Off-resonance suppression for multispectral MR imaging near metallic implants

Purpose Metal artifact reduction in MRI within clinically feasible scan‐times without through‐plane aliasing. Theory and Methods Existing metal artifact reduction techniques include view angle tilting (VAT), which resolves in‐plane distortions, and multispectral imaging (MSI) techniques, such as slice encoding for metal artifact correction (SEMAC) and multi‐acquisition with variable resonances image combination (MAVRIC), that further reduce image distortions, but significantly increase scan‐time. Scan‐time depends on anatomy size and anticipated total spectral content of the signal. Signals outside the anticipated spatial region may cause through‐plane back‐folding. Off‐resonance suppression (ORS), using different gradient amplitudes for excitation and refocusing, is proposed to provide well‐defined spatial‐spectral selectivity in MSI to allow scan‐time reduction and flexibility of scan‐orientation. Comparisons of MSI techniques with and without ORS were made in phantom and volunteer experiments. Results Off‐resonance suppressed SEMAC (ORS‐SEMAC) and outer‐region suppressed MAVRIC (ORS‐MAVRIC) required limited through‐plane phase encoding steps compared with original MSI. Whereas SEMAC (scan time: 5′46″) and MAVRIC (4′12″) suffered from through‐plane aliasing, ORS‐SEMAC and ORS‐MAVRIC allowed alias‐free imaging in the same scan‐times. Conclusion ORS can be used in MSI to limit the selected spatial‐spectral region and contribute to metal artifact reduction in clinically feasible scan‐times while avoiding slice aliasing. Magn Reson Med 73:233–243, 2015. © 2014 Wiley Periodicals, Inc.