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 essential for the guidance of interventional procedures by magnetic resonance imaging (MRI), because the accurate localization of each instrument is mandatory for this. In the past, this problem has been addressed by several groups, using theoretical, as well as experimental approaches. The artifacts associated with MRI are caused by geometry distortion and intravoxel dephasing. Usually, both effects mingle in the image, and depending on the pulse sequences and its parameters used for data acquisition, these effects are reflected in the image with different magnitude. Here, the authors shortly present the well-known mathematical background of the two underlying effects. Mathematically, both can be treated separately. Here, authors propose a new electromagnetic model which also allows to experimentally separate the effects better than by comparing spin-echo and gradient-echo images of the same object. With this new model, both effects-geometry distortion and intravoxel dephasing-are demonstrated separately using the same gradient-echo pulse sequence for all scans and adjusting the fields of the model properly. Furthermore, as this model allows to adjust both effects independently, it is used to study different weightings of both effects when they appear simultaneously in the image.