🩺 Introduction
Diffusion-weighted MRI (DWI) measures the random motion of water molecules (Brownian motion) within tissues. Pathological processes that alter tissue microstructure affect water diffusion, which is detected as signal changes on DWI and quantified using Apparent Diffusion Coefficient (ADC) maps.
⚡ Physics Simplified
- DWI sequence = T2-weighted sequence + diffusion-sensitive gradients.
- Restricted diffusion = bright on DWI, dark on ADC.
- T2 shine-through = lesion bright on both DWI & ADC (not true restriction).
🖼️ Interpretation of DWI
- High signal on DWI + low signal on ADC → True restricted diffusion.
- Seen in: acute infarct, abscess, high-cellularity tumors.
- High signal on DWI + high/normal signal on ADC → T2 shine-through.
- Seen in: cysts, edema, some benign lesions.
- Low signal on DWI + high ADC → Free diffusion (normal CSF).
🧩 Clinical Uses of DWI
1️⃣ Neuroradiology
- Acute ischemic stroke → Most important use; DWI positive within minutes.
- Brain abscess → restricted diffusion in pus cavity.
- Tumor grading → high-grade tumors = more restricted diffusion (low ADC).
- White matter diseases (e.g., demyelination, encephalitis).
- Neurodegenerative diseases → DWI helps in Creutzfeldt-Jakob disease.
2️⃣ Oncologic Imaging
- Tumor detection, characterization, and therapy response.
- High-cellularity tumors = restricted diffusion.
- Monitoring response → rising ADC suggests necrosis/response.
3️⃣ Body Imaging
- Liver → focal lesion characterization, metastasis detection, HCC evaluation.
- Kidneys → differentiating cysts from abscesses/tumors.
- Prostate → key sequence in PI-RADS; restricted diffusion suggests clinically significant cancer.
- Breast → adjunct to contrast MRI in cancer detection.
- Whole-body DWI → staging and follow-up in oncology.
4️⃣ Infection & Inflammation
- Abscess vs necrotic tumor → abscess cavity = restricted diffusion.
- Inflammatory bowel disease → active segments show restricted diffusion.
🧠 Teaching Pearls
- Always check ADC maps to confirm true restriction.
- Restricted diffusion = high cellularity or pus.
- T2 shine-through = artifact; don’t mistake for pathology.
- DWI is essential in stroke, oncology, and abscess imaging.
✅ Conclusion
Diffusion-weighted MRI is a powerful tool that provides functional information beyond anatomy. It has revolutionized imaging in stroke, oncology, infection, and body imaging. Correct interpretation requires correlation of DWI with ADC maps to avoid pitfalls.
