Will all strokes be visible on a computed tomography (CT) scan?

Not All Strokes Show Up on CT Scans

No, not all strokes will be visible on CT scans, particularly in the hyperacute phase of ischemic stroke and in cases of small or posterior fossa infarcts. CT has significant limitations in detecting early ischemic changes, with many strokes remaining invisible during the first hours after onset.

Types of Strokes and CT Visibility

Ischemic Strokes

• Early detection limitations: In the first 3 hours after onset, approximately 70% of ischemic strokes show no changes on non-contrast CT 1
• Timing factors: After the 3-hour mark, the odds of detecting ischemic changes increase by about 1% per minute 1
• Small infarcts: CT has limited sensitivity for detecting small acute infarcts 2
• Posterior fossa strokes: Infarcts in the posterior fossa (brainstem, cerebellum) are particularly difficult to visualize on CT due to bone artifacts 2

Hemorrhagic Strokes

• High visibility: CT is generally considered highly sensitive for detecting acute intracerebral hemorrhage 3
• Small hemorrhages: Microbleeds may not be visible on CT but can be detected on MRI gradient-echo sequences 3

Early Ischemic Signs on CT

When ischemic changes do appear on CT, they may include:

• Hyperdense middle cerebral artery sign: Visible in one-third to one-half of angiographically proven thrombosis cases 3
• Loss of gray-white differentiation: Subtle early sign that can be missed 2, 4
• Sulcal effacement: Another subtle sign that may be overlooked 4
• Loss of insular ribbon: Present in about 28.7% of early ischemic strokes 5
• Hypodensity of lentiform nucleus: Seen in approximately 20.7% of early cases 5

Comparative Imaging Modalities

MRI Advantages

• Higher sensitivity: MRI, particularly diffusion-weighted imaging (DWI), can detect ischemic changes within minutes of onset 6
• Microbleed detection: Gradient-echo MRI sequences can detect small hemorrhages not visible on CT 3
• Hemorrhagic transformation: MRI may detect hemorrhagic transformation earlier than CT in some cases 6

CT Perfusion and Angiography

• Enhanced detection: CT perfusion and CT angiography significantly improve stroke detection compared to non-contrast CT alone 7
• Wake-up strokes: Advanced imaging with CT perfusion is particularly valuable for wake-up strokes to assess salvageable brain tissue 2

Clinical Implications

• Initial screening: Despite limitations, non-contrast CT remains the first-line imaging test for suspected stroke to rule out hemorrhage 2
• Treatment decisions: CT's inability to detect early ischemic changes should not delay treatment decisions for thrombolysis within the appropriate time window 2
• Follow-up imaging: CT becomes more sensitive for evaluating the extent of ischemic changes on follow-up imaging 3

Common Pitfalls

• Overreliance on negative CT: A normal CT scan does not rule out acute stroke, especially within the first few hours
• Posterior circulation strokes: These are particularly prone to being missed on CT
• Small vessel strokes: Lacunar infarcts may not be visible on initial CT
• Contrast timing: For CT angiography and perfusion studies, proper timing of contrast administration is critical for accurate assessment

For optimal stroke detection and management, a multi-modal approach using non-contrast CT followed by CT angiography and/or CT perfusion (or MRI when available and not contraindicated) provides the most comprehensive assessment.