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.

Brain Anatomy

🧩 Key Structures

🎞️ Imaging Planes & Modalities

📏 Landmarks & Normal Values

🔀 Anatomical Variants

🧠 Pathology Correlation

💡 Pitfalls & Mimics