page_title: MRI artifact
section_title: Wrap-around
hierarchical_section_title: MRI artifact / Signal processing dependent artifacts / Wrap-around
caption_attribution_description: English: Head MRI with wrap around artifacts. For context, see: MRI artifact
context_page_description: An MRI artifact is a visual artifact in magnetic resonance imaging. It is a feature appearing in an image that is not present in the original object. Many different artifacts can occur
  during MRI, some affecting the diagnostic quality, while others may be confused with pathology. Artifacts can be classified as patient-related, signal processing-dependent and hardware-related.
context_section_description: A wrap-around artifact also known as an aliasing artifact, is a result of mismapping
  of anatomy that lies outside the
  field of view but within the slice volume.
  The selected field of view is
  smaller than the size of the imaged
  object. The anatomy is usually displaced
  to the opposite side of the
  image (Figs 6 and 7). It can be caused
  by non-linear gradients or by undersampling
  of the frequencies contained
  within the return signal.
  The sampling rate must be twice
  the maximal frequency that occurs in
  the object (Nyquist sampling limit). If
  not, the Fourier transform will assign
  very low values to the frequency signals
  greater than the Nyquist limit.
  These frequencies will then ‘wrap
  around’ to the opposite side of the
  image, masquerading as low-frequency
  signals. In the frequency encode
  direction a filter can be applied to the
  acquired signal to eliminate frequencies
  greater than the Nyquist frequency.
  In the phase encode direction, artifacts
  can be reduced by an increasing
  number of phase encode steps
  (increased image time). For correction,
  a larger field of view may be chosen.
caption: Fig. 6. Wrap-around artifacts.[1]
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