Velocity encoding and velocity compensation for multi-spoke RF excitation
Purpose - To investigate velocity encoded and velocity compensated variants of multi-spoke RF pulses that can be used for flip-angle homogenization at ultra-high fields (UHF). Attention is paid to the velocity encoding for each individual spoke pulse and to displacement artifacts that arise in Fouri...
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| Main Authors: | , , , , |
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| Format: | Article (Journal) |
| Language: | English |
| Published: |
2020
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| In: |
Magnetic resonance imaging
Year: 2019, Volume: 66, Pages: 69-85 |
| ISSN: | 1873-5894 |
| DOI: | 10.1016/j.mri.2019.11.007 |
| Online Access: | Verlag, lizenzpflichtig, Volltext: https://doi.org/10.1016/j.mri.2019.11.007 Verlag, lizenzpflichtig, Volltext: http://www.sciencedirect.com/science/article/pii/S0730725X19302978 
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| Author Notes: | Simon Schmidt, Sebastian Flassbeck, Peter Bachert, Mark E. Ladd, Sebastian Schmitter |
| Summary: | Purpose - To investigate velocity encoded and velocity compensated variants of multi-spoke RF pulses that can be used for flip-angle homogenization at ultra-high fields (UHF). Attention is paid to the velocity encoding for each individual spoke pulse and to displacement artifacts that arise in Fourier transform imaging in the presence of flow. - Theory and methods - A gradient waveform design for multi-spoke excitation providing an algorithm for minimal TE was proposed that allows two different encodings. Such schemes were compared to an encoding approach that applies an established scheme to multi-spoke excitations. The impact on image quality and quantitative velocity maps was evaluated in phantoms using single- and two-spoke excitations. Additional validation measurements were obtained in-vivo at 7 T. - Results - Phantom experiments showed that keeping the first gradient moment constant for all k-space lines eliminates any displacements in phase-encoding and slice-selection direction for all spoke pulses but leads to artifacts for non-zero velocity components along readout direction. Introducing variable but well-defined first gradient moments in the phase-encoding direction creates displacements along the velocity vector and thus minimizes velocity-induced geometrical distortions. Phase-resolved mean volume flow in the ascending and descending aorta obtained from two-spoke excitation showed excellent agreement with single-spoke excitation over the cardiac cycle (mean difference 0.8 ± 16.2 ml/s). - Conclusions - The use of single- and multi-spoke RF pulses for flow quantification at 7 T with controlled displacement artifacts has been successfully demonstrated. The presented techniques form the basis for correct velocity quantification and compensation not only for conventional but also for multi-spoke RF pulses allowing in-plane B1+ homogenization using parallel transmission at UHF. |
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| Item Description: | Available online 13 November 2019 Gesehen am 01.04.2020 |
| Physical Description: | Online Resource |
| ISSN: | 1873-5894 |
| DOI: | 10.1016/j.mri.2019.11.007 |