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Neurological

Brain Anatomy

The brain comprises the cerebral hemispheres, deep gray nuclei, brainstem, and cerebellum, all supplied by the anterior and posterior circulations via the circle of Willis. MRI is the primary tool for parenchymal detail, while CT dominates acute stroke and trauma triage. Recognizing normal gray-white differentiation, vascular territories, and midline symmetry underpins detection of infarct, mass, and hemorrhage.

Key Structures

  • Cerebral lobes: frontal (motor, executive), parietal (sensory, integration), temporal (auditory, memory/hippocampus), occipital (vision), separated by central and Sylvian sulci
  • Deep gray nuclei: caudate, putamen, and globus pallidus (basal ganglia) with the internal capsule between them; thalamus and hypothalamus around the third ventricle
  • White matter tracts: corpus callosum (genu, body, splenium), internal capsule (anterior limb, genu, posterior limb) carrying corticospinal fibers
  • Brainstem: midbrain (cerebral peduncles, tectum, substantia nigra), pons (transverse fibers, fourth ventricle floor), medulla (pyramids, olives)
  • Cerebellum: vermis and hemispheres with folia, connected by superior/middle/inferior peduncles; tonsils near the foramen magnum
  • Ventricular system: paired lateral ventricles, third ventricle, aqueduct of Sylvius, fourth ventricle draining via foramina of Luschka and Magendie
  • Vasculature: circle of Willis linking ICAs and vertebrobasilar system via anterior/posterior communicating arteries; ACA, MCA, PCA territories

Imaging Planes & Modalities

  • MRI multi-sequence: T1 (anatomy), T2/FLAIR (edema, gliosis, demyelination), DWI/ADC (acute infarct, abscess, hypercellular tumor), GRE/SWI (blood/mineral), post-contrast T1
  • Axial is the default review plane; coronal best shows the hippocampi and temporal lobes; sagittal displays midline structures, corpus callosum, and brainstem
  • Non-contrast CT: first-line for acute hemorrhage and trauma; assesses gray-white loss, hyperdense vessel sign, and mass effect
  • CT/MR angiography for the circle of Willis, occlusions, aneurysms, and dissections; MR/CT venography for sinus thrombosis
  • MR perfusion and DWI mismatch guide stroke treatment; MR spectroscopy characterizes tumors and metabolic disease
  • Time-of-flight MRA provides flow-based vessel imaging without contrast

Landmarks & Normal Values

  • Foramen of Monro connects lateral to third ventricles; the pineal and habenular calcifications are normal midline markers
  • Evans index (frontal horn width to maximal inner skull width) >0.3 suggests ventriculomegaly
  • Normal fourth ventricle is midline; tonsils normally lie <5 mm below the foramen magnum (below is Chiari I)
  • The internal capsule posterior limb carries the corticospinal tract; its lesions cause pure motor deficits
  • Vascular territories: ACA (medial frontal/parietal), MCA (lateral hemisphere/basal ganglia), PCA (occipital, medial temporal, thalamus)
  • Cortical thickness ~2-4 mm; symmetric gray-white differentiation is expected

Anatomical Variants

  • Circle of Willis variants are common: hypoplastic/absent A1 or posterior communicating arteries, fetal-type PCA (PCA from ICA)
  • Persistent trigeminal artery and other carotid-basilar anastomoses
  • Cavum septum pellucidum and cavum vergae as normal midline CSF spaces
  • Asymmetric lateral ventricles and mega cisterna magna as normal variants
  • Venous variants including asymmetric transverse sinuses and prominent arachnoid granulations (mimic filling defects)
  • Prominent perivascular (Virchow-Robin) spaces that follow CSF signal and should not be mistaken for lacunes

Pathology Correlation

  • Vascular territory recognition distinguishes embolic (territorial) from small-vessel (lacunar) and watershed infarcts
  • Basal ganglia and thalamic locations are typical for hypertensive hemorrhage and deep lacunes
  • Herniation syndromes (subfalcine, uncal/transtentorial, tonsillar) follow the dural and foraminal anatomy
  • Periventricular and callosal FLAIR lesions suggest demyelination; specific tract involvement predicts deficits
  • Aneurysms cluster at circle of Willis branch points (anterior communicating, MCA bifurcation, posterior communicating)
  • Hippocampal volume/signal asymmetry correlates with mesial temporal sclerosis and epilepsy

Pitfalls & Mimics

  • Symmetric basal ganglia and dentate calcification is usually physiologic and should not be called pathologic
  • Pseudo-subarachnoid hemorrhage appearance from diffuse edema or low hematocrit; correlate clinically
  • Volume averaging near the skull base creates artifactual brainstem hypodensity on CT
  • Arachnoid granulations and asymmetric/hypoplastic sinuses mimic dural venous sinus thrombosis
  • T2 shine-through can be misread as restricted diffusion; always correlate DWI with the ADC map
  • Perivascular spaces mimicking lacunar infarcts or cystic lesions

References

  • Osborn. Osborn's Brain: Imaging, Pathology, and Anatomy
  • Naidich et al. Imaging of the Brain
  • Harnsberger. Diagnostic Imaging: Brain
  • Duvernoy. The Human Brain: Surface, Blood Supply, and Three-Dimensional Sectional Anatomy

⚕️ Educational reference — correlate with clinical context and confirm with a qualified radiologist.