Brain MRI Anatomy, Landmarking & Positioning

Brain MRI Anatomy, Landmarking & Positioning

Gross Cranial Neuroanatomy

A comprehensive understanding of cranial anatomy enables the Imaging Assistant to anticipate imaging coverage, assist in slice prescription verification, and recognize normal anatomical structures across standard imaging planes.

The Cerebrum & Cerebral Lobes

The cerebrum comprises two paired cerebral hemispheres separated along the midline by the deep longitudinal cerebral fissure, which houses the dural reflection known as the falx cerebri. The cerebral hemispheres are divided into four primary lobes:

• Frontal Lobe: Positioned anterior to the central sulcus (fissure of Rolando) and superior to the lateral sulcus (Sylvian fissure). Responsible for executive function, voluntary motor control (precentral gyrus / primary motor cortex), expressive language (Broca's area in the dominant hemisphere), and personality.
• Parietal Lobe: Located posterior to the central sulcus and superior to the lateral sulcus. Houses the postcentral gyrus (primary somatosensory cortex) and integrates sensory inputs including touch, proprioception, and spatial orientation.
• Temporal Lobe: Situated inferior to the lateral sulcus. Contains structures essential for auditory processing (superior temporal gyrus), receptive language comprehension (Wernicke's area in the dominant hemisphere), and memory consolidation (hippocampus and parahippocampal gyrus in the mesial temporal lobe).
• Occipital Lobe: Positioned at the posterior pole of the cranial vault, demarcated anteriorly by the parieto-occipital sulcus. Contains the primary visual cortex (calcarine fissure) and visual association areas.

Deep Grey Matter & White Matter Tracts

• Corpus Callosum: The largest commissural white matter tract connecting homologous regions of the left and right cerebral hemispheres. Divided anatomically from anterior to posterior into the rostrum, genu, body, and splenium. It forms the roof of the lateral ventricles.
• Basal Ganglia: Deep subcortical nuclear masses critical for motor coordination, procedural learning, and habit execution. Major structures include the caudate nucleus (with its prominent head abutting the frontal horn of the lateral ventricle), the putamen, and the globus pallidus (which together form the lentiform nucleus).
• Thalamus: Paired ovoid sensory and motor relay nuclei bordering the third ventricle.

The Brainstem & Cerebellum

• Brainstem: Extends inferomedially to connect the cerebral hemispheres to the spinal cord, containing autonomic respiratory and cardiovascular centers and cranial nerve nuclei (CN III–XII). Composed of three craniocaudal segments:
  1. Midbrain (Mesencephalon): Contains the cerebral peduncles, substantia nigra, red nuclei, and the tectum (superior and inferior colliculi). It is traversed by the narrow cerebral aqueduct of Sylvius.
  2. Pons: Characterized by prominent transverse pontine fibers, forming the anterior wall of the fourth ventricle and bridging to the cerebellum via the middle cerebellar peduncles.
  3. Medulla Oblongata: The most caudal segment, extending from the pontomedullary junction through the foramen magnum to transition seamlessly into the cervical spinal cord. Houses the anterior pyramids and inferior olivary nuclei.
• Cerebellum: Positioned within the posterior cranial fossa inferior to the tentorium cerebelli and posterior to the brainstem. Composed of two cerebellar hemispheres connected by the midline vermis, and the inferior cerebellar tonsils. In Chiari malformations, herniation of these cerebellar tonsils below the level of the foramen magnum constitutes a critical diagnostic finding.

The Ventricular System & CSF Pathways

Cerebrospinal fluid (CSF) is synthesized primarily by the choroid plexus within the ventricles, circulating through a continuous interconnected network:

1. Paired Lateral Ventricles: Located in each hemisphere, featuring frontal (anterior), temporal (inferior), and occipital (posterior) horns, and a central body and atrium (trigone).
2. Interventricular Foramina of Monro: Channels connecting each lateral ventricle to the slit-like midline third ventricle.
3. Third Ventricle: Positioned between the medial walls of the bilateral thalami and hypothalami.
4. Cerebral Aqueduct (Aqueduct of Sylvius): Narrow midbrain conduit linking the third ventricle to the fourth ventricle. Common site of non-communicating (obstructive) hydrocephalus.
5. Fourth Ventricle: Diamond-shaped cavity situated between the dorsal pons/medulla and ventral cerebellum. CSF exits into the subarachnoid space via the paired lateral foramina of Luschka and the single midline foramen of Magendie.

The Circle of Willis

An essential arterial anastomotic polygon situated at the skull base (interpeduncular cistern) supplying blood to the brain:

• Anterior Circulation: Bilateral internal carotid arteries (ICAs), giving rise to anterior cerebral arteries (ACAs) connected by the single anterior communicating artery (A-Comm).
• Posterior Circulation: Bilateral vertebral arteries join to form the basilar artery, which bifurcates into the bilateral posterior cerebral arteries (PCAs).
• Connecting Branches: Bilateral posterior communicating arteries (P-Comm) connect the ICAs to the PCAs, establishing collateral flow in the event of vessel occlusion or aneurysm formation.