Objective: Evaluate for abdominal aortic aneurysm

Anatomy:

The abdominal aorta travels retroperitoneally just 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:

By Edoarado [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

By Edoarado [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

 

Aneurysms will appear as areas of dilatation along the aorta itself. These are most commonly fusiform, as below, but can be saccular as well. 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 dimension.

By BruceBlaus (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

By BruceBlaus (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

By Bakerstmd (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

By Bakerstmd (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

 

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

 

The Aortic Exam

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:

 
Derivative of work by By BruceBlaus (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

Derivative of work by By BruceBlaus (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

  1. Orient the probe in the transverse plane, with the probe marker facing to the patient’s right. Place the probe just inferior to the xiphioid process

  2. Adjust the depth such that you can identify the vertebral shadow in the far field of your image. This will appear as a densely hyperechoic, concave down shadow (frowny-face)

  3. Identify the Aorta, which will appear as a hypoechoic, pulsatile, thick walled circular structure just anterior to the vertebral shadow. (The aorta will tend to run just slightly right of midline on the screen. The IVC will appear just to its left, and will have compressible walls)

  4. Identify the celiac trunk, which will appear as an anterior branch off the aorta. The celiac will then branch into the hepatic and splenic arteries in the “seagull sign”. (You may not always be able to visualize this area, as it is often too proximal. It is useful for orienting yourself to the level of aorta you are imaging, but not necessary for a scan to be 'adequate'.)

  5. Fan/Slide the probe inferiorly. Note the branching  of the SMA which will appear as a small circular structure with hyperechoic walls, just anterior to the aorta. You may also see the left renal vein passing between aorta and SMA, as well as the splenic vein anterior to SMA'=

  6. Measure the aorta in its longest diameter at this level (outer wall to outer wall)

Transverse view of proximal aorta.

Note concave down shadow of vertebral body in far field, with pulsatile, thick walled aorta anterior. The IVC is visible just to the left of the aorta on the screen. 

 

Transverse view of proximal aorta with doppler

Note that both IVC and aorta have pulsatile flow. One must rely on anatomical position and ultrasound appearance of the wall to differentiate the two.

Proximal aorta at the level of Celiac Trunk/Superior Mesenteric artery. Image transverses proximal to distal in the transverse plane.

The celiac trunk is visible proximally with initial branch coming from anterior aorta. It gives off the hepatic and splenic arteries in the 'seagull sign'

The SMA originates just distal to the celiac trunk. It is visible in transverse view just anterior to the aorta. Note the left renal vein communicating with the IVC after it passes horizontally between the aorta and SMA.

 

 

Another, clearer example of the 'seagull sign' of the celiac trunk as it gives of hepatic and splenic arteries

 

Mid Aorta

 

Longitudinal view at same level as above

Note Celiac trunk branching off the Aorta proximally

Just below the celiac origin, the SMA is clearly visible as it branches off the aorta and takes a path pointing inferiorly.

 

 
Derivative of work by By BruceBlaus (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

Derivative of work by By BruceBlaus (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

 

 

  1. Continue sweeping the probe caudally in the transverse plane, keeping the aorta in view the entire time

  2. Ideally, visualize the renal arteries as you slide down the aorta. (This is often difficult)

  3. Obtain a measurement of the mid aorta in its longest dimension. (If you can visualize the renal arteries, care should be taken to obtain measurement infra-renally, as this is the most common location for aneurysms to occur)

 

 

 I

 

 

 

Image obtained while sweeping transversely down the aorta. Note lack of clearly visible branches. This lack of clearly identifying landmarks is typical for the midportion of the aorta.

If you look closely, a portion of the L renal artery is visible as it branches off of the aorta!

 

 

 

 

 

Distal Aorta

Derivative of work by By BruceBlaus (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

Derivative of work by By BruceBlaus (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

 
  1. Continue sweeping the probe caudally towards the umbilicus, keeping the aorta in view the entire time

  2. Watch the aorta carefully for its bifurcation into the iliac branches.

  3. Measure the diameter in the transverse plane both before and after its bifurcation.

 

 

 

 

 

 

Image obtained while sweeping probe inferiorly at the level of the distal aorta, through the bifurcation into the common iliac arteries.

 

 

 

 

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)

 

When imaging in the longitudinal plane, take special care to ensure that you are imaging the aorta and not the IVC

This image which fans from aorta to IVC as the probe is fanned from patients left to right. It shows that even a slight angle change of the probe can lead to false visualization of the IVC.

Always look for wall pulsatility, thick muscular wall of the aorta, and branching vessels to ensure that you are in the correct plane.

 

 

 

 

 

Image Interpretation:

 

Pattie's 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.

- 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:

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


References