See Digitization Noise Artifact.

The portion of the MRI equipment that detects and amplifies the RF signals picked up by the receiver coil. Includes a preamplifier, MR signal amplifier, and demodulator. Phase sensitive detectors (electronic device in which the output is dependent on the instantaneous difference in phase between two input signals) are used to down-convert the MR signal to audio-frequencies prior to digitization.

See also MRI Equipment.

Quick Overview
Please note that there are different common names for this artifact.

A data truncation artifact may occur when the interface between high and low signal intensities is encountered in one imaging plane. The 2D-FT techniques transform the MR signal to spatial intensity image data with frequency and phase information encoding each axis in the plane of the scan. This artifact is found in both frequency and phase axes. Artifactual ripples adjacent to edges in an image or sharp features in a spectrum, caused by omission of higher frequency terms in Fourier transformation, particularly with the use of zero filling to replace unsampled higher frequencies.
Complex shapes are specified by series of sine and cosine waves of various frequencies, phase and amplitude. Some shapes are more difficult to encode than others. The most difficult shapes to represent with Fourier series of terms are waveforms with instantaneous transitions, tissue discontinuities or edges. The low-frequency components of the series describe the overall shape of the step function. Higher frequency components are needed to describe the corners if the step function more accurately. If not enough samples are taken, these areas cannot be accurately represented. The truncation of the infinite data series results in a ringing artifact because of the inability to accurately approximate this tissue discontinuity with a shorter truncated data set. Therefore, the ringing that occurs at all tissue boundaries on MR is called truncation artifact.

This problem can be easily resolved by taking more samples - a higher acquisition matrix and/or a smaller FOV. See Gibbs Artifact and Gibbs Phenomenon.

A wrap around artifact (also called backfolding artifact or aliasing artifact) is produced by inadequate sampling or digitization. Wrap around artifacts result from digitizing fewer than two samples per period in a periodic function. Aliasing can occur in MR imaging whenever the scanned area extends beyond the field of view. These areas extending beyond the field of view boundaries are aliased back into the image to appear at artifactual locations.