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Middle cerebral artery horizontal segment (M1)

The horizontal segment (M1) of the middle cerebral artery (MCA) is the proximal portion of the MCA extending from its origin at the internal carotid artery bifurcation to its division into superior and inferior trunks at the limen insulae. It courses laterally within the sylvian fissure, giving off important lateral lenticulostriate arteries that supply the basal ganglia, internal capsule, and deep white matter. The M1 segment is clinically significant because it is a common site for atherosclerotic stenosis, embolic occlusion, and aneurysm formation, often resulting in large territory infarctions.

Synonyms

  • MCA M1 segment

  • Horizontal segment of MCA

  • Proximal middle cerebral artery

Function

  • Supplies blood to deep brain structures via lateral lenticulostriate arteries

  • Conveys flow to cortical branches of the MCA (frontal, parietal, temporal, opercular arteries)

  • Maintains perfusion of primary motor, sensory, and language areas through its downstream branches

  • Provides collateral circulation via anastomoses with anterior and posterior cerebral artery branches

MRI Appearance

T1-weighted images (non-contrast):

  • Appears as a signal void (black flow void) along the sylvian fissure due to fast blood flow

  • Vessel wall not well visualized without advanced sequences

T2-weighted images:

  • Also appears as a signal void, with surrounding CSF in the sylvian fissure as bright hyperintensity

  • Pathological changes such as thrombosis may be detected if flow void is lost

MRA (Magnetic Resonance Angiography):

  • TOF-MRA or contrast-enhanced MRA clearly demonstrates the origin, course, and caliber of the M1 segment

  • Normal artery appears as a well-defined enhancing vessel

  • MRA is excellent for detecting stenosis, occlusion, aneurysm, or collateral circulation

T1 Post-Contrast (Gadolinium-enhanced MRI):

  • Vessel lumen enhances strongly with contrast

  • Vessel wall imaging may demonstrate enhancement in cases of vasculitis, dissection, or plaque neovascularity

CT Appearance

Non-contrast CT:

  • M1 is not directly visible but may be inferred by sulcal landmarks

  • Acute thrombosis may appear as a hyperdense MCA sign at the M1 segment, an early indicator of large vessel occlusion

CT Angiography (CTA):

  • Provides excellent visualization of the origin, course, branching, and pathology of the M1 segment

  • Contrast opacifies the lumen, showing stenosis, occlusion, aneurysm, or vascular malformations

  • CTA is the modality of choice for stroke evaluation, surgical planning, and endovascular interventions

MRI images

Middle cerebral artery horizontal segment (M1)