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Posterior cerebral artery (P1 Segment)

The P1 segment of the posterior cerebral artery (PCA) is the proximal portion of the PCA, extending from the termination of the basilar artery at its bifurcation to the origin of the posterior communicating artery (PCoA). It lies in the interpeduncular cistern, coursing laterally and slightly posteriorly around the midbrain. The P1 segment supplies critical deep brain structures via its perforating branches, including the posterior thalamus, subthalamus, midbrain, and hypothalamus. It is an essential component of the Circle of Willis, connecting posterior circulation with anterior circulation through the PCoA.

Synonyms

  • PCA P1 segment

  • Proximal posterior cerebral artery

  • Pre-communicating segment of PCA

Function

  • Provides arterial supply to the midbrain, posterior thalamus, and hypothalamus via perforators

  • Serves as a key conduit in the Circle of Willis, linking basilar artery to anterior circulation through the PCoA

  • Contributes to collateral flow in cases of vertebrobasilar or carotid artery compromise

  • Supplies part of the posterior cerebral hemispheric territory indirectly via distal branches

MRI Appearance

T1-weighted images (non-contrast):

  • Appears as a flow void (signal loss) in the interpeduncular cistern, lateral to the midbrain

  • Vessel lumen not well visualized without angiography

T2-weighted images:

  • Also appears as a flow void, confirming vascular flow

  • Abnormal adjacent parenchymal hyperintensity may indicate ischemia or infarction in PCA territory

MRA (Magnetic Resonance Angiography):

  • TOF-MRA or contrast-enhanced MRA depicts the P1 segment from the basilar bifurcation to the PCoA origin

  • Normal appearance: bright, well-opacified lumen

  • Useful for detecting stenosis, occlusion, aneurysm, or hypoplasia of the P1 segment

T1 Post-Contrast (Gadolinium-enhanced MRI):

  • Vessel lumen demonstrates strong enhancement

  • Enhancing wall abnormalities may suggest vasculitis, dissection, or aneurysm

CT Appearance

Non-contrast CT:

  • Artery is not normally visualized directly

  • Acute thrombosis may present as a hyperdense vessel sign at the basilar bifurcation

CT Angiography (CTA):

  • Clearly demonstrates the P1 segment origin at the basilar artery bifurcation and course to the PCoA

  • Contrast highlights the lumen, allowing evaluation for stenosis, occlusion, aneurysm, or vascular malformations

  • CTA is highly sensitive for aneurysm detection at the basilar tip and P1–PCoA junction

MRI images

Posterior Cerebral Artery (P1 Segment