CT Imaging for Ischemic Stroke Diagnosis
Emergent noncontrast CT (NCCT) of the brain is the recommended initial imaging study for diagnosing ischemic stroke. 1, 2
Primary Imaging: Noncontrast CT Head
All patients with suspected acute ischemic stroke should receive emergent noncontrast CT as the first-line imaging modality. 1, 2 This recommendation is based on:
- Exclusion of intracranial hemorrhage, which is the critical first step before any thrombolytic therapy can be considered 1, 2
- Identification of stroke mimics such as brain tumors or other nonvascular causes of neurological symptoms 1, 2
- Detection of early ischemic changes including hyperdense middle cerebral artery sign, loss of gray-white differentiation, sulcal effacement, and lentiform nucleus changes 1, 3
The noncontrast CT should consist of contiguous 5-10mm thick sections from the vertex to the foramen magnum, parallel to the canthal-meatal line, and ideally performed within 1 hour of initiating thrombolytic therapy. 1
When Contrast CT Is NOT Recommended
CT with IV contrast alone has no role in acute stroke evaluation and should not be performed. 2 Contrast administration may obscure early hemorrhagic complications and provides no additional diagnostic benefit in the acute setting unless specifically needed for CT angiography or CT perfusion. 1, 2
Adding CT Angiography (CTA)
CTA head with IV contrast should be added immediately after noncontrast CT if large vessel occlusion (LVO) is suspected or if endovascular therapy is being considered. 1, 2 The specific indications include:
- Patients who are candidates for endovascular thrombectomy, where CTA provides rapid detection of intracranial large vessel occlusions with high sensitivity and specificity 1, 2
- Vascular imaging of the head and neck to evaluate stroke mechanism and assess future stroke risk 1
- CTA should cover the entire cerebrovascular axis from the aortic arch to the vertex when feasible 1
Time-Based Imaging Algorithm
For patients presenting within 0-4.5 hours (IV tPA window):
- Noncontrast CT is sufficient to exclude hemorrhage and determine extent of ischemic changes 1
- Add CTA if endovascular therapy is considered 1, 2
For patients presenting within 6 hours:
- CT perfusion is usually not necessary as an initial examination 2
- Noncontrast CT plus CTA is the standard approach 2
For patients presenting 6-24 hours after onset:
- CT perfusion with IV contrast may be required to determine endovascular therapy eligibility in anterior circulation strokes with confirmed LVO 2
- This helps identify salvageable brain tissue (penumbra) versus irreversibly damaged tissue 4, 5
Critical Pitfalls to Avoid
Do not delay thrombolytic treatment to obtain advanced imaging studies. 1 The performance of additional neuroimaging tests beyond noncontrast CT should not delay IV rtPA administration in eligible patients within the treatment window. 1
Early ischemic changes on CT are often subtle and easily missed. 3, 6 Using specialized "stroke windows" (adjusted window settings) significantly improves detection of early ischemic changes from 18% to 70% sensitivity. 6 Early infarct signs involving more than one-third of the MCA territory indicate higher risk of hemorrhagic transformation with thrombolysis. 1
MRI is an acceptable alternative to CT for excluding hemorrhage and determining ischemic changes in the 0-4.5 hour window, but CT remains more widely available and faster to obtain. 1, 5