Medical Imaging Artifacts Overview

Artifacts and Their Compensations

• Artifacts are false features in medical images, arising during the imaging process.
• Their causes can be rectified when known.

Outline of Presentation

• Artifacts include:
  • Ghosts/Motion Artifacts
  • Aliasing/Wraparound
  • Chemical Shift Artifacts
  • Gibbs or Truncation artifacts
  • Magnetic Susceptibility Artifact
  • Zipper Artifacts
  • Shading Artifacts
  • Cross Excitation

Ghosts/Motion Artifacts

• Ghosts are replicas of objects within the image.
• Caused by body parts moving during scanning.
• Movement during image acquisition can lead to mismapping.
• Periodic movements (breathing, heartbeats) produce ghosting
• Nonperiodic movement causes image smearing.
• Ghosting occurs along phase-encoding axis.
• Corrective steps:
  • Patient stabilization with straps and cushions
  • Shorter scanning sequences
  • Cardiac gating (mandatory for cardiac motion)
  • Respiratory gating/compensation (for respiratory motion)
  • Spatial Presaturation (SAT) technique to reduce blood flow motion artifacts

Aliasing/Wraparound

• Anatomy outside the field of view (FOV) appears within the image.
• Occurs when FOV is smaller than the anatomy being imaged.
• Aliasing happens along frequency and phase encoding axes.
• Corrective steps:
  • Increase field of view (FOV)
  • Filter frequency-encoded direction
  • Increase FOV along phase-encoding direction

Chemical Shift Artifacts

• Occur at the interface between water and fat due to different proton precessional frequencies.
• Expressed in parts per million (ppm).
• Water protons have a higher frequency than fat.
• Causes signal voids or misregistration, displaying as a dark region on one side of the tissue and a bright signal on the other side of the water-fat interface.
• More pronounced in higher field strengths.
• Corrected by fat suppression techniques.

Gibbs or Truncation Artifacts

• Bright and dark lines parallel to intensity change boundaries.
• Seen in CSF, spinal cord, fat, and muscle.
• Occur along phase-encoding direction.
• Corrections include increasing the matrix size and using filters.

Magnetic Susceptibility Artifacts

• Magnetic susceptibility: the ability of substances to become magnetized.
• Different magnetization degrees between tissues can lead to signal dephasing and loss
• High susceptibility differences, such as between soft tissues and air, produce signal loss and distortion of tissue boundaries in close proximity to air sinuses.
• Other causes: metal objects.
• Corrective steps:
  • Using SE sequences
  • Removing metal objects

Zipper Artifacts

• Caused by external RF signals interfering with the image.
• Often related to hardware/software problems or opening the scanner door during scanning.
• Appears as perpendicular parallel lines to the frequency axis.
• Corrective measures: reporting system-generated artifacts to engineers

Shading Artifacts

• Uneven contrast with signal intensity loss in specific regions of the image.
• Caused by uneven excitation of nuclei, improper coil loading, or magnetic field inhomogeneity.
• Corrected by properly loading the coil and shim corrections to reduce inhomogeneity.

Cross Excitation

• Adjacent slices can receive RF energy and be excited.
• The excited nuclei in adjacent slices do not have enough longitudinal magnetization for proper tilting.
• This reduces signal intensity in the adjacent slices.
• Remedy: increasing interslice gap.