Phase-uncertainty quality map for two-point Dixon fat–water separation

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This work investigates and compares two different phase-correction algorithms for Dixon fat–water separation and two different quality maps (QM) for region-growing: the original QM, based on phase gradients, and a QM based on phase uncertainty, proposed in this article. A spoiled dual-gradient-echo sequence was employed at 1.5 T to acquire in-phase and out-of-phase images of joints, parotid glands, abdomen and test objects. All 97 datasets were processed eight times each: with two different phase correction algorithms (original and hierarchical phase correction), with two different QM, and with/without removing linear component of the phase drifts associated with dual-echo acquisitions and bipolar readout gradient waveforms. The linear component of the phase drift along the readout direction was found to reach 4.1° pixel−1, depending on the geometric parameters. Pre-processing to remove linear phase shifts has little impact on outcome. The hierarchic phase-correction algorithm outperformed the original phase-correction algorithm in all applications. The proposed phase-uncertainty QM provides a small performance improvement in clinical images, but can be vulnerable to flow-related phase shifts in bright vessels. Overall the most successful phase-correction technique employed phase-uncertainty QMs and hierarchic algorithms, with pre-processing to correct the linear phase drift associated with dual-echo acquisitions and bipolar readout gradient waveform.