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

The received radio frequency signal is too strong, parts of the signal get lost by converting from analog to digital, resulting in a washed out image.

Auto-prescanning usually adjusts the amplification at the receiver in a way, that the received signal could be processed without any loss. Else the receiver gain must be corrected manually.

See also Data Clipping Artifact, Artifact Overview and Artifacts Reduction Index.

Quick Overview

Radio frequency quadrature artifacts occur when the detector channels of the quadrature detector are disturbed.
A DC offset of one amplifier output can e.g., produce a bright point in the center of the field of view (see also central point artifact), or a higher gain of one detector channel can generate diagonally ghosting.

Radio frequency quadrature artifacts are technical faults and must be eliminated by the service.

(SNR or S/N) The signal to noise ratio is used in MRI to describe the relative contributions to a detected signal of the true signal and random superimposed signals ('background noise') - a criterion for image quality.
One common method to increase the SNR is to average several measurements of the signal, on the expectation that random contributions will tend to cancel out. The SNR can also be improved by sampling larger volumes (increasing the field of view and slice thickness with a corresponding loss of spatial resolution) or, within limits, by increasing the strength of the magnetic field used. Surface coils can also be used to improve local signal intensity. The SNR will depend, in part, on the electrical properties of the sample or patient being studied. The SNR increases in proportion to voxel volume (1/resolution), the square root of the number of acquisitions (NEX), and the square root of the number of scans (phase encodings). SNR decreases with the field of view squared (FOV2) and wider bandwidths. See also Signal Intensity and Spin Density.

Measuring SNR:
Record the mean value of a small ROI placed in the most homogeneous area of tissue with high signal intensity (e.g. white matter in thalamus). Calculate the standard deviation for the largest possible ROI placed outside the object in the image background (avoid ghosting/aliasing or eye movement artifact regions).
The SNR is then:
Mean Signal/Standard Deviation of Background Noise

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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.