Finding the right tests…
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Finding the right tests…
MRI
Diffusion weighted imaging (DWI) is an MRI sequence, not a scan of one body part. It's sensitive to how freely water molecules move inside tissue, and it can be added to many different MRI exams. Its best-known use is spotting acute stroke: within minutes of a stroke starting, the affected brain tissue swells at the cellular level and water movement becomes restricted, which shows up clearly on DWI even when the rest of the MRI looks normal. DWI is also used outside the brain, where tumors tend to restrict water movement because they're packed more densely with cells than normal tissue, which helps with detecting and grading some cancers.
DWI produces images plus an apparent diffusion coefficient (ADC) map, showing where water movement is restricted. In the brain this can flag very early ischemic stroke; elsewhere it can flag tissue that behaves like a tumor. Your doctor interprets these findings alongside your symptoms and other imaging.
DWI doesn't image a fixed body part the way a knee MRI or a liver MRI does. It's a technique, a way of making the scanner sensitive to water diffusion, that gets added into many different types of MRI exams depending on what your doctor is looking for.
When brain tissue is starved of blood, cells begin to swell within minutes, and this restricts how water can move inside them. DWI picks up this restricted movement earlier than other MRI sequences and often earlier than a CT scan. One study found DWI detected acute stroke changes within 3 hours of onset in about 73% of cases, compared with about 12% for CT scans done in the same window, which is why DWI is a core part of urgent stroke imaging.
- Tumors are usually packed with more cells than surrounding normal tissue, which also restricts water movement and shows up on DWI - It can help suggest whether a lesion is more or less likely to be aggressive - It's used to help judge whether a tumor is responding to treatment, since responding tissue tends to show less restriction over time
Because DWI findings can overlap between different conditions, your doctor reads them together with the rest of your scan and your clinical history, not in isolation.
It's usually one sequence within a larger MRI exam, not a stand-alone scan. Your doctor adds it in when it's useful for what they're checking.
Brain cells affected by a stroke swell within minutes, which restricts how water moves inside them. DWI is sensitive enough to catch this change earlier than most other imaging, including CT.
Yes. It's also used in other parts of the body, since tumors tend to restrict water movement in a similar way, which helps in detecting and characterizing them.
No. It's an MRI sequence, so it uses magnetic fields and radio waves, not X-rays.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
No single sequence gives a certain answer. DWI findings are interpreted together with the rest of your MRI and your clinical picture, and further tests may still be needed.
It can add some time since it's an additional sequence, though the amount varies by exam. Ask your doctor or the centre what to expect for your specific scan.
Tissue responding to treatment tends to show less restricted water movement over time. Repeat DWI scans can help track whether a tumor is responding.