Quick Overview

A combination of B1 inhomogeneity, poor slice profile, and insufficient spoiler gradients between the refocusing pulse and the readout interval of a spin echo sequence results in a FID signal being detected along with the echo.
Since the FID is not phase encoded (normally the phase encoding occurs before the refocusing pulse), it is not dispersed along the phase encoding axis, but appears as a line across the center of the image.

If the problem persists, it must be addressed by a service representative.

Motion of material being imaged, particularly flowing blood, can result in many possible effects in the images.
Fast moving blood produces flow voids, blood flowing in to the outer slices of an imaging volume produces high signals (flow related enhancement, entry slice phenomenon), pulsatile flow creates ghost images of the vessel extending across the image in the phase encoding direction (image misregistration).
Flow-related dephasing occurring when spin isochromats are moving with different velocities in an external gradient field G so that they acquire different phases. When these phases vary by more then 180° within a voxel, substantial spin dephasing results leading to considerable intravascular signal loss.
These effects can be understood as caused by time of flight effects (washout or washin due to motion of nuclei between two consecutive spatially selective RF excitations, repeated in times on the order of, or shorter than the relaxation times of blood) or phase shifts (delay between phase encoding and frequency encoding) that can be acquired by excited spins moving along magnetic field gradients.
The inconsistency of the signal resulting from pulsatile flow can lead to artifacts in the image. The flow effects can also be exploited for MR angiography or flow measurements.

See also Flow Artifact.

Fractional Nex imaging (GE Healthcare term for imaging with a Nex value less than 1) benefits from the conjugate symmetry of the k-space to reduce the number of phase encoding acquisitions. With fractional Nex imaging (similar to partial Fourier or Half Scan), just over half of the data are acquired and the data from the lower part of k-space are used to fill the upper part, without sampling the upper part. Fractional Nex imaging sequences use a number of excitations values between 0.5 and 1. These values are a bit misleading, because the number of phase encoding steps is reduced, and not the NEX.
Fractional Nex imaging reduces the scan time considerable, by preserving the same contrast between the tissues. The effect by acquiring fewer data points is that the signal to noise ratio decreases.

See also acronyms for 'partial averaging//fractional Nex imaging' from different manufacturers.

Quick Overview

Ghosting artifacts are in the most cases caused by movements (e.g., respiratory motion, bowel motion, arterial pulsations, swallowing, and heartbeat) and appear in the phase encoding direction.

Ghosting artifacts can be reduced by respiratory and cardiac triggering, the use of breath holding pulse sequences, flow compensation or presaturation pulses, depending on their origin. To reduce bowel motion also pharmaceuticals, such as glucagon or scopolamine are useful. This will decrease artifacts from both peristalsis and breathing.

See also Motion Artifact, Phase Encoded Motion Artifact, Cardiac Motion Artifact, and Artifact Reduction - Motion.

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

Patient motion is the largest physiological effect that causes artifacts, often resulting from involuntary movements (e.g. respiration, cardiac motion and blood flow, eye movements and swallowing) and minor subject movements.
Movement of the object being imaged during the sequence results in inconsistencies in phase and amplitude, which lead to blurring and ghosting. The nature of the artifact depends on the timing of the motion with respect to the acquisition. Causes of motion artifacts can also be mechanical vibrations, cryogen boiling, large iron objects moving in the fringe field (e.g. an elevator), loose connections anywhere, pulse timing variations, as well as sample motion. These artifacts appear in the phase encoding direction, independent of the direction of the motion.

Motion artifacts can be flipped 90° by swapping the phase//frequency encoding directions.
The artifacts can be reduced by using breath holding, cardiac synchronization or respiratory compensation techniques: triggering, gating, retrospective triggering or phase encoding artifact reduction. Flow effects can be reduced by using gradient moment nulling of the first order of flow, gradient moment rephasing or flow compensation, depending of the MRI system.
Peristaltic motion can be reduced with the intravenous injection of an anti-spasmodic (e.g. Buscopan).
By using multiple averages, respiratory motion can be reduced in the same way that multiple averages increase the signal to noise ratio. Noticeable motion averaging is seen when four averages are obtained, six averages are often as good as respiratory compensation techniques and higher averages will continue to improve image quality.
In some cases will help a presaturation of the anatomy that was generating the motion.

See also Phase Encoded Motion Artifact.