ADC map
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Written from published radiology references and reviewed for plain language. Not written or signed by a physician, and not a diagnosis. How we write these pages →
In plain language
A picture calculated from the diffusion sequence, used as its cross-check: genuinely restricted tissue is bright on diffusion and dark here. Radiologists read the two together because diffusion images alone can shine for uninteresting reasons.
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Where you'll see this
MRI technique and findings, alongside DWI.
Your report may word it “ADC” or “Apparent diffusion coefficient”. These mean the same thing.
Questions worth asking your doctor
- Did this scan use the right technique to answer my question?
- Would contrast, or a different sequence, show something this one could not?
- Do I need another scan, or is this one enough?
How often does this turn up?
The ADC map is produced automatically whenever diffusion-weighted imaging is done, so it appears on essentially every brain MRI. It is not a picture of anatomy but a calculation, made from the diffusion images, showing how freely water is moving in each part of the tissue. It exists for one reason: to stop the diffusion images being read wrong.
Does it mean something is wrong?
On its own the ADC map is never the finding — it is the control. The rule radiologists apply is simple: genuinely restricted tissue is bright on diffusion and dark on ADC. An area bright on both is an artefact called T2 shine-through, and mistaking it for restriction would mean reporting a stroke that is not there. So when a report mentions the ADC map, it is usually documenting that the check was done and what it showed, which is a sign of careful reading rather than of a problem.
What to do with it
Nothing follows from the map itself. It is listed with the technique or referenced inside a finding, and it is the finding that drives anything further. If your report describes an area as bright on diffusion with corresponding low ADC, the radiologist is telling the referring doctor that the restriction is real rather than an artefact.
Frequently asked questions
- What does low ADC mean?
- That water movement in that area is genuinely restricted. Combined with a bright signal on the diffusion images, it confirms true restriction rather than an artefact — which in the brain is the pattern that identifies an acute stroke.
- What is T2 shine-through?
- An area that looks bright on diffusion images for a reason unrelated to restricted movement. It is the trap the ADC map exists to catch: if the area is also bright on ADC rather than dark, the brightness is shine-through and not restriction.
- Is the ADC map a separate scan?
- No, and it adds no time in the scanner. It is calculated from the diffusion images that were already acquired, which is why it appears automatically whenever diffusion-weighted imaging is part of the protocol.
How the scan was taken
Where this word comes up
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