Comparison of positive contrast MRI methods for depiction of field disturbing objects/paramagnetic contrast agents

Abstract

Local field inhomogeneities, that extend over areas largerthan one voxel size, change the applied imaging gradient and result in an echoshift in the readout direction and phase encoding direction. In this article,the field inhomogeneities that are caused by the presence of(super)paramagnetic (SPIO) particles in the human body are discussed. Thepresence of these particles eventually causes signal loss. Positive contrastmethods are an attempt to invert the dark contrast of the field perturbingstructures. Two categories of positive contrast techniques are presented:preprocessing and postprocessing methods. Among the acquisition methods thewhite marker techniques and the inversion recovery with on-resonance watersuppression (IRON) techniques will be treated. White marker conserves signalfrom regions with field inhomogeneities while the background is cancelled. IRONmethod suppresses the water protons which frequency is in the range of thesaturation radio frequency (RF) pulse. On the other hand, susceptibilitygradient mapping (SGM) is a postprocessing technique in which the positions ofthe echo shifts in k-space are calculated to determine the susceptibilityinduced gradient. For comparison the white marker and IRON technique can besimulated by postprocessing. The choice for a positive contrast techniquedepends on the application.

It is a trade-off between the advantages and disadvantagesof each technique. However, postprocessing techniques are more promising forfuture research