CT abdominal aorta can be performed with or without contrast. The decision is based on the indication, clinical indications provided, and vascular access. Various scanning methods can be utilised depending on the scanner and patient demographics.
NB: This article relates to general protocol design, as the specifics depend on various factors such as scanner limitations, patient demographics, radiologist, and referrer preferences.
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Indications
The indications for a CT of the abdominal aorta vary depending on an emergency versus outpatient presentation 1.
Generally, the abdominal aorta is included in standard trauma imaging (chest-abdomen-pelvis), which includes an arterial chest and portal venous abdomen. Thus, specific abdominal aortic imaging is only requested when high suspicion exists for a particular pathology.
Specific abdominal aortic pathologies that are investigated through imaging in an emergency setting include 1:
abdominal aortic trauma
active haemorrhage
penetrating trauma
Outpatient environments focus on elective indications, including 2:
assessment and follow-up aortic aneurysms
pre - and post-procedure evaluation of endovascular aneurysm repair
Purpose
When the primary concern is aortic injury then it is helpful to perform a multiphase examination. In these situations, a non-contrast chest, abdomen and pelvis scan is followed by an angiogram, and potentially a delayed scan. This technique helps to identify acute haemorrhage (on the non-contrast study) and also enables differentiation of the true and false lumen in dissection 3.
This protocol is often utilised in the trauma settings in conjunction with the thoracic aorta to visualise the entire vascular tree and extends from the aortic arch to the ilio-femoral bifurcation 1. Inclusion of the renal arteries is essential to evaluate potential trauma to the kidneys.
In other trauma presentations, generally, a non-contrast or portal venous phase provides sufficient detail for the evaluation of the abdominal aorta. In elective studies, such as pre-operative assessments for abdominal aneurysms a multiphase technique is adopted.
The multiphasic imaging technique is common in aortic angiograms to demonstrate the vasculature best. This includes:
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non-contrast
best demonstrates intramural haematoma or thrombosed false lumen, which can indicate acute haemorrhaging within the aortic wall 1
additional information, such as calcified plaque, can also be visualised, which is essential for endovascular aortic repair planning 2
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arterial phase
arterial opacification of the vessel is essential in assessing intravascular pathology, particularly dissection (differentiates lumens) and active extravasation beyond the aortic wall 1
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delayed phase
to obtain adequate filling of an aneurysm or assess a delayed leak 2
Technique
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patient position
supine with patient arms above their head
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scout
diaphragm to iliofemoral arteries, inclusive of renal arteries and the bifurcation point
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scan extent
diaphragm to iliofemoral arteries, inclusive of renal arteries and the bifurcation point
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scan direction
craniocaudal
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contrast injection considerations
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monitoring slice (region of interest) - varies depending on scanner capabilities and department protocols
descending aorta at the level of the carina or renal arteries
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threshold
dependent on scanner but usually 150 HU 2
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injection
75-100 mL of non-ionic iodinated contrast with a 100 mL saline chaser at 4.5/5 mL/s
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multiphasic approach
non-contrast (optional)
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arterial
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contrast volume: 70-90 ml at 5 ml/s with a saline chaser of 50 ml
approximately 1 ml of contrast per kg
bolus track: descending aorta
scan delay: 8 seconds post injection for sufficient arterial filling
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portal venous
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contrast volume: 70-90 ml at 2-4 ml/s with a saline chaser of 50 ml
approximately 1 ml of contrast per kg
bolus track: descending aorta
scan delay: 70 seconds from the time of the injection 4
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scan delay
dependent on the phase used
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respiration phase
single breath-hold: inspiration
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multiplanar reconstructions
axial images: axial to the body axis
coronal images: coronal to the body axis
sagittal images: sagittal to the body axis
slice thickness: soft tissue ≤3 mm
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maximum intensity projection
axial images: axial to the body axis
coronal images: coronal to the body axis
sagittal images: sagittal to the body axis
slice thickness: soft tissue ≤3 mm
3D reconstruction using soft tissue thins
Practical points
patient positioning in all planes is essential for appropriate dose modulation
when thoracic aortic injuries are suspected such as an aortic dissection a gated chest and abdomen CT angiography is recommended 2
a split bolus technique is sometimes utilised to acquire an arterial and venous phase in the one acquisition, hence reducing radiation dose 2
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