Computationally Efficient Combination of Multi-channel Phase Data From Multi-echo Acquisitions (ASPIRE).

PURPOSE: To develop a simple method for combining multi-echo phase information from a number of coils in an array that requires no volume coil or additional scans and yields signal-to-noise ratio-optimal images that reflect only ΔB0-related phase.

THEORY AND METHODS: Two SNR optimal coil combination methods were developed which retrieve the ΔB0-related phase by determining the coil-dependent phase offsets. The first variant, MCPC-3D-S, requires the unwrapping of one phase image; the second variant, ASPIRE, allows unwrapping to be avoided if two echoes j and k satisfy the echo time relation m⋅TEk=(m+1)⋅TEj, where m is an integer, making this a particularly fast and robust approach. Both developed methods constitute improvements over a prior method, MCPC-3D, in terms of robustness and computational expense.

RESULTS: In the brain at 7 T, phase matching and contrast-to-noise ratio were higher with MCPC-3D-S and ASPIRE than with phase difference reconstruction, and similar to the reference coil-dependent Roemer combination. Unlike the Roemer and virtual reference coil methods, the proposed approaches also eliminated all non- ΔB0-related phase.

CONCLUSION: MCPC-3D-S is an improvement over prior multi-echo methods, which is useful if the ASPIRE echo time condition cannot be fulfilled. ASPIRE is a particularly fast and robust approach that runs on the scanner's reconstructor in a small fraction of the acquisition time. Magn Reson Med 79:2996-3006, 2018. © 2017 International Society for Magnetic Resonance in Medicine.