CT Imaging to Rule Out Stroke
Start with noncontrast CT head immediately—this is the essential first imaging study to rule out stroke by excluding intracranial hemorrhage and identifying early ischemic changes. 1
Initial Imaging Protocol
Noncontrast CT (NCCT) head is the gold standard initial study for all patients with suspected acute stroke, regardless of presentation timing. 2, 1 This study serves multiple critical functions:
- Excludes intracranial hemorrhage, which is absolutely essential before any thrombolytic therapy can be considered 2, 1
- Identifies early ischemic changes using scoring systems like ASPECTS (Alberta Stroke Program Early CT Score) to estimate irreversible tissue damage 2, 1
- Rules out stroke mimics such as tumors, infections, or other pathology that may present with similar neurological deficits 1
- Provides rapid results that are available 24/7, inexpensive, and easy to interpret even with limited expertise 2
Technical Specifications for NCCT
The scan should include contiguous, discrete (nonhelical) images from the vertex of the calvarium through the foramen magnum, with 5mm thick sections preferred (maximum 10mm). 2 The scan plane should be parallel to the canthal meatal line. 2
When to Add CT Angiography (CTA)
Immediately follow NCCT with CTA head and neck if large vessel occlusion (LVO) is suspected, particularly in patients with significant neurological deficits (NIHSS ≥6). 2, 1
- CTA with IV contrast detects intracranial large vessel occlusions with high sensitivity and specificity, which is critical for endovascular therapy decisions 2, 1
- CTA provides vascular mapping from the aortic arch to the vertex, including information about vessel tortuosity and collateral flow that guides procedural planning 2
- Multiphase CTA is preferred as it allows assessment of collateral circulation, which helps estimate treatment risks and benefits 2
Timing-Based Imaging Considerations
Within 6 Hours of Symptom Onset
- NCCT followed by CTA is sufficient for clinical decision-making in most cases 2, 1
- CT perfusion (CTP) is not necessary within this early window, as treatment decisions can be made based on NCCT and CTA alone 2, 1
Beyond 6 Hours or Unknown Onset
- Add CT perfusion with IV contrast if large vessel occlusion is confirmed and you need to determine eligibility for endovascular therapy 2, 1
- CTP helps identify salvageable tissue (penumbra) versus irreversible core infarction in late-window patients 2, 1
What NOT to Order
Never order CT head with IV contrast alone—this has no role in acute stroke evaluation and may actually obscure hemorrhage. 2, 1 The American College of Radiology explicitly rates this as "usually not appropriate" with a rating of 3 out of 9. 2
Special Context: Post-Coronary Angiography
In your specific scenario of anisocoria after coronary angiography, the same algorithm applies:
- Start with NCCT head to exclude hemorrhage (particularly important given recent contrast exposure and potential anticoagulation) 1
- Follow with CTA if stroke is confirmed and LVO is suspected 1
- The anisocoria may represent brainstem or posterior circulation involvement, making vascular imaging particularly important 2
Common Pitfalls to Avoid
- Don't wait for MRI when stroke is suspected—while MRI is more sensitive for acute infarction, CT is faster, more widely available, and sufficient for initial triage decisions 2
- Don't skip NCCT and go straight to CTA—you must exclude hemorrhage first before considering any contrast administration 1
- Don't order CTP routinely in early-window patients—this adds time, cost, and technical complexity without changing management in the first 6 hours 2
- Don't let "normal" NCCT reassure you—early ischemic changes may be subtle or absent in the first few hours, but clinical suspicion should still prompt CTA if LVO is possible 3, 4