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How to Scan the Aorta

In many patients, due to body habitus and bowel/stomach gas, the normal aorta is difficult to visualise. Thankfully, in a patient presenting with a ruptured aneurysm, the dilated aorta is very easy to visualise on US. However, keep practising on patients without pathology so you get your technique and understanding of anatomy perfect before the hypotensive patient comes in. 

The indications for bedside ultrasound of the abdominal aorta is to look for an aneurysm or a dissection. Always look at the aorta in the 50+ year old with abdominal pain or loin pain or unexplained hypotension. 

Use the curvilinear transducer with the transducer marker to the patient's right. Start with the transducer transverse on the epigastrium with a depth of about 20cm. Hold the transducer with the thumb and the first and second fingers. Rest the other fingers and the heel of the hand on the patient's abdomen for stability. Aim the transducer face down into the bed as the aorta is just anterior to the vertebrae. 

The key to abdominal aorta US is to apply continuous graded downward pressure. This pushes the bowel gas out of the way like a curtain opening an US window to the aorta. The pressure needs to be continuous, otherwise the bowel gas just come right back up. 

Curvilinear transducer in the epigastrium: intermittent downward pressure causing the bowel gas to move back into the field of view

Curvilinear transducer in the epigastrium: continuous downward pressure causing the bowel gas to move out of the way. 

Once the bowel gas is out of the way, look for a vertebral body.  The vertebral body is seen on US as a hyperechoic convex line with shadowing posteriorly. On the screen, the aorta is anterior and slightly to the right of this; the IVC is to the left (or the probe marker side - WHICH IS THE RIGHT SIDE OF THE PATIENT). There should be no structures between the aorta and the vertebral body. This is very important as other vessels/structures may look like the aorta but do not have this association with the vertebral body. 

Once you see the vertebral body and shadow, decrease the depth so that the aorta is near the middle of the screen. 

Aim to visualise the aorta in transverse from the coeliac trunk to the iliac bifurcation. Typical landmarks for this are: coeliac trunk and SMA usually in the epigastrium, iliacs usually at the umbilicus. 

COELIAC TRUNK

This is the first branch of the aorta visualised with abdominal US. It gives off the splenic and hepatic arteries giving the appearance of a seagul in flight (seagull sign). 

SMA

The superior mesenteric artery is the next anterior branch of the aorta. It has its own mesentery so has a distinctive hyperechoic rim. The splenic vein is anterior to the SMA and the L renal vein is posterior to it. 

Patients may have aneurysms of the SMA which can rupture. The normal SMA should be <1cm.

ILIACS

The iliacs are seen side by side at the umbilicus. These also should be <1cm.  

AORTA LONG

Once you have a good view of the aorta in the axial plane, at about the midpoint between the epigastrium and the umbilicus, rotate your transducer so that the marker is cranial and visualise the aorta in the saggital (longitudinal) plane. As the vertebrae are posterior to the aorta, an on axis image should show lovely hyperechoic triangles of the vertebral spine posterior to the aorta. 

In this view, it's easy to slip and image the IVC instead of the aorta. Remember that the aorta has thick hyperechoic wall, has anterior branches and pulsates. Whereas the IVC has thin walls, has no anterior branches,  varies in size with respiration and does not have vertebral spines posteriorly.

MEASURING THE AORTIC DIAMETER

The normal aorta should be <3cm and should taper distally. Ensure that the aortic walls are crisp white (on axis imaging) and measure the diameter either in transverse or longitudinal from the outer edge of the anterior wall to the outer edge of the posterior wall. This is important because some aortic aneurysms have a hypoechoic thrombus attached to the wall. So if you only measure inner wall to inner wall (ie the diameter of the anechoic area), you will underestimate the size of the aneurysm. 

TROUBLESHOOTING

If you simply cannot image the aorta from the anterior abdomen, you could roll the patient left lateral decubitus so that the bowel gas moves to the RUQ or move the transducer to the LUQ. In the LUQ have the transducer as posterior as possible and fan slightly anteriorly to visualise the aorta.

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