How to evaluate lower extremity aneurysms on ultrasound

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.

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.

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.

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.

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.