CT angiography of the circle of Willis (protocol)

CT angiography of the circle of Willis (also known as CTA COW) is a technique that allows visualisation of the arterial structure of the brain; specifically the circle of Willis. While digital subtraction angiography (DSA) remains the gold standard for the diagnosis of intracranial aneurysms especially, CTA is a less invasive, cost-effective, and more widely available technique ¹. 

NB: This article is intended to outline some general principles of protocol design. The specifics will vary depending on CT hardware and software, radiologists' and referrers' preference, institutional protocols, patient factors (e.g. allergy) and time constraints.

Indications:

CT angiography of the circle of Willis is indicated when characterisation of cerebral arterial circulation is required. Indications include:

• subarachnoid haemorrhage (SAH) – when subarachnoid blood is visualised on non-contrast imaging or a high index of suspicion remains to detect responsible aneurysms. 70%-85% of spontaneous SAH are caused by ruptured intracranial aneurysms ². CTA negative SAH will require further imaging with DSA
• arteriovenous malformation – Diagnosis, monitoring, and planning. Cannot be completely ruled out on CT angiography of the circle of Willis in patients with high suspicion and requires DSA studies to rule out completely ³
• intracerebral aneurysm – monitoring or concern in patients with significant family history. While screening is usually not recommended in children or adolescents, patients with two first-degree relatives with an intracerebral aneurysm or autosomal dominant polycystic kidney disease may be considered to undergo screening ³. Screening of patients with no risk factors is not recommended ⁴
• Stroke

Purpose

The purpose of a CT angiogram of the COW is to achieve maximum opacification of the circle of Willis in order to identify vascular structure abnormalities or bleeding subarachnoid vessels. Ideally, there should be minimal contrast within the dural venous sinuses; such an instance indicates that the timing is somewhat late.

Contraindications

• Previous severe reactions to iodinated contrast
• Patient non-compliance (movement)

Technique

• patient position

  • ​supine with their arms by the patient’s side

  • Hard palate perpendicular to the table (chin down)

• scout
  • C2 to vertex
• scan extent
  • C2 to vertex
• scan direction
  • caudocranial
• contrast injection considerations
  • monitoring slice
    • level of C2
  • threshold
    • Manualmanual trigger when contrast is seen within vertebral/carotid arteries
  • injection 
    • 18g-20g in antecubital fossa (preferably)
    • 60ml of non-ionic iodinated contrast with 50ml saline bolus at 4.5/5ml/s
• scan delay
  • minimal scan delay – wait for approximately 5s before contrast monitoring

Practical points

Take care when monitoring for contrast to appear before manually triggering the helical acquisition. Structures such as the styloid processes may be mistaken for arterial blush if the operator is overly zealous. Pay attention to any bony opacities on the monitoring slice before the injection of contrast to ensure there is no confusion as contrast begins to perfuse the arterial circulation.

While contrast within the venous system denotes that the scan is late, this usually does not warrant repeat imaging if the circle of Willis is well opacified. Some venous opacification is unavoidable as the circulation time from arteries to veins is around 3-6 seconds ⁵.

If unsure whether to include the carotid arteries and perform a CTA carotids, seek a radiologist's opinion.

Whether or not to include a non-contrast and/or post-contrast brain will depend on local protocol and indications for the scan.  

Postprocessing:

The processing of data is similar to that of a non-contrast CT brain in regard to image orientation. Axial, coronal, and sagittal multiplanar reformat may be generated at thicknesses to the preference of local departments. Windowing should ideally differentiate between IV contrast, calcified plaques, and soft tissues – (e.g WW650 WL150).

Additional post-processing may include maximum intensity projections, curved reformats, and shaded surface display volume rendering (SS-VRT).