ARTEFACTS Flashcards
Phase Mismapping
It is caused by moving anatomy or fluid across the image in the phase direction e.g. chest wall during inspiration.
REM:
Swap frequency and phase to avoid artefact over anatomy of interest.
Aliasing or Wrap Around
Anatomy that exists outside the FOV are folded or wrapped over the anatomy inside the FOV.
Cause
REM:
Increasing the FOV so that all the anatomy falls within it – that will reduce the spacial resolution.
Chemical Shift Artifact
dark edge is produced at the interface between fat and water and occurs in the frequency direction only.
This artefact is caused by the different chemical environments of fat and water.
The only way to eliminate this is to use chemical saturation.
Chemical Misregistration
When the signals of fat and water are in phase they add constructively but when they are out of phase they cancel each other out.
REM:
Choose a TE that matches the periodicity of fat and water at the field strength of the magnet.
Truncation, Gibbs or Ringing Artifact
This is a banding artefact at the interfaces between high and low signal e.g. at the edges of the brain where high signal from fat in the scalp lies next to the low signal from the skull.
Magnetic Susceptibility Artefact
This artefact distorts the image and has large signal voids.
Cause
It is usually caused by metal objects within the imaging volume e.g. metal screws, braces surgical clips etc
Zipper artifact
This artefact appears on the MR image as a dense line in the frequency direction.
It is caused by an RF leak in the RF shielding of the room, when RF is entering the scan room at a certain frequency and interfering with the inherently weak signal coming from the patient.
Remedy
Ring the engineer to locate the leak and repair it.
Shading Artifact
The appearance of shading is a difference or loss of signal intensity across the image.
Cause
The main cause is the uneven excitation of nuclei within the patient due to RF pulses applied and other flip angles then 900 and 1800.
Moirés Artifact
This artefact is usually seen on the edges of the FOV as a black and white banding in a GRE sequence when a large FOV is used.
Remedy
Use a spin echo sequence instead and ensure the patients arms are in the FOV or put the arms above the patients head to ensure they are away from the anatomy being imaged.
Magic Angle
This artefact is seen in certain tendons as a high signal intensity that may mimic pathology.
It is caused when certain structures with tightly bound fibers lie at an angle of 550 to the main magnetic field altering its signal intensity
