What does it mean when a CT (Computed Tomography) scan cannot exclude an acute infarct?

Understanding "Cannot Exclude an Acute Infarct" on CT Scan

When a CT scan report states "cannot exclude an acute infarct," it means that while no definitive evidence of an acute stroke is visible on the scan, the limitations of CT imaging prevent the radiologist from completely ruling out the possibility of an early ischemic stroke. 1

Why CT Scans May Not Show Early Infarcts

• CT scans have suboptimal detection rates for early ischemic changes, especially within the first few hours after stroke onset 1
• During the first hours after acute ischemic stroke, the non-contrast CT (NECT) may show minimal or no abnormalities, even when an infarct is present 2, 3
• CT has limited sensitivity for detecting small infarcts or those in the posterior fossa 1
• The sensitivity of CT for detecting early signs of infarction ranges from 20% to 87%, with an average of 66% 4

What This Means for Patient Care

• When a CT "cannot exclude" an infarct, clinicians should:
  • Consider the patient's clinical presentation alongside the imaging findings 1
  • Recognize that MRI with diffusion-weighted imaging (DWI) is far superior to CT for detecting acute ischemia with very high sensitivity and specificity 1
  • Understand that a negative CT does not rule out stroke if clinical suspicion is high 5

Early CT Signs of Infarction That May Be Subtle or Absent

• Hyperdense middle cerebral artery sign (HMCAS), indicating arterial occlusion 2, 3
• Reduced contrast attenuation of cerebral parenchyma 2
• Attenuation of the lentiform nucleus (ALN) 2, 3
• Loss of the insular ribbon (LIR) 2, 3
• Hemispheric sulcus effacement (HSE) 2, 3

Clinical Implications

• For patients within 3 hours of symptom onset who are candidates for thrombolytic therapy, either NECT or MRI is recommended before intravenous tPA administration to exclude intracranial hemorrhage 1
• For patients beyond 3 hours from symptom onset, MR-DWI or CT angiography source images (CTA-SI) should be performed along with vascular imaging and perfusion studies 1
• When a CT "cannot exclude" infarct, additional imaging may be warranted:
  • MRI with DWI is the most sensitive and specific technique for demonstrating acute infarction within minutes after its occurrence 1
  • CTA-SI appears to be as good as DWI at detecting acute ischemia, with the exception of small foci and those in the posterior fossa 1

Common Pitfalls to Avoid

• Assuming a normal CT excludes stroke - up to 24% of patients with a clinical diagnosis of ischemic stroke may have a negative MRI even 4-6 weeks after the event 5
• Delaying appropriate treatment due to lack of CT findings - early signs of infarct on CT, regardless of their extent, are not a contraindication to thrombolytic treatment within the appropriate time window 1
• Relying solely on CT when higher sensitivity imaging is available and time permits 1
• Failing to consider that interobserver agreement for early CT signs of infarction is variable (kappa statistics range from 0.14 to 0.78) 4

Next Steps When CT "Cannot Exclude" Infarct

• If the patient is within the time window for thrombolysis (≤4.5 hours) and has no contraindications, consider proceeding with treatment even without definitive CT findings 1
• Consider advanced imaging if available and if it won't delay treatment:
  • MRI with DWI sequence 1
  • CT perfusion (CTP) to assess cerebral blood flow 1
  • CT angiography (CTA) to evaluate for vessel occlusion 1
• Monitor the patient closely for evolving neurological deficits 1
• Consider repeat imaging if the clinical picture changes or remains unclear 1

Remember that the phrase "cannot exclude an acute infarct" reflects the limitations of CT technology rather than a specific finding, and clinical judgment remains essential in these cases.