CT angiography of the cerebral arteries (protocol)

CT angiography of the cerebral arteries (also known as a CTA carotids or an arch to vertex angiogram) is a noninvasive technique allows visualization of the internal and external carotid arteries and vertebral arteries and can include just the intracranial compartment or also extend down to the arch of the aorta.

By using multidetector CT (MDCT) after intravenous contrast administration, the vessels become enhanced with contrast allow them to be differentiated from adjacent tissues.

Following image acquisition, post-processing techniques are applied for better 3D visualization  The overarching goal of this examination is an optimal enhancement of the vesselscarotid arteries with little to no venous component, the technical aspect of the examination will vary from site to site. 

Indications

CTA of the cerebral arteries is indicated in a wide variety of clinical scenarios including:

• ischaemic stroke to detect occlusion and their abnormalities.thrombosis
• transient ischemic attack to detect carotid artery stenosis
• subarachnoid haemorrhage for detection of aneurysms
• cerebral parenchymal haemorrhage to assess for the presence of a vascular malformation or ongoing bleeding (spot sign)

Anatomical considerations

The brain is supplied by two sets of vessels. The anterior circulation roughly supplies the anterior 2/3 while the posterior circulation supplies the posterior 1/3 of the brain.

The anterior circulation consists of the carotid arteries. The right common carotid artery arises from the brachiocephalic trunk while the left common carotid artery arises directly from the aorta. They ascend behind the sternoclavicular joints, lateral to the thyroid gland and at the level of the upper border of the thyroid cartilage, each one divides into external and internal carotid arteries. The internal carotid artery ascends to enter the skull through the carotid canal and bifurcates into the anterior and middle cerebral arteries.

The posterior circulation consists of the vertebrobasilar system. Each vertebral artery arises from the 1^st part of the corresponding subclavian artery. At the neck, it ascends inside the transverse foramina from C6 to C2 where it emerges and enters the cranium through the foramen magnum. Both vertebral arteries unite to form the basilar artery which passes in front of the pons and ends by bifurcation to posterior cerebral arteries on both sides.

IndicationsTechnique

CTA

• patient position
  • supine with their arms by their side
• scout
  • mid chest to vertex
• scan extent
  • aortic arch to vertex
• contrast injection considerations
  • monitoring slice (region of interest) 
    • descending aorta
  • threshold
    • 100 HU
  • injection 
    • 50-75 ml of non-iconic iodinated contrast with a 100 ml saline chaser at 4.5/5 ml/s
• scan delay
  • minimal scan delay 

Practical points

Some centres may place a monitoring region of interest on the cerebralcarotid arteries is indicated, the delay from engaging the scan to the scanner moving into its initial inferior starting point can result in a widevenous component. Tracking at the descending aorta attempts to maximise scan start efficiency and minimise venous interference.

Post-processing

CTA images are usually presented as axial and coronal and/or sagittal multiplanar reformat of a variety of clinical scenarios includingthicknesses depending on local preference.

Additional post-processing techniques include:

• maximum intensity projection MIP to detect occlusion and thrombosis: displays pixels with higher CT value
• curved planar reformats to detect carotid artery stenosis: delineates the entire course of the vessel and can be used when the vessel is tortuous
• shaded surface display volume rendering (SS-VRT): only the pixels above a user-defined threshold value are displayed as a 3D. It is used mainly for detection of aneurysms
• cerebral parenchymal haemorrhage to assess for the presence of a vascular malformation or ongoing bleeding (spot sign)pre-operative planning

Contraindications

• known hypersensitivity to iodinated contrast media

Advantages

CTA has a number of advantages over catheter angiography including the ability not only to evaluate the vessels from their origin at the aortic arch to the intracranial portion but also assess non-vascular neck structures and brain parenchyma. 

It is also less expensive and at lower risk to the patient.

Disadvantages

There are few disadvantages compared to catheter angiography, however, the inability to select a single vessel and the fact that it images the vessel at only one time does limit the ability to evaluate flow related features of complex malformations. It also has lower resolution than catheter angiography making the assessment of subtle wall changes, such as those seen in dissection or vasculitis, more difficult to identify. 

Compared to MR angiography, its main disadvantage is the need for both ionising radiation and intravenous contrast.