Metal Artifact Reduction Sequence (MARS)

The metal artifact reduction sequence (MARS) is a modified spin-echo pulse sequence that uses view angle tilting (application of the slice-select gradient at the same time as the frequency encoding gradient). Additional modifications are also made to optimize imaging.

All together, the MARS technique involves:

• View angle tilting: Used to remvoe geometric distortion. The slice-select gradient is applied at the same time as the frequency encoding gradient (i.e., during signal readout), giving the spins the same narrow frequency band and removing distortion due to magnetic field inhomogeneity. This comes at the cost of introducing blurring in the frequency encode direction.
• Increasing the slice-select gradient strength: This makes the imaging gradients as large as possible relative to the susceptibility-induced gradients produced by metal. Increasing the slice-select gradient strength also results in a decrease in slice thickness.
• Increasing the frequency encoding gradient strength: This makes the imaging gradients as large as possible relative to the susceptibility-induced gradients produced by the metal implants. Also decreases geometric distortion in the image. Increasing the frequency encoding gradient strength, however, requires a larger bandwidth, which reduces the signal-to-noise ratio (30% to 50% lower, depending on the field-of-view).

References

• Cho ZH, Kim DJ, Kim YK. Total inhomogeneity correction including chemical shifts and susceptibility by view angle tilting. Med Phys. 1988 Jan-Feb;15(1):7-11.
• Lee MJ, Janzen DL, Munk PL, MacKay A, Xiang QS, McGowen A. Quantitative assessment of an MR technique for reducing metal artifact: application to spin-echo imaging in a phantom. Skeletal Radiol. 2001 Jul;30(7):398-401.
• Olsen RV, Munk PL, Lee MJ, Janzen DL, MacKay AL, Xiang QS, Masri B. Metal artifact reduction sequence: early clinical applications. Radiographics. 2000 May-Jun;20(3):699-712.