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Zero‐gradient‐excitation ramped hybrid encoding (zG RF ‐RHE) sodium MRI
Author(s) -
Blunck Yasmin,
Moffat Bradford A.,
Kolbe Scott C.,
Ordidge Roger J.,
Cleary Jon O.,
Johnston Leigh A.
Publication year - 2019
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.27484
Subject(s) - flip angle , excitation , imaging phantom , signal (programming language) , encoding (memory) , nuclear magnetic resonance , physics , artifact (error) , materials science , optics , computer science , magnetic resonance imaging , artificial intelligence , medicine , quantum mechanics , radiology , programming language
Purpose Fast bi‐exponential transverse signal decay compounds sodium image quality. This work aims at enhancing image characteristics using a special case of ramped hybrid encoding (RHE). Zero‐gradient‐excitation (zG RF )‐RHE provides (1) gradient‐free excitation for high flip angle, artifact‐free excitation profiles and (2) gradient ramping during dead‐time for the optimization of encoding time (t enc ) to reduce T 2 * signal decay influence during acquisition. Methods Radial zG RF ‐RHE and standard radial UTE were investigated over a range of receiver bandwidths in simulations, phantom and in vivo brain experiments. Central k‐space in zG RF ‐RHE was acquired through single point measurements at the minimum achievable TE. T 2 * blurring artifacts and image SNR and CNR were assessed. Results zG RF ‐RHE enabled 90° flip angle artifact‐free excitation, whereas gradient pre‐ramping provided greater t enc efficiency for any readout bandwidths. Experiments confirmed simulation results, revealing sharper edge characteristics particularly at short readout durations (T RO ). Significant SNR improvements of up to 4.8% were observed for longer T RO . Conclusion zG RF ‐RHE allows for artifact‐free high flip angle excitation with time‐efficient encoding improving on image characteristics. This hybrid encoding concept with gradient pre‐ramping is trajectory independent and can be introduced in any center‐out UTE trajectory design.