Quick Overview

Uneven intensity or brightness may occasionally be noted on high field MRI e.g. of the brain. There are various causes of localized inhomogeneous brightness across the MRI images such as improper tuning of the RF transceiver, unbalanced deposition of the RF energy due to incorrect geometry and localized attenuation due to positioning or anatomical variants.

Tuning of the RF transceiver, a homogeneity correction filter and better positioning of the scanned object and/or coil in the scanner.

A water-cooled refrigeration system or heat pump used to reduce the shield temperatures of magnet LHe or LN2 vessels. Lower shield temperatures reduce costly magnet boil offs.

Current-carrying gradient coils with reduced gradient fringe field inside of the magnet cryostat structures like cryoshields and He-vessel. The shielding can be accomplished by secondary actively driven coils or by passive screens, which are inductively coupled to the gradient coils. In both cases eddy currents outside of the gradient system will be reduced.

See also Eddy Currents.

See Magnetic Shielding, Cryoshielding and Faraday Shield.

Paramagnetic compounds designed to induce a shift in the resonance frequency of nuclei with which they interact. For example, many rare earth's have been used as shift reagents for positive metal ions such as sodium and potassium.