What is the preferred imaging modality between Diffusion-Weighted Imaging (DWI) and T2-weighted imaging for diagnosing acute ischemic stroke?

DWI is Superior to T2-Weighted Imaging for Diagnosing Acute Ischemic Stroke

Diffusion-weighted imaging (DWI) is significantly better than T2-weighted MRI and much better than CT for detecting an ischemic focus within 6 hours of stroke onset. 1

Comparative Effectiveness of Imaging Modalities

DWI Superiority

• DWI has emerged as the most sensitive and specific imaging technique for acute ischemia, with sensitivity of 88-100% and specificity of 95-100%, far exceeding NECT or any other MRI sequences 1, 2
• DWI can detect restricted diffusion as extracellular water moves into the intracellular environment during ischemia, making abnormal areas of ischemia readily visible within minutes of symptom onset 1, 2
• Multiple studies have demonstrated that DWI is significantly better than FLAIR and T2-weighted MRI for detecting an ischemic focus within 6 hours of symptom onset 1
• A randomized crossover comparison of DWI and CT within 6 hours of symptom onset demonstrated a sensitivity/specificity for DWI of 91%/95% versus 61%/65% for CT 1

Mechanism of DWI Detection

• DWI detects cytotoxic edema that occurs when extracellular water moves into the intracellular environment during ischemia, accompanied by swelling of cells and narrowing of extracellular spaces 1, 3
• The apparent diffusion coefficient (ADC) map, which demonstrates restricted diffusion as low intensity, greatly increases the specificity of DWI by eliminating T2 "shine-through" effects 1, 2

Clinical Applications and Recommendations

Acute Stroke Management

• For patients within 3 hours of symptom onset, either NECT or MRI is recommended before intravenous tPA administration to exclude intracranial hemorrhage 1
• MR-DWI surpasses NECT and other MR sequences for the detection of acute ischemia and can be used if it does not unduly delay the administration of intravenous tPA 1
• For patients beyond 3 hours from symptom onset, MR-DWI should be performed along with vascular imaging and perfusion studies, particularly if mechanical thrombectomy or intra-arterial thrombolytic therapy is contemplated 1

TIA Evaluation

• In patients with TIAs, multimodal MRI with DWI is preferred, and NECT should be performed only if MRI is not available 1
• DWI can demonstrate lesions in approximately 40% of patients with TIAs 1
• DWI positivity in patients with TIAs is associated with a higher risk for recurrent ischemic events 1, 2, 4

Practical Considerations

Time Efficiency

• A streamlined MRI protocol including DWI, FLAIR, gradient echo, and MR perfusion can be performed in approximately 10 minutes, making it competitive with CT in terms of acquisition time 2
• Core imaging sequences should include DWI, FLAIR, susceptibility scans (either SWI or GRE), and T1-weighted and T2-weighted scans 1

Special Situations

• MRI with DWI is most sensitive for acute stroke if completed within the first 1-2 weeks after stroke symptoms or sudden change in cognition or behavior 1
• DWI can detect small cortical lesions and subcortical lesions, including those in the brain stem or cerebellum, areas often poorly visualized with standard MRI sequences and non-enhanced CT 2
• DWI can identify subclinical satellite ischemic lesions that provide valuable information on stroke mechanism 2

Potential Limitations and Pitfalls

• MRI may not be available or may be contraindicated in some patients, in which case CT becomes a reasonable alternative 1
• DWI can occasionally show regions of hyperintensity which do not progress to infarction, suggesting that in some cases it may detect compromised but potentially salvageable tissue 5
• The presence of MRI-detected microbleeds, in the absence of unenhanced CT-detected hemorrhage, is not a contraindication to intravenous tPA within 3 hours of stroke onset in patients with a small number of microbleeds 1