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Maxwell-compensated Design of Asymmetric Gradient Waveforms for Tensor-valued Diffusion Encoding

1Department of Radiology, Brigham and Women's Hospital, Harvard medical School, Boston, MA, USA.
2Clinical Sciences, Lund, Lund University, Lund, Sweden.
Publication Date:
Magn Reson Med
Volume Number:
Issue Number:
Magn Reson Med. 2019 Oct;82(4):1424-37.
PubMed ID:
Maxwell terms, asymmetric gradient waveforms, concomitant gradients, tensor-valued diffusion encoding
Appears in Collections:
P41 EB015902/EB/NIBIB NIH HHS/United States
2016-03443/Swedish Research Council/
2016-04482/Swedish Research Council/
AM13-0090/Swedish Foundation for Strategic Research/
MN15/Random Walk Imaging/
R01 MH074794/MH/NIMH NIH HHS/United States
Generated Citation:
Szczepankiewicz F., Westin C-F., Nilsson M. Maxwell-compensated Design of Asymmetric Gradient Waveforms for Tensor-valued Diffusion Encoding. Magn Reson Med. 2019 Oct;82(4):1424-37. PMID: 31148245. PMCID: PMC6626569.
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Diffusion encoding with asymmetric gradient waveforms is appealing because the asymmetry provides superior efficiency. However, concomitant gradients may cause a residual gradient moment at the end of the waveform, which can cause significant signal error and image artifacts. The purpose of this study was to develop an asymmetric waveform designs for tensor-valued diffusion encoding that is not sensitive to concomitant gradients. METHODS: The "Maxwell index" was proposed as a scalar invariant to capture the effect of concomitant gradients. Optimization of "Maxwell-compensated" waveforms was performed in which this index was constrained. Resulting waveforms were compared to waveforms from literature, in terms of the measured and predicted impact of concomitant gradients, by numerical analysis as well as experiments in a phantom and in a healthy human brain. RESULTS: Maxwell-compensated waveforms with Maxwell indices below 100 (mT/m)2 ms showed negligible signal bias in both numerical analysis and experiments. By contrast, several waveforms from literature showed gross signal bias under the same conditions, leading to a signal bias that was large enough to markedly affect parameter maps. Experimental results were accurately predicted by theory. CONCLUSION: Constraining the Maxwell index in the optimization of asymmetric gradient waveforms yields efficient diffusion encoding that negates the effects of concomitant fields while enabling arbitrary shapes of the b-tensor. This waveform design is especially useful in combination with strong gradients, long encoding times, thick slices, simultaneous multi-slice acquisition, and large FOVs.