Unlike ringing, aliasing is a true artifact because it is never possible to perform exact recovery of the initial data from their

FIGURE 1 Ringing. Especially with high-quality interpolation, oscillations may appear after horizontal translation by half a pixel. (Left) Original MRI. (Right) Translated MRI.

FIGURE 2 Aliasing. (Top) Low quality introduces a lot of aliasing. (Bottom) Better quality results in less aliasing. (Both) At too coarse scale, the structural appearance of the bundles of cells is lost.

aliased version. Aliasing is related to the discrete nature of the samples. When it is desired to represent a coarser version of the data using fewer samples, the optimal procedure is first to create a precise representation of the coarse data that uses every available sample, and then only to downsample this coarse representation. In some sense, aliasing appears when this procedure is not followed, or when there is a mismatch between the coarseness of the intermediate representation and the degree of downsampling (not coarse enough or too much downsampling). Typical visual signatures of aliasing are moire effects and the loss of texture. Figure 2 illustrates aliasing.

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