How to Read a Head CT: Systematic Approach

Creator: Progress Note
Published: 2026-08-29
Keywords: how to read a head CT, how to read a head CT, head CT interpretation, CT hemorrhage vs ischemia, blood cisterns brain bone, noncontrast head CT

bolt The 60-second version

check_circleNoncontrast head CT is fast and excellent for detecting acute blood and bone injury.
check_circleA systematic approach prevents missed findings.
check_circleAcute blood appears bright (hyperdense) on CT.
check_circleCT is done first in stroke to exclude hemorrhage before thrombolysis.
check_circleLook for midline shift and effaced cisterns as signs of mass effect.

Why CT First

The noncontrast head CT is fast, widely available, and excellent at detecting acute hemorrhage, fractures, and large mass effect, which is why it is the first imaging test in acute neurologic emergencies such as head trauma and suspected stroke. In ischemic stroke it is used chiefly to exclude hemorrhage before giving thrombolytics, since early ischemic changes can be subtle. MRI is more sensitive for early ischemia and posterior fossa detail.

A Systematic Approach

A common mnemonic is "Blood Can Be Very Bad": Blood (hemorrhage), Cisterns (are they open or effaced?), Brain (gray-white differentiation, symmetry, mass effect, hydrocephalus), Ventricles (size and symmetry), and Bone (fractures on bone windows). Working through the same sequence every time, and comparing left with right for symmetry, reduces missed findings.

Recognizing Hemorrhage

Acute blood is hyperdense (bright white) on CT. The pattern points to the type: a crescent-shaped collection crossing suture lines is a subdural hematoma (bridging vein tear); a lens-shaped (biconvex) collection not crossing sutures is an epidural hematoma (often arterial); blood in the sulci and basal cisterns suggests subarachnoid hemorrhage; and blood within the brain parenchyma is an intracerebral hemorrhage. Blood becomes less dense over days.

Mass Effect and Ischemia

Assess for mass effect: midline shift, effacement (compression) of the sulci and basal cisterns, and hydrocephalus indicate raised pressure and are urgent. In ischemic stroke, early CT signs are subtle — loss of gray-white differentiation, sulcal effacement, and a hyperdense vessel sign — and a completed infarct later appears as a hypodense (dark) region. Always interpret the scan alongside the clinical picture.

Frequently asked questions

How do you read a head CT systematically?expand_more

A common approach uses the mnemonic "Blood Can Be Very Bad": check for Blood (hemorrhage), Cisterns (open or effaced), Brain (gray-white differentiation, symmetry, mass effect), Ventricles (size), and Bone (fractures on bone windows). Following the same sequence and comparing sides reduces missed findings.

Why is a CT done first in stroke?expand_more

A noncontrast head CT is done first in suspected stroke mainly to exclude intracranial hemorrhage, because thrombolytic treatment for ischemic stroke would be dangerous if there were bleeding. Early ischemic changes are subtle on CT, while blood is readily seen.

How does blood look on a head CT?expand_more

Acute blood appears bright (hyperdense) on CT. A crescent shape crossing suture lines suggests a subdural hematoma, a lens (biconvex) shape suggests an epidural hematoma, blood in the sulci and cisterns suggests subarachnoid hemorrhage, and blood within the brain is an intracerebral hemorrhage.

What are signs of mass effect on a head CT?expand_more

Signs of mass effect include midline shift, effacement (compression) of the sulci and basal cisterns, and hydrocephalus. These indicate raised intracranial pressure and are urgent findings that require prompt action.

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