Circle of Willis
What is the Circle of Willis?
A ring of arteries, located at the base of the brain, where the anterior hemisphere communicates with the posterior and the right side with the left.
What is the location of the PCoA?
It courses vertically between the MCA and PCA
What is the origin of the PCoA?
Cerebral ICA
What is another name for the cervical ICA?
Terminal ICA
How do collateral pathways form?
It may occur via preexisting vessels or via new branches formed between adjacent blood vessels when there is a blockage. The blood tends to flow into the vessels that will carry the blood to its destination the quickest.
What is the importance of the Circle of Willis?
The Circle of Willis allows for collateral circulation in the brain if there is an occlusion.
What is the location of the MCA?
Inferior to the ACA and superior to the PCoA, coursing horizontally
What is the origin of the MCA?
Terminal ICA
What are the segments of the MCA?
M1: Horizontal segment of MCA
M2: Sylvian segment of MCA
M3: Cortical of MCA
Identify the top three collateral route systems
Crossover collateralization: ACoA
External-to-internal collateralization: ECA to ICA via OA
Posterior-to-anterior collateralization: PCoA
What percent diameter reduction and cross-section affects the Circle of Willis?
60% diameter reduction and 80% area reduction
What is the location of the ACA?
Superior to the MCA, coursing vertically
What is the origin of the ACA?
Terminal ICA
What are the segments of the ACA?
A1: pre-communicating segment, joins contralateral A1 via ACoA
A2: post-communicating segment, distal to ACoA and joins PCA branches
A3-5: distal segments
How do collateral pathways affect flow?
They can help maintain normal blood flow, but maximal flow capacity is usually reduced.
What vessels make up the Circle of Willis?
A1 segment of ACA (2)
ACoA (1)
PCoAs (2)
P1 segment of PCA (2)
Terminal cerebral ICA (2)
What is the location of the ACoA?
What is the purpose of the ACoA?
To join the right and left anterior cerebral circulation
What are the terminal vessels of the basilar artery?
Posterior Cerebral Arteries
What is the location of the basilar artery?
Inferior to the PCA and superior to the vertebral arteries, coursing vertically
What percentage of patients have the "classic" Circle of Willis?
20-25%
What is the location of the PCA?
Inferior to the PCoA and superior to the basilar artery, coursing horizontally
What is the origin of the PCA?
Basilar bifurcation
What are the branches and termination vessels of the terminal ICA?
Branches: OA and PCoA
Termination vessels: ACA and MCA
What is the origin of the basilar artery?
unity of the right and left vertebral arteries