Basal Ganglia Anatomy and Imaging

Overview

This lecture covers the anatomy, imaging identification, and clinical significance of the basal ganglia (basal nuclei) using MRI and CT scans.

Orientation and Basal Nuclei Location

• Basal nuclei are best visualized at the level of the lateral and third ventricles on axial MRI.
• The internal capsule separates the head of the caudate nucleus from the lentiform nucleus (putamen and globus pallidus).
• The thalami are located on either side of the third ventricle.

Structures of the Basal Nuclei

• The caudate nucleus has a head (bulky, anterior), body (runs lateral to ventricles), and tail (thin, follows lateral ventricles to temporal horns).
• The putamen lies lateral to the internal capsule and is bordered by the external capsule and globus pallidus.
• The globus pallidus (medial to putamen) has external (lateral) and internal (medial) segments and, together with putamen, forms the lentiform nucleus.
• The caudate and putamen together are called the neostriatum or dorsal striatum, acting as a functional unit.

Additional Basal Nuclei Components

• The subthalamic nuclei are situated just below and anterior to the thalamus, superior and slightly anterior to the red nuclei.
• The substantia nigra (dopamine-rich) lies anterior to red nuclei and posterior to cerebral peduncles in the midbrain.

Imaging Modalities and Identification

• On T1 MRI, gray matter (basal nuclei) appears darker than white matter; on T2, gray matter is lighter and CSF is bright.
• The substantia nigra is best visualized on susceptibility-weighted imaging (SWI) due to its dopamine content.
• On CT, basal nuclei appear lighter than surrounding white matter; loss of gray-white differentiation may suggest infarct.

Clinical Relevance

• Early ischemic changes can be seen as loss of gray-white differentiation at the caudate head on CT.
• The caudate head is supplied by branches of the anterior cerebral artery (A1/A2).
• Diseases can cause abnormal brightness of the globus pallidus or subthalamic nuclei on imaging.

Key Terms & Definitions

• Basal Nuclei (Basal Ganglia) — Gray matter structures in the CNS involved in motor control.
• Caudate Nucleus — C-shaped basal nucleus with head, body, and tail segments.
• Putamen — Lateral portion of the lentiform nucleus, functionally paired with caudate.
• Globus Pallidus — Medial segment of the lentiform nucleus, subdivided into external and internal parts.
• Neostriatum (Dorsal Striatum) — Collective term for caudate and putamen, functioning together.
• Subthalamic Nucleus — Small nucleus located below the thalamus.
• Substantia Nigra — Midbrain nucleus rich in dopamine, implicated in Parkinson’s disease.
• Internal Capsule — White matter tract separating basal nuclei structures.
• Cisterns (Quadrigeminal, Interpeduncular, Ambient) — CSF-filled spaces near the midbrain.

Action Items / Next Steps

• Review labeled MRI and CT images of the basal ganglia and associated structures.
• Practice identifying basal nuclei and related anatomy on different imaging modalities.
• Read about clinical syndromes associated with basal ganglia infarcts and movement disorders.