Real-time 4D phase unwrapping applied to magnetic resonance elastography

Purpose Phase amplitude is a source of signal in magnetic resonance elastography (MRE) experiments but its exploitation in experimental design has been limited due to the challenges of phase wrap. This study addressed this aspect of MRE through new developments in algorithms, heuristic strategy, and user interface. Methods A test dataset with systematic variation of three parameters - nested wrap, gradient, and noise level - was developed to choose phase-unwrapping algorithms and to analyze their performance. A new application, PhaseTools, was developed that implemented three phase-unwrapping algorithms that adhere to a "real-time" criterion of less than 3 min for a four-dimensional MRE acquisition. Two of the algorithms extend previously published algorithms and one was newly developed. The algorithms were then applied to five datasets from MRE, two typical cases and three edge cases that were particularly challenging in one of the three parameters. Results The performance of the PhaseTools algorithms on the test dataset was comparable to two widely cited algorithms that take hours or days to complete. Guidelines for the optimal use of each algorithm are established. Conclusion PhaseTools enables the substantial increase of signal-to-noise in MRE experiments at negligible additional computational cost. PhaseTools is freely released with this study, making robust real-time phase unwrapping available to any group using phase-based imaging. Magn Reson Med 73:2321-2331, 2015.