What is the earliest time after onset of an acute stroke that a lesion can be visualized on non‑contrast CT?

Early CT Detection of Acute Stroke Lesions

Ischemic stroke lesions typically become visible on non-contrast CT within 3-6 hours after symptom onset, though early subtle changes may appear within the first 1-3 hours in some cases. However, the sensitivity of CT in the hyperacute phase is quite limited, and many strokes remain invisible on initial imaging.

Timeline of CT Visibility

Hyperacute Phase (0-6 hours)

• Standard non-contrast CT shows abnormalities in fewer than 50% of patients within the first few hours after stroke onset 1
• Early ischemic changes that may be visible include:
  • Loss of gray-white matter differentiation 1
  • Hypodensity or hypoattenuation 1
  • Sulcal effacement or compression of CSF spaces (focal and/or diffuse brain swelling) 1
  • Hyperdense middle cerebral artery sign (indicating acute thrombus), visible in approximately 54% of patients with proximal MCA occlusion 1

Detection Accuracy

• Physician ability to reliably recognize early CT changes is variable, with accuracy for detecting ischemic areas involving more than one-third of the MCA territory approximately 70-80% 1
• The sensitivity of CT for acute ischemic stroke within the first 3 hours is only 16%, compared to 77% for diffusion-weighted MRI 2

Clinical Implications for Imaging Timing

Immediate Imaging Requirements

• All patients with suspected acute stroke should undergo brain imaging with non-contrast CT or MRI immediately 1
• For thrombolysis candidates, CT examination should be completed within 25 minutes of emergency department arrival, with interpretation within an additional 20 minutes (total door-to-interpretation time of 45 minutes) 1
• For potential endovascular thrombectomy candidates presenting within 6 hours, immediate non-contrast CT and CT angiography from arch-to-vertex should be performed without delay 1

Follow-up Imaging

• A repeat CT or MRI should be performed at 24 hours after initial stroke or thrombolytic therapy, regardless of clinical stability 2
• Approximately 25% of patients with acute stroke who have a normal initial CT will demonstrate acute or subacute infarction on MRI performed within 1-2 days 2
• Non-contrast CT becomes more sensitive for detecting the extent of ischemic changes on follow-up imaging than in the hyperacute setting 1

Important Caveats

Limitations of Early CT

• The primary purpose of immediate CT in acute stroke is to exclude hemorrhage before thrombolytic therapy, not to definitively visualize the ischemic lesion 1
• Posterior fossa strokes (brainstem and cerebellum) are particularly difficult to visualize on CT and may require MRI for detection 1
• Small cortical or subcortical lesions may not be visible on CT even hours after onset 1

MRI Superiority

• Diffusion-weighted MRI can visualize ischemic regions within minutes of symptom onset, with sensitivity of 88-100% and specificity of 95-100% even at very early time points 1
• MRI is more sensitive than standard non-contrast CT in identifying both new and preexisting ischemic lesions 1
• However, even DWI can be falsely negative in approximately 50% of small posterior fossa strokes within the first 48 hours 3

Practical Approach

In real-world practice, do not delay treatment decisions waiting for a lesion to become visible on CT. The absence of a visible infarct on early CT does not exclude acute ischemic stroke and should not prevent appropriate acute treatment if the patient meets clinical criteria and hemorrhage has been excluded 1.