CT Basics for Stroke

🧠 Introduction

• 💡 Stroke medicine evolved rapidly with the introduction of CT scanning, enabling clinicians to distinguish between ischaemic and haemorrhagic strokes quickly and safely. Non-contrast CT (NCCT) is the initial investigation of choice for nearly all suspected strokes.
• ⚙️ CT imaging is central to acute stroke management — it determines stroke type, rules out mimics, and guides emergency therapies such as thrombolysis or thrombectomy.
• 📜 Before CT, differentiating stroke subtypes relied on clinical signs or even post-mortem findings. Early diagnostic methods (e.g. pineal calcification displacement) were crude and unreliable.

📚 History of CT Imaging

• 🧑‍🔬 Sir Godfrey Hounsfield developed the first CT scanner in 1972 — funded by EMI’s Beatles record profits 🎸.
• 🧩 The first clinical CT scan in 1971 (Atkinson Morley Hospital, London) diagnosed a cerebral cyst — a landmark moment in neuroradiology.
• 🏥 By the late 1980s, CT became routine in major hospitals, revolutionising stroke diagnosis and management.

⚙️ Technical Aspects of CT

• 🌀 A CT scanner uses a rotating X-ray tube and detectors to capture cross-sectional images of the body, reconstructed by a computer into slices.
• 🔁 Helical (spiral) CT introduced in the 1990s enabled continuous scanning for 3D reconstructions — essential for vascular imaging.
• 📏 Image density is expressed in Hounsfield Units (HU):
  • Air = -1000 HU
  • Water = 0 HU
  • Grey matter ≈ +35 HU
  • Acute haemorrhage ≈ +70 HU
  • Bone = +400 to +3000 HU

☢️ Radiation Exposure

• CT involves ionising radiation and is regulated under IRMER in the UK — only to be performed when clinically justified.
• 🧮 A typical CT head ≈ 2 mSv → roughly equivalent to 100 chest X-rays.
• 🌍 Natural background radiation ≈ 3.1 mSv/year for comparison.

🏥 CT in Acute Stroke

• CT is first-line for suspected stroke — rules out haemorrhage before thrombolysis or thrombectomy.
• Indications: sudden focal neurological deficit, decreased consciousness, headache, or anticoagulant use.
• ⚠️ Early CT may appear normal within first 6 hours of ischaemia — correlate with clinical findings.

🔍 CT Interpretation Tips

🧩 Review CT systematically from vertex to base. Look for hyperdensities (clots), hypodensities (oedema or infarction), and mass effect. Always exclude mimics such as subdural haematoma or tumour before giving thrombolysis.

🧠 Early Signs of Ischaemia on CT

• ⚡ Hyperdense MCA sign: Thrombus in middle cerebral artery — earliest visible clue.
• 🌫️ Loss of grey-white differentiation: Early cytotoxic oedema, especially in cortex or basal ganglia.
• 🌀 Insular ribbon sign: Blurring between insular cortex and adjacent white matter.
• 🌊 Effacement of sulci: Local swelling and oedema flattening cortical grooves.

🧮 ASPECTS Score

The Alberta Stroke Program Early CT Score (ASPECTS) assesses early ischaemic change in MCA territory. • Score = 10 (normal), decreasing with extent of infarction. • ASPECTS ≤ 7 → poorer prognosis & higher risk of post-thrombolysis haemorrhage.

🩸 CTA & CT Perfusion (CTP)

• 🧩 CT Angiography (CTA): Maps arteries from aortic arch → Circle of Willis; identifies clots, dissections, or stenosis. Essential before thrombectomy.
• 🎯 CT Perfusion (CTP): Measures blood flow (CBF), volume (CBV), and mean transit time (MTT). Differentiates core infarct vs. penumbra (salvageable tissue).

⚠️ Limitations of CT

• ❌ May miss small brainstem strokes or subtle early cortical ischaemia.
• 🧩 False negatives possible in first few hours; false positives from leukoaraiosis in elderly.

🕓 Later CT Signs (6–24 hours)

• ⬜ Hypodense areas become obvious due to cytotoxic and vasogenic oedema.
• 💔 Haemorrhagic transformation may occur, especially in large infarcts or with anticoagulant use.

💡 Teaching tip: - NCCT remains the first-line scan for stroke — fast, reliable, and accessible. - Subtle early changes demand a trained eye and careful side-by-side comparison. - Combine imaging findings with clinical localisation for accurate decision-making.

📚 References

• ASPECTS for Assessing CT Scans in Acute Stroke — AJNR
• Barber PA et al., The Lancet — Validity of ASPECTS Score (2000)
• ASPECTS Official Resources & Training