Introduction
- Acute ischemic stroke results from sudden interruption of cerebral blood flow, leading to neuronal injury and infarction.
- Imaging is crucial for early detection, differentiating ischemic from hemorrhagic stroke, determining the extent, and guiding therapy (e.g., thrombolysis, thrombectomy).
- The time factor is critical—“Time is Brain”—as irreversible neuronal death can occur within minutes to hours.
Non-Contrast CT (NCCT)
Role
- First-line imaging in acute stroke due to speed, availability, and ability to exclude hemorrhage.
- Often normal in first few hours—subtle signs must be recognized.
Acute Ischemic Signs on CT
Early signs (within 6 hours)
- Loss of gray–white matter differentiation
- Effacement of cortical sulci.
- Blurring of basal ganglia margins.
- Loss of insular ribbon sign
- Particularly in MCA infarcts due to vulnerability of the insular cortex.
- Obscuration of lentiform nucleus
- Hyperdense artery sign
- Hyperdense MCA sign = acute intraluminal thrombus.
- Seen within minutes.
- Sulcal effacement
- ASPECTS score
- Alberta Stroke Program Early CT Score; each MCA territory region scored to assess infarct core.
Late signs (after 24 hours)
- Well-defined hypodensity in vascular territory.
- Mass effect with midline shift in large infarcts.
- Effacement of ventricles.
MRI in Acute Ischemic Stroke
Role
- Most sensitive for early ischemic changes.
- Detects infarcts within minutes of onset, before CT becomes positive.
- Used when diagnosis is uncertain, in posterior fossa stroke, or in young patients.
Key MRI Sequences
1. Diffusion-Weighted Imaging (DWI)
- Earliest abnormality—detects cytotoxic edema within minutes.
- Acute infarct = hyperintense on DWI, hypointense on ADC map (restricted diffusion).
- Stays positive for ~10–14 days.
2. Apparent Diffusion Coefficient (ADC)
- Confirms true restricted diffusion.
- ADC drop in acute phase, pseudonormalizes at 1–2 weeks.
3. Fluid-Attenuated Inversion Recovery (FLAIR)
- Becomes abnormal after ~3–6 hours.
- Hyperintensity in affected cortex/white matter; useful for “DWI-FLAIR mismatch” .
4. T2-weighted Imaging
- Hyperintensity develops later; less sensitive in early phase.
5. Gradient Echo / Susceptibility-Weighted Imaging (GRE/SWI)
- Detects hemorrhagic transformation or microbleeds.
- Can detect blooming from thrombus (“susceptibility vessel sign”).
6. MR Angiography (MRA)
- Detects arterial occlusion, stenosis.
CT vs MRI – Comparative Table
| Feature | CT (NCCT) | MRI (DWI/MRI protocol) |
|---|---|---|
| Speed | Very fast (few minutes) | Slower (~15–30 min) |
| Availability | Widely available | Limited in some centers |
| Sensitivity (early) | Low in first 3–6 hrs | Very high (DWI detects in minutes) |
| Hemorrhage detection | Excellent | Good (SWI/GRE), but CT preferred for acute |
| Posterior fossa | Poor sensitivity | Excellent |
| Therapy decision | Primary tool for IV tPA decision | Useful for wake-up strokes / unclear onset |
Imaging Timeline in Acute Stroke
- < 6 hrs: CT often normal → look for subtle signs. MRI DWI positive.
- 6–24 hrs: CT hypodensity appears. MRI clearly shows infarct.
- > 24 hrs: Both CT and MRI show well-defined infarct.
Teaching Points
- Always exclude hemorrhage first before thrombolysis.
- DWI is gold standard for early detection; NCCT is gold standard for excluding hemorrhage.
- Posterior fossa infarcts: MRI far superior.
- Hyperdense MCA sign is an early CT marker of large-vessel occlusion.
