What is the role of Diffusion-Weighted Imaging (DWI), Fluid-Attenuated Inversion Recovery (FLAIR), and Apparent Diffusion Coefficient (ADC) sequences in acute ischemic stroke diagnosis and management?

Role of DWI, FLAIR, and ADC Sequences in Acute Ischemic Stroke Diagnosis and Management

Diffusion-weighted imaging (DWI) is the most sensitive and specific imaging technique for detecting acute ischemic stroke, with sensitivity of 88-100% and specificity of 95-100%, making it superior to both FLAIR and conventional CT for early stroke detection. 1

Diffusion-Weighted Imaging (DWI)

• DWI detects 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
• DWI allows precise identification of lesion size, site, and age, which correlates significantly with clinical outcomes and final infarct volume 2, 3
• 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 (NECT) 1
• DWI can identify subclinical satellite ischemic lesions that provide valuable information on stroke mechanism 1
• In TIA patients, DWI positivity (present in approximately 39% of cases) indicates higher risk for recurrent ischemic events 1, 4

Apparent Diffusion Coefficient (ADC)

• ADC maps are essential companions to DWI as they eliminate T2 "shine-through" effect, greatly increasing the specificity of the technique 1
• ADC values in ischemic lesions are typically 29% lower than in normal-appearing brain tissue 2
• ADC ratios correlate significantly with clinical outcome, making them valuable prognostic indicators 2, 5
• ADC values may help differentiate tissue destined to infarct from potentially salvageable tissue with reperfusion therapy 5
• Early after ischemia onset, the visible diffusion lesion includes both regions of irreversible infarction (with more severe ADC changes) and regions of salvageable penumbra (with less severe ADC changes) 1

Fluid-Attenuated Inversion Recovery (FLAIR)

• Standard FLAIR sequences are relatively insensitive to acute ischemic changes compared to DWI 1
• FLAIR detects approximately 91% of ischemic lesions, while DWI detects 98% 2
• Vascular hyperintensities on FLAIR sequences can indicate slow-flowing blood passing through leptomeningeal collaterals 1
• DWI-FLAIR mismatch (positive DWI with negative FLAIR) can help identify patients within the 4.5-hour treatment window for thrombolysis when time of onset is unknown 6
• FLAIR visibility of lesions increases with longer time from stroke onset, lower patient age, and larger DWI lesion volume 6

Clinical Applications in Stroke Management

• For patients within the 4.5-hour window (IV tPA candidates), either NECT or MRI is recommended to exclude intracranial hemorrhage before IV tPA administration 4
• DWI is superior to conventional imaging in patients with minor strokes and those imaged later after symptom onset 7
• Initial DWI lesion volume is a strong predictor of clinical outcome - in first-ever stroke patients, a lesion volume ≤22 mL predicts good outcome with 75% sensitivity and 100% specificity 2
• The artery susceptibility sign on gradient echo sequences is the MR correlate of the hyperdense MCA seen on NECT, with MRI detecting clot in 82% of cases versus 54% with NECT 1
• In patients beyond 6 hours from symptom onset, multimodal imaging with perfusion assessment (including DWI) is essential for treatment selection 4

Practical Considerations and Pitfalls

• While MRI with DWI is more sensitive than NECT for early detection of ischemic changes, it should not delay treatment if not immediately available 4
• 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 1
• The main limitation to routine MRI use in acute stroke is not acquisition time but access to scanners on an emergency basis 1
• Focusing solely on structural imaging without perfusion assessment may miss the opportunity to identify salvageable tissue, especially in patients presenting beyond the standard treatment windows 4
• Magnetic susceptibility imaging (T2*-weighted sequences) can reliably detect intracranial hemorrhage, allowing MRI to be used as the sole initial imaging modality for evaluating acute stroke patients 1