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| Abdominal Aorta |
What are the indications for an abdominal aorta scan?
Indications for abdominal aorta ultrasound scan:
- Pulsatile abdominal mass.
- Pain in the midline of the abdomen.
- Poor circulation in the legs.
- Recent abdominal trauma.
- Suspected idiopathic aortitis (patient under the age of 40 with vascular symptoms relevant to the aorta or major branches).
Preparation of the patient for an abdominal aorta ultrasound scan
Preparation of the patient
For an abdominal aorta scan, The patient should take nothing by mouth for 8 hours preceding the examination. If the fluid is desirable, only water should be given. If the symptoms are acute, proceed with the examination.
For Infants- clinical condition permitting- should be given nothing by mouth for 3 hours preceding the examination.
Position of the patient
For an abdominal aorta scan, The patient should lie comfortably on his/her back (supine). The head may rest on a small pillow and, if there is much abdominal tenderness, a pillow may also be placed under the patient's knees.
Apply coupling agent down the midline of the abdomen over a width of 15cm from the ribs to the pubic symphysis.
Scanning is best performed with the patient holding the breath in, but he/she may breathe quietly until a specific region needs more careful examination.
Choice of transducer
For abdominal aorta ultrasound scans, use a 3.5 MHz transducer for adults.
Use a 5 MHz transducer for children or thin adults.
Setting the correct gain
Start by placing the transducer centrally at the top of the abdomen (the xiphoid angle).
Angle the beam to the patient's right side to image the liver; adjust the gain to obtain the best image.
What is the scanning technique for an abdominal aorta scan?
Move the transducer back to the midline and move it slowly towards the left until a pulsatile tubular structure is located. Follow this downwards to below the umbilicus, where the structure can be seen to divide: this is the bifurcation of the aorta.
Use transverse imaging to measure the cross-sectional diameter of the aorta at various levels, Image the iliac arteries by angling slightly to the right or left just below the bifurcation of the aorta.
Whenever a localized irregularity or variation is seen in the aorta, Scan transversely at that level and closely above and below it. In elderly patients, the course of the aorta may vary and there can be some displacement or change in direction, but the diameter of the aorta should not change significantly. If the aorta cannot be identified, scan through the back towards the left kidney.
If there is intervening bowel gas
If there is intervening bowel gas, apply gentle pressure and angle the transducer; use an oblique or lateral projection if necessary, and scan from either side of the spine. Occasionally an erect scan may be used to displace the gas-filled bowel.
How to measure the normal cross-sectional diameter of the adult aorta?
The normal cross-sectional diameter of the adult aorta, measured as the maximum internal diameter, varies from about 3cm at the xiphoid to about 1 cm at the bifurcation. Transverse and ventricle diameters should be the same.
Measurements should be taken at various points down the length of the aorta. Any significant increase in diameter causally (towards the feet) is abnormal.
Measurements of normal aorta
- Upper- 2-3 cm
- Middle- 1.5-2.5 cm
- Lower- 1-2 cm
- Iliac- 1cm or less
Aortic displacement of the abdomen
The aorta may be displaced by scoliosis, a retroperitoneal mass, or by para-aortic lymph nodes; these can mimic an aneurysm in some patients. Careful transverse scans will be needed to identify the pulsating aorta: lymph nodes or other extra-aortic masses will be seen posteriorly or surrounding the aorta.
If any cross-section of the aorta is more than 5 cm in diameter, an urgent referral for clinical assessment is required. There is a high risk that an aorta of this size will rupture.
Aortic dissection of an abdomen
The aorta may dissect at any level and over a short or long section. Dissection occurs most commonly in the thoracic aorta, which is difficult to image with ultrasound. The image of dissection may suggest a double aorta or double lumen. The presence of an intraluminal clot (thrombus) can also be very , misleading because the lumen will then be narrowed.
Whenever there is a change in aortic diameter, either enlargement or narrowing, dissection should be suspected. Longitudinal and transverse scans are essential to display the full length of the dissections; oblique scans are also necessary to show clearly the full extent.
When an aortic aneurysm or dissection is diagnosed, the renal arteries must be located prior to surgery to see whether they are involved. If possible, the state of the iliac arteries should be demonstrated.
The clinical recognition of a highly pulsatile mass in the midline of the abdomen is an indication for an ultrasound scan.
Narrowing of the aorta
Any localized narrowing of the aorta is significant and should be visualized and measured in both diameters, with both longitudinal and transverse scans to show the extent of the narrowing.
Atheromatous calcification should be assessed throughout the length of the aorta. Whenever possible, the aorta should be traced pat the bifurcation into the left and right iliac arteries, which should also be examined for narrowing or widening.
In elderly patients, the aorta may be tortuous or narrowed because of arteriosclerosis, which may be focal or diffuse. Classification of the aortic wall will produce focal areas of acoustic shadowing on the scan. A thrombus may develop, especially at the bifurcation of the aorta, followed by occlusion of the vessel. Doppler ultrasound or aortography (contrast radiography) may be necessary. Each part of the aorta should be examined before stenosis or dilatation is diagnosed.
When the patient has had surgical repair of the aorta (Aortic prosthesis)
When the patient has had surgical repair of the aorta, it is important to assess the position and calibre of the prosthesis and, using transverse sections, to exclude dissections or leakage. Fluid adjacent to a recently inserted graft may be due to haemorrhage, but can also be due to localized post-surgical oedema or infection. Correlation with the clinical condition of the patient and follow-up ultrasound scans are essential. In all cases, the full length of the prosthesis must be examined, together with the aorta and below it.
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