Aliasing (wrap-around) in MRI: cause and fixes
Anatomy outside the field of view folds back into the image. These are the fixes and what each one costs.
Aliasing (wrap-around)
Anatomy outside the FOV appears folded onto the opposite side of the image.
Tissue outside the FOV in the phase direction is still excited and receives phase values identical to tissue inside the FOV. In 3D it also occurs in the slice direction.
- Phase oversampling (No Phase Wrap / Foldover suppression)
- Enlarge the FOV
- Swap phase to the shorter anatomical axis
- Saturation band on the outside tissue
- Use a coil that does not receive from the outside tissue; add slice oversampling in 3D
Oversampling adds time or reduces averages. A larger FOV lowers resolution unless the matrix is raised.
Phase oversampling (No Phase Wrap)
Prevents wrap-around when anatomy extends beyond the FOV in the phase direction.
The scanner acquires a larger FOV in the phase direction and discards the extra part.
Extra phase lines add time unless averages are reduced to compensate. GE No Phase Wrap doubles the phase FOV and halves NEX, so time and SNR stay about the same.
Use only as much oversampling as the anatomy needs. A saturation band over the outside tissue is an alternative when time is tight.
This page is one entry from the MRI Technologist Reference
The full reference has 31 protocols, 9 dynamic study timelines, 19 artifacts, 16 scanner problems and 15 safety topics, as a PDF in English and Arabic plus an interactive tool.
Educational reference. It does not replace your site protocols, the radiologist's instructions, the MR safety officer, or the manufacturer's instructions for any implant, device or contrast agent. Parameter values are typical starting points at 1.5 T and must be adapted to the scanner, coil and patient.