What is the next step if CT (Computed Tomography) and CTA (Computed Tomography Angiography) are negative in a patient with suspected stroke, should I do an MRI (Magnetic Resonance Imaging)?

MRI After Negative CT and CTA in Suspected Stroke

Yes, you should obtain an MRI if CT and CTA are negative in a patient with suspected stroke, as follow-up MRI is reasonable to confirm the diagnosis and can detect acute ischemic infarcts missed by CT in approximately one-quarter of cases. 1

Why MRI is Indicated After Negative CT/CTA

The diagnostic gap is substantial: About 25% of acute stroke cases with initially negative CT will show evidence of acute or subacute infarction on MRI. 1 In one prospective study, MRI detected acute ischemic stroke in 46% of patients compared to only 10% detected by CT, with this advantage persisting even when CT was performed within 3 hours of symptom onset (46% vs 7%). 2

Guideline support is clear: The 2021 American Heart Association/American Stroke Association guidelines provide a Class 2a recommendation (Level B-NR evidence) stating that in patients suspected of having ischemic stroke, if CT or MRI does not demonstrate symptomatic cerebral infarct, follow-up CT or MRI of the brain is reasonable to confirm diagnosis. 1

Specific Clinical Scenarios

For Suspected Ischemic Stroke

• Perform follow-up MRI within 1-2 days if initial CT/CTA is negative but clinical suspicion remains high 1
• MRI with diffusion-weighted imaging (DWI) has 83% sensitivity for acute stroke versus 26% for CT 2
• This is particularly important for posterior circulation strokes, where follow-up MRI may be appropriate even when initial MRI is negative 1

For Suspected TIA

• Follow-up MRI is specifically reasonable in patients suspected of having had a TIA when initial head imaging does not demonstrate symptomatic cerebral infarct 1
• MRI helps predict risk of early stroke and supports the diagnosis 1
• Approximately one-third of patients with stroke symptoms lasting <24 hours have a DWI-positive lesion 1

Important Caveats and Pitfalls

MRI is not 100% sensitive: Even with MRI's superior sensitivity, absence of an infarct on MRI is not uncommon after clinical diagnosis of ischemic stroke. In one study, 24% of patients with clinically diagnosed ischemic stroke had negative MRI at 4-6 weeks, with 60% of these having a "likely" diagnosis of stroke by expert panel. 3

Timing matters: MRI sensitivity is highest in the acute phase and declines over time. 3 Therefore:

• Obtain MRI as soon as feasible after negative CT/CTA
• Do not rule out stroke based solely on negative MRI beyond the acute stage 3

Clinical context is paramount: Patients with MRI-negative strokes typically have:

• Lower stroke severity at admission 3
• Shorter hospital stays 3
• Better functional status at discharge 3

Small strokes may be missed: MRI can fail to detect strokes localized to cortex, brainstem, or subcortical/lacunar areas, even when performed within 24 hours of symptom onset. 4

Practical Algorithm

1. Initial negative CT/CTA with persistent clinical suspicion → Proceed to MRI with DWI, FLAIR, and gradient-echo sequences 1, 5

2. If MRI also negative but high clinical suspicion persists → Do not rule out stroke; consider:

   • Repeat MRI in 1-2 days (particularly for posterior circulation) 1
   • Complete diagnostic workup including vascular imaging (MRA), cardiac evaluation, and laboratory studies 1

3. Focus on secondary prevention regardless → Even with negative imaging, if clinical diagnosis of TIA/stroke is likely, initiate appropriate secondary prevention strategies 1