Zero‐gradient‐excitation ramped hybrid encoding (zG_RF‐RHE) sodium MRI

Abstract

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 (zGRF)‐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 (tenc) to reduce T2* signal decay influence during acquisition. Methods Radial zGRF‐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 zGRF‐RHE was acquired through single point measurements at the minimum achievable TE. T2* blurring artifacts and image SNR and CNR were assessed. Results zGRF ‐RHE enabled 90° flip angle artifact‐free excitation, whereas gradient pre‐ramping provided greater tenc efficiency for any readout bandwidths. Experiments confirmed simulation results, revealing sharper edge characteristics particularly at short readout durations (TRO). Significant SNR improvements of up to 4.8% were observed for longer TRO. Conclusion zGRF ‐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.

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