Friday, August 15, 2008

Volume Averaging Artifact

"Partial volume artifact" and "volume averaging" are terms that are thrown about willy-nilly by radiologists, because we run into this phenomenon every day. Here, we aim to give a brief explanation of this phenomenon.

This artifact occurs when an object is only partially within the slice (i.e. volume) that is imaged. When this occurs, the pixel value at that location is the average of the object and its surroundings.

To reify this concept, let us conduct an experiment. We will take a piece of plastic shaped like this:

Place this plastic piece in a water bath, and put the water bath into an MRI scanner. Next, perform two MRI pulse sequences, varying only slice thickness, keeping all other parameters the same:

(click on image to enlarge)

Each row contains consecutive slices through our plastic piece. At 0.7 mm slice thickness, the margins of the plastic object are relatively sharp, particularly at the center of the object.

At 1.2 mm slice thickness, however, we see something different. As we get to the edge of the object, there is marked blurring (red arrows). Why does this occur?

Remember that in this area, the slice encompasses both the object and the surrounding water. Thus, the signal value of the pixels in this area is the average of the pixel value of the object and the pixel value of the surrounding water. Voila, volume averaging artifact.

This has real world implications in radiology- to minimize volume averaging artifact, we want the smallest possible pixels. Unfortunately, as we make pixels smaller, the signal from those pixels also decreases. Thus, one must balance the desire for high resolution with the need for adequate signal. If done correctly, optimal image quality and diagnostic information is achieved.

Vic David MD

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