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Reading Brain CT and MRI: Bleed vs. Infarct with Temporal Staging

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Part 1 - How to Approach a Brain CT

The Systematic "ABCDE" Window Approach

• Brain window (WW ~80, WL ~35): evaluates parenchyma, gray-white differentiation, blood
• Bone window (WW ~2000, WL ~300): evaluates skull fractures, calvarium

CT Density (Hounsfield Units - HU)

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Part 2 - CT Appearance: Hemorrhage vs. Infarct

Hemorrhage on CT

"Fresh intracranial hemorrhage coagulates nearly immediately and therefore shows up on CT scans as hyperdense areas relative to brain. Fresh hemorrhage may appear about as white as bone."

• Neuroanatomy through Clinical Cases, 3rd Ed.

"The area of ischemic injury on brain CT scan appears as a relative hypodensity, in contrast to brain hemorrhage, which appears hyperdense compared with the surrounding brain."

• Goldman-Cecil Medicine

Infarct on CT

1. Loss of gray-white differentiation - the insular cortex and basal ganglia lose their normal contrast with white matter
2. "Insular ribbon sign" - loss of the normally sharp insular cortex outline (sensitive for MCA territory ischemia)
3. Sulcal effacement - gyri swell and compress the sulci
4. Hyperdense vessel sign - a dense MCA or basilar artery indicates acute thrombosis
5. "One-third rule" - if more than 1/3 of the MCA territory is already hypodense, thrombolysis is contraindicated (increased hemorrhage risk)

"Brain swelling on CT without accompanying low density does not always progress to infarction. Such cases may be due to abnormal perfusion, but a compensatory increase in CBV rather than a reduction."

• Grainger & Allison's Diagnostic Radiology

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Part 3 - MRI Approach and Sequences

The DWI-ADC Rule

• Acute infarct: DWI bright + ADC dark = true restricted diffusion = infarction
• Chronic lesion (gliosis): DWI dark + ADC bright = T2 shine-through without restriction
• T2 shine-through (subacute): DWI bright + ADC bright/normal = not infarction, just T2 effect

"DWI has a preeminent role in acute stroke imaging due to its extremely high sensitivity and specificity, with parenchymal hyperintensity as early as 5 minutes following the onset of infarction."

• Grainger & Allison's Diagnostic Radiology

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Part 4 - MRI Timeline of Intracerebral Hemorrhage

"The actual sequence of changes can be fairly complicated and variable, depending on brain microenvironment and individual scanners."

• Be = acute (Black on T2 at 1-3 days, deoxy)
• A = acute-subacute transition (starts to brighten T1)
• Bright White = subacute (methemoglobin, bright on both)
• Rim = chronic (dark hemosiderin rim on T2/SWI)

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Part 5 - MRI Timeline of Ischemic Infarction

"Initially, the hyperintense signal on DWI is caused by decreased water diffusivity due to swelling of the ischemic nerve cells (for the first 5-7 days); then it increasingly results from the abnormal T2 properties of the infarcted tissue (T2 shine-through). For this reason, a reliable estimation of the age of the ischemic lesion is not possible by looking at DWI images alone."

• Bradley and Daroff's Neurology in Clinical Practice

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Part 6 - Key Differentiating Features at a Glance

CT: Bleed vs. Infarct

MRI: Quick Reference Summary

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Part 7 - Practical Workflow in Emergency Setting

1. First: Non-contrast CT - rule out hemorrhage, mass lesion, or hemorrhagic transformation before giving thrombolytics
2. If CT negative but stroke suspected within 6 hours: MRI with DWI/ADC is the gold standard - bright DWI + dark ADC = confirmed ischemic infarct
3. CTA (CT Angiography): Aortic arch to intracranial vessels - look for large vessel occlusion (LVO), hyperdense vessel sign, and collateral flow
4. CT perfusion / MR perfusion: Identifies the penumbra (salvageable ischemic tissue = MTT prolonged but CBV preserved) vs. core infarct (CBV reduced)
5. SWI/GRE: Detects microhemorrhages (amyloid angiopathy), thrombosed veins, and acute thrombus in vessels with "blooming" effect

"DWI is the most sensitive imaging sequence for the detection of acute ischemic infarction and can show infarcted tissue within minutes of stroke onset, considerably earlier than CT and other MRI sequences."

• Adams and Victor's Principles of Neurology, 12th Ed.

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Quick Memory Tables

Hemorrhage on CT (Rule of 2 weeks)

• <7 days: White (hyperdense)
• ~7-14 days: Isodense (can be invisible - look for mass effect)
• >14 days: Dark (hypodense), may calcify or become cystic

FLAIR in Stroke

• Infarct <4.5 hours: DWI bright but FLAIR normal or subtle = safe thrombolysis window ("DWI-FLAIR mismatch" for wake-up strokes)
• Infarct >4.5 hours: Both DWI and FLAIR bright
• SAH: Hyperintense CSF in sulci on FLAIR (FLAIR more sensitive than CT at 24-48 hrs)

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