MRI artifacts

MRI artifacts are numerous and give an insight into the physics behind each sequence. Some artifacts affect the quality of the MRI exam while others do not affect the diagnostic quality but may be confused with pathology.

When encountering an unfamiliar artifact, it is useful to systematically examine general features of the artifact to try and understand its general class. These features include:

  • type of sequence, e.g. fast spin echo, gradient, volumetric acquisition
  • direction of phase and frequency
  • fat or fluid signal
  • presence of anatomy outside the image field
  • presence of metallic foreign bodies
  • Classification of the artifact type may give one an idea about how to try to fix it.

    Artifacts are caused by a variety of factors that may be patient-related such as voluntary and physiologic motion, metallic implants or foreign bodies. Finite sampling, k-space encoding, and Fourier transformation may cause aliasing and Gibbs artifact. Characteristics of pulse sequences may cause black boundary, Moiré, and phase-encoding artifacts. Hardware issues may cause central point and RF overflow artifacts.

    Remember that artifacts are not all bad, and that occasionally they are intentionally exploited, e.g. susceptibility artifact.

    The artifacts

    Many artifacts have a characteristic appearance and with experience they can be readily identified.

    MRI hardware and room shielding
    MRI software
    Patient and physiologic motion
    Tissue heterogeneity and foreign bodies
    Fourier transform and Nyqvist sampling theorem