% setup MRI-education-resources path and requirements
cd ../
startup
loading image
loading signal
Artifacts#
This notebook includes a simulation of various MRI artifacts, as well as a high-level Artifact Comparison below. The wikipedia entry https://en.wikipedia.org/wiki/MRI_artifact is also very comprehensive.
Learning Goals#
Identify artifacts and how to mitigate them
Introduction#
Many of the artifacts that occur in MRI can be understood and analyzed using the k-space perspective. In particular, they can be understood as the k-space data being modified by some function. This is described mathematically in MRI Signal Equation and K-space in the K-space Data Weighting section.
Artifact Comparison#
MRI artifacts can generally be categorized as originating from the sample, the sequence, or the system (Source in table below). The table below provides a high-level comparison of these artifacts.
Artifact Name |
Source |
Appearance |
What to do? |
Frequency or Phase encoding |
|---|---|---|---|---|
Aliasing |
Sequence - FOV too small, or parallel imaging failed |
Signal folds across image |
Increase FOV, swap PE/FE, reacquire sensitivity maps or reduce acceleration factor (parallel imaging) |
Phase encoding |
Gibbs Ringing/Truncation |
Sequence - due to Fourier encoding of edges |
Ripples/ringing at sharp edges |
Filtering of data, decrease voxel size |
N/A |
Boundary Artifacts/Partial Volume |
Sequence and Sample - opposing phase of \(M_{XY}\) within a voxel due to inversion or chemical shift |
Artificial dark lines at tissue boundaries |
fat suppresion, fat/water imaging, change inversion time |
N/A |
Motion/Flow |
Sample - signal modulated during spatial encoding |
Copies or “Ghosts” of image regions that are changing due to motion or flow |
Breath-hold, gating, triggering, flow compensation, spatial saturation, swap PE/FE |
Phase encoding |
Image Displacement/Distortion |
Sample - chemical shift (fat) and off-resonance (e.g. magnetic susceptibility differences) |
Shifts of tissue signals in image, pile-up and stretch artifacts |
Increase bandwidths, shimming, fat suppression, fat/water imaging, swap PE/FE |
Frequency encoding (2DFT), Phase encoding (EPI) |
Slice Displacement |
Sample - chemical shift (fat) and off-resonance (e.g. magnetic susceptibility differences) |
Slice location shifts |
Increase RF bandwidth, shimming, use 3D scanning |
N/A |
T2* |
Sample - magnetic suscpetibility differences (e.g. implants, air) |
Signal loss |
Use spin-echoes, shorten TE, shimming |
N/A |
Dielectric Shading |
System - RF propogates unevenly, creating B1 inhomogeneity |
Shading across entire image, particularly with large FOVs and higher B0 |
B1-insensitive pulse sequences (e.g. adiabatic pulses), image bias field corrections |
N/A |
RF interference/Zipper |
System - unwanted frequencies received in data |
Line of noisy signal on top of image |
RF shielding (e.g. close door), check RF hardware, check room lights |
At a specific frequency encoding location |
Gradient non-linearity |
System - magnetic field gradients are not linear, especially away from isocenter |
Distortion of subject, particularly near edges of large FOV |
Gradient warping correction |
N/A |
Spike Noise |
System - intermittent unwanted signal |
Specific spatial frequency on top of entire image |
check RF hardware (loose connections, broken components) |
N/A |