INDICATIONS:

Evaluate for abdominal aortic aneurysm

Abnormal:

• >3cm diameter for abdominal aorta

• >1.5cm diameter for common iliac arteries

REQUIRED IMAGES:



Anatomy:

The abdominal aorta travels retroperitoneally just anterior and left of the spine. It enters the abdomen at the level of the xiphoid, tapering in diameter distally until it branches into the iliac arteries just inferior to the level of the umbilicus. On its path it gives off several branches at predictable levels:

Aneurysms will appear as areas of dilatation along the aorta itself. These are most commonly fusiform, as below, but can be saccular as well.  Fusiform aneurysms are focal areas of circumferential widening, while saccular aneurysms are an outpouching of the aortic wall. A third type of aneurysm, a pseudoaneurysm, can occur in conjunction with aortic dissection. This can form an outpouching of the aorta as the damage to the vessel allows blood to collect between the two outer layers of the aortic wall

This is why it is important to image the aorta in two planes, as each of these can be easily missed if only viewed in one plane.

Our goal in aortic ultrasound is to visualize the entirity of the abdominal aorta, beginning at the level of the celiac artery, all the way to its bifurcation into the common iliac arteries. We will measure the diameter of the aorta at 3 different levels to assess for any areas of dilatation. 

 A diameter of the abdominal aorta >3cm or diameter of iliac artery >1.5cm indicates presence of an aneurysm.

An adequate exam should contain:

1. Transverse views of the proximal, mid, distal aorta, as well as the bifurcation into iliac arteries

2. Longitudinal view of aorta at proximal, mid, and distal levels

HOW TO SCAN AORTA:

Probe selection

The probe of choice is the curvilinear probe as it has lower frequency and can penetrate deeply. Alternatively, one can also use a phased array probe if available. 

 This exam is performed in 3 sections: Proximal, Mid, and Distal/Bifurcating Aorta

Proximal Aorta:

 Distal Aorta

 Longitudinal

This can be performed along with each segment, or following completion of imaging in the transverse plane. It is usually easier to complete this separately, given the constant, even pressure often needed for adequate imaging of the aorta. Obtaining a transverse image and releasing pressure can allow bowel (and air!) to move and obscure a previously good imaging window.

As such, we recommend that once an adequate image is obtained in the transverse plane, the user sweeps the probe through the entire length of the aorta as directed above, before progressing to longitudinal views.

1. Return to the proximal aorta, turn probe 90 degrees in the longitudinal plane, such that probe indicator faces patient’s head

2. Repeat scan in the longitudinal plane, fanning through the aorta, making sure to capture the aorta in its midline (maximal diameter)

Longitudinal view extending from proximal to mid abdominal aorta

IMAGE INTERPRETATION

Abdominal Aortic Aneurysm (AAA)

Although aortic aneurysms can occur in any portion of the aorta, the most common location is in the abdominal portion. Luckily, this is also the portion that most amenable to evaluation via ultrasound. As such, the above exam is indicated to evaluate for exactly this pathology.

Abdominal aortic aneurysms can occur for various reasons. Risk factors include: male gender, age>50, HTN, smoking history, and history of connective tissue disorder such as marfans or Ehlers- Danlos. They also do tend to run in families for unclear reasons.  Patients with AAA often asymptomatic and pain free initially, however can have pain, neurologic or vascular compromise as the aneurysm enlarges. Rupture becomes more likely as size increases and is rapidly fatal when present.

Abdominal aortic aneurysm is defined as:

• >3cm diameter for abdominal aorta

• >1.5cm diameter for common iliac arteries

Examples of AAA visible in transverse section. Note rim-like mural thrombi which surround the aortic lumen. This exemplifies the importance of the need to measure outside wall to outside wall when measuring aortic diameter.

AAA in the mid abdominal aorta viewed in longitudinal section. Also visible is mixed echogenicity mural thrombus which is most prominent anteriorly (superior aspect of image)

While ultrasound alone cannot determine whether a AAA is ruptured, a positive finding on aortic ultrasound should be taken in clinical context. In a patient with unstable vitals and appropriate history, they should be presumed as ruptured until proven otherwise. Assessment for peritoneal fluid may be negative as these patients often hemorrhage in the retroperitoneal space.

AORTIC DISSECTION

Aortic dissection occurs when blood enters the space between the intima and media layers of the arterial wall. This causes the creation of a flap of tissue within the vessel. As the blood enters the media of the vessel at high pressure, it further dissects through this tissue plane, often creating a false lumen.

[A- Aorta, M-Media, I-Intima, FL-False lumen, TL-True Lumen]

Ultrasound is not indicated as a primary modality for the evaluation of aortic dissection given inadequate sensitivity (about 67-80%). However, there is high specificity (99-100%). As such, it is worthwhile to note the appearance of dissection when seen on ultrasound. An ultrasound which does not show a dissection does NOT rule out this disease process, however. Similarly, this exam only focuses on evaluation of the abdominal aorta, and thus we cannot make any determinations about the thoracic aorta.

The Sanford classification separates aortic dissection into two types based on location.

Type A dissection originates from the most proximal aspect of the aorta (proximal to left subclavian artery). It can, however, extend as far as the descending and abdominal aorta.

Type B dissection originates anywhere distal to the left subclavian artery. 

The presence of dissection on ultrasound can appear as a free flap visible within the lumen of the aorta. Depending on the plane in which the flap occurs it may be visible on transverse, longitudinal or both views. 

Aortic Dissection present in proximal aspect of abdominal aorta. Image alternates from transverse view to longitudinal view. Note hyperechoic line representing dissection flap in most proximal aspect of the aortic lumen.

Aortic dissection noted again in longitudinal view in proximal aorta. Hyperechoic line in aortic lumen can be seen pulsating with heartbeat.

ANY patient with a finding of dissection flap on aortic evaluation should undergo rapid cardiac ultrasonography to rule out proximal dissection with cardiac tamponade.

PATTIES PEARLS

-Too much bowel gas? Try applying more pressure or rolling patient into left lateral decubitus

-You can an attempt to place probe in right anterior axillary line and use the liver as a window (similar to FAST view) but this will only assist in imaging the proximal aorta

- Aneurysms >5cm requires emergent work-up, aneurysms 3-5cm in the right clinical context can likely be referred for outpatient workup.

- Aneurysms most commonly occur infra-renal

- Watch out for intraluminal thrombus which can appear hyperechoic. That's why we measure outer wall to outer wall!

-Don’t mistake the IVC for aorta, especially in the longitudinal plane. Note that the IVC often has pulsatile flow and doppler may not be helpful.

Written by: Aalap Shah, MD

Peer reviewed by: Lori Stolz, MD, RDMS

All images obtained by Aalap Shah, MD and Pattie Smith, RDMS unless otherwise noted

References

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