. (Illustration 1) The surface anatomy landmarks are the xiphoid process and the umbilicus (Illustration 2).
- Illustration 1. Overview of the abdominal aorta with major branches (Source: https://commons.wikimedia.org/wiki/File:Aorta_branches.jpg)
- Illustration 2. Scanning technique. Probe is just inferior to the xiphoid process, indicator towards the patient’s right (star) for a transverse view. Probe is moved inferior to the umbilicus.
Figure 1. Transverse image of the aorta shows a classic example of the seagull sign. The celiac trunk branches into the hepatic and splenic arteries.
Figure 2. Aorta imaged in long axis with the celiac trunk and SMA branches
III. Scanning Technique, Normal Findings and Common Variants
Sonographic Technique
- 5 MHz transducer is adequate for most abdominal scanning, including aorta. Lower frequency may be needed in large patients, and higher frequency will give more detail in thin patients.
- Aorta and iliac arteries are measured from outer wall to outer wall. Normal abdominal aorta diameter is less than 3 cm. AAA is defined as greater than 3 cm.
Start in transverse plane, high in the epigastrium, using liver as a sonographic “window.” Identify the vertebral body (a dark, rounded shape, with dense shadow)
- Identify aorta on the patient’s left, and the inferior vena cava (IVC) on the patient’s right anterior to vertebral body.
- In real-time obtain transverse clips of the aorta from the celiac to the bifurcation.
Video 1. Transverse sweep of normal aorta from proximal to iliac artery bifurcation
Proximal measurements should be obtained around the celiac trunk or liver tip.
Figure 3. Transverse view of the proximal aorta
Figure 4. Transverse view of the mid aorta
Figure 5. Transverse view of the distal aorta
Special Techniques for Ultrasound Evaluation
The transverse colon should be expected to be encountered running across the upper abdomen immediately inferior to the liver edge. It is usually fairly wide and filled with gas. In order to obtain complete exam of the aorta without “skipped areas” it may require separate views from above (with the transducer directed inferiorly, using the liver as a window) and below, with the probe fanning in a cephalad direction. At any location, if bowel gas makes it difficult to obtain images, some or all the following can help:
- Apply gentle downward pressure, known as graded compression. It may take 1-2 minutes of compression to allow bowel to move aside.
- Move the probe slightly off midline and angle towards midline.
- Reposition patient in the right decubitus position.
- Obtain right coronal views using the liver as a window (Figure 6). The probe is placed with indicator towards patient’s head, position in mid axillary line at or below costal margin, directed slightly anterior. In most patients this only affords very limited views of the upper aorta, and it is technically challenging. In cases where it is possible, the IVC and aorta can be seen running parallel, one in front of the other with the aorta lying “deep” on the screen to the IVC.
Figure 6. Probe positioning for visualization of the aorta from the right upper quadrant
Video 3. The IVC and aorta visualized through the liver in the right upper quadrant
IV. Pathology
- AAA is described as being a focal dilatation of the abdominal aorta of 150% of normal.12-13 Although there is no established definition of aneurysm size, conventionally, an AAA is diagnosed when the diameter exceeds 3.0 cm.14
Figure 7. Transverse view of a 4.1 cm AAA
Video 4. Transverse view of large AAA
Video 5. Longitudinal view of large AAA
Figure 8. Iliac artery aneurysm
V. Pearls and Pitfalls
- The only contraindication of scanning for AAA is delay to surgical intervention.
- Obtain measurements of aorta from outer wall to outer wall. Since aneurysms will often contain a thrombus, and with time this becomes calcified and hyperechoic, one may accidentally mistake the inner rim of the thrombus for the aortic wall. Doing this will lead a falsely decreased measurement of the true aortic diameter, possible causing the aneurysm to be missed completely.
Figure 9. Transverse view of a 7 cm AAA with intraluminal clot (* on the lumen, “v” denotes vertebral body)
The attempt should be made to create transverse cuts with respect to the aorta, even when it is tortuous, ectatic and deformed by advanced atherosclerosis. In such circumstances, any single longitudinal image will not be complete, and will section parts of the vessel obliquely. In such vessels, any view can both overestimate and underestimate the size of the vessel.
A small aneurysm does not preclude rupture:17,18 Any symptoms consistent with aortic pain in a patient with an aortic diameter greater than 3.0 cm should have this diagnosis (or alternative vascular catastrophes) ruled out.
Do not mistake the IVC for the aorta. It is important to identify the branches of the aorta. The IVC will run through the liver with the hepatic vein draining into it. You can also identify the IVC entering into the right atrium (Figure 9).
Figure 10. Note the difference between the aorta and the IVC
VI. References
- Wilmink AB, Quick CG. Epidemiology and potential for prevention of abdominal aortic aneurysm. Brit J Surg.1998;85:55–62.
- S. Preventive Services Task Force.Screening for abdominal aortic aneurysm: recommendation statement. Ann Int Med. 2005;142(3):198-202.
- Hermsen K, Chong WK.Ultrasound evaluation of abdominal aortic and iliac aneurysms and mesenteric ischemia. Radiol Clin N Am. 2004;42:365-81.
- Fink HA, Lederle FA, Roth CS, et al.The accuracy of physical examination to detect abdominal aortic aneurysm. Arch Int Med. 2000;160(6):833-6.
- Lin PH, Bush RL, McCoy SA, et al. A prospective study of a hand-held ultrasound device in abdominal aortic aneurysm evaluation. Am J Surg. 2003;186(5):455-9.
- Riegert-Johnson DL, Bruce CJ, et al.Residents can be trained to detect abdominal aortic aneurysms using personal ultrasound imagers: a pilot study. J Am Soc Echocardiogr. 2005;18(5)394-7.
- Kuhn M, Bonnin RL, Davey MJ, et al.Emergency department ultrasound scanning for abdominal aortic aneurysm: accessible, accurate, and advantageous. Ann Emerg Med. 2000;36(3):219-23.
- Constantino TG, Bruno ED, Handly N, et al. Accuracy of emergency medicine ultrasound in the evaluation of abdominal aneurysm. J Emerg Med. 2005;29:455-460.
- Reed MJ, Cheung L. Emergency department led emergency ultrasound may improve the time to diagnosis in patients presenting with a ruptured abdominal aortic aneurysm. European Journal of Emergency Medicine. 2014; 21(4):272-275.
- Ernst CB. Abdominal Aortic Aneurysm. N Engl J Med.1993;328:1167-72.
- American College of Emergency Physicians. Ultrasound guidelines: Emergency, Point-of-care, and clinical ultrasound guidelines in medicine [policy statement]. Approved April 2023.
- Johnston KW, Rutherford RB, Tilson MD, et al.Suggested standards for reporting on arterial aneurysms. Subcommittee on reporting standards for arterial aneurysms, Ad Hoc committee on reporting standards, Society for Vascular Surgery and North American Chapter, International Society of Cardiovascular Surgery. J Vasc Surg. 1991;13:444–50.
- Sox HJ, Huber JM (eds.) Guide to Clinical Preventive Services, Second Edition, Section I, Chapter 6. Copyright©, Columbia-Presbyterian Medical Center.
- Sakalihasan N, Limet R, Defawe OD.Abdominal Aortic Aneurysm. Lancet. 2005;365:1577-89.
- Vardulaki KA, Prevost TC, Walker NM, et al. Growth rates and risk of rupture of abdominal aortic aneurysms. Br J Surg. 1998;85:1674-1680.
- Bacharach JM, Slovut DP. State of the art: management of iliac artery aneurysmal disease. Catheter Cardiovasc Interv 2008; 71:708.
- Miller J, Grimes P, Miller J. Case report of an intraperitoneal ruptured abdominal aortic aneurysm diagnosed with bedside sonography. Acad Emerg Med. 1999;6(6),661–4.
- Darling RC, Messina CR, Brewster DC, et al.Autopsy study of unoperated abdominal aortic aneurysms. Circulation. 1977;56(3) Supp II:161-4.
