How do you differentiate between ischemic and hemorrhagic stroke on a computed tomography (CT) scan?

Differentiating Ischemic from Hemorrhagic Stroke on CT Scan

Hemorrhagic stroke appears as bright white (hyperdense) areas on non-contrast CT, while acute ischemic stroke typically shows subtle or no changes in the first hours, eventually developing as darker (hypodense) regions.

Primary CT Findings

Hemorrhagic Stroke

• Intracerebral hemorrhage appears as a hyperdense (bright white) mass within the brain parenchyma on non-contrast CT, making it immediately distinguishable from ischemic stroke 1
• Subarachnoid hemorrhage demonstrates high-density blood in the subarachnoid spaces, cisterns, and sulci 1
• CT has extremely high sensitivity for detecting acute hemorrhage, which is why it remains the gold standard for excluding hemorrhage before thrombolytic therapy 1

Ischemic Stroke

• In the hyperacute phase (first 3-6 hours), CT is often normal or shows only subtle early signs because ischemic changes require time to develop as tissue water content increases 1, 2
• The hyperdense middle cerebral artery (MCA) sign appears as increased density within the occluded vessel, representing acute thrombus, and is seen in one-third to one-half of angiographically proven MCA occlusions 1, 3
• Early parenchymal changes include loss of gray-white matter differentiation, subtle hypodensity, attenuation of the lentiform nucleus, loss of the insular ribbon, and sulcal effacement 1, 3
• CT sensitivity for detecting acute ischemic changes in the first hours is only 32-36%, which is a critical limitation 2

Critical Time-Dependent Considerations

The appearance of ischemic stroke on CT evolves over time, creating a diagnostic challenge in the hyperacute window when treatment decisions are most critical:

• Within 0-6 hours: CT may be completely normal or show only subtle early signs; the primary role is hemorrhage exclusion 1
• Within 6-24 hours: Hypodensity becomes more apparent as cytotoxic edema develops 1, 4
• After 24 hours: Well-defined hypodense infarct with mass effect becomes clearly visible 4

Practical Diagnostic Algorithm

For acute stroke patients, follow this systematic CT interpretation approach:

1. First, scan for any hyperdense (bright) areas indicating hemorrhage—if present, this is hemorrhagic stroke 1
2. If no hemorrhage, look for the hyperdense vessel sign (bright MCA or other vessel) suggesting acute thrombotic occlusion 1, 3
3. Assess for early ischemic signs: loss of gray-white differentiation, subtle hypodensity, insular ribbon loss, or sulcal effacement 1
4. Quantify extent: If early signs involve more than one-third of the MCA territory, this indicates large infarct with higher hemorrhagic transformation risk 1

Advanced Imaging Considerations

When CT findings are equivocal or for improved diagnostic accuracy, multimodal imaging provides superior information:

• CT angiography (CTA) identifies vessel occlusion and can be performed immediately after non-contrast CT without delaying treatment 1, 5
• CT perfusion differentiates infarcted core from salvageable penumbra, particularly valuable for patients beyond 6 hours from onset 5, 4
• MRI with diffusion-weighted imaging (DWI) achieves 88-100% sensitivity and 95-100% specificity for acute ischemia, far superior to CT's 32-36% sensitivity 1, 2
• Gradient-echo MRI sequences are at least as accurate as CT for detecting acute hemorrhage (96% concordance) and superior for detecting chronic microbleeds 1, 6

Critical Pitfalls to Avoid

Several common errors can lead to misdiagnosis or treatment delays:

• Never assume a normal CT excludes acute ischemic stroke—CT sensitivity in the first hours is poor, and treatment decisions for IV tPA should be based on clinical presentation and time window, not CT visibility of infarct 1, 2
• Do not confuse chronic hypodensity from old infarcts with acute hemorrhage—old strokes appear dark (hypodense), not bright 4, 7
• Avoid delaying thrombolytic therapy to obtain advanced imaging if the patient is within the treatment window and non-contrast CT excludes hemorrhage 1, 5
• Do not rely on stroke severity alone to differentiate types—there is substantial overlap in clinical presentation between hemorrhagic and ischemic strokes 8
• Physician accuracy for detecting early ischemic changes is only 70-80%, so when in doubt, consider advanced imaging or expert consultation 1, 2

Special Clinical Scenarios

For patients being considered for endovascular therapy, vascular imaging (CTA or MRA) should be obtained during initial evaluation to identify large vessel occlusions, as this changes management from IV therapy alone to potential thrombectomy 1, 5

In rural or resource-limited settings where CT is unavailable, immediate transfer to a facility with imaging capabilities is mandatory—clinical features alone have inadequate sensitivity and specificity to guide treatment, and empiric antithrombotic therapy without imaging risks catastrophic hemorrhage expansion 8