Summary of "Introduction to CT Head: Approach and Principles"
Introduction to CT Head: Approach and Principles
This guide presents a practical, high-yield approach to non-contrast CT head aimed at learners (especially non-radiologists and new radiology residents). Emphasis is placed on a reproducible checklist and windowing strategy to avoid missing management-changing pathology.
Core purpose
- Teach a practical, high-yield approach to non-contrast CT head.
- Emphasize the three clinical findings you must never miss:
- Mass effect/herniation
- Acute hemorrhage
- Acute ischemic stroke
- Show why proper windowing and a consistent scan checklist are essential.
Key imaging physics and densities (Hounsfield units, HU)
- HU scale basics:
- Air ≈ -1000
- Water = 0
- Cortical bone / metal ≈ +1000
- Brain/cranial ballpark densities (approximate; for intuition):
- CSF ≈ near water (~5–15 HU)
- Fat ≈ -100 HU
- White matter ≈ ~25 HU
- Grey matter ≈ ~35–40 HU (slightly more dense than white matter)
- Acutely clotted blood ≈ ~80 HU (appears bright on CT)
- Important principle: brain structures occupy a narrow HU range — small differences matter, which is why windowing is critical.
Essential anatomy (practical, high-yield)
- Orientation on axial CT:
- Patient right = image left
- Anterior = toward forehead
- Posterior = toward occiput
- Major lobes: frontal, parietal, temporal, occipital.
- Cortical landmarks: Sylvian fissure, central sulcus (reverse omega sign; “mustache” marginal sulcus).
- Deep gray structures (important for stroke): caudate head, putamen, globus pallidus, thalamus.
- White-matter landmarks: internal capsule, external capsule, corona radiata, centrum semiovale.
- Posterior fossa: cerebellar hemispheres, vermis, cerebellar tonsils, brainstem (midbrain — “Mickey Mouse head”; pons — football/Darth Vader-like).
- CSF flow pathway: lateral ventricles → foramen of Monro → 3rd ventricle → cerebral aqueduct → 4th ventricle → foramina of Luschka and Magendie → subarachnoid space. Blockage produces upstream ventricular dilatation.
- Basal cisterns: pre-pontine, suprasellar (star-shaped landmark), ambient, quadrigeminal — important for detecting subarachnoid blood and uncal herniation.
- Vascular structures visible on non-contrast CT: internal carotids, MCA/ACA territories (anterior circulation); vertebral/basilar and PCA (posterior circulation); dural venous sinuses (superior sagittal, transverse, sigmoid → jugular).
Three clinical pathologies and how they appear
1) Hemorrhage
- Appearance:
- Acute (clotted) blood = hyperdense/bright (~80 HU).
- Major extra-axial patterns:
- Epidural: biconvex (lentiform), does NOT cross suture lines.
- Subdural: crescentic, can cross sutures but generally not the falx/midline.
- Subarachnoid hemorrhage: high density within basal cisterns and sulci (e.g., suprasellar cistern looks star-shaped).
- Intracerebral hemorrhage: bright parenchymal collection.
- Time-course of blood density:
- Hyperacute: unclotted/ongoing bleeding — relatively lower density; may show mixed-density “swirl.”
- Acute: clotted and hyperdense.
- Subacute: density decreases toward brain parenchyma; can be hard to see.
- Chronic: approaches CSF density.
- Pitfalls / mimics:
- Calcified structures: choroid plexus, pineal gland, globus pallidus, dentate nucleus.
- Pseudo-subarachnoid hemorrhage: diffusely low brain density with relatively bright vessels — don’t assume all bright = blood.
2) Ischemic stroke (non-contrast CT signs)
- Early signs:
- Hyperdense vessel sign: bright segment representing thrombus (e.g., M1/M2, basilar).
- Loss of grey–white differentiation: cytotoxic edema blurs grey from white (look at insular ribbon, cortical grey in MCA/ACA territories, deep nuclei).
- Evolution:
- Early subtle loss of grey/white → progressive hypoattenuation.
- Mass effect typically peaks around days 3–5 (max edema).
- Chronic infarct → encephalomalacia and ex-vacuo ventriculomegaly (negative mass effect).
3) Mass effect and herniation
- What to look for:
- Sulcal effacement (loss of cortical sulci prominence).
- Midline shift (measure from falx attachments to septum pellucidum or other midline structures).
- Effacement/obliteration of basal cisterns (e.g., suprasellar cistern for uncal herniation).
- Tonsillar herniation at foramen magnum (use sagittal reformats if in doubt).
- Common herniation syndromes:
- Subfalcine (cingulate) herniation — brain under falx, often with midline shift.
- Uncal (uncinate / transtentorial) herniation — uncus compresses midbrain; check suprasellar cistern and CN III signs.
- Descending (transtentorial) and ascending (upward) herniations — watch for peduncular compression, PCA infarcts, Duret hemorrhages.
- Tonsillar herniation — cerebellar tonsils through foramen magnum; risk to brainstem (respiratory/cardiac centers).
- Secondary complications to seek:
- Foramen of Monro compression → acute hydrocephalus.
- ACA compression (subfalcine).
- Third-nerve palsy, peduncular compression, midbrain hemorrhages, PCA infarction, brainstem compression → potential cardiorespiratory collapse.
Windowing: how and why to use it
- Rationale: CT displays limited gray shades; windowing maps a chosen HU range to visible shades to accentuate features of interest.
- Common windows and uses:
- Brain window (default): width ≈ 70, level ≈ 30 — general anatomy, gross mass effect.
- Blood window: wider and centered higher (example width 180, level 80) — separates bone from clot; useful for extra-axial blood.
- Stroke window: narrower/shifted to accentuate grey vs white to spot early infarction.
- Soft-tissue window: evaluate orbits, neck/scalp soft tissues.
- Bone window: assess fractures and bony detail.
- Practice point: always switch windows during dedicated searches — especially use a blood window when looking for extra-axial hemorrhage.
Practical, reproducible conceptual approach (recommended checklist)
- Quick scrolls (one or two passes) to identify anything large/obvious.
- Dedicated search — Mass effect / herniation:
- Compare sulci bilaterally for effacement.
- Evaluate midline for shift.
- Inspect suprasellar cisterns for uncal herniation.
- Inspect foramen magnum for tonsillar herniation (get sagittal reformats if needed).
- Dedicated search — Hemorrhage:
- Inspect basal cisterns and ventricles for high density (start with brain window, then blood window).
- Check dependent sulci (Sylvian fissures, occipital horns) and extra-axial spaces.
- Use a blood window to detect subtle extra-axial collections adjacent to bone.
- Keep mimics and chronic vs acute densities in mind.
- Dedicated search — Ischemic stroke:
- Look for hyperdense vessel sign (M1/M2, basilar).
- Use stroke window to evaluate grey–white differentiation in ACA/MCA territories and deep gray structures; check the insular ribbon.
- Include PCA, cerebellum, and brainstem in the search.
- Bones and soft tissues:
- Review bone window for fractures; check sinuses and mastoid air cells for blood/fluid.
- Inspect soft tissues, orbits, retrobulbar fat, optic nerves, and carotid canals.
- If vascular or subtle vascular signs are suspected → consider CTA / CT perfusion next.
- For radiology trainees: extend the checklist to a complete survey of visible arteries, venous sinuses, skull base fat pads, sinuses, orbital structures, full bone survey, and known blind spots.
Blind spots / common pitfalls
- Extra-axial hemorrhage adjacent to bone can be hidden on a brain window — always check a blood window.
- Dependent sulci (posterior Sylvian fissure, occipital horns) and tentorial / falx regions can be missed.
- Supratentorial cisterns and the foramen magnum: tonsillar herniation may be subtle on axial slices — use sagittal reformats.
- Small hyperdense vessel thrombus may be subtle or blurred on thick slices — thin slices or CTA may reveal it.
- Calcifications (choroid plexus, pineal, globus pallidus) can mimic hemorrhage.
- Pseudo-subarachnoid hemorrhage may occur in diffusely hypodense brain.
Clinical examples highlighted
- Subtle right MCA infarct detected primarily by loss of grey–white differentiation (hyperdense vessel not obvious on thick slices).
- Classic hypertensive intracerebral hemorrhage in the basal ganglia.
- Subdural hematoma visible only on blood window (nearly invisible on brain window).
- Trauma case with posterior fossa hemorrhage producing transtentorial and tonsillar herniation requiring urgent decompression.
- Isolated left internal carotid artery dissection identified by hyperdense mural hematoma in the carotid canal region (confirmed on CTA).
Take-home points
Always check for the three high-yield, management-changing findings: mass effect/herniation, hemorrhage, and ischemic stroke.
- Learn relative densities (CSF ~ water; grey > white; acute blood brightest) and use windowing to make those differences visible.
- Use a short, repeatable checklist when opening any acute head CT so you don’t miss subtle but critical findings.
- Non-radiologists should focus on the conceptual, focused approach; radiology trainees should use a more detailed, systematic search that includes subtle areas and vascular/venous evaluation.
Resources and references
- “A Practical Introduction to CT” — first talk in the series (recommended prerequisite).
- e-Anatomy (Imaios) — detailed cross-sectional anatomy.
- headneckbrainspine.com — practice scrolling through images and anatomy.
- RadPrimer and STATdx (or similar neuroradiology references) — useful for complications of herniation.
- Articles / PDFs on the evolution of stroke (recommended for deeper reading).
Speakers / sources
- Primary speaker: the lecture presenter (unnamed in subtitles) delivering the “Introduction to CT Head” talk.
- External resources referenced: A Practical Introduction to CT, e-Anatomy (Imaios), headneckbrainspine.com, RadPrimer / STATdx.
Category
Educational
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