How to evaluate lower extremity aneurysms on ultrasound

26th Feb 2021

During the ultrasound evaluation of a patient, it’s important to check for aneurysms. Lower extremity arterial aneurysms are seen more often in the popliteal artery and less often in the common femoral artery (CFA). 

Genetic disorders (such as Marfan’s syndrome) increase the chance of an aneurysm in other vessels. In patients without a genetic predisposition, the popliteal artery and the CFA are the most commonly affected vessels. This is believed to be because these two vessels are located in joint creases. So, they undergo frequent compression and can incur wall damage from repetitive microtrauma.

If a patient is found to have an abdominal aortic aneurysm, you’ll also want to spot check the CFA and popliteal artery for any evidence of aneurysms.

Lower leg arteries with common femoral artery and popliteal artery labeled. Illustration.

Figure 1. The two most common locations where a lower extremity arterial aneurysm can occur are within the popliteal artery and the common femoral artery (CFA).

 

Guidelines for defining an aneurysm on ultrasound

Aneurysms can be defined in two different ways on ultrasound:

  1. Comparison
  2. Direct measurement 

The most important way to define an aneurysm is by comparison. In the lower extremities, arteries are considered aneurysmal if the vessel’s diameter increases by 1.5 times compared to the segment proximal to the suspected aneurysm.  

With the direct measurement method, 10 mm is considered aneurysmal for the popliteal artery and 15 mm is considered mildly aneurysmal for the CFA.  

Aneurysm identified on longitudinal and transverse ultrasound using the comparison and direct measurement methods. Ultrasound images.

Figure 2. Aneurysms can be identified on ultrasound by comparison or direct measurement. With the comparison method, a bulge is considered an aneurysm if it is 1.5 times greater in diameter than the proximal segment. With direct measurement, a bulge is considered an aneurysm if it is greater than or equal to 10 mm in the popliteal artery or 15 mm in the common femoral artery.

 

Steps for measuring an aneurysm on ultrasound

There are four steps for measuring an aneurysm on ultrasound:

  1. Make the aneurysm as circular as possible in the transverse view. 
  2. Place the calipers from the outer wall to the opposite outer wall and include all of the mural thrombus. Measure the diameter of the aneurysm from anterior to posterior.
  3. Measure the nearest normal proximal segment for comparison. 
  4. Switch to a longitudinal view and measure the aneurysm to confirm the transverse measurement. 

Aneurysm on a transverse view ultrasound with the measurement from outer wall to outer wall highlighted. Ultrasound image.

Figure 3. When measuring an aneurysm on ultrasound in a transverse view, measure anterior to posterior from outer wall to outer wall and include the mural thrombus, if present.

When an artery has a stent graft placed to exclude a thrombotic aneurysm, you should still measure the aneurysm from the outer wall to the opposite outer wall in the transverse view. Also, check for an endoleak (a leak through the graft or at the graft end), which allows blood flow into the excluded aneurysm sac. An endoleak continually increases the size of the aneurysm and the risk of rupture.

A stent graft within a thrombotic arterial aneurysm that is displaying an endoleak. Illustration.

Figure 4. An artery with a stent graft that has been placed to exclude a thrombotic aneurysm should be checked with ultrasound for an endoleak. Endoleaks allow blood flow into the aneurysm sac and increase the chance of rupture. 

When taking measurements in a longitudinal view, also take anterior to posterior measurements (e.g., from the top of the screen to the bottom). The measurements should be taken as perpendicular as possible to the vessel walls to get the most accurate diameter. 

Aneurysm on ultrasound in a longitudinal view showing a measurement at 90 degrees to the vessel walls. Ultrasound image.

Figure 5. When measuring an aneurysm in a longitudinal view, measure from top to bottom (e.g., anterior to posterior) and take the measurement as perpendicular to the vessel walls as possible.

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