Ch14: Neuroimaging — Brain Tumors, Stroke & Hemorrhage（颅脑疾病影像）

# Ch14: Neuroimaging — Brain Tumors, Stroke & Hemorrhage（颅脑疾病影像）

Preparatory Mindset

Brain disease imaging splits into tumors, vascular disease (infarction, hemorrhage, aneurysm/AVM), demyelination, degeneration, infection and trauma. The first question for any intracranial mass: is it intra-axial (within the brain — metastasis or astrocytoma in 75% of adult cases) or extra-axial (outside the brain — meningioma or schwannoma)? Extra-axial lesions push the brain away: look for CSF cleft, displaced subarachnoid vessels, gray matter between lesion and white matter, and a widened subarachnoid space.

Core Concepts

Intra-axial vs extra-axial

• Extra-axial signs: CSF cleft between lesion and brain; subarachnoid vessels displaced; gray matter between lesion and white matter; subarachnoid space widened (lesion pushes brain away). Meningioma has a broad dural base / dural tail of enhancement and adjacent bone hyperostosis.
• Intra-axial: arises within brain parenchyma (astrocytoma/GBM, metastasis, oligodendroglioma).

Meningioma (extra-axial, most common non-glial intracranial tumor)

• Arises from meninges of the vault, falx or tentorium; commonest sites: parasagittal, cerebral convexities, sphenoid ridges.
• WHO: Grade I typical/benign 88–94%; Grade II atypical 5–7%; Grade III anaplastic 1–2%.
• CT: hyperdense mass, calcification; homogenous enhancement on CECT; adjacent bone hyperostosis.
• MRI: iso- or slightly hypointense on T1WI and T2WI; broad-based dural attachment; dural tail sign; homogenous enhancing globose mass.

Glioma / astrocytoma (intra-axial)

• Glioma = non-specific term for tumors from glial cells (astrocytes — astrocytoma is the commonest).
• Low-grade astrocytoma: hyperintense on T2, no or mild enhancement; imaging may be subtle.
• Glioblastoma multiforme (GBM) — most common primary CNS malignancy: large size, crosses the midline, ill-defined outer margin, strong enhancement with irregular central necrosis (cauliflower pattern), peripheral white matter edema; subcortical white matter location.
• Oligodendroglioma: like low-grade astrocytoma but always contains calcification; mild hyperintensity T1, hyperintensity T2, no/mild enhancement.

Vestibular schwannoma (acoustic neuroma) (extra-axial)

• Arises on the nerve within the internal auditory canal, extends medially into the cerebellopontine angle (CPA).
• 80% of CPA masses; solid nodular with cystic degeneration; enlargement of the ipsilateral internal auditory canal.

Pituitary tumors

• Macroadenoma (>1 cm) vs microadenoma (<1 cm).
• Macroadenoma extends superiorly; indentation at the diaphragm sella → snowman appearance; compresses the optic chiasm; cavernous sinus invasion.
• Microadenoma: ~2/3 hypoattenuating on dynamic contrast-enhanced CT/MRI (enhances less than normal gland, best seen 30–60 min after contrast); patients present with hyperprolactinemia.

Brain metastases

• 25–30% of intracranial tumors in hospital patients.
• 80% from 5 primaries: lung, renal cell, breast, melanoma, GI adenocarcinomas (mostly colorectal).
• 80% localize to cerebral hemispheres, 15% cerebellum, 3% basal ganglia; often at the gray-white matter junction.
• "Small nodule, large edema" sign; typically sphere and well circumscribed (vs GBM's complex shape); rim-enhancing with necrotic center; abscess shows central restricted diffusion (DWI).
• Metastasis vs GBM: metastasis at gray-white junction, sphere, well circumscribed, less deep brain involvement; GBM in subcortical white matter, complex shape, extends to ependymal surface.

Cerebral hemorrhage

• Hypertension is the most common cause (basal ganglia, cerebellum, brainstem).
• Clinical presentation depends on region and size; may break into the ventricle.
• Intracerebral hemorrhage (ICH): hyperdense on CT; hypertensive basal ganglia hemorrhage is a master case.

Cerebral infarction (stroke)

• MCA is the most commonly affected territory.
• CT: extensive low attenuation in the MCA distribution (extent); acute infarct: restricted diffusion on DWI (bright), dark on ADC.
• Old infarction → encephalomalacia with compensatory dilatation of the adjacent lateral ventricle.
• CTA/MRA to identify occlusion; perfusion for penumbra.

Aneurysm

• Abnormal focal dilation of an artery; MRA demonstrates (e.g. right MCA aneurysm near its origin); subarachnoid hemorrhage from aneurysm (basilar artery aneurysm example).

Trauma — intracranial hematomas

• Epidural hematoma (EDH): extra-axial blood external to the dura; lentiform (biconvex) shape; does not cross cranial sutures; associated skull fracture; often middle meningeal artery injury.
• Subdural hematoma (SDH): crescentic, beneath the dura; can cross cranial sutures (but not midline falx); acute = hyperdense crescent; chronic = hypodense; mixed density = acute + chronic components; huge SDH → midline shift + ventricular compression.
• Contusion/laceration: patchy heterogeneous high density (hemorrhage) mixed with low density (edema) — common in significant head injury.

Brain abscess

• Collection of pus enclosed within a capsule.
• MRI: well-circumscribed lobulated lesion, high signal on FLAIR; central restricted diffusion on DWI (dark on ADC); enhancing wall (ring enhancement).
• DDx from necrotic tumor: abscess = central restricted diffusion.

High-Yield Points

• Key Point: Extra-axial: meningioma (dural tail, hyperostosis) / schwannoma (CPA, IAC widening); intra-axial: metastasis / astrocytoma.
• Key Point: GBM = ring-enhancing mass crossing midline with central necrosis + edema; most common primary CNS malignancy.
• Key Point: Metastasis = gray-white junction, sphere, "small nodule large edema", 80% from lung/renal/breast/melanoma/colorectal.
• Key Point: Pituitary macroadenoma = snowman + optic chiasm compression; microadenoma = hypoenhancing on dynamic scans (hyperprolactinemia).
• Key Point: EDH = lentiform, doesn't cross sutures; SDH = crescentic, crosses sutures.
• Key Point: Acute infarct = DWI bright/ADC dark (restricted diffusion); MCA territory most common.
• Key Point: Hypertensive hemorrhage: basal ganglia/cerebellum/brainstem; hyperdense on CT.
• Key Point: Abscess = ring enhancement + central restricted diffusion.

Topic Summary

Brain disease imaging: classify masses by intra- vs extra-axial location (metastasis/astrocytoma vs meningioma/schwannoma). Master cases: meningioma (dural tail + hyperostosis), GBM (ring enhancement + necrosis + midline crossing), vestibular schwannoma (CPA + IAC widening), pituitary adenoma (snowman/optic chiasm; dynamic enhancement for microadenoma), metastases (gray-white junction, small nodule large edema), hypertensive hemorrhage (basal ganglia), MCA infarction (DWI restricted), EDH (lentiform) vs SDH (crescentic), and brain abscess (ring enhancement + central restricted diffusion).

Illustrations（图解速览）

Case gallery for this chapter — identify the imaging technique, location, features and diagnosis for each: