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 non-invasive technique that 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. The overriding goal of this examination is an optimal enhancement of the carotid arteries with little to no venous component; the technical aspect of the examination will vary from site to site. 

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

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

• ischemic stroke to detect occlusion and thrombosis

• transient ischemic attack to detect carotid artery stenosis

• subarachnoid hemorrhage for detection of aneurysms

• cerebral parenchymal hemorrhage to assess for the presence of a vascular malformation or ongoing bleeding (spot sign)

Purpose

The CTA of the cerebral arteries is performed to demonstrate the full patency of the cerebral arteries via contrast enhancement.  It is important to time the scan as accurately as possible (see practical points) to ensure maximal opacification and decrease venous contamination. 

Contraindications 

• previous severe reactions to iodinated contrast media

• non-compliance

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.

Technique

• patient position

  • supine with their arms by their side

• scout

  • mid-chest to vertex

• scan extent

  • aortic arch to vertex

• scan direction

  • caudocranial 

  • can be performed craniocaudal to minimize venous contamination in the head portion of the scan, often utilized in slower scanners

• contrast injection considerations

  • monitoring slice (region of interest) 

    • descending aorta

  • threshold

    • 100 HU

  • injection 

    • 50-75 ml of non-ionic iodinated contrast with a 100 ml saline chaser at 4.5/5 ml/s

• scan delay

  • minimal scan delay 

• ​respiration phase

  • ​suspended 

Practical points

Some centers may place a monitoring region of interest on the carotid arteries, granted the scanner has a short diagnostic delay this is adequate. However, scanners with a longer diagnostic delay, combined with a transit time to the initial stages of the scan (inferior movement from carotid to arch) may result in an overly venous scan

Tracking at the descending aorta attempts to maximize scan start efficiency and minimize venous contamination.

Post-processing

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

Additional post-processing techniques include:

• maximum intensity projection MIP: displays pixels with higher CT value

• curved planar reformats: 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 pre-operative planning

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 to assess non-vascular neck structures and brain parenchyma. Additionally, CTA allows multiplanar reconstruction and better appreciation of plaque characteristics and morphology ⁷.

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

Disadvantages

There are a 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 a 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 ionizing radiation and intravenous contrast.