Vascular Territories in the Brain: Mapping & Pathology Insights

Understanding how blood flow is distributed in the brain is essential for diagnosing and managing cerebrovascular diseases. Each major artery supplies specific regions, and disruptions in these vascular territories lead to distinct neurological deficits.

Advancements in imaging have improved the ability to map these territories with precision, offering deeper insights into both typical anatomy and pathological changes.

Key Cerebral Arteries

The brain’s vascular network is primarily supplied by three major arteries: the anterior cerebral artery (ACA), middle cerebral artery (MCA), and posterior cerebral artery (PCA). Each serves distinct regions, and their occlusion leads to characteristic neurological impairments. Understanding these distributions is crucial for interpreting stroke patterns, guiding surgical interventions, and optimizing imaging techniques.

Anterior Cerebral Artery

The ACA arises from the internal carotid artery and extends along the medial surface of the cerebral hemispheres. It primarily supplies the medial portions of the frontal and parietal lobes, as well as the anterior corpus callosum. The artery is divided into several segments (A1–A5), with the A2 segment giving rise to the pericallosal and callosomarginal arteries, which perfuse the superior frontal gyrus and paracentral lobule.

Occlusion of the ACA often results in contralateral lower limb weakness due to ischemia in the paracentral lobule, which houses the motor cortex for the leg. Damage to the medial prefrontal cortex can lead to apathy, impaired judgment, and executive dysfunction. A study published in Stroke (2021) found that ACA infarcts account for approximately 5% of ischemic strokes, with embolism from the heart or large arteries being the most common cause.

Middle Cerebral Artery

The MCA is the largest of the three cerebral arteries, supplying the lateral aspects of the frontal, temporal, and parietal lobes, as well as the basal ganglia and internal capsule via its lenticulostriate branches. It originates from the internal carotid artery and bifurcates into superior and inferior divisions, which perfuse different cortical areas.

MCA strokes account for nearly 40% of ischemic strokes, according to a meta-analysis in The Lancet Neurology (2022). Infarction in this territory can lead to hemiparesis, predominantly affecting the face and upper limb, contralateral sensory loss, and, if the dominant hemisphere is involved, aphasia. Damage to the nondominant hemisphere may result in neglect and visuospatial deficits. The lenticulostriate arteries, which lack collateral circulation, are particularly vulnerable to small vessel disease, making them a frequent site for lacunar infarcts. These infarcts can manifest as pure motor strokes or sensorimotor syndromes.

Posterior Cerebral Artery

The PCA originates from the basilar artery and supplies the occipital lobe, inferior temporal lobe, thalamus, and parts of the midbrain. It is divided into P1–P4 segments, with the P2 segment giving rise to thalamogeniculate and posterior choroidal branches, which perfuse the thalamus and hippocampus.

Ischemia in this territory commonly results in homonymous hemianopia due to occipital cortex involvement, with macular sparing often present due to collateral supply from the MCA. If the thalamus is affected, patients may develop thalamic pain syndrome (Dejerine-Roussy syndrome), characterized by persistent, severe pain on the affected side. According to a review in Brain (2023), PCA strokes represent about 10% of ischemic strokes, with embolism from the heart or vertebrobasilar system being a leading cause. Bilateral PCA infarcts, though rare, can lead to cortical blindness.

Watershed Zones

Watershed zones, also known as border zones, are regions of the brain at the junctions between two major arterial territories. These areas are particularly vulnerable to ischemia because they rely on the most distal branches of adjacent arteries, making them susceptible to reductions in systemic perfusion. Unlike infarcts caused by direct arterial occlusion, watershed infarcts often arise from systemic hypoperfusion or embolic showers.

Two primary types of watershed infarcts have been described: cortical and subcortical. Cortical watershed infarcts occur at the brain’s surface, typically between the ACA and MCA territories or between the MCA and PCA territories. These infarcts often present with “man-in-the-barrel” syndrome, characterized by proximal limb weakness that spares the hands and feet. A study in Neurology (2021) found that patients with cortical watershed infarcts frequently exhibit impaired executive function and attention due to involvement of frontal and parietal association areas.

Subcortical watershed infarcts occur in the deep white matter between the penetrating branches of the MCA and the deep branches of the ACA or PCA. These infarcts are often associated with prolonged hypotension or cardiac arrest. Clinically, they can manifest with motor impairments resembling lacunar syndromes but are more likely to present with additional cognitive deficits due to the involvement of association fibers in the centrum semiovale. A meta-analysis in Stroke (2022) reported that subcortical watershed infarcts are strongly linked to chronic cerebral hypoperfusion in conditions such as carotid artery stenosis.

Unlike territorial strokes, which result from major arterial occlusion, watershed infarcts often arise from a drop in cerebral blood flow, making them more common in settings of severe hypotension, cardiac surgery, or prolonged hypoxia. Embolic contributions have also been noted, particularly in patients with atrial fibrillation. Advanced imaging techniques, such as perfusion MRI and arterial spin labeling, have improved the identification of watershed infarcts by detecting subtle reductions in cerebral blood flow before irreversible tissue damage occurs.

3D MRI Techniques for Mapping Territories

Advancements in magnetic resonance imaging (MRI) have revolutionized the ability to delineate vascular territories in the brain with remarkable precision. Traditional imaging modalities, such as computed tomography angiography (CTA) and two-dimensional MRI sequences, provide structural insights but often lack the spatial resolution necessary to map the intricate boundaries of cerebral perfusion. The emergence of three-dimensional MRI techniques has bridged this gap, enabling detailed visualization of arterial distributions and their variations across individuals.

Time-of-flight (TOF) and contrast-enhanced magnetic resonance angiography (MRA) generate volumetric reconstructions of the brain’s arterial network, allowing non-invasive mapping of major cerebral vessels and their branching patterns. When combined with arterial spin labeling (ASL), which quantifies cerebral perfusion without contrast agents, these imaging methods highlight territorial boundaries based on blood flow distribution. This has proven particularly useful in identifying subtle perfusion deficits that may not be apparent on conventional imaging.

Functional MRI (fMRI) has played a growing role in understanding vascular territories by tracking changes in oxygenation-dependent signals. This has been instrumental in preoperative planning for neurosurgical procedures, where preserving eloquent brain regions supplied by specific arteries is paramount. Techniques such as dynamic susceptibility contrast (DSC) and dynamic contrast-enhanced (DCE) MRI further refine this process by assessing cerebral hemodynamics in real time.

Typical Variations in Vascular Supply

The cerebral vasculature exhibits considerable anatomical variability, with differences in arterial branching patterns and collateral circulation influencing susceptibility to ischemic events. One of the most well-documented variations involves the circle of Willis, a critical arterial structure that provides redundancy in cerebral blood supply. While a complete and well-balanced circle of Willis is present in only about 50% of individuals, anatomical deviations such as hypoplastic or absent communicating arteries can significantly alter perfusion dynamics.

The posterior cerebral artery (PCA) classically arises from the basilar artery, but in approximately 20% of cases, it originates directly from the internal carotid artery—a configuration known as the fetal-type PCA. This variant alters the relative contributions of the anterior and posterior circulations.

The lenticulostriate arteries, which supply deep subcortical structures, also exhibit significant variability in number and diameter. These small penetrating vessels originate from the MCA and are end-arteries, meaning they lack collateral connections. Individuals with fewer or narrower lenticulostriate branches may be more predisposed to lacunar infarcts, which are commonly associated with chronic hypertension and small vessel disease.

Identifying Territory-Specific Patterns in Pathology

Different vascular territories in the brain give rise to distinct pathological patterns when blood supply is disrupted. Stroke syndromes provide some of the most well-characterized examples, with deficits aligning closely with the regions supplied by the affected artery.

ACA infarcts typically result in contralateral lower limb weakness and may also present with abulia or impaired motivation. MCA strokes frequently produce hemiparesis affecting the face and upper limb, often accompanied by aphasia if the dominant hemisphere is involved. PCA infarcts primarily cause visual disturbances, such as homonymous hemianopia, but may also lead to memory impairment if the hippocampus is affected.

Beyond ischemic stroke, other pathologies demonstrate territory-specific patterns. Neurodegenerative diseases, such as Alzheimer’s, often show early involvement of PCA-supplied regions. Small vessel disease predominantly affects deep white matter, contributing to vascular cognitive impairment. Recognizing these patterns enhances diagnostic accuracy and informs targeted treatment strategies.