Why does a negative Magnetic Resonance Imaging (MRI) not rule out a cerebrovascular accident (CVA) or stroke?

From the Guidelines

A negative MRI does not rule out a stroke because the most recent and highest quality study, published in 2024 1, highlights the limitations of MRI in detecting certain types of strokes, particularly small infarcts and those that occurred within the first few hours after symptom onset.

Key Points to Consider

• The study found that false-negative MRIs with small strokes occurred 6 to 48 hours after the onset of vestibular symptoms, emphasizing the need for careful clinical assessment and potentially repeat imaging.
• Diffusion-weighted imaging (DWI) sequences, which are most sensitive for acute ischemia, may not show changes until 3-6 hours after stroke onset, as noted in earlier studies 1.
• Certain stroke mimics like transient ischemic attacks (TIAs) may cause temporary symptoms without leaving permanent tissue damage detectable on imaging, further complicating diagnosis.
• Clinical assessment remains crucial in stroke diagnosis, and a negative MRI should not override strong clinical suspicion of stroke, especially when symptoms are recent.

Recommendations for Clinical Practice

• In cases where stroke is still suspected despite negative initial imaging, repeat MRI after 24-48 hours, additional imaging modalities like CT perfusion, or close clinical monitoring may be warranted.
• The use of MRI with diffusion-weighted imaging is particularly helpful in evaluating patients with low-risk TIA and mild neurological symptoms, as suggested by the 2021 guideline 1.
• Earlier MRI studies in posterior fossa stroke showed a high false-negative rate on DWI when performed within the first 24 hours of symptom onset, but this has improved with better imaging techniques over time 1.

From the Research

Reasons for a Negative MRI Not Ruling Out Stroke

• A negative Magnetic Resonance Imaging (MRI) does not rule out a cerebrovascular accident (CVA) or stroke due to several factors, including the timing of the MRI scan and the sensitivity of the imaging technique 2, 3.
• The sensitivity of MRI in detecting a lesion shortly after ischemic stroke rapidly declines as time progresses, which can lead to false-negative results 3.
• Studies have shown that a significant proportion of patients with clinically confirmed stroke can have negative diffusion-weighted imaging (DWI) MRI, ranging from 21% to 33% 4, 5.
• Patients with negative DWI-MRI can still have a high risk of stroke recurrence, cognitive impairment, and poor functional outcomes, similar to those with positive DWI-MRI 4.
• The absence of an infarct on MRI is not uncommon, even in patients with a likely diagnosis of ischemic stroke, and neurologists should be cautious when ruling out the diagnosis based on MRI alone 3.

Factors Associated with Negative MRI

• Less severe stroke, younger age, female gender, and increased time from stroke onset to scan are associated with negative DWI 5.
• Younger age and female gender are also associated with negative MRI (DWI or T2 or FLAIR) scans 5.
• The technical spatial resolution of MRI and the involvement of small-sized lesions can also contribute to false-negative results 6.

Clinical Implications

• A negative MRI or DWI does not exclude the diagnosis of stroke, and clinical judgment should be used in conjunction with imaging results to make a diagnosis 5.
• Patients with negative DWI-MRI should still receive secondary prevention and be closely monitored for stroke recurrence and other complications 4.
• The high rate of negative MRI and DWI among patients with minor stroke has important management and research implications, highlighting the need for careful clinical evaluation and follow-up 5.