Gadolinium oxide (Gd2O3) is a prototype paramagnetic agent for contrast enhanced MRI. Gd2O3 particles have very high relaxivity. With its high magnetic moment, gadolinium reduces the relaxation time and enhance MR signal intensity. All gadolinium compounds are highly toxic. Very stable complexes are developed to eliminate the toxicity. See also Paramagnetic Substance, Paramagnetism, Paramagnetic Contrast Agents and Contrast Agents, the info sheet gives an overview and more in-dept information about different types of MRI Contrast media.

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 MRI artifact.

The Gibbs or ringing artifact appears as a series of lines in the MR image parallel to abrupt and intense changes in the object at this location. This artifact does not occur visibly on smooth objects. This artifact is caused by the Gibbs phenomenon, an overshoot or ringing of Fourier series occurring at discontinuities.
In the spinal cord, a small syrinx can be simulated by the Gibbs phenomenon. Gibbs artifacts are also seen in other regions, for example the brain//skull interface.
Fine lines visible in an image may be due to undersampling of the high spatial frequencies, respectively incomplete digitization of the echo.
With more encoding steps the Gibbs artifacts is less intense and narrower. Therefore, e.g. the artifact is more intense in the 256 point dimension of a 256x512 acquisition matrix.

This problem can only be resolved by smoothing filters (LanczosSigmaFactor, 2-D Exponential Filtering, Gegenbauer Reconstruction etc.) or with a higher acquisition matrix and/or a smaller FOV, to smooth the object.

See also Gibbs Phenomenon and Apodization.

(HS) A method in which approximately one half of the acquisition matrix in the phase encoding direction is acquired. Half scan is possible because of symmetry in acquired data. Since negative values of phase encoded measurements are identical to corresponding positive values, only a little over half (more than 62.5%) of a scan actually needs to be acquired to replicate an entire scan. This results in a reduction in scan time at the expense of signal to noise ratio. The time reduction can be nearly a factor of two, but full resolution is maintained.
Half scan can be used when scan times are long, the signal to noise ratio is not critical and where full spatial resolution is required. Half scan is particularly appropriate for scans with a large field of view and relatively thick slices; and, in 3D scans with many slices. In some fast scanning techniques the use of Half scan enables a shorter TE thus improving contrast. For this reason, the Half scan parameter is located in the contrast menu.

More information about scan time reduction; see also partial fourier technique.

The period of time required to collect the image data. This time does not include the time necessary to reconstruct the image. The total time for performing a scan must take into consideration the additional image reconstruction time when determining how quickly the image may be viewed.