Metal artifact reduction sequence

A metal artifact reduction sequence (MARS) is intended to reduce the size and intensity of susceptibility artifacts resulting from magnetic field distortion.

A variety of techniques are used for reducing metal artifacts at MRI, both for addressing artifacts due to the presence of metal in the image plane (in-plane artifacts) and for artifacts due to metal in an adjacent plane (through-plane artifacts).

In-plane artifact reduction

A number of simple changes to the scan protocol can greatly reduce artifacts. Examples are ¹:

• lower magnetic field strength: 1.5 T rather than 3 T
• increase bandwidth during slice selection and readout
• increase matrix: 512 pixel
• maintain good signal to noise ratio by increasing number of excitations (NEX)
• spin echo instead of gradient echo where possible
• STIR for fat suppression (spectral frequency selective fat suppression performs better in a homogeneous field)
• shorter echo spacing
• smaller water-fat shift
• thinner slices
• view-angle-tilting (VAT)

Through-plane artifact reduction

Multiacquisition variable-resonance image combination (MAVRIC) is a specialised sequence designed by GE to minimise metallic artifact around metallic prostheses ³. It relies on 3D FSE sequences, using multiple different overlapping volumes at different frequency offsets.

Another technique developed by Siemens used for addressing through-plane metal artifacts is Slice-Encoding for Metal Artifact Correction (SEMAC), where an additional slice-encoding gradient is added to a standard fast-spin echo sequence ⁴. The combination of the MAVRIC and SEMAC technique is known as MAVRIC-SL ⁵.

See also

• 1.5 T vs 3 T MRI