Common Artefacts
Artefacts are distortions or structures that are not anatomically present but are visible in a MRI examination. This page will demonstrate examples of the most common artefacts seen in T1-weighted, T2-weighted, fMRI and DTI scans from 3T scanners. It will also detail known causes, preventive measures and possible fixes.
- RF overflow artefact cause a nonuniform, washed-out appearance to an image.
- This artifact occurs when the signal received by the scanner from the patient is too intense to be accurately digitized by the analog-to-digital converter.
- Autoprescanning usually adjusts the receiver gain to prevent this from occurring but if the artifact still occurs, the receiver gain can be decreased manually.
- Post-processing methods also exist but may be time consuming.
- The central point artefact is a focal dot of increased signal in the centre of an image.
- It is caused by a constant offset of the DC voltage in the receiver.
- After Fourier transformation, this constant offset gives the bright dot in the centre of the image.
- The first axial MRI image of the head shows a central point artefact projecting in the pons in the centre of the image.
- Head movement during the acquisition of a scan can cause distortions within the image.
- The distortions can be severe (A) or more subtle (B).
- While it is most important to make sure the participant does not move their head during scans, there are many pre- and post- processing methods to decrease the impact of head movement.
- The optimal method depends of the type of scan you are using and the purpose of your analysis.
- Herringbone artefact, also called as crisscross artefact or corduroy artefact. It appears as a fabric of herring bone.
- They can be subtle or more pronounced.
- The artefact is scattered all over the image in a single slice or multiple slices.
- It is caused by electromagnetic spikes by gradient coils fluctuating power supply or RF pulse discrepancies.
- Zipper refers to a type of MRI artefact where one or more spurious bands of electronic noise extend perpendicular to the frequency encode direction and is present in all images of a series.
- RF from some radio transmitters will cause zipper artefacts that are oriented perpendicular to the frequency axis of your image.
- Frequently there is more than one artefact line on an image from this cause corresponding to different radio frequencies.
- Other equipment and software problems can cause zippers in either axis.
- Susceptibility artefacts are especially encountered while imaging near metallic orthopedic hardware or dental work (e.g. braces), and result from local magnetic field inhomogeneities introduced by the metallic object into the otherwise homogeneous external magnetic field B0.
- In the figures on the left, the nose folds into the back of the skull.
- In the figure on the right, the ear wraps into the side of the skull.
- Wrap around generally occurs when the subject’s head size is larger than the acquisition box.
- If the acquisition box does not fully cover the subject’s head (including the nose), increase the FOV and increase the number of phase encoding steps accordingly so as not to change the pixel resolution of the image.
- Reposition the T1 acquisition box to cover the entire head (including the nose) and rescan.
- Low signal to noise ratios can result in lack of contrast between gray and white matter as well as the high noise signal in the background of the image.
- These can be subtle so look carefully.
- Acquisitions with low SNR can result from incorrect sequence parameters and incorrect placement of the head coil (make sure the head coil is plugged in correctly!).
