Velocity encoding and velocity compensation for multi-spoke RF excitation

• Verfasst von: Schmidt, Simon [VerfasserIn]
• Verfasst von: Flassbeck, Sebastian [VerfasserIn]
• Verfasst von: Bachert, Peter [VerfasserIn]
• Verfasst von: Ladd, Mark E. [VerfasserIn]
• Verfasst von: Schmitter, Sebastian [VerfasserIn]
• Titel: Velocity encoding and velocity compensation for multi-spoke RF excitation
• Verf.angabe: Simon Schmidt, Sebastian Flassbeck, Peter Bachert, Mark E. Ladd, Sebastian Schmitter
• Jahr: 2020
• Jahr des Originals: 2019
• Umfang: 17 S.
• Fussnoten: Available online 13 November 2019 ; Gesehen am 01.04.2020
• Titel Quelle: Enthalten in: Magnetic resonance imaging
• Ort Quelle: Amsterdam [u.a.] : Elsevier Science, 1982
• Jahr Quelle: 2020
• Band/Heft Quelle: 66(2020), Seite 69-85
• ISSN Quelle: 1873-5894
• DOI: doi:10.1016/j.mri.2019.11.007
• Datenträger: Online-Ressource
• Sprache: eng
• Sach-SW: 4D-flow
• Sach-SW: Displacement artifact
• Sach-SW: Flow quantification
• Sach-SW: Multi-spoke RF pulses
• Sach-SW: Parallel transmit
• Sach-SW: RF shimming
• K10plus-PPN: 1693668785

Abstract

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.