Partial volume averaging
Read Your Scan Editorial Team·Last updated
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
When one slice contains two different tissues, the scanner reports their average, which can invent an appearance belonging to neither. It is why a thin slice through the edge of a structure sometimes suggests something that thinner slices show is not there.
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Where you'll see this
CT and MRI findings, offered as the innocent explanation for a borderline appearance.
Your report may word it “Volume averaging” or “Partial volume effect”. These mean the same thing.
Questions worth asking your doctor
- Was any part of the scan unreadable because of this?
- Does it need to be repeated, and if so how soon?
- Could this have hidden something, or only made the pictures look odd?
How often does this turn up?
Partial volume averaging is a technical artefact present in all cross-sectional imaging, and radiologists mention it when it might explain an appearance. Each image is a slice with real thickness, so when two different tissues both fall within one slice, the scanner reports their average rather than either one — which can create something that looks like a finding but is not.
Does it mean something is wrong?
It is an explanation rather than a finding, and it is usually invoked to dismiss something rather than to raise it. It is most likely where structures are small relative to the slice thickness, or at boundaries between very different tissues — the top of the liver against the lung, or a small structure at the edge of an organ. A radiologist naming it is saying this appearance may be an artefact of how the images were built, which is a reason to be less concerned rather than more.
What to do with it
Where the appearance matters, the answer is usually thinner slices or a different plane, both of which reduce the effect and frequently make an apparent finding disappear entirely. Modern scanners use thinner slices than older ones, so the artefact is less prominent than it once was. Where it explains an ambiguity fully, nothing further is needed.
Frequently asked questions
- Does partial volume averaging mean the scan was done badly?
- No. It is inherent to slice-based imaging rather than a fault, and it exists on every CT and MRI. Radiologists account for it routinely and name it when it might account for an appearance.
- Can it hide something?
- It can blur small structures and make them harder to characterise, which is why thin slices are used where small detail matters. It can equally create an appearance that is not real, which is the more common reason it gets mentioned.
- How is it avoided?
- Thinner slices and imaging in an additional plane. Where an ambiguous appearance matters, repeating that part of the study with thinner sections usually resolves it one way or the other.
Picture problems, not body problems
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