Phase-corrected bipolar gradients in multi-echo gradient-echo sequences for quantitative susceptibility mapping
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Large echo spacing of unipolar readout gradients in current multi-echo gradient-echo (GRE) sequences for mapping fields in quantitative susceptibility mapping (QSM) can be reduced using bipolar readout gradients thereby improving acquisition efficiency.
Materials and methods
Phase discrepancies between odd and even echoes in the bipolar readout gradients caused by non-ideal gradient behaviors were measured, modeled as polynomials in space and corrected for accordingly in field mapping. The bipolar approach for multi-echo GRE field mapping was compared with the unipolar approach for QSM.
The odd–even-echo phase discrepancies were approximately constant along the phase encoding direction and linear along the readout and slice-selection directions. A simple linear phase correction in all three spatial directions was shown to enable accurate QSM of the human brain using a bipolar multi-echo GRE sequence. Bipolar multi-echo acquisition provides QSM in good quantitative agreement with unipolar acquisition while also reducing noise.
With a linear phase correction between odd–even echoes, bipolar readout gradients can be used in multi-echo GRE sequences for QSM.
KeywordsQuantitative susceptibility mapping Bipolar gradient Phase correction
This study was supported in part by grants from The National Natural Science Foundation of China (81271533) and by NIH R01EB013443, R01NS073270, and R43EB015293.
Conflict of interest
The authors declare that they have no conflicts of interest.
The study was approved by the internal institutional review board that oversees compliance with the Declaration of Helsinki. Written informed consent was obtained from all subjects prior to imaging.
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