# Ch13: Neuroimaging — Methods & Normal Anatomy（神经影像方法与正常解剖）
Preparatory Mindset
Neuroimaging begins with choosing the right method: plain CT is the first-line test for acute head trauma and acute intracranial hemorrhage; CT is low-sensitivity for early ischemic stroke; MRI is much better for brain pathology, especially at the skull base (pituitary, posterior fossa); DWI is the key sequence for acute infarction. You must master the normal CT densities and MRI signals of brain tissue, blood, CSF, fat, bone and calcification, and the basic abnormal manifestations (mass effect, ventricular enlargement).
Core Concepts
Examination methods
- X-ray plain film: finds some lesions (fracture, myeloma) but too non-specific for diagnosis; rarely performed except in brain injury or myeloma.
- Plain CT: multiple slices (20–30 slices, 3–5 mm thickness), multiple planes (axial routine; coronal/sagittal supplement); brain window + bone window; displays tumors, hemorrhage, infarction and bone abnormalities.
- CECT (contrast-enhanced CT): displays lesions that break down the blood-brain barrier (ischemia, inflammation, neoplasms); extra-axial lesions lacking BBB enhance avidly; increases conspicuity (location, appearance, number, anatomy).
- CTA/CTV/CT perfusion: CTA — arterial occlusions, aneurysms, AVMs; CTV — venous sinus thrombosis; CT perfusion — presence and extent of infarction/ischemia.
- Plain MRI: much better visualization, especially skull base (pituitary, posterior fossa); multiple sequences (T1WI/T2WI/FLAIR/DWI).
- CEMR: additional info; usually high signal on T1WI.
- MRA/MRV: recognizes flowing blood without contrast; vascular occlusion, aneurysm, AVM, DVT.
- Functional MRI (fMRI): DTI/tractography (fiber tracts), MRS (metabolic info — NAA ≈2 ppm; differentiates tumor origin), MR perfusion (PWI/ASL — tissue microcirculation), BOLD (blood-oxygenation-level-dependent; brain function, psychiatric disease).
Normal CT densities
| Structure | CT value |
|---|---|
| White matter | 20–30 HU (hypodense to gray) |
| Gray matter | 30–40 HU (hyperdense to white) |
| Ventricle & cistern (CSF) | 0–10 HU |
| Skull | >250 HU |
| Physiologic calcification | pineal, falx cerebri, choroid plexus, basal ganglia, cerebellar dentate nucleus |
Normal MRI signal intensity
| T1WI | T2WI | |
|---|---|---|
| White matter | High | Low |
| Gray matter | Low | High |
| CSF | Low | High |
| Fat tissue | High | High |
| Skull table | Low | Low |
| Diploe | High | High |
| Vessel (flow void) | Low | Low |
| Calcification | Low | Low |
Normal anatomy landmarks
- Plain film: inner/outer table dense with spongy diploe; sutures = saw-tooth; convolutional impressions; middle meningeal artery impression (streaks, wide to narrow); sella 7–16 mm long, 7–14 mm wide, 8–10 mm deep; internal auditory meatus ~6 mm wide, symmetrical; physiologic pineal calcification.
- CT/MRI: anterior/posterior horn of lateral ventricle, corpus callosum (genu, splenium), internal capsule, thalamus, basal ganglia (caudate, putamen, globus pallidus), brainstem (midbrain, pons, medulla), cerebellum (hemispheres, vermis, dentate), fourth ventricle, straight sinus.
- Pituitary: gland + stalk + optic chiasm; posterior pituitary hyperintense on T1.
Abnormal CT density / MRI signal
- Low density (CT): cytotoxic edema (infarcts), vasogenic edema (around neoplasms), cyst, encephalomalacia.
- High density (CT): hemorrhage, abscess, calcification, contrast enhancement.
- Physiologic enhancement: choroid, anterior pituitary, arteries, dural venous sinuses. Pathologic: metastasis, some primary gliomas, meningioma, abscess, acute demyelination.
- MRI signal abnormality: edema (low T1/high T2); deoxyhemoglobin (iso/mild low T1, low T2); methemoglobin (high T1, low T2); hemosiderin (low T1, low T2); fat (high/high); calcification (low/low); protein-containing (high/high); cyst (low/high).
Basic abnormal manifestations
- Mass effect: displacement/compression of lateral ventricles; shift of midline structures (septum pellucidum, third ventricle, pineal) away from the lesion; ventricular dilatation if CSF obstructed; effacement of basal cisterns (uncal or tonsillar herniation).
- Enlargement of ventricles: obstructive hydrocephalus (CSF pathway obstruction) vs atrophy (surrounding brain tissue loss with compensatory dilatation).
Types of edema
- Vasogenic edema: extends through white matter, spares overlying gray matter cortex (around neoplasms).
- Cytotoxic edema: wedge-shaped, involves both gray cortex and underlying white matter (ischemic stroke, MCA territory).
Specific brain disorders (overview)
- Vascular disease: stroke (infarction, hemorrhage); vascular malformation (aneurysm, AVM).
- Brain tumor: extra-axial (meningioma, acoustic neuroma, metastasis) vs intra-axial (glioma).
- Demyelinating: leukoaraiosis, multiple sclerosis.
- Degenerative: aging, Alzheimer's.
- Infection: encephalitis, abscess.
- Brain injury: extracerebral hematoma (EDH/SDH), intracerebral hematoma, fracture.
High-Yield Points
- Key Point: Acute head trauma/intracranial hemorrhage → plain (non-contrast) CT.
- Key Point: Early ischemic stroke → CT low sensitivity; MRI DWI shows restricted diffusion.
- Key Point: CSF = 0–10 HU; white matter 20–30; gray matter 30–40; skull >250.
- Key Point: MRI: water low T1/high T2; fat high/high; calcification low/low; acute hemorrhage = high density on CT.
- Key Point: FLAIR suppresses CSF; DWI/ADC for infarction and abscess.
- Key Point: Mass effect = midline shift + ventricular compression + herniation; hydrocephalus vs atrophy.
- Key Point: Vasogenic edema spares cortex (tumor); cytotoxic edema involves cortex + white matter (infarct).
LMCHK OSCE Practice（OSCE & LMCHK）
- "CT findings of stroke, subarachnoid hemorrhage" is a listed LMCHK non-interactive station (NS): acute ischemic stroke = low-density wedge in a vascular territory (DWI positive early); SAH = hyperdense blood in basal cisterns/sulci (aneurysm — basilar artery aneurysm example); EDH = lentiform, does not cross sutures; SDH = crescentic, crosses sutures.
- Practice describing: "Right MCA territory low attenuation with mass effect → acute/subacute cerebral infarction; CT initially normal in hyperacute phase — DWI is the confirmatory test."
- Hydrocephalus: dilated ventricles (obstructive) vs enlarged sulci (atrophy) — a frequent image station.
Topic Summary
Neuroimaging: plain CT first for trauma/acute hemorrhage; MRI (T1/T2/FLAIR/DWI) for parenchymal detail, especially posterior fossa/skull base; CTA/CTV/perfusion and fMRI (DTI, MRS, BOLD) for function and vasculature. Normal densities (white 20–30, gray 30–40, CSF 0–10 HU) and MRI signals (water, fat, calcification, hemorrhage stages) are exam staples. Basic abnormalities: mass effect (midline shift, herniation), ventricular enlargement (hydrocephalus vs atrophy), vasogenic vs cytotoxic edema. Master case types: stroke, hemorrhage, tumors, demyelination, infection, trauma.