Partial Volume Averaging Artifact on Radiology Reports: What It Means

Partial volume averaging is a common imaging artifact found on studies like CT or MRI.  This occurs when tissues get blended together within a single image slice or voxel. The scanner then reports an average value instead of the true value of each tissue. This is usually a technical limitation of how images are made, and radiologists are trained to recognize and work around it.

The simple idea: one pixel, many tissues

Imagine taking a photograph of a black-and-white checkerboard but using very large, blocky pixels. If one pixel covers both a black and a white square, that pixel will look gray. It’s not because the board has gray squares—it’s because the camera blended black and white together. CT and MRI can do the same thing when a voxel (a 3-D pixel) contains more than one type of tissue, such as soft tissue and fat, or bone and air. The result is an average number that doesn’t exactly match either tissue, which can make edges look blurry or create small false impressions.

Where it shows up most

• At sharp edges: Boundaries between very different materials—like bone next to soft tissue or soft tissue next to air—are classic spots for partial volume.

• In thin or curved structures: Tiny vessels, ducts, or the edges of organs.

• In small lesions: A very small nodule or cyst can look different depending on how it intersects with the slice thickness; it may seem denser or less dense than reality.

Why radiologists call it an “artifact”

An artifact is a feature created by the imaging process rather than the patient’s body. Partial volume averaging is not a disease. It’s a side effect of the “sampling” process—how the scanner divides the body into slices and voxels. Calling it an artifact helps separate technical effects from real findings that need treatment.

What causes partial volume averaging

• Slice thickness: Thicker slices include more anatomy within each voxel, increasing the chance that different tissues get averaged together.

• Object orientation: If a structure runs diagonally through the slice, more types of tissue fall into the same voxel, increasing averaging.

• Small structures: Sometimes the scanner can’t represent it cleanly; it becomes partly averaged with its surroundings.

How it changes the image (and the numbers)

On CT, tissue is measured in Hounsfield Units (HU). Partial volume can raise or lower the HU of a small region simply because a bit of bone, air, fat, or contrast agent shares the same voxel.

Common examples you might see in reports

• Lung nodules on an abdominal CT: When the lungs are only partly included and slices are thicker, a small calcified granuloma may look less dense, or a tiny vessel can mimic a nodule on a single slice. Dedicated thin-slice chest CT usually clears this up.

• Liver lesions: A small cyst can look less dense on one slice and slightly denser on another, depending on how it’s sampled.

• Bone edges: The border between dense bone and nearby soft tissue can seem fuzzy or slightly “thicker” due to averaging.

How radiologists reduce partial volume

• Thinner slices / high-resolution reconstructions: Thinner slices mean each voxel contains less mixed tissue, reducing averaging.

• Multiplanar reformats (MPR): Reconstructing the data in different planes (axial, sagittal, coronal, or oblique) helps confirm whether a finding is real or just an averaging effect on a single plane.

• Repeat or dedicated imaging when needed: If a finding is uncertain and clinically important, a targeted scan (for example, a dedicated thin-slice chest CT for a suspected lung nodule) may be recommended.

What this means for patients

• Partial volume is common and expected: It is a normal part of image formation, not a sign of poor quality care.

• Context matters: Radiologists compare multiple slices, planes, and sequences to decide whether something is real. They also consider your history, symptoms, and prior studies.

• Follow-up may be suggested: If partial volume could hide or mimic something important, the report may recommend a different technique (thinner slices, different sequence) or short-interval follow-up to be safe.

FAQs

Is partial volume dangerous?

No. It’s a technical effect. The concern is not harm, but possible confusion—blurring edges or making small things look different than they are. Radiologists are trained to spot it.

Why didn’t the scanner just use super-thin slices for everything?

There’s a balance. Thinner slices produce more images, more noise, and larger files, which can increase scan time, reading time, radiation dose (for CT, depending on protocol). Protocols aim for the right resolution for the clinical question.

Could partial volume make a benign thing look worrisome?

Occasionally. That’s why radiologists check the same area in multiple planes, look for consistency across slices, and compare with prior imaging.

Can partial volume hide disease?

It can blur very thin structures or tiny lesions. When a finding would change care, radiologists adjust technique (thinner slices, special sequences) or suggest tailored imaging to reduce that risk.

Tips to understand your report

• Look for the word “artifact”: If partial volume is mentioned, the radiologist is telling you a feature may reflect image blending rather than true anatomy.

• Note any recommendations: Requests for dedicated or follow-up imaging are usually aimed at removing uncertainty caused by artifacts.

• Ask about prior studies: Comparison with older scans often clarifies whether a subtle finding is real or stable.

How partial volume differs from other artifacts

• Motion artifact blurs images because the patient or organ moved.

• Beam hardening (CT) creates streaks near dense objects like metal or bone.

• Susceptibility (MRI) distorts areas near metal or air.
  Partial volume, by contrast, is about averaging of mixed tissues within a voxel, most noticeable along edges and in tiny structures.

What clinicians do next

If an apparent finding could affect decisions—like starting a treatment, doing a biopsy, or changing follow-up—your care team will typically confirm it with additional imaging tests (thinner slices, different sequences, or a targeted exam). This approach helps avoid unnecessary procedures and makes sure important abnormalities aren’t missed.

Conclusion

Partial volume averaging artifact is a normal imaging limitation, not a disease. It happens when a single voxel contains more than one type of tissue and the scanner shows an average. Radiologists anticipate it and use techniques to reduce it.  If your report mentions partial volume, it’s usually a sign your radiologist is being careful and precise.

References

https://radiopaedia.org/articles/partial-volume-averaging-ct-artifact-1

https://www.ncbi.nlm.nih.gov/books/NBK597347/

https://mriquestions.com/partial-volume-effects.html