Diffusion-weighted MRI (DWI) shows restricted diffusion in a region of the brain. True restricted diffusion on DWI requires confirmation on which sequence to rule out 'T2 shine-through'?
- A FLAIR sequence
- B Apparent Diffusion Coefficient (ADC) map showing low signal in the same region ✓
- C T1 post-contrast sequence
- D Gradient echo (GRE) sequence
Correct answer: B. Apparent Diffusion Coefficient (ADC) map showing low signal in the same region
Explanation
DWI signal is influenced by both diffusion restriction AND T2 prolongation (T2 shine-through). A lesion with long T2 may appear bright on DWI even without true restricted diffusion. True restricted diffusion requires: bright on DWI AND dark (low signal) on the corresponding ADC map. If ADC is high (bright) despite high DWI signal, this represents T2 shine-through from vasogenic oedema or other T2-bright lesions, NOT cytotoxic oedema or cellular restriction. ADC confirmation is therefore essential before diagnosing restricted diffusion.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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