Infographics / MRI Physics

General MRI artefacts (motion, aliasing, susceptibility, Gibbs)

Five schematic panels of the MRI artefacts named in the questions. Aliasing: an object larger than a too-small field of view folds back on the opposite side along the phase-encode direction. Chemical shift type 1: fat is misregistered a few pixels along the frequency-encode direction giving a bright and a dark band at a fat-water interface. Motion ghosting: discrete ghost copies of a structure repeat along the phase-encode direction. Susceptibility: a gradient-echo image shows a large signal void around metal while a spin-echo image shows a smaller one because its 180 degree pulse refocuses static dephasing. Gibbs ringing: a sharp intensity edge reconstructs with oscillatory overshoot because of finite k-space sampling.

MRI artefacts and their causes Five schematic panels of the MRI artefacts named in the questions. Aliasing: an object larger than a too-small field of view folds back on the opposite side along the phase-encode direction. Chemical shift type 1: fat is misregistered a few pixels along the frequency-encode direction giving a bright and a dark band at a fat-water interface. Motion ghosting: discrete ghost copies of a structure repeat along the phase-encode direction. Susceptibility: a gradient-echo image shows a large signal void around metal while a spin-echo image shows a smaller one because its 180 degree pulse refocuses static dephasing. Gibbs ringing: a sharp intensity edge reconstructs with oscillatory overshoot because of finite k-space sampling. MRI artefacts and their causes Aliasing (wrap-around) FOV phase encode FOV too small in the phase-encode direction: anatomy beyond the FOV is under-sampled (Δky too large) and folds to the opposite side. Chemical shift (type 1) fat (shifted) water bright dark frequency encode Fat resonates ~220 Hz below water at 1.5 T (~440 Hz at 3 T); frequency mapping mis- registers fat along the frequency-encode axis. Motion ghosting phase encode Movement between phase-encode steps (one per TR) adds phase errors, forming discrete ghost copies along the phase-encode axis. Susceptibility: GRE vs SE GRE — large void SE — reduced GRE lacks a 180° refocusing pulse, so T2* de- phasing at metal and air–tissue interfaces is not recovered; SE's 180° pulse refocuses it. Gibbs / truncation ringing position signal true edge overshoot Finite k-space sampling truncates the signal, so sharp edges reconstruct with oscillatory overshoot; increase matrix size (more encodes) to reduce it.
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