From the Guidelines
Yes, a stroke can be missed on MRI, particularly if standard MRI sequences are used, as they are relatively insensitive to the changes of acute ischemia within the first hours after onset of stroke. According to the guidelines for the early management of patients with ischemic stroke 1, the standard MRI sequences (T1-weighted, T2-weighted, and proton density) will show abnormalities in less than 50% of patients. However, diffusion-weighted imaging (DWI) allows visualization of ischemic regions within minutes of onset of symptoms, with a high sensitivity (88% to 100%) and specificity (95% to 100%) for detecting acute ischemia 1.
Limitations of MRI in Stroke Detection
Some of the limitations of MRI in the acute setting include:
- Cost
- Relatively limited availability of the test
- Patient contraindications such as claustrophobia, cardiac pacemakers, or metal implants 1
- Technical factors like patient movement during scanning, improper imaging parameters, or artifacts can obscure stroke findings
- Interpretation errors by radiologists can occur, especially with subtle findings
Importance of Clinical Assessment and Follow-up Imaging
Clinical assessment remains crucial, and when stroke is strongly suspected despite negative initial imaging, follow-up MRI after 24-48 hours may reveal abnormalities that weren't initially visible. The recommendations for imaging of acute ischemic stroke suggest that for patients beyond 3 hours from onset of symptoms, either MR-DWI or CTA-SI should be performed along with vascular imaging and perfusion studies 1.
Role of DWI in Stroke Detection
DWI sequences significantly improve stroke detection, showing abnormalities within minutes of onset, but even these aren't 100% sensitive, particularly for posterior circulation strokes. The use of MRI, particularly DWI, can provide helpful early prognostic information and help identify patients who might be successfully treated with thrombolytic therapy or other agents 1.
From the Research
Stroke Diagnosis and MRI
- A stroke may not always be apparent on Magnetic Resonance Imaging (MRI) 2, 3, 4.
- Studies have shown that MRI may not visualize all acute strokes, and some cases of stroke may have negative brain MRI scans 2.
- False-negative diffusion-weighted MR findings in acute ischemic stroke can occur, especially in cases of stroke in the posterior circulation or when MRI is obtained within 24 hours after symptom onset 3.
Factors Influencing MRI Detection of Stroke
- The size and location of the stroke lesion can affect its visibility on MRI, with smaller lesions or those in certain areas of the brain being more likely to be missed 3.
- The timing of the MRI scan can also impact its ability to detect a stroke, with sensitivity declining over time 4.
- Clinical characteristics such as stroke severity, duration of hospital stay, and functional status at discharge can differ between patients with positive and negative MRI findings 4.
Clinical Implications
- The absence of an infarct on MRI does not necessarily rule out a diagnosis of ischemic stroke, especially in cases where clinical characteristics suggest a likely diagnosis 4.
- Neurologists should be cautious when interpreting MRI results, particularly beyond the acute stroke stage, and consider clinical diagnosis and other factors when making treatment decisions 4.