What does Diffusion-Weighted Imaging (DWI) show on imaging?

What DWI Shows on Imaging

Diffusion-Weighted Imaging (DWI) detects restricted diffusion of water molecules caused by cytotoxic edema in acute ischemic brain tissue, appearing as hyperintense (bright) signal within minutes of stroke onset, making it the most sensitive and specific imaging technique available for acute ischemic stroke. 1, 2

Primary Pathophysiologic Finding

• DWI visualizes restricted diffusion that occurs when extracellular water moves into the intracellular environment during ischemia, accompanied by cellular swelling and narrowing of extracellular spaces 1
• The technique creates an isotropic DWI map that makes abnormal ischemic areas readily visible as bright (hyperintense) regions 1
• This signal change occurs within minutes of symptom onset, with sensitivity of 88-100% and specificity of 95-100% for acute ischemic stroke 2

Critical Technical Consideration: T2 Shine-Through

• DWI is T2-based, which means high T2 abnormalities like vasogenic edema can create false-positive bright signals (called "shine-through") 1
• Correlation with the Apparent Diffusion Coefficient (ADC) map is essential to confirm true restricted diffusion, which appears as low intensity (dark) on ADC maps 1, 2
• Alternatively, dividing the DWI value by the T2 value for each pixel creates an exponential image that eliminates T2 shine-through, greatly increasing specificity 1

Specific Lesions Detected

• Small cortical and subcortical lesions including those in the brainstem or cerebellum—areas poorly visualized with standard MRI sequences and non-enhanced CT 2
• Subclinical satellite ischemic lesions that provide valuable information on stroke mechanism 2
• Acute versus chronic infarcts can be differentiated, as DWI is remarkably sensitive for acute lesions while chronic infarcts have different signal characteristics 3

Temporal Evolution of DWI Signal

• Acute phase (minutes to days): Significantly reduced ADC values due to cytotoxic edema 3
• Subacute phase (~1 week): ADC returns to "pseudonormal" values 3
• Chronic phase: Elevated ADC values are seen 3
• Signal intensity differences: Even within 6 hours of symptom onset, stroke patients show significantly higher signal intensity (rAI=1.26) compared to TIA patients with DWI lesions (rAI=1.16, p=0.03) 4

Clinical Applications Beyond Acute Stroke

• In TIA patients, DWI positivity indicates higher risk for recurrent ischemic events 2
• DWI shows clinically appropriate lesions in 57% of minor stroke patients and 14% of TIA patients scanned 2-4 weeks after symptom onset 5
• Persistent DWI lesions are closely related to markers of severity, including persistent symptoms and neurological signs 5

Important Pitfalls to Recognize

• DWI lesions can be reversible in the early hours of stroke, and the entire lesion may not represent irreversible ischemic core 6
• False-negative DWI can occur, leading to "DWI-negative stroke" or missed stroke diagnosis 6
• Ischemic stroke mimics can produce DWI hyperintensity in non-cerebrovascular neurological conditions 6
• Early diffusion lesions include both regions of irreversible infarction and regions of salvageable penumbra with less severe ADC changes 2

Superiority Over Other Imaging Modalities

• DWI is significantly better than FLAIR and T2-weighted MRI for detecting ischemic foci within 6 hours of symptom onset 1
• DWI is far superior to CT, with sensitivity/specificity of 91%/95% versus 61%/65% for CT within 6 hours 1
• 100% sensitivity versus 75% for CT within 6 hours in head-to-head comparisons 1