Which statement accurately describes DWI positive/FLAIR negative mismatch?

Which statement accurately describes DWI positive/FLAIR negative mismatch?

• A. DWI positive/FLAIR positive mismatch is used to predict salvageable tissue
• B. DWI negative/FLAIR mismatch is used to select patients for thrombolysis
• C. DWI positive/FLAIR negative mismatch is used to select patients that may benefit from thrombolysis within 4.5 hours or with unclear onset
• D. DWI negative/FLAIR negative mismatch indicates no infarct

Answer: (C)

The important idea here is how timing shows up on MRI sequences. Diffusion-weighted imaging detects acute cellular injury within minutes of stroke onset, so a lesion on DWI indicates tissue that is already affected. FLAIR changes, however, take a bit longer to appear, typically becoming evident a few hours after onset. When you have a DWI lesion but no corresponding FLAIR signal yet—this DWI positive/FLAIR negative pattern—it suggests the stroke is very recent, likely within a window where clot-busting therapy can still be effective, even if you don’t know exactly when the event began. That’s why this mismatch is used to identify patients who may benefit from thrombolysis within 4.5 hours or in cases of unclear onset. If both DWI and FLAIR are positive, the infarct is older and the benefit from thrombolysis is less certain. If both are negative, there may be no acute infarct visible. The key takeaway is that DWI positive/FLAIR negative points to a recently onset stroke, making thrombolysis a possible option within the standard time window or when onset time is uncertain.

The important idea here is how timing shows up on MRI sequences. Diffusion-weighted imaging detects acute cellular injury within minutes of stroke onset, so a lesion on DWI indicates tissue that is already affected. FLAIR changes, however, take a bit longer to appear, typically becoming evident a few hours after onset. When you have a DWI lesion but no corresponding FLAIR signal yet—this DWI positive/FLAIR negative pattern—it suggests the stroke is very recent, likely within a window where clot-busting therapy can still be effective, even if you don’t know exactly when the event began. That’s why this mismatch is used to identify patients who may benefit from thrombolysis within 4.5 hours or in cases of unclear onset.

If both DWI and FLAIR are positive, the infarct is older and the benefit from thrombolysis is less certain. If both are negative, there may be no acute infarct visible. The key takeaway is that DWI positive/FLAIR negative points to a recently onset stroke, making thrombolysis a possible option within the standard time window or when onset time is uncertain.