Acceleration of slice encoding for metal artifact correction at 0.55 T using hexagonal sampling.

Purpose: To evaluate a hexagonal sampling approach for accelerated slice encoding for metal artifact correction (SEMAC) at 0.55 T. Contemporary mid-field systems (0.1 T-1.0 T) show tremendous potential for imaging near metal implants. However, the limited parallel-imaging options necessitate alternative methods for scan time reduction.

Methods: We apply retrospective hexagonal undersampling to current state-of-the-art SEMAC with 2-fold generalized autocalibrating partially parallel acquisitions-based parallel imaging at 0.55 T. The hexagonal sampling approach results in an additional 50% scan time reduction. Feasibility is evaluated with phantom experiments involving spinal fixation and total hip arthroplasty hardware, and in vivo experiments involving patients with spinal fusions with varying compositions and 1 volunteer with a total hip arthroplasty.

Results: Hexagonal sampling provides an additional 50% scan time reduction with compatible image quality. The two tradeoffs are (i) a small increase in signal void due to the loss of signal from one SEMAC spectral bin during post-acquisition filtering and (ii) position-dependent signal-to-noise-ratio reduction at locations close to the edge of the field of view in the phase-encoding direction.

Conclusion: We demonstrate that hexagonal sampling can provide 50% scan time reduction in addition to generalized autocalibrating partially parallel acquisitions/parallel imaging for SEMAC at 0.55 T without introducing substantial artifacts. This may be a valuable mechanism for reducing scan time at 0.55 T and other midfield strengths, where parallel-imaging acceleration is limited.