This happens whenever a structure is only partly within the imaging section, pixel or voxel. The consequence is that the signals of the structure and the adjacent or surrounding structures present in the section, pixel or voxel are averaged.
As a result erroneous pixel or voxel signal values result. This can lead to diagnostic misinterpretations.

See also Partial Volume Artifact.

(THK) The thickness of an imaging slice. As the slice profile may not be sharp edged, a criterion such as the distance between the points at half the sensitivity of the maximum (FWHM) or the equivalent rectangular width (the width of a rectangular slice profile with the same maximum height and same area) is used to determine thickness.

For the image quality its important to choose the best fitting slice thickness for an examination. When a small item is entirely contained within the slice thickness with other tissue of differing signal intensity then the resulting signal displayed on the image is a combination of these two intensities. If the slice is the same thickness or thinner than the small structure, only that structures signal intensity is displayed on the image. This partial volume averaging effect explains the vanishing of fine details by choosing slices too large for the scanned object.

See also Partial Volume Artifact.