A perspective on needle artifacts in MRI: An electromagnetic model for experimentally separating susceptibility effects

A thorough understanding of artifacts caused by metallic instruments is ess ential for the guidance of interventional procedures by magnetic resonance imaging (MRI), because the accurate localization of each instrument is mand atory for this. In the past, this problem has been addressed by several gro ups, using theoretical, as well as experimental approaches, The artifacts a ssociated with MRI are caused by geometry distortion and intravoxel dephasi ng, Usually, both effects mingle in the image, and depending on the pulse s equences and its parameters used for data acquisition, these effects are re flected in the image with different magnitude. In this paper we shortly pre sent the well-known mathematical background of the two underlying effects. Mathematically, both can be treated separately, Here, we propose a new elec tromagnetic model which also allows to experimentally separate the effects better than by comparing spin-echo and gradient-echo images of the same obj ect. With this new model, both effects-geometry distortion and intravoxel d ephasing-are demonstrated separately using the same gradient-echo pulse seq uence for all scans and adjusting the fields of the model properly. Further more, as this model allows to adjust both effects independently, it is used to study different weightings of both effects when they appear simultaneou sly in the image.